Production aspects of maize + sorghum intercropping systems in Central America


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Production aspects of maize + sorghum intercropping systems in Central America
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vii, 238 leaves : ill. ; 28 cm.
Arias Milla, Francisco Roberto, 1948-
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Intercropping -- Central America   ( lcsh )
Intercropping -- Nicaragua   ( lcsh )
Corn -- Central America   ( lcsh )
Sorghum -- Central America   ( lcsh )
Agronomy thesis Ph. D
Dissertations, Academic -- Agronomy -- UF
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )


Thesis (Ph. D.)--University of Florida, 1985.
Includes bibliographical references (leaves 223-237).
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Also available online.
Statement of Responsibility:
by Francisco Roberto Arias Milla.
General Note:
General Note:

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University of Florida
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I express sincere gratitude to Dr. Raymond Gallaher, chairman of

the supervisory committee. His guidance, dedication, and continuous

encouragement were valuable throughout my graduate program. I also thank

Dr. Victor E. Green, Dr. Clift Taylor, Dr. Mary Collins, and Dr. Maxie

McGhee for their teachings and guidance in the preparation of this


My studies would have been impossible without the financial

assistance of the Centro Agronomico Tropical de Investigacion y

Ensenanza (CATIE) and the W. K. Kellogg Foundation, to the staff members

of which I express my sincere gratitude.

Thanks are also due to Mr. David Block for his assistance in the

statistical analysis of the data, and to Mr. Jacobo Reyes Palma for his

valuable assistance in conducting the field work. Acknowledgment is due

to Mrs Beneranda Arias and Mrs. Oliviethe Ortiz for their assistance in

the tissue analysis. Words of gratitude are expressed to Dr. Raul Moreno

for his encouragement, and friendship.

To my mother, for her never ending sacrifices, her love, and

prayers, I owe much gratitude. I also thank my brothers Julio, Marina,

and Guillermo who have also contributed greatly towards the fulfillment

of my goals in life. To my wife, Beneranda, I express my gratitude for

her moral support, patience, and encouragement. I thank Roberto,

Liliana, and Veronica, without whose motivation and love this task would

have been less bearable.


ACKNOWLEDGMENTS . . . . . . . . . . . . ..

ABSTRACT . . . . . . . ..............................

CHAPTER 1. INTRODUCTION . . . . . . . . . . .

CHAPTER 2. LITERATURE REVIEW . . . . . . . . . ..

Growth . . . . . . . . . . . ............
Crop Growth Rate . . . . . . . o . . . .
Factors That Affect Growth . . . . . . . . ..
Leaf Area Index . . . . . . . . . . . .
Dry Matter Accumulation . . . . . . .........

Forage Quality . . . . . . . . . . .........
Crop Residues . . . . . . . . . .........
Energy . . . . . . . . . . .............







Nutrition . . . . . .. ....
Critical Levels . . . . .
Factors That Affect Concentration .
Nutrient Accumulation . . . .

Sulfur . . . . . . .
Importance of S . . . . .
Forms and Amount of S in the Soil
Sulfur Deficiency in Soils . .
Plant's Requirements and Content.
Absorption and Accumulation of S.
Effects of S Deficiency . . .
Interaction Between S and Other Nt
Crop Response to S Fertilizer .

* * *
* * *
* * *
* * *
* * *
* * *
* * *
itrients .
* * *


Field Procedures . . . . . . . . . . . .
Fertility Trials . . . . . . . . . . .
Growth Analysis . . . . . . . . . . . .
Survey of Sulfur Deficiency in Maize . . . . . ..

Laboratory Procedures . . . . . . . . . . .
Soil Analysis Methods . . . . . . . . . .
Plant Analysis Methods . . . . . . . . ..

SITUATIONAL ANALYSIS . . . . . . . . .. 78

Introduction . . . . . . . . . . . .. 78
Materials and Methods . . . . . . . . . ... . 80
Results and Discussion . . . . . . . . . .. 81
Bio-Physical Environment . . . . . . . . .... 84
Socio-Economic Environment . . . . . . . ... 87
Crop/Animal Interactions . . . . . . . . ... 103
Constraints .. ..... . . . . . . . . . . 105
Research Opportunities . . . . . . . . ... 107
Interventions . . . . . . . . . . . .. 108


Introduction . . . . . . . . . . . . .. 109
Materials and Methods . . . . . . . . . . .. II.I
Results and Discussion . . . . . . . . . .. 113
Percent Soil Moisture . . . . . . . . . .. 113
Dry Matter Accumulation . . . . . . . . .. 118
Leaf Area Index and Other Plant Characteristics . . .. .130

+ MILLET SYSTEMS . . . . . . . . . .. 131

Introduction . . . . . . . . . . . . . 131
Materials and Methods . . . . . . . . . ... 134
Field Procedures . . . . . . . . . . .. 134
Laboratory Procedures . . . . . . . . . .. 136
Results and Discussion . . . .. .................... 137
Percent Organic Matter, IVOMD, Metabolizable Energy,
and Nitrogen . . . . . . . . . . . .. 137
Phosphorus, K, Ca and Mg Accumulation . . . . . .. .157
Iron, Cu, Mn and Zn Accumulation and Distribution . . .. .174


Introduction . . . . . . . . . . . . .. 187
Materials and Methods . . . . . . . . . ... 191
Field Methods . . . . . . . . . . . .. 191
Laboratory Procedures . . . . . . . . . .. 193
Results and Discussion . . . . . . . . . .. 195
Experiment I . . . . . . . . . . . . 195
Experiment 2 . . . . . . . . . . . .. 205

CHAPTER 8. SUMMARY AND CONCLUSIONS . . . . . . . ... 220

BIBLIOGRAPHY . . . . . . . . . . . . . .. 223

BIOGRAPHICAL SKETCH . . . . . . . . . . . ... 238

Abstract of Dissertation Presented to the Graduate School of the
University of Florida in Partial Fulfillment of the Requirements for the
Degree of Doctor of Philosophy



Francisco Roberto Arias Milla

August 1985

Chairman: Raymond Noel Gallaher
Major Department: Agronomy

Farmers in the semi-arid areas of Central America have developed a

maize (Zea mays) + sorghum (Sorghum bicolor (L.) Moench)-animal mixed

production system in response to resource availability and family food

needs. The objectives of this study were to a) describe the maize + sor-

ghum system in its bio-physical and socio-economic environment, b) study

the relationship between soil moisture and dry matter accumulation, c)

describe dry matter, energy, and nutrient accumulation by the system,

and d) determine if S deficiency is a widespread problem in areas where

the system is used.

From a situational analysis it was found that marginal soils, ir-

regular rainfall pattern, lack of appropriate technology, and limited

resources are characteristics of farms in the areas where the system is

practiced. During a growing season a farmer on a 7 ha farm may invest up

to $200 (US), mainly on fertilizers (70% of the total cash investment)

and $45 (US) on animal feeds.

This research showed that late planting coupled with inadequate

soil moisture resulted in poor growth. Results suggest that grain yield

was more susceptible to water stress than total dry matter yield. The

highest crop growth rate observed (756 kg ha day ) was in maize plants

from the maize + millet system. Striking differences in dry matter

distribution were observed between the photosensitive and non-

photosensitive sorghums. At grain harvest, both sorghums had accumu-

lated similar amounts of dry matter (14.3 and 14.9 Mg ha respective-

ly). However, the distribution of the dry matter in the stems, leaves,

and heads of photosensitive sorghum and the non-photosensitive was 46,

37, 17 and 28, 17, and 55%, respectively.

Water stress affected IVOMD and nutrient concentration in all

crops. In the photosensitive sorghum, the stem had higher IVOMD than any

other plant component. In general, nutrient concentration declined with

maturity. However, leaf Ca increased with maturity. An imbalance between

K:Ca, K:Mg, and K:Ca+Mg was observed in maize plants from all systems.

Sulfur deficiency is a widespread problem in some areas of Nic-

aragua and Florida. Stunted chlorotic plants observed in both areas were

deficient in S and had a N:S and P:S imbalance. Sulfur deficiency appar-

ently caused maize leaves to be deficient in K, resulting in a K:Ca+Mg

imbalance, even though sufficient K was indicated in the whole plant



Historically, increases in production have been brought forth by

increasing the amount of land under cultivation or by increasing unit

yields of existing hectares through improved technology. These

constitute what Sanchez (1976) has called the area and space dimensions,

respectively. Except where limited by soil moisture supply or altitude,

the growing season in tropical latitudes is infinite and multiple

cropping systems have been used for centuries. Most low-income farmers

in the tropics practice cropping systems (i.e. intercropping, relay

and/or sequential cropping) that intensify production not only in space

but also in time.

Intercropping has been an important practice in many parts of the :

world, especially in the tropics, and there is little doubt that it will

remain so. It is only recently, however, that research has established

that intercropping can give higher yields than growing sole crops.

Wahua and Miller (1978a) explain why intercropping is popular among

small-scale farmers in tropical and sub-tropical environments. Some of

these reasons are built-in balanced nutritional supply of energy and

protein, profit and resource maximization, efficient water and light

utilization, inexpensive weed control, minimization of agricultural

risks, and improvement of soil fertility. Other authors (Mead and

Willey, 1980) consider that low-income farmers need to grow more than

one crop, whether intercropped or not, to spread labor peaks, to reduce


marketing risks, and to satisfy different dietary needs. Small-

scale farmers in Central America comprise the most important basic

food production group. They use production systems frequently referred

to as traditional in the literature (Rodriguez et al., 1977; Arias et

al., 1980; Rosales, 1980; Mateo et al., 1981). These systems have been

developed through the interaction of man and his environment, under

conditions involving high risk and restraints which limit crop


Several cropping systems can be found in the dry areas of the

Isthmus. Maize (Zea mays L.) intercropped with sorghum (Sorghum bicolor

(L.) Moench) (maize + sorghum) with varying degrees of competition in

time and space, is the most widely practiced cropping system in Central

America (Hawkins et al., 1983; Larios et al., 1983). Other relevant

systems include mono or relay crops of maize, or bean (Phaseolus spp.)

or cowpea (Vigna spp.); sorghum intercropped with bean, sesame (Sesamum

indicum L.); or sisal (Agave spp.) intercropped with maize, sorghum,

and/or bean.

It is accepted (De Wit and Hollman, 1970; Quimby, 1974) that

sorghum was introduced to the western hemisphere by African slaves and

Portuguese sailors in the 16th century. According to Martin (1975) it

was not until the 19th century that sorghum gained importance in the

United States. It has not been documented in the literature when the

system was first used in Central America, but it cannot be more than 350

years ago.

Agricultural research in the tropics has been conditioned by the

cropping systems of more developed areas, the origin of most research

workers, and until recently, rather cursory attention has been paid to

indigenous systems. Baker (1979) emphasizes that attempts to improve

production in the tropics have failed not because of farmer conservatism

or the lack of extension programs, but because it has not been realized

that subsistence agriculture is a tropical agro-ecosystem geared for a

low level of production and not simply a collection of crops and

animals to which inputs can be applied indiscriminately to obtain

immediate results.

A lack of appreciation of the personal nature of the subsistence

farmers' farming systems has resulted in a tendency for research to

produce solutions to the wrong problems. For example, agricultural

research in Central America has traditionally been dedicated to export

crops such as banana (Musa spp.) and coffee (Coffea spp.), while most of

the available technology for increasing basic food crop productivity has

been adapted from temperate countries and thus has generally been

developed for sole cropping.

Mateo et al. (1981) and Larios et al.(1983) have pointed out that

in spite of the economic importance of the maize + sorghum this system

has received little attention in the work plans of local research

institutions. Reports in the literature (Arias et al., 1980; Mateo et

al., 1981; Fuentes and Salguero, 1983) indicate that of the total area

cultivated with sorghum in Guatemala, El Salvador, and Honduras, the

percentages intercropped with maize are, respectively, 80, 93 and 93.

Larios et al. (1983), concluded that knowledge of the process of

integrating component technologies into farming systems is lacking,

especially for small-scale farms. Due to the lack of available

technology adapted to conditions prevailing in the semi-arid regions of

Central America it is mandatory to conduct field experimentation that

will alleviate this deficiency. A summary of the scheme presented by

Arze et al. (1983) to accomplish this task follows:

1. Identify important cropping systems used in the semi-arid

regions of Central America.

2. Study the performance of the cropping patterns under different

environmental variables.

3. Determine the degree and form of relationship among these


4. Use the previous information to maintain, arrange or re-design

the system so that it operates optimally with respect to its objectives.

Several authors (Rodriguez et al., 1977, Arias et al., 1980; Mateo

et al., 1981; Guzman, 1982; Fuentes and Salguero, 1983; Hawkins et al.,

1983; Larios et al., 1983) have identified environmental stresses

limiting crop productivity in the semi-arid regions of Central America,

their listings including drought, nutrient deficiencies, particularly N,

P and S (CATIE, 1980, 1982a; Rico, 1982; Hawkins et al., 1983), and

water and wind erosion. These areas are primary sources of cereal

grains, oilseed crops, fruits, vegetables, table legumes, meats, and

dairy products for a large majority of the rural and urban population of

the Isthmus. Basic research projects to overcome environmental stress

limitations have been notably successful in many crops and can provide

data for future significant progress with adaptive research.

Maize + sorghum was selected as the subject of this research

project because of its importance in food production. The maize +

sorghum cropping system is predominant in semi-arid areas of Central

America at elevations below 1,000 m. The system apparently increases

the productivity and reduces risk of loss in areas with marginal

conditions for maize production. Shallow soils and variability of

rainfall make failure of the maize crop more common than failure of the

sorghum crop, which is of lower value to the farmer for both sale and


Present fertilizer use is oriented toward maize production, while

sorghum mostly is dependent on native soil fertility and residual

fertilizers applied to the previous crop. Whether any of the fertilizer

applied to the maize reaches the sorghum will perhaps be clarified by

the present experiments. Presently, few farmers apply any fertilizer to

the sorghum, despite the fact that under climatic constraints in which

they are operating, sorghum or millet has as much or higher yield

potential than maize.

The general objectives of this research were 1) to provide basic

information, 2) to improve traditional cropping systems, 3) to develop

new systems adapted to prevalent bio-physical conditions, and 4) to

apply new discoveries that will increase net family income and that are

appropriate to the economic resources available to farmers of the

semi-arid regions of Central America.

The specific objectives of this study are 1) to describe the maize

+ sorghum system in its bio-physical and socio-economic environment in

Central America, 2) to describe the growth pattern of the maize +

sorghum system and of potential substitutes, 3) to describe the pattern

of energy and nutrient accumulation of the system and of potential

substitutes, 4) to determine if variation of other components of the

system, such as substituting the traditional photo-sensitive sorghum

with improved non-photosensitive cultivars or millet (Pennisetum

americanum (L.) Leeke), will increase productivity, and fertilizer and


water use efficiency; 5) to study the relation between gravimetric soil

moisture and dry matter accumulation by the maize + sorghum, maize +

millet systems; 6) to determine the existence of soil S deficiency in

areas where the system is practiced in Nicaragua.



Crop Growth Rate

Goldsworthy and Colegrove (1974) found that crop growth rates (CGR)

in maize (Zea mays L.) declined rapidly when grain growth commenced. As

grain growth increased towards a maximum, about 100-110 days after sowing,

CGR decreased to near zero values. There are two possible explanations

for this pattern of change in dry weight. First, it is possible that a

large part of grain dry matter is derived from assimilates which

accumulate in plant parts other than grain and are then translocated to

the grain. Second, if as reported by Allison and Watson (1966) and by

Palmer et al. (1973) the dry matter that fills the grain is derived from

current assimilation, then presumably the large loss in weight from other

parts of the plant, mainly the stem, represents respiration losses that

are not replaced by current assimilation.

The rates of dry-weight production (500 kg ha week ) per unit leaf

area reported by Goldsworthy and Colegrove (1974) were high at silking.

