• TABLE OF CONTENTS
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 Front Cover
 Title Page
 Copyright
 Table of Contents
 List of Tables
 List of Figures
 Introduction
 Yield gap analysis
 Nutrients
 Planting date and crop establi...
 Water management
 Lodging
 Weed control
 Conclusion
 Reference
 New papers from the Natural Resources...






Group Title: Paper - Natural Resources Group - 98-01
Title: Increasing wheat yields sustainably through agronomic means
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Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00077523/00001
 Material Information
Title: Increasing wheat yields sustainably through agronomic means
Series Title: National Resources Group paper
Physical Description: v, 22 p. : ill. ; 28 cm.
Language: English
Creator: Hobbs, P. R
Sayre, Kenneth Dean, 1945-
Ortiz Monasterio Rosas, José Iván, 1958-
International Maize and Wheat Improvement Center
Publisher: International Maize and Wheat Improvement Center
Place of Publication: México D.F. México
Publication Date: 1998
 Subjects
Subject: Wheat -- Yields   ( lcsh )
Cropping systems   ( lcsh )
Agriculture -- Research   ( lcsh )
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Includes bibliographical references (p. 19-21).
Statement of Responsibility: P.R. Hobbs, K.D. Sayre, and J.I. Ortiz-Monasterio.
General Note: "CIMMYT sustainable maize and wheat systems for the poor"--Cover.
General Note: An errata slip is inserted.
Funding: Electronic resources created as part of a prototype UF Institutional Repository and Faculty Papers project by the University of Florida.
 Record Information
Bibliographic ID: UF00077523
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 39528614
issn - 1405-2830 ;

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Table of Contents
    Front Cover
        Front cover
    Title Page
        Page i
    Copyright
        Page ii
    Table of Contents
        Page iii
    List of Tables
        Page iv
    List of Figures
        Page v
    Introduction
        Page 1
    Yield gap analysis
        Page 1
    Nutrients
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
    Planting date and crop establishment
        Page 8
        Page 9
        Page 10
    Water management
        Page 11
        Page 12
        Page 13
    Lodging
        Page 14
        Page 15
        Page 16
    Weed control
        Page 17
    Conclusion
        Page 18
    Reference
        Page 19
        Page 20
        Page 21
    New papers from the Natural Resources Group
        Page 22
Full Text





II
CIMMYT


Increasing Wheat Yields

Sustainably through Agronomic Means


P.R. Hobbs, K.D. Sayre, and J.I. Ortiz-Monasterio *






Natural Resources Groupo
Paper 98-01





The authors are from the International Maize and Wheat Improvement Center (CIMMYT). Peter R. Hobbs is an
Agronomist with the Natural Resources Group and is based in Kathmandu, Nepal. Kenneth D. Sayre is Head, Crop
Management/Physiology, and J.I. Ortiz-Monasterio is an Agronomist; both are with the Wheat Program and based in
Mexico. The views presented in this paper are the authors' and do not necessarily reflect CIMMYT policy. This paper
was prepared for the International Group Meeting on "Wheat Research Needs Beyond 2000 AD," 12-14 August 1997,
Directorate of Wheat Research, Karnal, India.




























CIMMYT is an internationally funded, nonprofit scientific research and training organization.
Headquartered in Mexico, the Center works with agricultural research institutions worldwide to improve
the productivity and sustainability of maize and wheat systems for poor farmers in developing countries. It
is one of 16 similar centers supported by the Consultative Group on International Agricultural Research
(CGIAR). The CGIAR comprises over 50 partner countries, international and regional organizations, and
private foundations. It is co-sponsored by the Food and Agriculture Organization (FAO) of the United
Nations, the International Bank for Reconstruction and Development (World Bank), the United Nations
Development Programme (UNDP), and the United Nations Environment Programme (UNEP).