This, combined with the large leaf areas, accounted for the peak growth

rates observed. The rapid decline in CGR after silking was related to the

simultaneous and rapid decline in leaf area and net assimilation rate.


Vanderlip and Reeves (1972) have shown that during grain filling

there is a net reduction in stem weight, with grain accumulation occurring

at a greater rate than the rate of total dry matter accumulation.As the

grain approaches physiological maturity the stem again increases in


Goldsworthy (1970) concluded that a decrease in radiation and a

loss in dry weight from decay and detachment of dead sorghum (Sorghum

bicolor (L.) Moench) leaves were probably the most important of the

factors that contributed to the decrease in growth rate observed in the

middle of the season. Since net-assimilation rate is also dependent on

leaf area index (LAI), lower leaves were probably making little or no

contribution to dry weight increase at this time. A sharp decline in leaf

area and in net-assimilation rate accounts for the rapid fall in crop

growth rate of the non-photosensitive sorghum after heading, at the end of


Goldsworthy (1970) reported that a large proportion of the increase

in total dry weight of sorghum 'Farafara' (a photosensitive cultivar)

after heading was as dry weight in the stems. During the 3 weeks before

harvest (22-25 weeks after sowing), stems lost weight, and the losses in

weight were similar to the weights gained after head emergence. In

contrast, virtually all of the dry weight increase by 'NK-300' (a

non-photosensitive cultivar) after head emergence was in the heads. In

the first year of his study the weight of stems of the 'NK-300' reached a

maximum at heading and then remained constant until harvest, whereas in

the second year the stems lost weight before harvest, and it is probable

that, with a smaller supply of assimilate, carbohydrate normally respired


in the stem was diverted to the head and that this accounts for the loss

in stem weight in this instance.

Apparent photosynthesis was closely correlated with dry matter

accumulation by the shoots during grain fill. Consequently, the progress

of photosynthate accumulation can probably be viewed as the progress of

dry matter accumulation. Translocation was less inhibited than dry matter

accumulation or apparent photosynthesis under dry conditions. As a

result, grain yield, while significantly inhibited, was probably less

inhibited than it would have been if the translocation of reserves had not


Other authors have shown that maize stems often lose dry matter as

the grain matures (Johnson et al.,1966; Daynard et al., 1969; Hume and

Campbell, 1972), particularly when the environment becomes unfavorable

during grain fill. The dry weight appears to be soluble carbohydrates

that can be stored in the stem (Daynard et al., 1969; Hume and Campbell,

1972). This mobilization of stem reserves has not been observed in every

instance (Hanway, 1962a). This agrees with the conclusion by Duncan et

al. (1965) that even relatively short-term adjustment by stem reserves may

enable the maintenance of a high rate of grain filling while the

conditions for photosynthesis are temporarily unfavorable. Thus, in maize

stem, mobilization probably occurs when sink demand exceeds source


It has been shown that the dry matter stored in the grain of

sorghum (Stickler et al., 1961b; Goldsworthy, 1970), in the grain of maize

(Allison and Watson, 1966), and in rice (Enyi, 1962) is derived mainly

from assimilates produced after head emergence so that grain yield is

directly related to leaf area after the ears emerge. However, McPherson


and Boyer (1977) concluded that since grain yield was in excess of the

photosynthesis occurring during grain development, yield must have

reflected the amount of photosynthate accumulated by the crop over a

larger portion of the growing season than the grain filling period alone.

This is generally interpreted to mean (Yoshida, 1972) that dry matter

accumulated by the shoots determines yield.

Factors That Affect Growth

Environmental factors

Maize plants, wherever they are grown, should progress through all

the stages of development described by Hanway (1963). However, the

length of time between the stages and the identifying characteristics may

differ for different hybrids and for different environmental conditions.

Shaw and Thorne (1951) reported that the elapsed time between plant

emergence, stage 0, and silking, stage 5, is variable, and Tyner (1946)

reported that the period from silking to physiological maturity appears to

be relatively constant for different hybrids and different environmental

conditions. The intervals between the intermediate stages of growth

differ for different hybrids and with differences in environmental

conditions, especially temperature and fertilization (Hanway, 1963).

Sivakumar et al. (1979) suggested that plant growth is the result

of an effective integration of many factors. Restriction of growth may

occur due to the limitation of any one factor; for example, water deficits

in plants generally lead to reduced leaf water potentials and stomatal

closure, as manifested from an increased leaf resistance to transpiration.

The effects of depletion and replenishment of soil water on transpiration

are of specific importance to water use and its efficiency in crop

production. The relative rates of absorption and transpiration determine


a plant's internal water balance, which directly affects the physiological

and biochemical process of plant growth.

Plant nutrition

Hanway (1962b) indicated that variations in light, moisture, and

many other factors cause fluctuations in the growth rate. The growth rate

of maize under N sufficient conditions was 250 kg ha-1 day while the

growth rate for maize on the extremely N-deficient continuous maize plot

was much less (84 kg haI day- ). The fertility differences did not

markedly alter the relative proportion of each plant part.

Goldsworthy and Colegrove (1974) found production of dry weight

after silking to be related to the amount and duration of leaf area after

silking and to the efficiency of the leaf area. The ratio of grain weight

to leaf-area duration per unit area of land after heading is an index of

the efficiency with which the leaf area present after heading produces dry

matter for the grain (Watson et al., 1963).

Hanway (1962a) observed that extreme N and K deficiencies result in

premature death of several lower leaves. This shortens the period over

which these leaves carry on photosynthesis. In any case, the primary

effect of nutrient deficiencies appears to be on the amount of leaves

produced rather than on the net assimilation rate (rate of increase of dry

weight per unit leaf area).

Data presented by Hanway (1962a) suggested that although N, P, and

K concentration of maize leaves at the beginning and end of the grain

formation period varied markedly, the chemical composition of the leaves

had very little effect upon the rate of photosynthesis in the leaves.

However, extreme N and K deficiencies were observed to result in premature

death of several lower leaves on some of these plants. Nutrient


deficiencies are reflected in both leaf area and the chemical composition

of the leaves. The chemical composition of the leaves at silking time can

indicate which nutrient elements are deficient and which deficiencies have

resulted or will result in a reduced leaf area and, thereby, a reduction

in grain yield. These leaf analyses are very valuable diagnostic tools,

but it appears that their interpretation should be based upon their

relation to leaf area and not to net assimilation rate.

According to Nelson (1956) many investigators have found high

positive correlation between the percentages of N, P, and K in maize

leaves at silking time and the yield of grain. Nutrient deficiencies are

reflected in both leaf area and in the chemical composition of the leaves.

Thus, while grain yield is primarily a function of leaf area, leaf area

is a function of the nutrient status of the plant which is reflected in

the chemical composition of the leaves.

Maize under low fertility generally silks later and forms the

black layer earlier, resulting in rather large differences in filling days

and filling degree-days. Peaslee et al. (1971) found that P may tend to

shorten the grain filling period by accelerating the development of the

grain to the maturity stage. This tendency was apparently counteracted by

the tendency of P to also accelerate development to the grain initiation

stage. Earlier silking and lower moisture contents of the grain at

harvest were associated with additions of P fertilizer. However, these

were cases in which the levels of P in the soil were low and plant growth

response to P was marked. Peaslee et al. (1971) concluded that either P

or K was directly responsible for early initiation of the ear and/or delay

of the black layer formation by some specific function or they indirectly


influence plant development through the quantity of soluble carbohydrates

present in plants and their transformation into grain.


Almost every plant process is affected directly or indirectly by

water deficits. Some processes are quite sensitive to water stress, but

others are relatively insensitive. When plants are subjected to water

stress there is a decrease in photosynthesis and cell enlargement. There

is also considerable retention of carbohydrates in photosynthetic tissues.

Although translocation proceeds, its rate is reduced. Translocation is

rarely mentioned as a factor in reduced plant growth under limited

moisture. Translocation could be one of the chief physiological factors

limiting growth under unfavorable moisture conditions.

Shaw (1974) found that experimental maize yields were highly

correlated with a plant moisture stress index which was based on

calculations of daily ratios of actual to potential evapotranspiration for

the period of 40 days before to 45 days after silking. Although soil

moisture strongly interacts with temperature in plant growth processes, it

is much less important in plant phasic development.

Sivakumar and Shaw (1978) reported that the major components of

sorghum yield which were significantly affected by drought in the case of

the nonirrigated plots were tertiary branches per secondaries, seed number

per panicle, and seed size. The reduction in these components was 46, 26,

and 28%, respectively. Data presented in this study bring out the

importance of the availability of a few additional cm of water to a

sorghum crop under water stress and the benefits that should accrue from

such water applications.


The depressive effect of water stress on photosynthate

translocation reported by Brevedan and Hodges (1973) is in agreement with

the observation of several other authors (Hartt, 1969; Plaut and Reinhold,

1965; Wardlaw, 1967). Wardlaw (1967) found a continued movement of

assimilates from the leaf to the developing wheat (Triticum aestivum L.)

grain under water stress conditions. He also observed a lower velocity of

sugar transport from leaves of stressed plants than from well-watered


Data presented by Denmead and Shaw (1960) suggested that lower

assimilation in plants subjected to stress is partly due to smaller leaf

area, as indicated by the size of the ear leaf, and partly due to the

metabolic activity of the plants at different growth stages. When

compared with the reduction in assimilation caused by stress at other

growth stages, the reduction in assimilation resulting from stress at

silking is larger than the reduction in leaf area could indicate. The

data also suggested that there may be a tendency for recurring periods of

stress to have less and less detrimental effect on assimilation and yield.

Stress applied while the plant is actively expanding retards enlargement

of plant parts. Recovery when the stress is removed is not immediate but

growth rate appears to return to normal after a few days.

McPherson and Boyer (1977) reported that the physiological

mechanisms responsible for yield losses under dry conditions are unknown,

especially for grain crops. Drought causes massive losses in the yield of

crops, but the physiological mechanisms responsible for decreased yields

are poorly understood. Most aspects of the physiological behavior of

plants are known to be altered by the onset of dry conditions (Boyer,

1973; Hsiao, 1973). But photosynthesis and translocation, which are


important in grain crops, are especially sensitive (Hsiao, 1973; Boyer and

McPherson, 1975; Boyer 1976).

It is generally accepted that optimized grain filling requires

continued dry matter production and translocation of the product to the

grain. However, Brevedan and Hodges (1973) concluded that translocation

was more sensitive than photosynthesis to drought. Wardlaw (1967) studied

desiccated wheat during the grain filling stage and found that movement of

assimilates into conducting tissue was delayed in wilted leaves, but the

velocity of translocation was relatively unaffected.

Barlow and Boersma (1976) found that the partitioning of the total

dry matter into grain was affected by desiccation. In the controls, the

grain dry matter was only 63 to 76% of the total dry matter accumulated by

the shoots during grain fill. In the desiccated plants, however, grain

dry matter was 50% larger than the total dry matter accumulated during

grain fill. Thus, in the desiccated plants, grain development must have

occurred at the expense of dry matter stored in other parts of the plants.

Jurgens et al. (1978) concluded that while grain fill was seriously

inhibited by desiccation (grain yield was reduced to 42% of the control),

it was clearly maintained above that expected from dry matter accumulation

during the grain filling period. This occurred because translocation to

the grain continued at a modest rate even when there was no net

accumulation of photosynthetic material by the desiccated plants. Thus,

under field conditions photosynthesis was more affected than translocation

during desiccation. It appears that grain crops having the opportunity to

accumulate reserves under favorable moisture conditions are able to

preserve grain development if conditions later become unfavorable.


Assimilation after ear emergence, both in the leaves and in the ear

itself, is primarily responsible for accumulation of material in the grain.

Early stress, then, has an indirect effect on yield of grain through

reducing the size of the assimilatory surface at the time of ear

development. Stress imposed after the ear has emerged has a more direct

effect through reducing assimilation in this critical period when daily

assimilation rates are high and most of the assimilates are being used for

grain production. The relatively small effect of stress during the ear

stage suggests that the critical period would not extend longer than about

3 weeks after 75% silking.

Jurgens et al. (1978) found that the induction of low leaf water

potentials in desiccated plants caused an almost immediate decrease in

viable LAI. At maturity it was evident that the prolonged drought

treatment had markedly affected grain components. Yield, kernel weight,

and percent oil were all substantially reduced by desiccation while

percent protein increased. Grain production was relatively closely tied

to the total dry matter production for the season. In the controls grain

production was 51% the total dry matter production, and in desiccated

plants it was 39%. The desiccated plants produced less dry matter and

exhibited a slower gain in grain dry weight than the controls.

Reddy and Willey (1981) reported that the total water use

(i.e.transpiration plus evaporation from the soil surface) by sole millet

and sole groundnut over their full growing periods were 30.3 and 36.8 cm,

respectively. The total water use of 40.6 cm by the intercrop was greater

than either sole crop but it was 11% less than the total water use

expected if each component had used water at its sole crop efficiency.

Thus, the 28% higher dry matter yield of the intercrop could only be


partly explained on the basis of greater total water use and it must have

been partly due to an increase in total water use efficiency.

In an irrigation study Sivakumar et al. (1979) found that the

non-irrigated sorghum crop used 213 mm of water to produce 510 kg ha1 of

dry matter, whereas, the irrigated sorghum used 321 mm of water to produce

930 kg haI of dry matter. Szeicz et al. (1973) observed that average

sorghum crops in Texas use approximately 320 mm of water throughout the
growing season to produce around 0.8 kg dry matter m

Leaf Area Index

Dale et al. (1980) reported that growth and duration of green leaf

area of a crop determine the percentage of incident solar radiation

intercepted by the crop canopy and thereby influence canopy

photosynthesis, evapotranspiration, and final yields. Leaf area index is

defined as the ratio of the area of one side of the green leaves of a

plant to the area of soil surface allocated to the plant.

Shih et al. (1981) reported that LAI is often used as an indicator

of plant growth and for evaluating assimilation and transpiration rates in

plant physiological studies. This growth parameter is also frequently

used in agronomic studies to model yield and to make crop production

decisions. Ashley et al. (1965) found good correlation between LAI and

leaf dry weight of cotton (Gossypium hirsutum L.). Rhoads and Bloodworth

(1964) and Pearce et al. (1965) also found strong correlations between

leaf area and dry matter yield in cotton and orchard grass (Dactylis

glomerata L.).

According to Hanway (1963), the genetic characteristics of the

plant, day length and various environmental conditions prior to stage 1 of

growth, have determined the number of leaves that will develop on maize


plants. He also observed that except under very carefully controlled

conditions in fields or in experimental plots there will be differences in

growth between different plants that have received the same treatment.

The number of leaves per plant on plants of the same hybrid grown in the

same plot varied between 15 and 22 leaves per plant. All plants in a

given plot will develop at the same rate, so not all plants in a plot are

at the same stage of development at the same time (Hanway, 1963).

Hanway (1962a) suggested that the dry weight of the entire plant

and of the grain are directly related to and highly correlated with the

weights of the leaves in these plants. Since leaf growth in maize is

completed relatively early in the season, the linear rate of dry matter

accumulation over a major part of the growing season appears reasonable,

unless the net assimilation rate decreases with age of the leaves or

decreases with the seasonal trend in climatic conditions.

Hanway's (1962a) study of maize growth as related to soil fertility

showed that approximately 30% of the total leaf weight had been produced

45 days after planting and that during the following two weeks leaf growth

was very rapid and by 60 days after planting over 85% of the final leaf

weight had been produced. This would appear to be a critical stage in

leaf development when nutrient deficiencies might reduce the final weight

of leaves. Most of the grain was produced in a 30 to 35 day period. This

growth pattern was similar for plants from all the fertility levels except

that the silking and subsequent growth of the cob, shank, and grain was

delayed slightly in the N-deficient plants from the continuous maize plot.

Shih et al. (1981) reported that leaf area per stalk declined after

stalk length reached about 185 cm. This is consistent with the general


pattern of leaf area in sweet sorghum which starts declining after the

plant approaches 50% bloom.

Dry Matter Accumulation

Hanway (1962a) reported that the potential yield of maize grain

which is produced late in the season is determined by the leaf area, which

is always produced early in the season. However, less than this potential

yield of grain will actually be attained if a) the net assimilation rate

is decreased by any factor such as a moisture deficiency later in the

season or b) the leaf area is prematurely reduced by some factor that

results in premature death of leaves such as a nutrient deficiency or

insect, disease, or hail damage. If no other factor limits yield, one

would expect that increasing LAI should result in increased grain yield.

Data reported by Tollenaar and Daynard (1978) showed that grain

yield per plant was most affected by shading during the silking period,

indicating a sink limitation for grain yield in hot environments. A

source limitation in short-season regions may be the consequence of low

leaf area per plant.

Hanway and Russell (1969) reported relatively large differences

among maize hybrids in the length of the grain filling period. Daynard

and Duncan (1969) have observed such differences among hybrids and have

found a high correlation between the length of the grain filling period

and the yield.

The patterns of growth and dry matter distribution observed in

tropical cultivars grown in Mexico (Goldsworthy and Colegrove, 1974)

suggest that the capacity of the grain sink to accommodate assimilate can

limit grain production. Results of defoliation studies in Rhodesia

(Allison and Watson, 1966), which showed that a relatively large amount of


dry matter which could be translocated to the grain normally remains in

the stem, also indicate a sink limitation. Conversely, the decrease in

stem weight in the latter part of the grain-filling period, observed by

Daynard et al. (1969), seems to indicate that source may be limiting under

the environmental conditions prevailing in the northern periphery of the

maize-growing area of North America.

McPherson and Boyer (1977) pointed out that another potentially

more serious problem occurs if sink size has been affected by low leaf

water potential. If, for example, crop desiccation occurred during floral

development or pollination, irreversible loss of floral primordia (Moss

and Downey, 1971) or unsuccessful pollination could result. Thus, grain

yield would be limited more by the availability of developing grain than

by the availability of photosynthate (Denmead and Shaw, 1960; Classen and

Shaw, 1970), and prediction of yield based on photosynthesis would be in

error. Such a limitation would be less important where maize is grown for

silage, because shoot dry weight would reflect photosynthesis directly.

Moss (1962) and Allison and Watson (1966) have shown that when the

grain sink is missing, dry matter that would have passed to the grain

accumulates in the stem and husk. The presence of more barren plants

(Goldsworthy et al., 1974) probably explains why more dry weight

accumulated in the stems at Poza Rica than at Tlaltizapan, Mexico. It

would also account for the differences in the values for grain at the two

sites, since barren plants contribute to dry matter but not to grain


Goldsworthy et al. (1974) reported that dry weight also accumulated

in the stem of fertile plants and that the capacity of the ear to

accommodate the photosynthate produced was a further factor limiting


yield. Allison and Watson (1966) have shown that when the grain sink is

removed by preventing pollination the dry matter that would have passed to

the grain accumulates in the stover and that when the source of assimilate

is restricted by removing leaves, stem weight decreases as previously

stored dry matter moves to the grain.

Shih et al. (1981) found that the ratios between dry and fresh

phytomass were computed as 21.4, 35.1 and 35.4% for stalk, leaf, and

panicle, respectively. The ratios for leaves and panicles are similar,

but the ratio for stalks is lower. These ratios can be used to estimate

the dry weight yield based on the known fresh biomass, or vice versa.

Hanway (1962a) found that differences in soil fertility resulted in

different rates of dry matter accumulation but did not markedly influence

the relative proportions of the different parts. He reported crop growth

rates of maize of 250 kg ha dayI in N-sufficient plots. Hanway and

Russell (1969) found that the leaves, leaf sheaths, stalks, and husks of

maize attained their final mature weights at about stages 4.0, 4.5, 5.0,

and 5.5, respectively. All these plant parts continued to increase in

weight following these stages of development, but this continued increase

in weight was apparently an accumulation of materials which were later

translocated into the developing grain. At stage 6.5 the total

accumulated material in these plant parts was equivalent to about 20% of

the total mature dry weight of the nongrain parts of the plants. The cob

and ear shank attained their maximum dry weight at about stage 6.5 and

showed no later decrease in weight. An average of 42% of the total,

mature dry weights of the plants was grain and 58% was nongrain. However,

the relative proportions of grain and nongrain varied widely among the

different hybrids, years, and plant populations, with the grain varying


from 35 to 52% of the total plant weight. The rate of dry matter

accumulation in the grain was similar for all hybrids, years, and plant

populations varying from 163 to 181 kg ha day

Vanderlip and Arkin (1977) reported that on a daily basis, dry

matter in sorghum was allowed to be partitioned to the leaves up to 125%

of that neccesary for the leaf area developed that day. The same authors

found that at least 25% of the daily dry matter production was

partitioned to the roots, which under conditions of low photosynthate

production would cause an increase in specific leaf area.

Barlow and Boersma (1976) demonstrated that even grain dry matter

was only 63 to 76% of the total dry matter accumulated by the shoots

during grain fill. In the desiccated plants, however, grain dry matter

was 50% larger than the total dry matter accumulated and was accompanied

by a reduction in net photosynthate accumulation in the source leaf.

Goldsworthy (1970) reported that photosensitive sorghums sown at

the end of May formed from two to three times as much dry weight as 'NK

300' non-photosensitive which was sown 10 weeks later. The photosensitive

sorghum 'Farafara' had the largest total dry weight but the smallest

grain yield. About 70% of the total dry weight was in the stems and only

from 9 to 13% in the heads. In contrast, the dry weight of 'NK 300' was

only about one-third or one-half that of 'Farafara' but from 40 to 60% of

it was in the heads so that the grain yields were much larger than those

of 'Farafara'.

The reason, it seems, for the low grain yield of photosensitive

sorghum is that a substantial part of the assimilate formed after heading

accumulates in the stem, whereas most all of the dry matter produced

after heading in non-photosensitive moves to the head. The results of


these and of other experiments at Samaru suggest that the main reason for

the difference in yield between the photosensitive sorghum and

non-photosensitive sorghum is in the number of spikelets present at head

emergence; the number and/or potential size of the developing grains in

the photosensitive sorghum appears to be too small. Thus the dry weight

of the non-photosensitive sorghum 'NK 300' was much smaller than that of

the other sorghums mainly because of the inability of the head to accept

all the carbohydrate that the leaves can produce.

Goldsworthy (1970) observed that the heads of non-photosensitive

sorghum continued to gain weight 13 weeks before harvest, but those of

photosensitive sorghum did not and it is more probable that the loss in

weight from the stem was by respiration of labile carbohydrate that had

accumulated there. It may be that the developing head does not receive

the assimilate it needs before emergence and that it is unable to

accommodate all of the increased supply of assimilate from the leaves

after it emerges.

Blum (1970) concluded that when sorghum plants compete for water

the effect of competition on a photosensitive genotype is more severe

than on a non-photosensitive genotype, and that the highest yield is

obtained with an early maturing hybrid planted at relatively high plant

densities. Yielding potential was in direct relationship to duration of

growth under non-competitive conditions and in an inverse relationship

under extreme competition. This indicates that the importance ascribed to

a long duration of growth of cereals with respect to their yield

potentials does not hold under limited water supply. Blum (1970)

concluded that of his experiments demonstrated the superiority of an


early maturing hybrid under extreme plant competition for water is due,

at least partly, to advanced plant suppression by interplant competition.

Forage Quality

Many factors combine to determine the relative feeding value of

sorghum grain. Some of these are differences in tannin content, protein

content, amino acid composition, amount of floury and horny endosperm,

presence or absence of yellow endosperm, whether mixed with grain of

other species, whether processed in one or a number of ways such as

cooking, flaking, and/or steam rolling, whether weathered in the field

before being harvested, amount of damage of insects, presence of

aflatoxins or other molds, amount of rancidity of the oil, degree of

glume and other trash removal, presence of contaminants, and, most

important of all, degree of milling and type of animal to which it is to

be fed. Data presented by Hall et al. (1965) indicate that sorghum grain

is comparable to maize in digestibility of proximate components,

digestible energy, metabolizable energy, and nitrogen retention.

Eng et al. (1965) reported results which indicate wide variations

in the compositions and feeding values of sorghum grains. It appears that

such variations may be caused by geographical area, soil moisture, soil

fertility and variety of plants and might be important considerations in

the apparently divergent results which have been obtained by various

research workers when sorghum grain was compared to maize as an energy

source for fattening cattle. Further research work in this area is


Clark et al. (1965) found little difference in the carrying

capacity, milk production, or dry matter production of pearl millet and a


sorghum x sudangrass (Sorghum sudanense L.) hybrid when utilized as

pasture for lactating dairy cows. Johnson et al. (1966) studying changes

in dry matter and protein distribution in maize found that the protein

concentration of leaves declined rapidly and steadily until the final

mature stage. The protein concentration of the stalks was between 11 and

12% prior to tasseling, declined rapidly until 15 days after tasseling,

and declined only slightly throughout the remainder of ear growth and


Johnson et al. (1966) observed that percent ash, cellulose and

crude protein were significantly decreased with increasing maturity.

Digestibility of dry matter and organic matter was significantly affected

by maturity, increasing to maximum at the dough-dent stage and decreasing

slightly thereafter. Dry matter digestibility was 68% at the earliest

maturity stage. Increasing maturity significantly decreased digestibility

of cellulose and protein throughout the harvest period.

Rendig and Broadbent (1979) observed that concentration of crude

protein ranged from about 6% in maize grain from plots that received no

added N or 90 kg N ha to nearly 10% in plots receiving 180 kg ha -1. The

concentrations in the grain protein of triptophane, lysine, glycine,

arginine, and threonine were decreased, and the concentrations of

analine, phenylalanine, tyrosine, glutamic acid, and leucine increased by

applications of N.

Schmid et al. (1975) observed that cell walls of maize cultivars

were considerably more digestible than those of the sorghum cultivars.

These results indicated that low cell wall digestibility was a major

factor limiting dry matter digestibility of sorghum silages. The same

authors reported that head IVDMD of short sorghum hybrids with high grain


ratios remained constant or increased with advancing maturity, while that

of the tall hybrids with lower grain ratios decreased with maturity. Stem

IVDMD of the tallest hybrids increased with maturity while the other

hybrids decreased with maturity.

Although maturity is a factor affecting sorghum silage nutritive

value, its influence varies depending on forage type in that a decline in

cellulose digestibility with maturity may be detrimental to total

digestibility of low-grain sorghums, but not for those with high-grain

content. Apparently, the rapid increase in the amount of a highly

digestible starch during maturation in the high-grain sorghums

compensates for the decline in cellulose digestion (Schmid et al., 1975).

Cummins (1970) in a two-year study observed that in general the in

vitro dry matter digestibility (IVDMD) of maize leaves decreased with

maturity, although some year-to-year variation occurred. The IVDMD was
negatively correlated with maturity (R =0.67 and 0.88, respectively).

Both IVDMD and carbohydrate content of the stalks were closely related to

the rainfall distribution during the maturity period.

Johnson and McClure (1966) found highest total dry matter yield per

hectare to be between the dent and glaze stage of kernel development.

Although ears constituted 60% of the dry matter of the mature maize

plant, they did not reach this proportion until the dent stage of

development. IVDMD of maize stem cellulose was quite constant during ear

development but digestibility of leaf cellulose declined steadily. The

soluble carbohydrate in maize stem tissue increased rapidly from

tasseling to a maximum in late August (milk stage) and thereafter

declined with maturity. Crude protein content declined steadily in maize

leaves but changed very little in maize stalks from milk stage to final


maturity (Johnson et al., 1966). Protein content in the whole plant

declined slowly during ear maturation.

The IVDMD for the stover silage reported by Colenbrander et al.

(1971) ranged from 41 to 50%. Results from this experiment indicated that

a low quality roughage such as maize stover silage can provide an

alternative source of nutrients for growing dairy heifers. Green (1973),

studying the yield and digestibility of bird-resistant grain sorghum,

observed that IVDMD varied from 79.8 to 50.5%. All of the NBR Varieties

had higher values. The correlation coefficient calculated to ascertain

the relationship between yield and IVDMD for the 41 varieties was low

(r=+0.52) and not significant, indicating they were not related. Schmid

et al. (1975) reported that IVDMD values at the 4-week growth stage

ranged from 71.7 to 84.2% for five brown midrib sorghum mutants compared

to 67.1 to 78.3% for their normal counterparts.

Crop Residues

Plant populations, row-spacings, and soil fertility affect not only

grain yields, but also the yields and quality of residues. Residue yields

from maize are normally greater than from grain sorghum but lower in

crude protein. Crude protein concentration of grain sorghum residues was

consistently higher than that of maize while IVDMD values were

consistently lower in grain sorghum.

Crop residues of grain sorghum and maize have attracted attention

as an alternate economical forage resource for livestock utilization.

There are few data available on agronomic production factors affecting

yield and quality of crop residues (Perry and Olson, 1975).

Perry and Olson (1975), studying the effects of N fertilization on

yield and quality of maize and sorghum residues, observed that grain N


concentration increased with N fertilization in both crops. Grain sorghum

N concentration was generally equal to or greater than that of maize.

Residue yields of both crops were increased significantly by 90 kg ha-

with no further increase at the higher N rate. Maize grain:stover ratios

increased with increasing N levels. Crude protein of grain sorghum

residues was consistently higher than that of maize while IVDMD values

were consistently lower in grain sorghum. Crude protein increased

significantly in grain sorghum residue with each increasing N level while

little increase occurred in maize.

Martin and Wedin (1974) reported that row-crop residues, although

present in great quantities in the midwestern United States, are often

considered to be of low quality. Consequently, grain sorghum stover on

millions of hectares is not utilized by livestock. Stover often remains

as a leafy, succulent, growing plant following grain harvest and should

be considered as a feed source for ruminants being maintained for

reproductive purposes.

Under Iowa conditions (Perry and Olson, 1975) grain sorghum

continues growth during the interval between grain harvest and killing

frost. This unique characteristic allows the ratoon crop to maintain and

perhaps improve its nutritional composition before utilization for winter

grazing or silage harvest. Burns et al. (1970) postulated that forage

sorghum produces photosynthate after maturation of seeds and until frost

and accumulates a reservoir of water-soluble carbohydrates in the pith of

the stalks.

Perry and Olson (1975) observed that maize dry matter yields

decline as much as 30% within 100 days of harvest. Any decline in crude

protein and digestibility following grain harvest of maize and grain


sorghum appears to be associated with environmental factors. Martin and

Wedin (1974) observed that grain sorghum stover lost 28.3% of its

original dry matter yield by 76 days after grain harvest. The leaves lost

dry matter more rapidly.

Any consideration of grain sorghum stover as a feed for ruminants

is dependent on meeting all or some of the animal's nutrient

requirements. For example, a 500-kg, dry pregnant beef cow consuming 7.6

kg of dry matter requires 5.9% crude protein and 50.0% total digestible


Using percentage IVDMD as roughly equivalent to percent TDN, data

presented by Perry and Olson (1975) indicate that IVDMD in stover before

frost is ample to maintain a pregnant beef cow or other ruminants.

Grazing grain sorghum stover must be reconciled with the high prussic

acid potential in sorghum species.


Net energy has become widely accepted in recent years for

expressing the value of a ration and the energy requirements for feedlot

cattle (Croka and Wagner, 1975). The energy value of livestock feeds can

be expressed as digestible or metabolizable energy or as the cultural

energy necessary to produce the feed. Livestock feeds are priced

essentially on their digestible energy content, except for high protein

feeds which are fed in supplemental amounts.

Energy reserves may be considered as organic accumulates

synthesized by the plant which are available for plant growth,

development and metabolism (Matches, 1969). Although soluble

carbohydrates are found throughout grass plants, their concentrations are

usually greater in the stubble than in the roots or leaves. Much of the


fructosan in grasses is accumulated in the first internode of the stem.