Financial support for CIMMYT's research agenda currently comes from many sources, including the
governments of Australia, Austria, Belgium, Canada, China, Denmark, France, Germany, India, Iran, Italy,
Japan, the Republic of Korea, Mexico, the Netherlands, Norway, the Philippines, Spain, Switzerland, the
United Kingdom, and the USA, and from the European Union, the Ford Foundation, the Inter-American
Development Bank, the Kellogg Foundation, the OPEC Fund for International Development, the Rockefeller
Foundation, the Sasakawa Africa Association, UNDP, and the World Bank.

Responsibility for this publication rests solely with CIMMYT.

Printed in Mexico.

Correct citation: Hobbs, P.R., K.D. Sayre, and J.I. Ortiz-Monasterio. 1998. Increasing Wheat Yields Sustainably
through Agronomic Means. NRG Paper 98-01. Mexico, D.F.: Mexico.

Abstract: This paper examines common factors that constrain wheat yields: insufficient nutrients (using
nitrogen as an example); problems of late planting and poor crop establishment; suboptimal water
management; lodging; and weeds. The authors suggest agronomic practices, including tillage practices,
rotations, and input management options that can ameliorate important constraints and sustainably
improve yields. Examples are drawn largely from rice-wheat systems in the Indo-Gangetic Plains and from
wheat systems in northwestern Mexico. These examples indicate that there is still considerable potential for
raising wheat yields in a sustainable manner and meeting rapidly expanding demand for wheat in
developing countries.

ISSN: 1405-2830
AGROVOC descriptors: Wheats; farming systems; crop management; cropping patterns; planting date;
nutritional requirements; fertilizer application; lodging; plant water relations; weed control; yields;
developing countries; research projects; innovation adoption
AGRIS category codes: F01 Crop husbandry
F08 Cropping patterns and systems
Dewey decimal classification: 633.115











Contents

Page

iv Tables
v Figures
1 Introduction
1 Yield Gap Analysis
2 Nutrients
8 Planting Date and Crop Establishment
11 Water Management
14 Lodging
17 Weed Control
18 Conclusion
19 References










Tables

Page

3 Table 1. Kilograms of nitrogen in the above-ground wheat biomass at maturity for different
harvest index values and different yield levels
3 Table 2. Amount of nitrogen that must be supplied by the soil to the above-ground biomass
(grain and straw) at a harvest index of 0.41
4 Table 3. External nitrogen that needs to be applied to wheat to obtain various grain yields,
calculated at different nitrogen recovery rates
5 Table 4. Effect on wheat yield (kg/ha) of applying poultry manure to wheat, with and
without susboiling, CIANO Station, Sonora, northwestern Mexico
7 Table 5. Comparison of the application of 150 kg N/ha at planting and first node on grain
yield and grain protein percentage for various wheat genotypes
10 Table 6. Date from a wheat establishment trial following rice, Bhairahawa Agricultural Farm,
Nepal, 1993/94
10 Table 7. Comparison of zero tillage and farmers' practice for establishing wheat after rice in
locations in the Pakistan Punjab where planting dates for the two methods differed
13 Table 8. Effect of bed size configuration on wheat grain yield, Punjab Agricultural
University, Ludhiana, India, 1995/95
13 Table 9. Grain yield (kg/ha at 12% moisture) for conventional versus bed planting at high
and low seed rates, CIANO Station, Sonora, northwestern Mexico, 1993/94
14 Table 10. Wheat yield (kg/ha) averaged over four years for tillage/straw management and
nitrogen management treatments for a bed-planted wheat (W) and maize (M)
rotation
16 Table 11. Strategies for applying 225 units of nitrogen fertilizer and resulting effect on lodging










Figures


2 Figure 1. Wheat yield data, Yaqui Valley, northwestern Mexico, 1990-95
3 Figure 2. Percentage of nitrogen in wheat grain and straw at different yield levels
6 Figure 3. Response of grain yield to different rates and timing of nitrogen application
7 Figure 4. Response of flour protein to different rates and timing of nitrogen application
7 Figure 5. Response of percent apparent fertilizer recovery to different rates and timing of
nitrogen application
8 Figure 6. Nitrogen response for wheat under conventional and zero tillage from seven
experiments in farmers' fields, Punjab, Pakistan
8 Figure 7. Effect of planting date on wheat yield, by variety, Punjab, India
8 Figure 8. Effect of planting date on wheat yield, 1987/88 to 1990/91
17 Figure 9. Effect of Ethephon on yields of two wheat varieties