Within the stubble there may be a gradation in concentration of

carbohydrate reserves. In bromegrass (Bromus sp.) and timothy (Phleus

pratense) at anthesis concentrations of fructosans and total water-

soluble carbohydrates were greatest in the internode, leaf blade, and

sheath tissue closest to the stem base.

Bolsen et al. (1975) observed that sorghum stover and maize stalks

are energy-containing by-products of grain production. Both crop

residues make acceptable silage and both supply the energy needed in

maintenance rations for beef cows or ewes. Maize stalk silage can be

used in rations for growing dairy heifers if additional energy is

provided (Colenbrander et al., 1971).

On an energy basis, maize silage is especially valuable for milk

production. It has been reported that the energy value of maize silage

was closely related to total grain yield and proportion of ears to

stalks (Bryant et al., 1966). Sorghum has become increasingly important

in recent years as an energy source in high concentrate rations for

feedlot cattle in the southwest (Croka and Wagner, 1975b).

Crop residues have potential as fertilizer, as fuel, and as

livestock feed. Comparative values for these uses can be determined on

the basis of the amount of energy saved by substituting these materials

for conventional sources of feed, fertilizer, or fuel.

Goodrich et al. (1975) found that energy losses during the

ensiling process average 68% of dry matter losses, apparently because of

the loss of volatile energy-containing compounds during drying, which

inflated the dry matter loss values. Also, decarboxylation reactions

would result in greater losses of dry matter than energy. Energy losses


increased from 21.5 to 33.1%. The energy loss for maize ensiled at

33.1% moisture was greater (p=0.05) than that for maize ensiled at 21.5%

moisture. Energy loss was not significantly influenced by kernel

preparation (3.0 and 2.5% energy loss for whole kernel and rolled maize)

or by time of ensiling (2.9 and 2.6% energy loss for maize ensiled at

harvest and maize ensiled after drying and reconstitution).


Critical Levels

According to Bates (1970) the diagnosis of nutrient deficiencies

and the prediction of fertilizer requirements from plant analysis are

based on a critical concentration of a nutrient or nutrient fraction

within the plant or some plant part, below which growth or crop yield is


In early studies by Macy (1936) the nutrient calibration curve

included the zones of minimum percentage, poverty adjustment, and luxury

consumption. He proposed a central concept stating that there is a

critical percentage of each nutrient in each kind of plant above which

there is luxury consumption and below which there is a poverty adjust-

ment which is almost proportional to the deficiency until a minimum

percentage is reached.

Ulrich (1952) defined critical nutrient concentration with respect

to plant growth in terms of 1) that which is just deficient for maximum

growth, 2) that which is just adequate for maximum growth, and 3) that

which separates the deficiency from the adequacy zones.

Factors That Affect Concentration

Bates (1970) reported that, next to the supply of elements, the

physiological age of tissue is probably the most important factor

affecting the mineral composition of a given species. There appears to

be general agreement with this statement. He further observed that the

pattern of nutrient content varies with the age of the species and with

the nutrient. Phosphorus concentration in a maize plant decreases with

age. This change in nutrient concentration with age is probably due to

both a changing nutrient content of a given tissue with age, the leaves

for example, and changing proportions of certain tissues with age, such

as an increasing proportion of stem and a decreasing proportion of leaf

tissue. Physiological age was particularly important for Ca and other

nutrients which are not readily translocated in the phloem. The

nutrient concentrations in plant samples can therefore be interpreted

only if the growth stage at sampling is defined.

Environmental and soil factors also influence the availability of

P and K and thus crop response to applied P and K. Volumetric water

content, soil bulk density, buffering capacity, concentration, counter

diffusing ions, and soil chemical reactions are important in determining

the amount of P and K that reach the root. These factors indicate

greater yield response to P and K applications in drier years. When

rates of K were split with application of P the response was dependent

on weather conditions (Reneau et al., 1983).

According to Bates (1970) although the concentration of nutrients

in plant tissue chosen to provide a constant physiological age changes

with the age of the plant, there is some question whether the critical

concentration changes with the age of the plant. It is commonly


accepted that critical concentrations vary from species to species

although it has been suggested that this may not be so for all


Nutrient Accumulation

Jacques et al. (1975) suggested that nutrient uptake precede dry

matter production because the nutrients are required for growth and dry

matter accumulation. More than half of the total nutrient uptake occurs

before maximum vegetative dry matter was produced. Nutrient

concentrations varied among sorghum plant parts and changed throughout

growth. Concentrations in most vegetative plant parts were highest

right after emergence, decreased until maximum vegetative dry weight had

been produced, and changed little, if at all, while grain developed.

Lockman (1972b), studying the mineral composition of sorghum,

found that N levels in plant samples were well correlated with yields at

all growth stages and in both years (1968-69). Dry weather during the

first year of his experiment decreased N levels slightly relative to the

normal moisture data from the same plots in the second year. Results

are quite convincing that the critical level for N will not drop below

3.0% even at late stages of growth.

Lockman (1972b) observed that P levels also were well correlated

with grain yields at all stages and in both years. Dry weather caused

higher P levels, which are considered to be simple accumulation effects

with less growth. Data presented by Lockman (1972b) indicated that K

levels were correlated with grain yield in only the seedling and

vegetative samples; K levels in grain sorghum plant samples decreased

almost linearly with age. Dry weather appreciably decreased relative K

levels in seedling samples in one year.


Lockman (1972b) reported that Ca levels were only moderately

correlated with yield, generally in a negative manner. Dry weather

samples had lower Ca levels at the seedling stage; however, Ca

accumulated in the later samples. Levels in grain sorghum were poorly

correlated with yield. Dry weather appeared to cause higher Mg levels

in late season samples.

Copper levels were not well correlated with yields. Higher Cu

levels were noted with later samples in the dry year. Iron levels in

grain sorghum samples generally were not well correlated with yield.

Whole-plant sample analyses indicate that the Fe:Mn ratio has to be

considered to accurately define Fe deficiency. Fe levels in bloom and

fruiting stage third-leaf samples were higher in a dry year in field

samples. Mn accumulated to relatively higher levels in the later

samples of the dry year. Zinc levels in grain sorghum plant samples

showed curvilinear correlation with grain yield. At low yield levels

the correlation was positive, but as yields increased beyond category,

the correlation became negative (Lockman, 1972b).

In their study of the efficiency of maize hybrids, Gallaher and

Jellum (1976) found that Mg deficiency in maize is a major problem in

many parts of the world because of widespread soil Mg deficiency.

Concentrations of K, Ca, and Mg in leaf tissue appeared to be positively

related to soil test.

Bates (1970) suggested that severe deficiency destroys the

potential for growth so that the plants stop growing completely but

continue to accumulate Zn. Plants usually resume growth when a nutrient

deficiency is corrected, but it is possible that a degree of deficiency

can be reached beyond which they are completely unable to recover.


From studies conducted to estimate the uptake of N, P, K, Ca, and

Mg by maize and grain sorghum harvested for silage Fribourg (1974)

concluded that the amounts contained in above-ground plant parts exhibit

considerable range: 34 to 220 kg ha- of N; 8 to 34 of P; 31 to 271 of

K; 8 to 55 of Ca; and 9 to 45 kg haI of Mg. This large range is not

unexpected, due to soil drainage and fertility.

According to Sayre (1948) the grain does not accumulate much K.

The maximum rate of accumulation and the time when the maximum amount

occurs in the leaves and stems is earlier than for N and P. There is a

small but consistent increase in the amount of K in the grain and a

rather marked loss from the other plant tissues, especially the stem.

Voss et al. (1970), studying factors that affect nutrient

concentration in maize, found that under nearly all combinations of

conditions the N:P ratio for these nutrient concentrations remained

nearly 10:1. Within the actual data for the individual plots, ratios

greater than this were observed, but there were few ratios smaller than

this, even under conditions of high applied and/or indigenous P and low

N levels. Thus, definite levels for leaf N and P at which maximum

predicted yields occurred could not be defined, but rather they varied

with soil. The authors concluded that nutrient levels of approximately

2.9% N and 0.28% P were predicted for 95% of maximum yields.

The maximum rate of P accumulation occurred at the same period as

nitrogen, but the total amount per plant continued to increase as long

as the plants were sampled. This shows that the plant continued to

absorb P from the soil all during the season, since the loss which

occurred from the leaves, stem, husks, and cob did not account for the

quantity which moved into the grain (Sayre, 1948). These data differ


from those of the other two elements in several ways. He also suggested

that N accumulation in the maize plant reached a maximum at silking time

and ceased about four weeks later in the season studied. Nitrogen

continued to move into the grain from other tissues until maturity.

There was an actual loss of potassium after that time, largely from the

leaves and stems of the plant. No marked accumulation of potassium

occurred in the grain.

Reneau et al. (1983) pointed out that interactions between P and K

(P:K) are vital information for obtaining maximum yields. The

literature in this area shows that limited progress had been made with

P:K so that viable interactions could not be proposed. They reported

that the concentrations of N, P, K, Ca, and Mg in forage sorghum were

influenced by either P or K application or both. Nitrogen, P, Ca, and

Mg concentrations were increased and K was decreased with P application.

Nitrogen, Ca, and Mg were decreased and K increased with increased K

application. This antagonistic effect of K on Ca and Mg uptake by

monocots is well documented (DeWit et al., 1963). Bar-Yosef (1971)

demonstrated that higher P concentration in solution would increase Ca

uptake. Conversely, Greenwood and Hallsworth (1960), with intact root

systems, reported no direct effect of P on Ca uptake and further

reported more severe deficiencies with high P levels. Although limited

data is available on the effect of P on Mg uptake, Truog et al. (1947)

reported a synergistic relationship between Mg and P in plants.

Several secondary reactions are shown by the data presented by

Lockman (1972a): decreased Mg with N deficiency; increased Zn with P

deficiency; Ca and Mg accumulation with K deficiency; increased P, Mn,

and Ca but less K with S deficiency; increased P with low Cu; increased


Mn, N, P, and Al with Fe deficiency; and N, P, and K accumulation with

Mn or Zn deficiency.

Average mineral composition of grain sorghum plant samples

reported by Lockman (1972a) was consistently affected by soil fertility

factors. Bloom and fruiting stage samples continued to reflect

fertilization practices for N and P, but not for K. The levels of Mg,

Fe, Mn, and Al were also affected by N-P-K fertilizers. Magnesium

levels were decreased in vegetative and later samples, which is likely

an antagonistic effect caused by K additions. Mn levels were increased

during the same periods, probably an effect of fertilizer acidity. Fe

and Al levels in seedling samples were higher without N-P-K fertilizers.

Limited nutrient criteria have been reported for grain sorghums.

Lockman (1972a) reported that in 1966, values of 1.90 to 2.37% N were

intermediate levels for second-leaf, bloom-stage grain sorghum samples.

Values of 1.60 to 1.76% N are cited as low, and a value of 1.57% N is

cited as deficient. Normal N level is 2.48% N for 42 day seedling

plants, with a value of 1.64% N being low for whole-plant N level. He
listed 10 mg kg Zn as being an intermediate level for second-leaf,

bloom-stage sorghum samples. Intermediate levels of boron were listed
as 16 to 138 mg kg in a mature leaf sample.

Locke et al. (1964), cited by Lockman (1972a) suggest that

critical levels for bloom-stage grain sorghum leaves are about 2% N and

between 0.17 and 0.21% P. He noted that 1.7% K is adequate, since no K

fertilizer responses were obtained with this level of K in bloom-stage


Results presented by Lockman (1972a) indicate that seasons

appreciably affect nutrient levels in grain sorghums but not always in


the same manner and degree as in maize samples. The dry year, 1967,

caused increased P, Ca, Mn, Mg, Cu, Fe, and Al levels in grain sorghum

third-leaves, perhaps from lack of growth dilution. However, maize in

the dry year had reduced levels of N and K and increased levels of P,

Cu, Fe, and Mn in the ear-leaf samples. In a dry year maize did contain

less N than sorghum. With better moisture, maize leaves contained as

much N as the sorghum or more.

Comparisons of nutrient levels in sorghum and maize presented by

Bennett (1971) showed that N and P were generally higher in the grain

sorghum, whereas Ca and Mg were generally higher in the maize. There

were no consistent differences in the K content of the two crops. Baker

et al. (1970) reported growth response of maize hybrids to different

levels of P in the soil. In addition to showing more rapid growth,

hybrid 1 removed more P from the tagged band, contained more dry matter

after 29 days, but was not different from the other hybrids with respect

to grain sorghum in the Kansas survey but higher in maize in this study.

According to Jones and Wild (1975) P deficiency occurs widely in

the savanna zone. Kang and Osiname (1979) reported that crop responses

have been obtained with small P applications in the range of 4 to 10 kg

P ha In the forest zone the main response is to N application and

less to P application. This is contrary to the long-held belief that P

is the major problem in most tropical soils. The lesser P response may

be attributed to a combination of factors, such as a) higher P status of

forest soils due to better nutrient recycling, b) release of organic P

during cropping of newly cleared land, and c) large quantities of P made

available in the plant ash by traditional clearing and burning of



Olagunde and Sorensen (1982) reported that in spite of the

substantial decreases in Mg, Ca, and P concentrations in sorghum there

seemed to be no relationship between K/Mg ratio and dry matter yield.

This constant amount of cations in the plants might explain why there

was no substantial change in dry matter yield. The authors suggested

that K, Ca, and Mg carry out plant functions which can be performed by

one cation in the absence of another.

Shukla and Mukhi (1979) noted that applications of Zn to maize

resulted in increased shoot Mg at all levels of K and Na. The

increasing K levels decreased Mg and the decrease was more when Zn or Na

was not applied. The results thus showed that K and Zn had antagonistic

and synergistic relationships, respectively, with Mg. The results also

evidenced antagonism between K and Ca, K and Mg, and synergism between

Mg and Zn.

Gallaher and Jellum (1976) found that leaf concentrations of Zn

and Fe were influenced by planting date for maize hybrids but

interactions between hybrids and planting date were found only for K,

2+ -1
Ca, Mg concentrations and the sum of the mmol(M ) Ca+Mg kg the
2+ -1
mmol(M ) K+Ca+Mg kg and the K:Ca and K:Ca+Mg ratios.

Potassium content in maize tissue and its balance with Ca and Mg

has had widespread interest. Macy (1936) stated that a critical

percentage of each nutrient in each plant species existed, above which

there was luxury consumption and below which there was poverty
-l -l
adjustment. Critical concentrations of 2 mg K g and 200 mg Mg g on

a fresh weight basis were established for optimum photosynthesis in



Gallaher et al. (1975) defined the critical elemental K

concentrations in maize tissue at two sampling dates as the point at

which yields no longer give a statistically significant increase from

further applications of fertilizer K. Concentrations of K above those

critical levels would be in the luxury consumption category because

yields were not significantly increased beyond those concentrations.

Beyond those critical levels K might, in some instances, induce Ca or Mg

deficiencies because of the interaction among cations. Therefore the

critical levels of Ca and Mg are defined as being at the same point

where the critical level of K occurred. The optimum balance of the 3

cations should occur at the critical concentration for K.

Gallaher et al. (1975) reported critical mmol(M2+) of young maize

plants taken 38 days after planting to be 91 to 78 mmol(M2+ ) K kg -1, 31
2+ -1 2+ -1I
to 28 mmol(M2 ) Ca kg- and 40 to 39 mmol(M2 ) Mg kg-. The dilution or

age effect is evident from the critical levels of K at the thinning

stage compared to the ear leaf at the silking stage of growth. The
critical mmol(M 2+) in the ear leaf 86 days after planting was 44 to 40
2+ 1 2+ 1 2+
mmol(M ) K kg 34 to 30 mmol(M2 ) Ca kgI, and 22 to 16 mmol(M2) Mg

kg The critical 44 to 40 mmol(M2) K kg-, in the ear leaf at the

silking stage of growth was in agreement with the critical percent of

1.75 for maize ear leaf tissue at the silking stage in the midwestern

United States.