Increasing Wheat Yields Sustainably
through Agronomic Means

P.R. Hobbs, K.D. Sayre and J.I. Ortiz-Monasterio


Introduction

Despite the impressive advances that have been
made over the years in improving the yields of
food crops, including wheat, there is little
reason to become complacent about the food
supply, especially in the developing world.
During the next three decades, the population
of developing countries will grow by at least
1.6%. As this growing population becomes
increasingly urban-based, as incomes rise, and
as consumers substitute out of rice and coarse
grain cereals, the demand for wheat will rise.
By 2020, two-thirds of the world's wheat
consumption will occur in developing countries
(CIMMYT 1997). To meet demand across the
Asian Subcontinent, we will have to maintain
wheat yield growth at 2.5% per year over the
next 30 years, because cropped area is expected
to remain minimal or even negative (Hobbs
and Morris 1996). Yields will not only have to
grow; they will have to grow without depleting
the natural resource base on which agriculture
depends.

This is no small challenge for agricultural
research, but there are reasons to be optimistic
that researchers will be able to develop
technologies that can improve wheat yields and
at the same time preserve the resource base.
Some of the most exciting opportunities for
sustainably improving wheat system
productivity have been developed through
crop management research, and they are
reviewed in this paper. We begin by describing
the gap between farmers' actual yields and
potential yields and the reasons for that yield
gap. Next, we review a series of factors that


influence yields: nutrients, planting date, crop
establishment, water management, lodging, and
weed control. We provide examples of how
agronomic practices can improve the efficiency
of each factor and ultimately increase yield in a
sustainable manner. In addition, we discuss
some potential interactions of alternative crop
management strategies and some of the
requirements for farmers to adopt new
management strategies.


Yield Gap Analysis

Wheat yields can be described in various ways:

1. The highest physiological yield where there
are no biotic or abiotic constraints (i.e., the
highest yield that could theoretically be
obtained). This yield is determined by solar
radiation and temperature and the genetic
ability of the plant to convert light energy
into dry matter and subsequently partition
this dry matter into harvested yield. Potential
yield can be calculated from radiation and
temperature data by models for different
locations. In northwestern Mexico, potential
yield could surpass 10,500 kg/ha, which has
been reported in some CIMMYT trials at the
CIANO experiment station.

2. The highest achievable yield obtained from
maximum yield experiments under field
conditions where all inputs are provided
without constraint and plants are protected
from lodging and biotic stresses (i.e., the
highest yield that has actually been
obtained). This figure may approach that in










Increasing Wheat Yields Sustainably
through Agronomic Means

P.R. Hobbs, K.D. Sayre and J.I. Ortiz-Monasterio


Introduction

Despite the impressive advances that have been
made over the years in improving the yields of
food crops, including wheat, there is little
reason to become complacent about the food
supply, especially in the developing world.
During the next three decades, the population
of developing countries will grow by at least
1.6%. As this growing population becomes
increasingly urban-based, as incomes rise, and
as consumers substitute out of rice and coarse
grain cereals, the demand for wheat will rise.
By 2020, two-thirds of the world's wheat
consumption will occur in developing countries
(CIMMYT 1997). To meet demand across the
Asian Subcontinent, we will have to maintain
wheat yield growth at 2.5% per year over the
next 30 years, because cropped area is expected
to remain minimal or even negative (Hobbs
and Morris 1996). Yields will not only have to
grow; they will have to grow without depleting
the natural resource base on which agriculture
depends.