Jacques et al. (1975) found Ca concentrations were much lower in

heads than in other plant parts both years and in the first year were

lower in grain than in threshed head parts. Calcium concentrations in

blades increased after maximum blade weight was reached. During grain

development Ca concentrations were higher in blades than in unformed


heads. Calcium utilized in calcium pectate formation in mature leaf

cells may have been responsible for the increased concentrations in

blades and culms. A greater percentage of the total Ca taken up in the

hybrids was accumulated in their blades than in the other plant parts.

Hanway (1962a) found that in maize, N accumulated in each plant

part as that part grew. He also observed that there was little

translocation from one plant part to another until after grain formation

began, and then N was translocated from all other plant parts to the

grain. Translocation of N from the cob, husk, and stalk appears to

precede that from the leaves. The leaves contained approximately 30% of

the N accumulated by the plant even though they constituted only about

13% of the final dry matter accumulation. At maturity the grain

contained approximately 66% of the total N in the plant. About 50% of

the N in the grain at maturity appeared to be N that had been lost

through translocation from other above-ground plant parts.

Jordan et al. (1950) observed a somewhat different pattern of N

uptake. In their study N fertilizer was side-dressed when the plants

were knee-high and the maximum rate of N absorption occurred immediately

following the application of N fertilizer. The pattern of N

accumulation by plants is undoubtedly influenced by the seasonal pattern

of N availability in the soil. K accumulation in this study continued

until a later stage of maturity and there was no loss of K from the

plants during the latter part of the season.

Hanway (1962a) suggested that continued mineral accumulation by

the plants later in the season is essential to prevent excessive loss

through translocation of N and K from the leaves which would result in

premature death of some of the leaves. Some loss of nutrients from the


leaves to the grain does not appear to be detrimental to the yield of

grain so long as this does not result in premature death of the leaves.

Jacques et al. (1975), comparing two sorghum hybrids, found that

Mg concentrations were lower in heads than in other plant parts.

Magnesium was evenly distributed in blades and sheaths of each hybrid.

For the two hybrids Mg accumulation in blades, sheaths, and culms was

similar to that of dry matter production. Both dry matter production

and evidence of translocation of Mg out of stems into the heads during

grain development was indicated, because the percentage of Mg decreased

in the stems and increased in the developing grain.

Jacques et al. (1975), studying nutrient uptake by different

sorghum hybrids, found that little difference between hybrids in Zn and

Cu uptakes occurred each year, but differences existed between hybrids

in Mn uptake. Stem tissue was initially high in Fe content but

decreased during both hybrids' vegetative growth. Concentrations in

head tissue and in head parts and grain were generally below 75 mg kg-

and relatively constant throughout the growing seasons, especially in

culm tissue. Differences were small among plant parts after vegetative

growth ceased, and concentrations in plant parts remained relatively

constant during grain development. Some translocation of Zn from

vegetative plant parts into the developing grain was suggested by a

decrease in Zn content in some cases, but for the most part Zn seemed to

be relatively immobile.



Importance of S

From the time of Liebig S has been known to be one of the elements

required for plant growth, but only recently has this element received

the attention it deserves as a plant nutrient. Despite this early

recognition, the importance of S as a limiting plant nutrient was

largely ignored until recently (Coleman, 1966; Caldwell et al., 1969).

In countries concerned with increasing food production, S is an

element that must not be overlooked. It is required not only for

increased total production, but what is probably far more important, it

is needed for increasing the quality of the protein present in the

foods that are produced (Coleman, 1966).

Allaway and Thompson (1966) indicated that the problem of

malnutrition due to deficiency and poor quality of protein in human

diets has been described by nutritionists and emphasized in the press.

The nutritional quality of a protein depends upon its amino acid

composition. Animals (including humans) must be supplied with the

S-containing amino acid methionine and the S-bearing vitamins biotin and


In areas where protein deficiency in human diets is a critical

problem, animal products are rarely consumed, and plants are the major

source of proteins. In addition, plants are frequently lower in total

protein than are animal products, so that the amounts of food required

to supply 1 g of S-amino acids are higher for plant products than for

animal products. The protein requirements of many areas will need to be

met by direct consumption of plant products. The appropriate ultimate

objective of S fertilization of soils is, therefore, to increase the


S-containing amino acid of human diets (Allaway and Thompson, 1966;

Coleman, 1966).

In a review article Coleman (1966) pointed out that S is needed in

crop production because certain plant functions require it for 1) the

synthesis of amino acids, cysteine, cystine, and methionine, and hence

for protein elaboration; 2) the activation of certain proteolytic

enzymes such as the papainases; 3) the synthesis of certain vitamins, of

glutathione, and of coenzyme A; 4) the formation of the glucoside oils

found in onion, garlic, and cruciferous plants; 5) the formation of

certain disulfide linkages that have been associated with the structural

characteristics of protoplasm; and 6) in some species the concentration

of sulfhydril (-SH) groups in plant tissue,which is related to an

increase in cold resistance.

The importance of S in animal nutrition has been summarized by

Allaway and Thompson (1966). They reported that the S-containing amino

acid requirement of chicks has been estimated at 0.8% of the total

ration when the ration is 23% protein. In the case of ruminant animals,

where synthesis of S-containing amino acids from inorganic S by rumen

microflora has been established, the total S content is normally used to

appraise S status of a ration. The optimum S level in lamb diets was

found to be approximately 0.17 dag kg-I S when methionine was used to

increase the S content of a low-S basal diet. The authors suggested a

N:S ratio of 15:1, or less, as an appropriate guide to the S adequacy of

a ration for dairy cows. A precise evaluation of the optimum N:S ratio

for ruminant animals is probably impossible, because ruminants adjust to

low levels of N and S through recycling processes.

Forms and Amount of S in the Soil

Tropical and temperate soils differ in both the total amount of S

and its form. Jordan and Reisenauer (1957) report average values of 540
and 210 mg kg S for Mollisol and Alfisol surface soils from temperate

areas. Generally, the total S content of tropical soils is lower

because of their lower organic matter content, and adsorbed S is often

the major reserve of this element.

Sulfur is found in soils as a variable mixture of primary

minerals, sulfate ions in solution, adsorbed sulfate, ester sulfate, and

organic S compounds. Blair et al. (1980) summarized the differences

between the forms of S present in tropical and temperate soils.

Adsorbed S is generally higher in tropical soils, as in this state it

cannot be leached from the profile.

Neller (1959) showed that extractable sulfate-S increased with an

increase in the clay content in 14 Florida Ultisols. The subsurface

horizons contained considerably more sulfate-S than the surface

horizons. The increase in sulfate-S with horizon depth is associated

with S adsorption by Fe and Al hydroxides and oxyhydroxides and with an

increase in 1:1 clay minerals in the argillic horizons.

More recently Mitchell and Blue (1981) showed that most S in

Florida Spodosols and Entisols is associated with organic matter, and

sulfate-S accounted for less than 7% of the total soil S in the entire

profile of selected Florida Spodosols. Sulfate-S accounted for

approximately 15% of the total S in the C horizons of Florida Entisols,

and 44% of the total S in the B2t horizons of 10 Utisols. Extractable S

in the surface (A or A ) horizon (0-14 cm) ranged from 1 mg kg-I in a

Myakka fine sand (Aeric Haplaquod) from Alachua County to 8 mg kg-I in a


Leon fine sand (Aeric Haplaquod) from Duval County. Extractable

sulfate-S did not vary as much as total S in the soils. The mean total

S in the nine Spodosols varied from 104 mg kgI in the surface horizons

and 92 mg kg in the spodic (B2H) horizons to 17 mg kg in the A2


Sulfur Deficiency in Soils

Blair et al. (1980) gathered information indicating that response

to S has been reported in 40 countries of the tropics. Coleman (1966)

and Spencer and Freney (1980) reported that S-deficient areas are rather

widespread throughout the world. For example, crop deficiencies of the

element have been reported from countries in Central and South Africa,

India, Brazil, Argentina, Central America, Europe, Australia, New

Zealand, Canada, and the United States.

Sulfur deficiency frequently occurs in soils derived from volcanic

parent-materials. In such soils, which are common in Central America

(Fritts 1970), the organic matter is closely associated with allophane

and the mineralization of the allophane-bound organic matter, i.e., the

rate of release of sulfate-S is very low. Plants on such soils are

often S deficient despite the fact that the soil is high in organic S.

According to Blair et al. (1980) the reasons for an S deficiency

in soils can be grouped broadly into three areas 1) inherently low

initial S status, 2) low availability of S-containing soil organic

matter, and 3) the result of agricultural practices. Sulfur deficiency

often develops in the tropics after a period of agricultural

exploitation. The major factors that contribute to its onset are crop

removal, organic matter losses, leaching and erosion losses, and

fertilizer use and management. The more intensive the cropping system,


the greater the product removal and S demand. For example S contents of

the rice grain vary from 0.034 dag kgI under deficiency conditions to

0.16 dag kg in a nonresponsive situation, and rice grain yields may

vary from 0.75 to 8 Mg ha -. A further factor to take into account when

considering the effect of crop removal on S demand is the zone of

removal in the soil.

The removal of crop residue contributes significantly to losses of

soil S in some situations (sorghum, millet). The recycling of S in crop

residues is important in livestock systems where the residue material is

used for animal feed or bedding. Since organic matter acts as a reserve

of S in soils, the losses have serious implications for the S-supplying

power of soils.

Coleman (1966) suggested that S deficiencies occur probably

because of a) the increased use of S-free fertilizers; b) the decreased

use of S as a fungicide and insecticide; and c) increased crop yields,

which means requirements of all of the essential plant nutrients in

larger amounts. Blair et al. (1980) indicated that the use of

non-S-containing phosphatic fertilizers may aggravate the S problem by

replacing adsorbed sulfate with phosphate. In this regard two aspects

of sulfate adsorption are important. First, phosphate ions will replace

sulfate ions. Bromfield (1974) estimated that sulfate ions are then

free in the soil solution and can be leached.

Adsorbed sulfate associated with the argillic horizons of Ultisols

is considered a primary source of plant-available S in soils of the

southeastern United States (Neller, 1959; Bardsley et al., 1964;).

Ensminger (1954) reported that in some areas S may leach out of the

surface horizons of coarse-textured soils but be retained by the lower


horizons. This is particularly true if the subsoil contains a large

amount of kaolinitic-type clays and of hydroxides of Fe and Al, and is

acid. Under such conditions plants may exhibit S deficiency during

early stages of growth. However, the plants will subsequently recover

when the roots enter the lower soil layers. The initial stunting of

growth caused by S shortage in the topsoil may, however, result in

reduced crop yields. Mitchell and Gallaher (1979) reported that deep

rooted plants are able to utilize adsorbed sulfate associated with the

clay in lower soil horizons, but seedlings may exhibit S-deficiency

symptoms when grown on sandy surface soils with no S fertilization.

These plants were definitely low in S with an average S concentration of
0.12 dag kg Soil analyses indicated increasing extractable sulfate-S

with depth in the horizon. They further suggest that increased

mineralization of organic S later in the season may have also

contributed to the improved S status in the plants.

Sulfur present in soils as sulfate undergoes many reactions

similar to those of nitrate and phosphate, and its strength of

adsorption to surfaces is intermediate between that of the two anions.

In experiments by Pearson et al (1962), 90% of the water-soluble bases

were leached as sulfate from Latosol and Ultisol profiles; chlorides

and nitrates accounted for only about 6% and 1%.

Organic S and ester sulfate, which may be thought of as reserve S,

are generally lower in the tropical soils. Environmental conditions in

the tropics are generally conducive to a rapid mineralization of organic

matter, which leads to high turnover rates of S. An exception is the

volcanic soils (Andepts), in which adsorption of organic matter on the

particles of allophane retards mineralization (Blair et al.,1980).


In many parts of the tropics burning of plant matter, which is an

integral part of farming, can be an avenue for S loss. When carbon is

burned off by combustion there is a concentration of S in the ash.

Blair et al. (1980) and Bromfield (1974) showed that the S content in

ash can be 2 to 10 times higher than that in dried plant material. In

areas of prevailing winds S loss may be high, but in regions of variable

winds the gains may equal the losses. One additional consequence of

burning is that organic forms of S are converted to inorganic forms

which, at the beginning of the rainy season when fields are bare, are

susceptible to leaching losses.

Blair et al. (1980) reported that sulfate which was mineralized

from organic matter moved down the profile and accumulated at lower

depths. Since a large proportion of the S present in the soil is in the

organic matter and this is often accumulated on the soil surface, losses

from wind and water erosion may be high in some circumstances.

Plant's Requirements and Content

Tisdale and Nelson (1964) concluded that S is required by many

crop plants to about the same extent as is P. As a general rule of

thumb, grass and cereal crops generally require smaller amounts of S

than do legume and cruciferous crops. Ensminger and Freney (1966) list

the effect of age on S contents in several species. Because it is

necessary to select a plant part that has a relatively constant S

content, the effect of maturity on the S content of the plant parts)

should be examined to determine sensitivity to sampling time.

The critical concentration of S in young maize plants has been
reported to be around 0.20 dag kg (Fox et al., 1964; Stewart and

Porter, 1969; Jones and Eck, 1973; Terman et al., 1973). Friedrich and


Schrader (1978) indicated that maize seedlings are not S-deficient

unless the concentration of S (dry weight basis) in the shoots is

approximately 0.10 dag kg-I or less. Blue et al. (1981), found that

maize plants without S fertilization were chlorotic and oven-dry herbage

contained only 0.10 dag kg total S; herbage from treatments with S
-1 -1
applied at 15 and 30 mg kg contained 0.19 and 0.23 dag kg ,

respectively. Oven-dry herbage yields were increased from 6.6 to 9.4 g
-i -
pot by the addition of 15 mg kg- of S, and there was no additional
yield increase from the 30 mg kg S treatment. Mitchell and Gallaher

(1979) reported that seven harvested crops in a maize/grain system and
the maize forage system removed an estimated 48 and 63 kg ha S,

respectively, during 2 years.

Fox et al (1977) have shown that the critical level of S in cowpea

varies between cultivars. In their solution culture experiment, the
critical value varied between 0.032 dag kg total S in cultivar 'Sitao

Pole' and 0.064 dag kg-I in 'TVU76-2E'.

Published data for rice (Blair et al., 1979) show grain S contents
varying from 0.134 dag kg under deficiency conditions up to 0.16 dag
kg in a non-responsive situation. Rice grain yields may vary from 750

kg ha up to 8,000 kg haI which gives a S removal varying from 0.26 up

to 12.8 kg ha Rice straw may contain similar amounts of S.

Absorption and Accumulation of S

Coleman (1966) reported that in addition to the S they receive

from precipitation, plants and soils absorb sulfite and perhaps other

sulfurous gases directly from the atmosphere. It has been known for

many years that sulfite is absorbed directly by plant leaves. Work by

Olson (1957) showed that plants supplied with adequate sulfate in


solution still obtained about 30% of their S from the atmosphere. When

plants were grown in a S-deficient nutrient solution, they obtained up

to 90% of their S from the atmosphere, but the total amount absorbed was

insufficient for normal growth. There is considerable evidence that

sulfurous gases in the atmosphere may be absorbed directly by soil.

However, Blue et al. (1981) reported that S additions to soils through

the atmosphere and rainfall are inadequate for intensive production


Jones et al. (1979) estimated that an average of 8.4 kg ha yr-

of atmospheric S was absorbed directly by the soil in South Carolina

from 1973 to 1977. No data are available on the amount of S that may be

absorbed directly by the plant foliage as sulfite.

Sulfate is absorbed by plants with more difficulty than other

anions and it has been shown that the uptake proceeds even more slowly

in the presence of more mobile anions such as chloride and nitrate.

Usually S and N are absorbed in the form of sulfate and nitrate from the

soil and subsequently undergo metabolic conversion into organic S and N.

Any quantity above that needed to supply sulfate and nitrate to the

metabolic process is stored up provisionally as inorganic sulfate and

nitrate in the plant (Spencer and Freney, 1980).