This is no small challenge for agricultural
research, but there are reasons to be optimistic
that researchers will be able to develop
technologies that can improve wheat yields and
at the same time preserve the resource base.
Some of the most exciting opportunities for
sustainably improving wheat system
productivity have been developed through
crop management research, and they are
reviewed in this paper. We begin by describing
the gap between farmers' actual yields and
potential yields and the reasons for that yield
gap. Next, we review a series of factors that


influence yields: nutrients, planting date, crop
establishment, water management, lodging, and
weed control. We provide examples of how
agronomic practices can improve the efficiency
of each factor and ultimately increase yield in a
sustainable manner. In addition, we discuss
some potential interactions of alternative crop
management strategies and some of the
requirements for farmers to adopt new
management strategies.


Yield Gap Analysis

Wheat yields can be described in various ways:

1. The highest physiological yield where there
are no biotic or abiotic constraints (i.e., the
highest yield that could theoretically be
obtained). This yield is determined by solar
radiation and temperature and the genetic
ability of the plant to convert light energy
into dry matter and subsequently partition
this dry matter into harvested yield. Potential
yield can be calculated from radiation and
temperature data by models for different
locations. In northwestern Mexico, potential
yield could surpass 10,500 kg/ha, which has
been reported in some CIMMYT trials at the
CIANO experiment station.

2. The highest achievable yield obtained from
maximum yield experiments under field
conditions where all inputs are provided
without constraint and plants are protected
from lodging and biotic stresses (i.e., the
highest yield that has actually been
obtained). This figure may approach that in











definition 1 but is usually less because some
biotic and/or abiotic constraint is present
during crop development. The improved
wheat variety Super Kauz yielded
8,845 kg/ha (averaged over six years,
1990-95) in CIMMYT maximum yield trials
at the CIANO station.

3. The average performance of "normally
managed" on-station trials. These trials
receive the recommended station fertilizer,
irrigation, weed control, and control of
other biotic stresses. The plants are not
protected from lodging. This yield is
sensitive to the level of management set by
experiment station managers. As a
consequence, yield gaps calculated on the
basis of this yield are a bit artificial. The
yield from CIMMYT trials at the CIANO
station for 1990-95 was 7,219 kg/ha.

4. The average yields obtained by farmers
over a certain period. Over 1990-95 in the
Yaqui Valley of northwestern Mexico,
farmers' yields averaged 4,843 kg/ha.

These four yields are shown in Figure 1, in
which gap I, gap II, and gap III represent the
differences between the four levels. The
objective of any agronomist is to reduce these


Average
farmer Gap III
Averageap
station Gap II
Highest


achievable
Maximum


0 2,000 4,000 6,000 8,000 10,000
Grain yield (kg/ha)

Figure 1. Wheat yield data, Yaqui Valley,
northwestern Mexico, 1990-95.


12,000


gaps so that farmers can obtain the highest and
most profitable yield possible. This can be
achieved by analyzing the constraints that
prevent higher yields and by developing
technical options capable of overcoming these
constraints in profitable ways. At the same
time, a good agronomist must evaluate the
long-term consequences of the different options
for natural resource quality and system
sustainability.

Sustainable improvement in crop yields
requires that all factors affecting yields be set at
optimal levels. Von Liebig's "Law of the
Minimum" (Paris 1992), a useful way of
expressing this relationship, states that yields
are constrained by the level of the most limiting
factor and that yield improvement depends on
the successive removal of binding constraints.
In the sections that follow, we will examine the
most common factors that constrain wheat
yields and suggest ways in which these yields
may sustainably be improved through the
amelioration of important constraints.
Examples are drawn largely from rice-wheat
systems in the Indo-Gangetic Plains and from
wheat systems in northwestern Mexico.



Nutrients

Von Liebig's "Law of the Minimum,"
introduced above, is helpful in understanding
the role of balanced nutrition in the sustainable
improvement of wheat yields. Wheat yields
will be constrained by the most limiting macro-
or micronutrient. In this section, we examine
the nitrogen content and needs of wheat for
different yield levels; other nutrients could be
examined in the same way. Balancing the
uptake of nutrients by the plant with those
supplied externally or from the soil is also
important for sustaining yield. If soils are
mined and more nutrients are removed from


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the system than are supplied externally, there
will eventually come a time when a nutrient
will become limiting and prevent the
expression of yield potential. Therefore, it is
important to improve nutrient use efficiency
and (over the long term) to match nutrient
uptake with nutrient supply consistent with a
desired level of yield.