Friedrich and Schrader (1978) suggested that N-reductase (NR), the

rate-limiting enzyme in nitrate-N assimilation, serves as a primary

regulatory coupling between nitrate and sulfate assimilation, while

Reuveny and Filner (1977) postulated that ATP-sulfurylase, the initial

enzyme in the pathway of sulfate assimilation, acts in synchrony with NR

to coordinate nitrate and sulfate assimilation in cultured cells.


However, Brunold and Schmidt (1976) proposed that adenosine-5'-

sulfotransferase, not ATP-sulfurylase, regulates sulfate assimilation in

chlorophyllous tissue.

Rabuffetti and Kamprath (1977) concluded that S accumulation in

maize grain is highly dependent upon the supply of N available for the

formation of amino acids. Sulfur accumulation in stover was enhanced by

both N and S application. On a Goldsboro soil, S application increased

S accumulation in the stover at all N rates. This occurred only at the

high N rates on the Wagram soil.

According to Friedrich and Schrader (1978) higher plants generally

accumulate N and S in amounts proportional to that incorporated into

protein. However, when plants are S-deficient, protein synthesis is

inhibited and nonprotein N is accumulated. Likewise, sulfate will often

accumulate in plants when the availability of N is limiting protein

synthesis. The observed interaction between N and S accumulation

suggests that nitrate and sulfate assimilation are closely linked


The pattern of remobilization of N and S reported by Friedrich and

Schrader (1979) is similar to the pattern of N redistribution observed

by previous researchers (Hanway, 1962c). The percentage of the labeled

N and S present at silking that was later remobilized was not constant

among fractions. The husk fraction remobilized more of its N and S than

did any other fraction.

Barrien and Wood (1939) studied the effects of N supply on the

amounts of protein-S and sulfate-S. The authors concluded that the most

striking feature of the curves for amount of protein-S plotted against

time is that they follow the trend of the curve for amount of dry


matter. An increase in N supply caused an increase in the amount of

protein-S. As in the case of protein-N, the highest N treatment caused

at first a depression in the amount of protein-S due to an effect of

treatment on growth rate.

Friedrich and Schrader (1979) reported that in maize a greater

proportion of N compared to S was remobilized from all fractions with

the exception of roots. This suggests that N is more mobile than S, at

least under the conditions of this experiment. The supply of S strongly

influences the efficiency of nitrate-N utilization in maize. Regardless

of the external supply of N during grain-filling, N and S absorbed prior

to silking will later comprise most of the N and S in the ear.

Sulfur absorbed by maize plants prior to silking was partitioned

more effectively into the ear than S absorbed after silking (49 vs.

23%). Although maize plants can absorb large quantities of S during

grain-filling, it is apparent that remobilization of S accumulated prior

to silking contributes more to ear development. Furthermore, the

remobilization of S is similar to N remobilization in that it occurs at

a constant rate that is not affected by N supply during grain-filling,

(Friedrich and Schrader, 1979)

Effects of S Deficiency

Sulfur affects not only the yield of crops, but in certain cases

the quality also. It is essential for the synthesis of methionine,

cystine, cysteine, and hence the elaboration of amino acids into a

high-quality protein. Methionine and cystine are indicators of protein

quality (Allaway and Thompson, 1966; Blair et al., 1980; Stewart and

Porter, 1969; Lancaster et al., 1971).


Friedrich and Schrader (1978) studied S deprivation and N

metabolism. In maize seedlings, leaf fresh weight was not affected by S

deficiency. However, stem fresh weight was reduced 24% compared to

normal plants. The authors concluded that it may be that S deficiency

was having a greater effect on the young, rapidly elongating culms and

unfurled leaves in the stem fraction than on the older leaf blades.

There are few data, however, that indicate to what degree yield and

quality are related.

According to Allaway and Thompson (1966) the importance to human

and non-ruminant animal nutrition of the S-containing amino acids cannot

be overstated. Many studies of the nutritive value of proteins have

shown that the lack of S-containing amino acids is the factor that

limits the biological value of the protein. The investigators making

this survey further concluded that a large segment of the world's

population is living on a diet that is strongly deficient in methionine.

If the animals are fed a ration low in total S, they will not make the

best utilization of the N in the diet. This means that meat, milk, or

wool production will be reduced.

Under conditions of S deficiency and high N fertilization, protein

synthesis is retarded by a lack of the S-containing amino acids,

cysteine and methionine, and this is reflected by marked accumulation of

unassimilated N in the plants as nitrate-N, amides, and free amino

acids. Such forage when fed to animals represents an unbalanced ration

in which N content may exceed requirement, which in turn can result in

nutritional disorders, especially in ruminants. High nitrate-N in

forage, for example, can cause nitrate poisoning and hypomagnesemia in

grazing dairy cattle. It arises from an inadequate absorption of Mg and


is probably associated with high ruminal ammonia (NH3) production.

Until recently, S had not been known to limit digestibility of inferior

quality standing pasture. It has since been shown that S fertilization

improved the intake and digestibility of inferior quality herbage.

Interaction Between S and Other Nutrients

According to Goh and Kee (1978) the total N (Nt):total S (St)

ratio in plants has been extensively studied because of its potential

use in assessing S deficiency in crops. Sulfur requirement is closely

associated with N metabolism, and high application of N fertilizer to

increase crop production may be detrimental and often wasteful if the

corresponding increase in S demand is not met. Stewart and Porter

(1969) found that to achieve maximum utilization of the added N, one

part of S must be added to 15 parts of N. Results presented by Goh and

Kee (1978) indicate 17 parts of N to one part of S.

Dijkshoorn et al. (1960) found that, on account of the ability of

grass to accumulate variable amounts of non-protein N-metabolites free

of S (such as glutamic and aspartic acid), the N:S ratio in the

non-protein organic substance is usually different from the protein

ratio Sp:Np and is subject to variation according to the nutritional

status of the plant. Therefore the ratio of organic S (So) to organic N

(No) in the total mass of forage So:No is also different from Sp:Np and

is some function of the nutritional status and the composition of the


In S-fertilization experiments in New Zealand, McNaught and

Christoffels (1961) reported N:S ratios of 17:18.5 for white clover and

11:12 in grasses at maximum yields. Pumphrey and Moore (1965) found

that a N:S ratio of 11 or less indicated an adequate S supply for


alfalfa. Stewart and Whitfield (1965) suggest a N:S ratio of 17 or less

in wheat clippings as indicative of adequate S nutrition. Thus, the N:S

ratios found desirable for optimum growth of plants are generally

slightly higher than the N:S ratio of 10:1 to 15:1 suggested by Allaway

and Thompson (1966) as optimum for ruminant animal nutrition. It would,

therefore, appear to be quite likely that certain forage plants may be

deficient in S for ruminant animals, even though the plants themselves

are growing at nearly maximum rates. When S is deficient, the ratio of

Nt:St will exceed the 15:1 required for protein synthesis, formation of

protein will diminish, and nonprotein N will accumulate.

The Np:Sp ratio has been reported to range from 11 for maize to 18

for legumes (Dijkshoorn and Van Wijk, 1960). Metson (1973) considered

the Np:Sp ratio to be more reliable than the total Nt:St ratio for

assessing the crop's N:S requirement because it is not influenced by the

accumulation of non-protein S and non-protein N. When S supply is

adequate the accumulation of non-protein S will cause the Nt:St ratio to

be lower than the Np:Sp ratio, whereas when S is deficient, non-protein

N will accumulate resulting in a higher Nt:St ratio.

Barrien and Wood (1939) observed a decline in the ratio of Np:Sp

in sudangrass leaves as the plants matured. They suggest that this

change might be due to the presence of a relatively stable S-rich

protein fraction that is not readily remobilized.

Under conditions of S deficiency the uptake of nitrate-N seems to

be affected less by a limited S supply than is the plant's capacity for

protein synthesis. Thus, because the ratio of Np:Sp in individual

proteins is fixed by the genetic code, nonprotein forms of N accumulate

when the availability of S limits protein synthesis. Similarly, sulfate


will accumulate in plants when the rate of uptake exceeds the amount

required for protein synthesis (Friedrich and Schrader, 1978).

The ratio of Nt:St in all vegetative fractions of maize plants

declined during grain-filling. Likewise, the Nt:St ratio was lower in

ears of N-deprived plants than in control plants; however, this ratio

was constant throughout grain-filling and was usually significantly

lower for N-deprived plants. Nitrogen supply had no effect on the

decline in Nt:St ratio in the leaves (Friedrich et al., 1979).

The accumulation of zein, a grain protein that is low in S-amino

acids, is known to be enhanced by increased N-fertilization. The ear

Nr:Sr ratios were much higher than the Nt:St ratios. This is due to the

larger proportion of ear S present as sulfate-S compared to the pro-

portion of total N present as nitrate-N (1 to 6 dag kg ). However,

this sulfate-S may have been in the cob, rather than in the grain.

Maize grain does not accumulate nitrate-N, (Friedrich and Schrader,


Goh and Kee (1978) found when N is added in high rates a reduction

in reducible S content occurred because of the incorporation of S into

plant proteins. In the high N low S treatments (N SO) over 95% of total
S in plants occurred as organic forms (total S, reducible S). Stewart

and Porter (1969) showed that when S is deficient nearly all the S

present in both herbage tops and roots occurred in the protein fraction.

Conversely, in the low N and high S treatments the reducible S fraction

consistently made up more than 20% of the total S.

Phosphorus-S interactions have been observed by Kamprath et al.

(1956) and Radet (1966). Caldwell et al. (1969) reported the S

treatments decreased the P content of the mature tissue. The S:P ratios


for second cutting alfalfa in 1966 ranged from 0.45:1 in untreated plots
to 1.42:1 in alfalfa which received 112 kg ha of elemental S annually.

There was no effect of S on the P content of maize in 1962. Sulfur

increased the S content of the leaves from 0.22 to 0.26 dag kg .

Phosphorus content decreased from 0.45 dag kg without S to 0.38 dag
kg when S was applied. Kamprath et al. (1957) reported that liming

and P fertilization reduced the retention of sulfate in the surface

horizons and as a result sulfate added to these soils would be leached

out of the plow layer into the B horizons, where it accumulates.

Work by Caldwell et al. (1969) demonstrated that the effect of S

on the K content of alfalfa varied. In one year increasing S resulted

in a decrease in the K content. In the second year the S-treated

alfalfa contained more K than the S controls. Apparently the healthier,

more vigorous plants on the treated plots were able to extract more K

from the soil than the weak, unthrifty, S-deficient plants.

Caldwell et al. (1969) found no discernible effects of S on the

Ca, Mg, and Fe content of alfalfa. Neither were there any significant

differences in the Cu, Zn, Al, and Sr contents of the alfalfa as a

result of the S fertilization. Average Cu content of the maize
decreased from 10 to 8 mg kg with S (significant at the .05

probability level).

Mitchell and Blue (1981a, b) found that both total S and

extractable sulfate-S were significantly correlated with organic C and

total N in the surface horizons of Florida soils. Only total S was

highly correlated with organic C and total N in the spodic horizon.

Crop Response to S Fertilizer

Tisdale and Nelson (1964) reported that numerous crop species have

been found to respond to applications of S under the usual field

conditions. Some of these are lucerne clovers, pasture grasses, cotton,

maize, peanut (Arachis hypogea L.), rice, jute, banana, small grains,

apple, stone and citrus fruit, cruciferous crops, tea, and coffee. It is

to be suspected that this response would be found in every crop.

According to Jordan and Bardsley (1959), crop deficiencies of S

can be corrected by the application of numerous S compounds or elemental
S. Normal superphosphate contains 11 to 13 dag kg S and ammonium
sulfate contains 24 dag kg S. The intentional application of S as a

fertilizer nutrient has never received wide acceptance among growers.

It has been present in most fertilizer materials as an anion associated

with the other macro and micronutrients or as a by-product of the

manufacturing process.

Recent work with supplemental S by Gaines and Phatak (1982)

studied the effect of additional S on maize, soybean, cowpea, tomato,

cotton, and okra. This work showed that yields of maize, soybean,

cowpea, and tomato tops were significantly increased by rates of up to
32 mg kg S, but yields of cotton and okra tops were unaffected by S

treatments. The results which were obtained by Rabuffetti and Kamprath

(1977) suggest that the addition of S to maize crops which were

adequately fertilized with N would be likely to improve the grain

quality of maize produced in Coastal Plain soils.

In a study testing the relation of S content of forage crops to

cattle fed on those forages, Lancaster et al. (1971) indicated that S

fertilization influenced rumen microbial activity when cattle were fed


four forage species but not in alfalfa. Gas production increased for

the grasses and decreased in the legumes with increased S application.

Total plant S and sulfate-S levels were increased with additional S

applied for all species; however, the percent protein decreased in the

grasses and increased in the legumes.

Lancaster et al. (1971) also found that the percentages of fiber

and lignin in the crops they studied were not influenced by S

fertilization, except for the second clipping of sudangrass and the 20
and 40 mg kg levels applied to alfalfa. The critical level of S in
these trials appears to be less than 10 mg kg S applied to the soil.

There were positive correlations between gas production and S

concentration for the grasses but there were negative correlations for

the legumes.

Caldwell et al. (1969) reported that the effect of S on the yield

of alfalfa was striking. Three times as much hay was produced by the

treated plots as by the check plots. The untreated alfalfa contained
from 0.146 to 0.221 dag kg S. Lancaster et al. (1971) reported that

the dry matter production in the grass species they studied was

increased by the addition of S, but for the legumes this increase was

not significant.

Rabuffetti and Kamprath (1977) reported that S had little effect

at low N rates on N accumulation in maize stover. However, they found

that at N rates of 168 and 224 kg haI there was an increase in N

accumulation in stover with S rates of 44 and 66 kg ha-I on the

Goldsboro soil and 33 and 66 kg haI on the Wagram soil. Total S

accumulation in grain was found to be increased by N application at both



Work by Blue et al. (1981) showed that increasing rates of applied
S caused striking maize growth responses to S applied at 10 mg kg.

Yields from two soils were increased approximately fourfold. Herbage S

concentrations were only 0.06 dag kgI from each soil without S

fertilizer; interestingly, they were increased to only 0.09 and 0.07 dag
kg respectively, from the Marion and Suwannee county soils fertilized

with 10 mg kgI of applied S. Stewart and Porter (1969) gave similar S

concentrations in maize plants of the same age with the additional point

that S requirement increased with increasing amounts of applied N.
However, S content of herbage was increased from 2 and 3 mg pot for

soils from Marion and Suwannee counties without applied S to 11 and 14
-l -1
mg pot respectively, by application of 10 mg kg of S to the soils.

Mitchell and Gallaher (1979) found that applied S from non-Mg

sources had no significant effect on the final grain yield of two maize

cultivars. All of the S treatments increased the S concentration of the

tissue over that of the check, but did not affect yield of grain or


A grain yield response by rice to S application was obtained at

three sites by Blair et al. (1979); responses to S in three experiments

reported ranged from 47 dag kgI or 1837 kg grain ha at one site and

up to 231% or 2,146 kg grain ha- at another site.

From work reported by Wagner and Jones (1968) it is evident that S

fertilization affected the quality of annual grassland forage as

measured in terms of protein level. Evans and Davis (1966) reported

that addition of a dietary level of sulfate to an in vitro system

improved cellulose digestibility. Jung and Reid (1966) obtained a


correlation coefficient of 0.82 when in vivo digestible dry matter was

compared with in vitro cellulose digestibility.


Field Procedures

Fertility Trials

Experiments involving three intercropping systems (Fig. 3-1) and

six fertilizer treatments (Table 3-1) were established at two sites in

Esteli, Nicaragua (Fig. 4-1) during the 1982-1983 growing season. Site 1

(Centro Experimental de Esteli) located 1 km north of the city of Esteli

on the Panamerican Highway at an elevation of 975 m. The average annual

precipitation is 1000 mm distributed in a bimodal pattern (CATIE,

1981a); the largest amounts fall during May, June, August, and September

(CATIE 1980, 1981). This rainfall pattern is the determining factor in

defining the growing season. The average annual temperature is 19C. The

soil may be classified as a Vertisol (CATIE, 1981). The field where the

experiment was established was previously planted to shade tobacco

(Nicotiana tabacum L.) but had been under fallow for the last 7 years.

Site 2 was a production cooperative (Sabana Larga) managed by 15

farmers and located 6 km southwest of the city of Esteli at an elevation

of 930 m. The average annual precipitation and temperature are 1247 mm

and 20.1 C, respectively (CATIE, 1980). The field had been under

continuous maize + beans (Phaseolus spp.) for the last 25 years.

There were 13 treatments with four replications in a randomized

complete block design at each site. The design incorporates all six

Table 3-1. Fertilizer rates and times of application evaluated at two
sites in Northern Nicaragua.

TRT -Crop- Days after planting Cl 25 days after
# Cl C2 10 25 planting C2

---------------------- kg ha --------------
1 M PS 30 0 0 0 0
2 M PS 30 30 40 0 0
3 M PS 30 0 35 0 0
4 M PS 30 0 70 0 0
5 M PS 30 30 40 0 35
6 M PS 30 30 40 30 35
7 M NS 30 30 40 0 0
8 M NS 30 30 40 0 35
9 M NS 30 30 40 30 35
10 M MI 30 0 0 0 0
11 M MI 30 30 40 0 0
12 M MI 30 30 40 0 0
13 M MI 30 30 40 0 0

M = maize, PS = photosensitive sorghum, NS = non-photosensitive sorgum,
MI = millet.


80cm M M M M M
--- M M M M M

80cm PS PS PS
P- M M M M M









80 cm Plants n-1
-NS---4 ---------------------------- NS
40cm M M M M M M M M M M M M
LNS ----------------------------------- NS
NS ------------------------------------NS
F---M M 1` M M M M M M M M
I NS ------------------------------------- NS
INS------------------------------------ NS
NS------------------------------------ NS

50cm 12 Plants m-
r-Ml- ---4-- ---------------------------- Mi
40cm M i M M M M M M M M M M
III--------------------------------------- MI
MI- M- I
i-I M nM M M MM K M M M 1
L MI- --i
-1------------------------------------- MI

Figure 3-1. Spatial arrangement of maize (M) + photosensitive
sorghum (PS), maize + non-photosensitive sorghum
(NS), and maize + millet (MI) intercropping







fertilizer treatments in the maize + photosensitive sorghum cropping

system, but only the high fertilizer treatments in the maize +

non-photosensitive sorghum and the maize + millet systems.

The individual plots were 9.8 by 10 m. Twelve rows of 'NB-3' maize

were planted no-tillage by hand in each plot (52,000 plants ha ) on 16

June. The second crops ('Criollo', photosensitive sorghum; 'Pioneer

895', non-photosensitive sorghum; and 'Gahi-3' millet) were interplanted

on 16 September according to the spatial arrangements depicted in Figure

3-1. The 'Criollo' was seeded between maize rows (40 cm from the maize

hills), in hills spaced 0.80 cm apart, and later thinned 24 days after
seeding to six plants per hill to obtain 75,000 plants ha 'Pioneer

895' and 'Gahi-3' were seeded in double rows (20 cm from the maize).

Twenty four days after seeding the rows were thinned to have 120,000

plants haI.

One week prior to planting the experimental plots were sprayed with
2 L ha of paraquat to kill the established weed population and

immediately after planting with a mixture of 1 L of atrazine + 0.75 L of

Lasso ha to prevent further weed infestations. Together with the seed

11 kg of carbofuran haI were incorporated in the soil to prevent damage

to the plants by soil insects. No further insect control was necessary

at either site. To simulate the conditions of the typical farmer all

other management activities were performed as described in Chapter 4.

The N, P, and K fertilizer applied at planting was incorporated

into the soil with a planting stick; later applications on N and P were

hand-drilled near the hills of the maize or photo-sensitive sorghum and

banded next to the rows of the non-photosensitive sorghum or millet and


covered with 3 to 5 cm of soil. The fertilizer treatments are described

in Table 3-1. No initial soil amendments were necessary.

Grain and stover were harvested from 8 m of the two central rows of

each plot. Grain moisture was measured with a Steinlite moisture meter

and yields calculated at 15.5% moisture content. The plant was separated

into components, and each part was then weighed. Dry matter yields were

calculated using subsamples dried to constant weight at 70C for 72

hours. All above ground plant material was removed from the plots,

separated into parts (leaf, stem, flower, ear, or head), and subsampled

for chemical analysis as described in the section of laboratory analysis

of this chapter.

One soil sample was collected from each plot prior to planting and

immediately after harvesting the last crop. All samples were air-dried,

sieved through a 2 mm stainless steel screen, and stored at ambient

temperatures until analyzed.

The following response variables were measured to determine the

effect of the fertilizer treatments on the systems under study: a) soil

pH (1:1) and extractable N, P, K, Ca, Mg, Fe, Cu, Mn, and Zn

concentration at the beginning and end of the experiment, b) maize and

sorghum grain yield, c) Dry matter production of the different plant

parts, d) Concentration of N, P, K, Ca, Mg, Fe, Cu, Mn, and Zn of the

different plant parts, e) combustible energy, f) percent organic matter,

and g) percent in vitro dry matter digestibility, (IVOMD). Statistical

analyses for these variables are presented in Tables 3-2, 3-3, 3-4, and


Table 3-2. Stastistical analysis model for maize data. Factorial (3x3)
for treatments 2, 5, 6, 7, 8, 9, 11, 12, and 13.

Source df
Replications 3 (r-l)
Treatments 12 (T-l)
SPECIES EFFECT (2 sorghums + millet) 2 (E-1)
Treats (2, 5, 6, 7, 8) vs (11, 12, 13) 1
Treats (2, 5, 6) vs (7, 8, 9) 1
Treats (2, 7, 11) vs (5, 6, 8, 9, 12, 13) 1
Treats (5, 8, 12) vs (6, 9, 13) 1
Treat 3 vs 4 1
Treats (1, 10) vs (3, 4) 1
Treat I vs 10 1
ERROR 36 (r-1)
TOTAL 47 (RT-1)

* The degrees of freedom are not orthogonal.

Table 3-3.

Statistical analysis model used for sorghum data. Factorial
(2x2) for treatments 1 through 9.

Source df

Treats (1, 2, 3, 4, 5, 6) vs (7, 8, 9)
Treats (2, 5, 6) vs (7, 8, 9)



(a-1) (b-1)

(r-i) (t-i)

* Degrees of freedom are not orthogonal.

Table 3-4.

Statistical analysis model used for millet data. Randomized
complete block design.

Source df

Treat (10) vs (11, 12, 13) 1
Treat (11) vs (12, 13) 1
ERROR 9 (r-1) (t-1)
TOTAL 15 (rt-1)

Table 3-5.

Statistical analysis model used in the growth analysis. For
treatment 6, 9, and 13.

Source df

ERRORS 36 (r-1) (t-l)
TOTAL 47 (rt-1)

Growth Analysis

To conduct an analysis of the growth of the systems under study in

the fertility trials, treatments 6, 9, and 13 were selected as

representatives of each system. Crop growth rate (CGR) of the 'NB-3' +

'Criollo', 'NB-3' + 'Pioneer 895', and 'NB-3' + 'Gahi-3' systems were

estimated on different phases of growth (Table 3-6). Total above-ground

growth was harvested from six hills of 'NB-3' and 'Criollo' and 1 m row

length from 'Pioneer 895' and 'Gahi-3' every sampling stage. Sampling

began when the crops were thinned, and at 21 and 24 days after seeding

for the first and second crops, respectively. At each sampling stage

plants were harvested, separated by components (leaf, stem, flower,

head, and/or ears) and weighed in the field. Subsamples were weighed and

oven dried at 70C for 72 hours (when constant weight was reached) then

weighed again to determine dry matter content. The change in average

plant dry weight on the nth (day n + t) day since the previous harvest

(on day t) was divided by n to estimate crop growth rate (CGR) expressed

in kg ha -dayI for each day in the period. Sub-samples were ground in a

Wiley mill to pass a 1 mm stainless steel screen, and stored in

air-tight bags until analyzed.

Green leaf area measurements were made at 50% bloom, soft-dough,

and at black-layer stages. All the leaves of the 12 maize plants from

each of the three plots were measured from base to tip and at the point

of maximum width. Leaf area was converted to leaf area indices (LAI) as

described by Dale et al (1980).

Daily precipitation data (Fig. 5-2) were obtained by averaging

readings from four rain gauges placed in the the four replications. Soil

samples from three 15-cm sections to a depth of 45 cm were taken on a

Table 3-6. Sampling procedure for growth analysis.

Sampling Days after
Parts stage Cl planting

WP At thinning 21
L + S 1.0 m tall 65
L + S + F Full silk 73
L + S + F + E Soft-dough stage 99
L + S + F + E Black layer 120
L + S + F + E Harvest 160
L + S + 30 days after harvest 194

Sampling Days after
Parts stage PS NS MI

WP Thinning 24 24 24
L + S 0.75 m tall 45 45
L + S + H Full bloom 99 52 45
L + S + H Soft-dough stage 120 81
L + S + H Harvest 160 101 45/85
L + S 30 days after harvest 193 136

Cl = Maize, PS = 'Criollo', NS = 'Pioneer 895', MI = 'Gahi-3'.


weekly basis to determine percent soil moisture on a volumetric basis.

Soil from each increment was placed in a previously weighed can, then

weighed, oven dried at 105C for 24 hours, and weighed again. Assuming

constant weight, percent soil moisture was determined by difference.

Survey of Sulfur Deficiency in Maize

Experiment 1

Sixty day-old no-tillage maize was grown in a 65 ha field in

Alachua County, Florida. Plants showed various degrees of stunting and

ranged from dark green healthy plants to light green or yellowish

stunted plants. Plant height ranged from approximately 30 cm to 120 cm.

The stunted plants exhibited intervenal chlorosis, the degree of which

diminished as plant height increased. The hypothesis proposed stated

that the problem was likely associated with soil characteristics and the

solution could be obtained through soil-plant analysis.

A completely randomized experimental design was used that included

three replications of five maize treatments. Treatments included 30, 60,

75, 90, and 120 cm tall plants. Ten whole-plant samples were taken at

random for each replication, as well as the associated youngest mature

leaf. Soil samples were taken at several depths within 25 cm of the

treatment plants.

Plants and youngest mature leaves were washed in distilled water,

dried at 70C in a forced air oven, and ground in a Wiley mill to pass a

1 mm stainless steel screen. Soils were sampled in 15 cm increments to

45 cm, then later to 90 cm, air dried, ground by mortar and pestle, and

screened on a 2 mm stainless steel screen. Plant and soil samples were

analyzed as described in the section of laboratory procedures of this


---'----- ___________________.....

Experiment 2

Eighteen maize fields in northern Nicaragua (Esteli and Matagalpa)

that were between 40 and 50 days old were selected as experimental

fields. A second criterion of selection was the ocurrence of S deficient

and sufficient healthy looking plants (based on the criteria established

in experiment 1).

Once inside the experimental field, four deficient and four

sufficient plants were selected. Each pair of plants within a field was

considered to form a replication of a randomized complete block design.

The youngest fully expanded mature leaf was collected from each plant,

described, and measured in length and width. The leaves were weighed,

oven dried, ground in a Wiley mill to pass a 1 mm stainless steel

screen, and stored in air-tight bags for analysis.

Four soil samples (from 0 to 30 cm depth) were taken within a 25 cm

circumference around each plant. The samples for each plant were mixed

to form one sample per plant per replication. Samples were air dried,

sieved to pass a 2 mm stainless steel screen and, stored for analysis.

Response variables measured in plants were a) plant height, b) leaf

length, c) leaf width at the widest point, d) dry weight per leaf, e)

leaf concentrations of S, N, P, K, Ca, Mg, Cu, Fe, Mn, Zn. Soil response

variables measured were concentration of the same nutrients measured in

the leaf tissue.

Laboratory Procedures

Soil Analysis Methods

For all experiments, N analysis employed a microKjeldahl procedure

(Bremner, 1960) as modified by Gallaher et al. (1976). A l.0-g sample


was placed in 100-ml digestion tube to which 3.2 g of catalyst (90%

anhydrous K SO4, 10% anhydrous CuSO 4), 10 ml concentrated H SO4 and 2 ml

of 30% H202 were added. Samples were then digested in an aluminum block

digester (Gallaher et al., 1976) for 2.5 hours at 375C. Upon cooling,

solutions were diluted to 75 ml with deionized water. Nitrogen

concentrations of these prepared solutions were determined using a

Technicon AutoAnalyzer II.

All soil P, K, Ca, Mg, Fe, Cu, Mn, and Zn analyses were conducted

using procedures recommended by the University of Florida's Soil Testing

Laboratory. Five grams of air-dried soil were extracted with 0.05 N HCl

+ 0.025 N H 2SO4 at a soil:solution ratio of 1 to 4 (W:V) for 5 minutes.

Soil P was then analyzed using colorimetry. Potassium was determined by

atomic emission spectrophotometry. Calcium, Mg, Fe, Cu, Mn, and Zn were

determined by atomic absorption spectrophotometry. Soil pH was

determined using a 2:1 water:soil ratio.

Soil S was determined by the method described by Bardsley and

Lancaster (1965). Ten grams of 20-mesh soil were placed in a 50-ml

Erlenmeyer flask and extracted with 39 g of NH4 C2H302 diluted in one L

of 0.025 N acetic acid for 30 minutes, 0.25 g of washed activated

charcoal was added and extracted for 3 additional minutes. The soil

suspension was filtered using a sulfate-free Whatman No. 42 filter

paper. Ten milliliters of the filtrate were pipetted into a 50-ml

Erlenmeyer flask to which 1 ml of acid seed solution (6 N HCl + 20 mg
kg of S as K 2SO 4) was added, swirled and 0.5 g of BaCl 22H 0 crystals

were added. This solution was left standing for 1 minute, then swirled

to dissolve all the crystals. Soil S concentration was then determined

using a Perkin-Elmer/Coleman 54 spectrophotometer.

Plant Analysis Methods

Nitrogen analysis of plant material employed the microKjeldahl

procedure modified by Gallaher et al. (1976). A 0.1-g sample was placed

in a 100-ml digestion tube to which two boiling chips, 3.2 g of catalyst

(90% anhydrous K 2SO4, 10% anhydrous CuSO4), 10 ml of concentrated H 2SO4

and 2 ml of H2 02 were added. Samples were then digested in an aluminum

block digester (Gallaher et al. (1976) for 2.5 hrs. Upon cooling,

solutions were diluted to 75 ml with deionized water. Nitrogen

concentration of these solutions were determined on a Technicon

AutoAnalyzer II.

Phosphorus, K, Ca, Mg, Fe, Cu, Mn, and Zn concentrations were

determined by a mineral analysis procedure in which 1.0 g samples were

placed in 50-ml pyrex beakers and ashed in a muffle furnace at 480C

for a minimum of 4 hrs. After cooling each was treated with 2 ml of

concentrated HC1 and heated to dryness on a hot plate. An additional 2

ml of concentrated HCl + water was added to the dry beakers followed by

reheating to boiling and then diluting to 100 ml volume with deionized

water. Solutions were analyzed for P using colorimetry on an

Autoanalyzer. Potassium was determined by atomic emission

spectrophotometry. Calcium, Mg, Fe, Cu, Mn, and Zn were determined by

atomic absorption spectrophotometry.

IVOMD of plant material was determined by the Tilley and Terry

(1963) two-stage procedure adapted by Moore et al. (1972). For

measurements of combustible energy, approximately 0.5 g of sample was

pelleted in a cylinder press and weighed to the nearest 0.00001 g.

Samples were then placed in clean combustion boats. Combustible energy


values were obtained using a computerized Parr adiabatic calorimeter,

using standard ASTM methods (ASTM, 1979).