The percentage of nitrogen in the wheat straw
and grain increases as yield increases, which
means that more nitrogen is needed per unit of
dry weight as yield increases. The data in Table
1 show the amount of nitrogen in the above-
ground biomass at maturity under different
harvest index values for various yield levels.
Figure 2 shows the percentage of nitrogen in
wheat grain and straw at different yield levels.

Some of this nitrogen is obtained from the soil.
Each soil has a specific nitrogen supplying
capacity (SNSC), depending on soil organic
matter content, the mineralization rate of this
organic matter, and the availability of the
already mineralized nitrogen (nitrate and
ammonium) stored in the soil. Perhaps the best


Table 1. Kilograms of nitrogen in the above-
ground wheat biomass at maturity for
different harvest index values and different
yield levels

Harvest index
Yield Grain-N Straw-N
(kg/ha) 0.3 0.4 0.5 (%) (%)

1,000 19 17 16 1.45 0.18
2,000 41 37 35 1.53 0.22
3,000 69 62 58 1.68 0.26
4,000 95 86 81 1.75 0.27
5,000 147 132 123 2.10 0.36
6,000 197 174 160 2.19 0.47
7,000 235 207 190 2.22 0.49
8,000 283 246 224 2.25 0.55
Note: Data for grain-N and straw-N are taken from
CIMMYT trials in Obregon, Mexico, except for data
for the 1,000 and 8,000 kg/ha yield levels, which
are estimated based on the data trends.


indicator of SNSC is an estimate of the amount
of nitrogen that a wheat crop will take up by
maturity, when no nitrogen fertilizer is applied.
This quantity can be estimated, without having
actually to measure the amount of nitrogen in
the above-ground biomass, by knowing the
yield when no nitrogen fertilizer is applied
(Tables 1 and 2). Table 2 shows the most
common wheat yield levels when no fertilizer
is applied, 1.5-3.0 t/ha. For example, if the
zero-N plot yields 2 t/ha, the soil provides 42
kg/ha of nitrogen. For yields above 3 t/ha, the
amount of nitrogen that a soil needs to supply
when no nitrogen fertilizer is applied can also
be calculated, but at these higher yield levels,
an external application of nitrogen is usually
needed.


2.5

2.5 ----------- -------------------
5 Grain-N

1 1


0.5


1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000
Wheat yield (kg/ha)

Figure 2. Percentage of nitrogen in wheat
grain and straw at different yield levels.


Table 2. Amount of nitrogen that must be
supplied by the soil to the above-ground
biomass (grain and straw) at a harvest index
of 0.41

Yield N in grain N in straw N in above-ground
(kg/ha) (kg/ha) (kg/ha) biomass (kg/ha)

1,500 22 7 29
2,000 31 11 42
2,500 40 15 55
3,000 50 19 69

Note: Calculations based on data presented in Table 1.










Table 3 shows how much nitrogen has to be
supplied externally to obtain various wheat
yields. The data are presented using different
nitrogen recovery percentages (the percentage
of nitrogen that is recovered by the above-
ground plant parts from the external nitrogen
applied) and assuming that the SNSC was
equivalent to 2,000 kg/ha (42 kg N/ha).
Yamaguchi (1991) reviewed the literature of
15N experiments in wheat and maize (largely
from agricultural systems in developed
countries) and found a range of 24-85%
recovery by the crop; the average value was
57%. On the other hand, estimates of recovery
in wheat systems in developing countries have
been lower. In the Yaqui Valley of Mexico,
estimates from farmers' fields and experiment
station trials in which nitrogen rates were
similar to those used by farmers showed
fertilizer recoveries between 35% and 50%
(Ortiz-Monasterio et al. 1994b). Byerlee and
Siddiq (1994) estimated nitrogen recoveries in
wheat in Pakistan at around 30%.