A sample of 0.3 g + 0.05 g of plant tissue was weighed in a clean

boat. The samples were spiked with 0.5 g of vanadium pentoxide (V2 0 5).

Sulfur concentrations were then determined using a Leco S Determinator

model SC132 at 540 nm.



In Central America, sorghum (Sorghum bicolor (L.) Moench) is

generally cultivated in association or in sequence with maize (Zea mays

L.). These two cropping systems are used by low-income farmers on

marginal agricultural land located in the semi-arid regions. Farmers use

these systems to stabilize production, reduce the risk of maize yield

loss caused by irregularities in climate, and as a response to scarcity

of resources. Monocropping sorghum systems are found on excellent

agricultural land where mechanization is possible and are practiced by

farmers of bountiful resources.

These production schemes have different purposes. In multiple

cropping systems sorghum produced by low-income farmers serves as a

staple food for human and animal consumption, while monocropping systems

produce testa-colored sorghums, used only in animal feed or for forage.

Based on this differentiation, production problems can be grouped in two

categories: those of sequential and intercropping systems and those of

monocropping systems.

Available information to improve the maize + sorghum cropping

system is limited or has not been adequately diffused to low income

farmers. This scarcity exists because national research programs have

generally been designed with a reductionist philosophy (research by



product), making it difficult for scientists to analyze interactions or

competition within the system.

According to Arze et al. (1983), in a research scheme for sorghum

cropping systems it is necessary to analyze the problem in a series of

logical and sequential phases, beginning with broad aspects and ending

with specific matters. This analysis permits the hierarchic

identification of relations between components of the system and the

determination of restrictions to crop production. The identification of

these restrictions is a basic element for the successful design of

research; it will also enhance the possibilities of accomplishing

specific proposed objectives.

In a general way, and considering sequence, the phases designed by

Arze et al. (1983) can be summarized as follows: 1) definition of the

problem, 2) characterization and diagnosis, 3) design of the research,

4) implementation, 5) validation, and 6) diffusion.

Few attempts have been made in the area to describe the maize +

sorghum system. Some of the most recent attempts include those conducted

in different areas of El Salvador by Rodriguez et al. (1977), Guillen et

al. (1978); Alvarado et al. (1978), Alegria et al. (1979); and Arias et

al. (1980). Kass (1980) and Fuentes and Salguero (1983) give a brief

description of the system in Guatemala. In Honduras, Mateo et al. (1981)

described the system in some detail. Hawkins et al. (1983) and Larios et

al. (1983) have provided the only recent descriptions of what may be

considered the typical maize + sorghum cropping system in Central


The objectives of this research were 1) to locate and describe the

maize + sorghum system in Central America, 2) to describe the relations


between the maize + sorghum systems and its bio-physical and

socio-economic environment, 3) to describe interactions with other

production systems, and 4) to identify constraints and research

opportunities to alleviate these constraints.

Materials and Methods

Between November 1981 and March 1983 several trips were made to

Guatemala, El Salvador, Honduras, and Nicaragua. During these visits the

primary areas of production in each country were visited to obtain first

hand observation of the bio-physical and socio-economic environments of

the system. Informal interviews were held with several randomly selected

farmers, with extension agents, and with research staff of each area.

These visits provided an opportunity to gather more secondary

information in each country.

Due to economic and time limitations the search of information to

meet the proposed objectives was limited to the topics listed in the

following outline:

I. Bio-physical characteristics.

A. Soil and climatic factors.

1. Edapho-climatic characterization.

2. Identify relations between climate and the

hydro-edaphocelations in the area.

B. Agro-biological factors.

1. Characterization of the maize + sorghum/animal system.

2. Analysis of constraints to the system.

C. Eco-physiological relations of the system.

1. Photoperiodism.


2. Rainfall and soil moisture.

3. Temperature.

II. Socio-economic Characteristics.

A. Social aspects.

1. Structure of the production system.

2. Social systems.

B. Economics.

1. Economic parameters of production.

2. Use of available resources.

Results and Discussion

In the semi-arid regions of Central America the owners of small and

medium size farms have developed a maize + sorghum/animal production

system (maize + sorghum/animal) in response to the predominant

environmental characteristics (Arias et al., 1980). Management of the

system and the structure of its components are based primarily on

environmental variations and economic parameters. Larios et al. (1983)

found few studies of the relationship among components in this system or

of the analysis of inputs and outputs. The search for technological

alternatives requires from the researcher an understanding of the system

and an analysis and selection of the alternatives within the farmer's

possibilities that will cause greater development.

The system described here has been generally called "Maize/sorghum

intercrop". A list of common names used in Central America follows:


Maiz y sorgo El Salvador and Guatemala (Arias et

al.,1980;Kass, 1980)


Maiz y maicillo El Salvador, Guatemala, and Honduras

(Arias et al., 1980; Rosales, 1980;

Mateo et al., 1981; Fuentes and

Salguero, 1983)

Maiz y million Nicaragua (CATIE, 1980; Pineda et al., 1979)

Although maize and sorghum are the most important food crops in the

semi-arid regions of Central America (Larios et al., 1983), other crops

such as bean (Phaseolus spp.) (Guillen et al., 1978), cowpea (Vigna

spp.) (Alegria et al., 1979), and fruits are important in specific

areas. Sesame (Sesamum indicum L.) (CATIE, 1982a) and flaxseed (Linum

usitatissimum L.) (CATIE, 1980) are widely cultivated and compete with

maize and sorghum for land and other resources.

Larios et al. (1983) reported that in the countries where the

system exists it interacts with animal production systems. Therefore, a

more complete and descriptive name should include its animal

componentss. A list of animal components of the systems found in these

areas follows:


Cattle (dual purpose) Meat, dairy, and power (Juarez

et al., 1979; Mateo et al., 1981

Swine Meat and lard (Rodriguez et al., 1977)

Poultry Meat and eggs (Guillen et al., 1978; Kass,



The maize + sorghum/animal production system is limited generally

to the foothills near the Pacific coastal plains, rolling lands and

valleys of the interior of Central America as depicted in Figure 4-1.

Figure 4-1.

Distribution of maize + sorghum systems in Central
America (Drawn with information from Arias et al.,
1980; Mateo et al., 1981; and Hawkins et al.,


Hawkins et al. (1983) identified northern El Salvador as the area where

the system is most widely cultivated (240,000 ha). Reports in the

literature (Arias et al., 1980; Mateo et al., 1981; Fuentes and

Salguero, 1983) indicated that of the total area cultivated with sorghum

in Guatemala, El Salvador, and Honduras, 80, 93, and 93%, respectively

are intercropped with maize.

Bio-Physical Environment


The Central American Isthmus extends from east to west with the

Caribbean Sea at the north and the Pacific Ocean at the south (Fig.

4-1). Hot humid lowlands predominate on the Atlantic coast. The interior

is composed of mountains and valleys. A wide belt of steppe conditions

is found in these areas where the maize + sorghum system is found.

Precipitation stays below 1,000 mm yrI in the interior valleys. This

dryness is probably caused by mountain valley winds rather than by the

shielding effect of the mountains. In the Pacific coastal foothills

annual rainfall may range from 1,400 to 2,000 mm (Alvarado et al., 1978;

CATIE, 1980), and in some areas, such as northern El Salvador it may be

greater than 2,000 mm (Guillen et al., 1978). Annual rainfall is

distributed in a bimodal pattern (CATIE, 1980; CATIE, 1982a). The dry

season begins in November and ends in April or May, and the wet season

is interrupted by a dry period called "canicula" in July or August

(Rodriguez et al., 1977; Guillen et al., 1978). Results of studies

conducted by CATIE, (1980); and Guzman (1982) indicate that potential

annual evapotranspiration is high. The observed range in some areas is

between 1,000 and 2,000 mm (Fig. 4-2). This results in a soil moisture

deficit through May and the depletion of soil reserve in July. In the

E 600
E 500-

'- 400.




Figure 4-2.

S Jan Mar May Jul .Sep Nov

Water deficient periods. La Trompina, El
Salvador (CATIE, El Salvador, unpublished


semi-humid areas an excess of water occurs in September, contributing to

an increase in the availability of residual moisture through December.

The number of months in a year with a moisture deficit in the semi-arid

and semi-humid regions ranges from 7 to 10 and from 5 to 6,


Southeastern Guatemala, southern Honduras, northeastern El

Salvador, and northwestern Nicaragua correspond to what have been

described as semi-arid regions (Larios et al., 1983). The agroclimatic

characteristics of these regions are similar to other semi-arid regions

of the world. These are summarized by Larios et al. (1983) as follows:

1. The beginning of the rainy season is uncertain.

2. More than 90% of annual precipitation occurs during the wet

season, which lasts generally from four to seven months.

3. Precipitation during the wet season is often extremely variable,

not only from year to year but also within seasons.

4. Mean daily rainfall intensities are two to four times greater

than in many temperate regions. The short duration intensities

frequently exceed the water intake capacity of the soil.

Mateo et al., (1981) claimed that the areas where maize + sorghum

is found most frequently correspond to what Holdridge classified as 1)

Bh-S(c), humid sub-tropical forest with biotemperature above 24C, 2)

Bs-T, dry tropical forest with biotemperatures below 24C but with

annual average air temperatures above 24C, and 3) Bs-S, dry

subtropical forest. Arias et al. (1980) and Larios et al. (1983) agree

that the cultivation of sorghum is related to biotemperatures or air

average annual temperatures above 24C.

Predominant soil types

The soils where the maize + sorghum cropping system is cultivated

can be classified into one of the following orders: Alfisols, Entisols,

Inceptisols, Vertisols or Mollisols (CATIE, 1980; Kass 1980; CATIE

1982a; Rico, 1982). Figure 4-3 shows an environmental profile for the

association of maize and sorghum in El Salvador, Nicaragua, and

Honduras; considering annual rainfall, altitude, slope, and soil

fertility and depth. The typical landscape is made up of steep lands,

slopes up to 50%, shallow soils (30 cm maximum) with prevalence of loose

stone or shale. It is highly susceptible to erosion (Arias et al.,

1980; CATIE, 1980).

Socio-Economic Environment

Family composition

Larios et al. (1983) reported that the average farm family is

comprised of seven members, approximately 75% of whom are under 30 years

of age; in some areas the population is somewhat younger (40% under 12

years). Assuming the inputs of women and children are 0.7 and 0.5,

respectively, man's working day, the average farmer has a daily
equivalent of 5 man days in his family.


Education levels vary from country to country. In some literacy can

be higher than 80%; in others it may be lower than 60%. Among children

it is generally higher than in parental groups (Larios et al., 1983).


Farm size is considered to range from small to medium (0.25 to 70

ha.). Land tenure is unsatisfactory (Green, 1974). In some cases 75% of

the farmers occupy 25% of the land (Hawkins et al., 1983; Larios et al.,

0 800 12001(

0 5001000

0.8 8.30 30

Very Moderate
* low low

0 10

Figure 4-3.

!000 2400 Annual ralnfoll,mm

2000 Alt Itudem

I Slope,%


Soil fertility

Soil depth cm

Environmental profile of the maize sorghum system
in three countries of Central America (El Salvador,
Nicaragua, Honduras) (Larios et al., 1983).


1983). This situation is rapidly changing through agrarian reform plans

(personal observation by the author).

Cash flow in and out of the farms is very difficult to quantify,

especially that spent on food and clothing. Farm expenses and farm

activities are closely related to farm size (Fig. 4-4). In crop

production, a low-income farmer on a 7 ha farm may invest up to $200

(US) mainly on fertilizers (70%) and other field supplies and about $45

(US) on animal feeds. Cash flow into the farms comes from activities on

and off the farm, as depicted in Figure 4-4. On small-scale farms most

of the income (approximately 75%) is obtained from the sale of excess

grains (maize and sorghum), dairy products, meat, and draft animals

(CATIE, El Salvador, unpublished data, 1982). In their characterization

studies Arias et al. (1980) and Larios et al. (1983) emphasize the

importance of the animal component in generating income increases as

farm size augments (Fig. 4-5). Cattle are more common on larger farms,

while swine and poultry can be important sources of income among the

smaller farms.

Facilities and equipment

The value of fixed capital (housing, storage, fencing, and animal

shelter) is affected also by farm size; on the average, farmers of these

areas report holdings worth approximately $830 (US) (CATIE, 1980).

Juarez et al. (1979) report that farm equipment is limited to sprayers,

hoes, shovels, and "macanas" or "bordones" (a handweeding instrument

used for planting beans and sorghum into stands of maize). The average

value of this equipment varies from $18 to 40 (US). Very few farmers

own transport facilities other than an oxcart.


L6 h' Il hoa

60 ho.


Figure 4-4. Percentage of income derived from farm activities
in different farm sizes (unpublished data, CATIE,
El Salvador).

70- Cattle -
Annual Crops -
60 Rented Land




/o /
<20-, -


0 7 14 21 2835 42 49 56

Figure 4-5. Variation of activities as farm size increases.
Tejutla, El Salvador (Unpublished data, CATIE,
El Salvador).

Farming systems

On small-scale farms crop production systems constitute the main

activities, since they are related to family subsistence. As farm size

increases the area dedicated to crop production is comparatively small,

about four or five hectares, the maximum surface which can be managed by

a farmer and his family (Larios et al., 1983). As farms become larger in

size, the areas dedicated to cattle activities tend to increase,

especially as related to crop production areas (Fig. 4-5).

On farms larger than 50 ha, crop production systems increase since

the owner rents part of the land to landless farmers to be planted with

maize and sorghum. In return the farm owner will receive cash, part of

the crop, different forms of labor, and/or combinations of these

arrangements. The cultivated areas managed directly by the farm owner

tend to diminish to a minimum level of subsistence.

Hawkins et al. (1983) and Larios et al. (1983) were able to

establish a direct relation between farm size and the availability of

soil moisture. Large farms are normally located in areas with less

available soil moisture whereas small-scale farms are concentrated in

areas where more soil moisture is available. In the low-income farmer's

production systems maize is the main crop, normally associated with

beans or sorghum. Pigs and poultry are the main income producing

animals. The increase in the hydric deficit modifies the cropping

patterns; maize is displaced in importance by sorghum, forage, pasture,

or sisal. Swine and poultry are kept on most farms, and the number of

cattle is increased. The relations among farm size, hydric deficit, and

farming systems are shown in Figure 4-6, all within a representative

area for the maize + sorghum/animal production system.

Crop production systems

Studies by Guzman (1982), Hawkins et al. (1983), and Larios et al.

(1983) indicated that the location of the cropping system (Fig. 4-1) is

very much related to the frequency of uncertain rainfall periods

interrupted by a "canicula" (Fig. 4-2). Those farmers who practice

sorghum-based cropping systems have adopted cropping patterns that

diminish risk but ensure food for their families (Clara et al., 1983).

Figure 4-7 depicts the relation between rainfall patterns and relative

growth of the maize + sorghum system. Arias et al. (1980), Mateo et al.

(1981), and Hawkins et al. (1983) identified four cropping patterns as

the most often used in the system (Fig. 4-7): a) maize and sorghum

planted simultaneously in May; b) maize planted in May, sorghum planted

25 to 30 days later (at sidedressing and hilling up of the maize); c)

maize planted in May, sorghum in July (during anthesis of maize); and d)

maize planted in May and sorghum in August (at bending-over of maize).

The patterns used are closely related to the cultivar and available soil

moisture in the area. The crop spacings found more frequently are a)

single rows of maize interplanted with sorghum, b) single rows of maize

and sorghum both sharing the same hill, and c) sorghum broadcast in

maize stands (Fig. 4-8).

The early maturing maize cultivars (46 days to tasseling and 80 to

harvest) have a greater probability of escaping the "canicula".

Cultivars such as 'Criollo' perform better when cultivated in

association with sorghum (Clara et al., 1983). Moreover, these varieties

can be seeded for grain and/or forage. Sorghum cultivars used are

day-length sensitive; when planted in May, they have a long vegetative

growth period. In August and afterwards the competition with maize is