The nitrogen fertilizer recommendation for
wheat over most of the Asian Subcontinent is
120 kg N/ha. Even assuming a nitrogen
recovery of 0.8, this rate is obviously not
enough to get a grain yield of much more than
5 t/ha (see Table 3). This may explain why

Table 3. External nitrogen that needs to be
applied to wheat to obtain various grain
yields, calculated at different nitrogen
recovery rates

Nitrogen recovery percentage
Yield
(kg/ha) 30 50 65 80
5,000 300 180 138 112
6,000 447 264 203 165
7,000 550 330 254 206
8,000 680 408 314 255
Note: Calculations assume a harvest index of 0.4 and soil
nitrogen supplying capacity equivalent to a yield of
2,000 kg/ha (42 kg N/ha).


scientists have trouble getting higher
maximum yields in this region: they do not
apply enough nitrogen. Although it is often
said that wheat does not respond to nitrogen
levels greater than 120 kg/ha, recent
experiments show that wheat does respond
economically to nitrogen above this level. If no
yield response is seen at higher levels of
nitrogen application, the lack of response may
be related to constraints such as water stress,
lodging, and biotic factors (discussed later in
this paper).

Ortiz-Monasterio et al. (1994a) compared solar
radiation and temperature in India (Ludhiana)
and northwestern Mexico (the Yaqui Valley)
using the photothermal quotient (PTQ), the
mean solar irradiance divided by mean
temperature minus the base temperature, for
both locations. At similar levels of solar
radiation and temperature, the number of
grains per square meter, which is highly
correlated with yield in both locations, was
much lower in Ludhiana than the Yaqui
Valley. Ortiz-Monasterio et al. (1994a) have
suggested that nitrogen might have been the
factor limiting yields in Ludhiana. The
implication is that higher yields will require
higher levels of nitrogen than are currently
applied (or even recommended), combined
with practices that remove other constraints,
such as lodging, that tend to appear when
nitrogen levels are high. Another consideration
is that continuous selection for large-grained
(high thousand-grain weight) varieties in India
may itself limit yield potential, given the
strong relationship between yield and grain
number per square meter.

Increasing the SNSC is important for
improving yields, in part because it helps
improve the efficiency of applied nitrogen.
However, raising the SNSC is not easy. One
way to achieve it is by applying organic











fertilizers, such as manures or crop residues, or
by rotating wheat with leguminous plants. The
release pattern of these nitrogen sources tends
to match the timing of nitrogen uptake by
crops, resulting in higher efficiencies. In many
developing countries, manure is an important
source of pollution, but the costs of hauling
manure away and applying it to a field often
exceed the value of the manure as a fertilizer -
unless manure improves soil quality as well as
supplying nutrients. This often is the case.
Organic matter lowers soil bulk density and
increases water holding capacity, infiltration
rate, and aggregate stability. Manures are
essentially slow release fertilizers, providing a
stable supply of the ammonium ions needed
for high yield. Organic phosphates and
chelated micronutrients found in manures are
slowly released to the plant. They can also
provide biotic factors that antagonize root
diseases. Thus manures essentially improve the
properties of soil as a growing medium. Data
presented in Table 4 present the effect of using
poultry manure and subsoiling on wheat in
northwestern Mexico compared to the control,

Table 4. Effect on wheat yield (kg/ha) of
applying poultry manure to wheat, with and
without subsoiling, CIANO Station, Sonora,
northwestern Mexico

Mean,
With Without poultry
Factor subsoiling subsoiling manure
With poultry manure 8,005 7,575 7,790 a
Without poultry manure 7,075 6,915 6,995 b
Mean, subsoiling 7,540 a 7,245 b
Note: Results are from an 11-year trial (1987-97).
Subsoiling was to a depth of 60 cm with 75 cm
between subsoil shanks. Poultry manure was
applied at a rate to supply 150 kg N/ha. Crop
rotation was Sesbania (green manure) wheat.
Means in rows or columns followed by the same
letter do not differ significantly at LSD (0.05). The
subsoil poultry manure interaction was significant
at 0.001 level and the interaction LSD (0.05) = 636
kg/ha.


on which these two management practices were
not used. These practices can raise yields by
nearly 800 and 300 kg/ha, respectively.

Unfortunately, supplies of organic fertilizers are
declining in many parts of the world, even in
China, where organic fertilizers have always
been a major component of fertilizer strategies.
In South Asia, some reasons for this decline are:

* Animal numbers and therefore manure
supplies are declining as tractors are
increasingly used for farm operations. The
cost of maintaining a pair of bullocks for a
year is becoming prohibitive.
* Animal manure is used increasingly as
cooking fuel (Fujisaka, Harrington, and
Hobbs 1994; Harrington et al. 1993).
* Higher labor costs make moving the heavy
manure to fields, especially those far from the
homestead, uneconomic.

In the rice-rice and rice-wheat systems of South
Asia, long-term fertilizer experiments on
research stations have indicated that yields and
total factor productivity have declined over time
for both crops when grown at constant
recommended fertilizer levels (Cassman and
Pingali 1995; Cassman et al. 1996; Pagiola 1995;
Nambiar 1995; Regmi 1994). Farmers also report
the need to increase the amount of fertilizer in
order to maintain yields at previous levels.
Agronomists need to understand the reasons for
these productivity declines, including the
biophysical processes that underpin them, if they
are to succeed in reversing them.

A decline in soil organic matter (SOM) or
changes in SOM quality have been hypothesized
to cause this sustainability problem (Cassman et
al. 1995). Soil organic matter is declining
throughout the rice-wheat belt of the Indo-
Gangetic Plains of South Asia (Nambiar 1994).
Applications of farm yard manure reduce the










rate at which SOM declines. Experiments at the
International Rice Research Institute (IRRI),
though, indicate that maintaining or building
up SOM does not necessarily result in a higher
or sustained nitrogen supply. The total size of
the SOM in soils may be less important than the
size and activity levels of the active fraction that
is involved in nutrient cycling. Cassman et al.
(1995 and 1996) and Olk et al. (1996) have
hypothesized that changes in the quality of
SOM as a result of continuous flooding in rice-
rice systems lead to decreased SNSC. Is the
same problem occurring in rice-wheat systems,
in which the soil is not continuously in a
reduced state because of flooding? The roles of
SOM and SNSC in sustaining high wheat yields
require much more research.

Another way to improve nitrogen availability is
to increase the use efficiency of externally
applied nitrogen. Developing countries cannot
achieve food security without some external
application of nutrients, including nitrogen. At
the same time, the public health and
environmental costs of nitrate pollution of
groundwater and of nitrous oxide emissions
into the atmosphere are important and cannot
be ignored. The goal of an agronomist must be
to enhance the use efficiency of applied
nutrients, while minimizing undesirable
environmental impacts.

Improvements in matching the nitrogen
demand from the crop with the nitrogen supply
can increase nitrogen recovery efficiency. This
can be accomplished in different ways. One way
is to use split fertilizer applications. Another
method relies on chemicals, such as nitrapyrin,
which inhibit the conversion of ammonium to
nitrate in the soil; this method has shown some
promise, but the results are inconsistent,
varying largely from soil to soil. Two other
options are the use of urease inhibitors (to slow
the hydrolysis of urea, which is particularly


effective in surface applications) or slow-release
fertilizers (generally covered with sulfur or
polyurethane, which take several weeks to
break down).

Still another possibility is to apply nitrogen in a
band below the ground rather than broadcasting
it on the soil surface. In many wheat-growing
areas of the world, most nitrogen applied after
planting is applied as a top-dress on the soil
surface. It would be difficult to change this
practice in fields commonly planted in dense
stands. However, the use of bed-planting
systems, which are described later in this paper,
would enable farmers to improve fertilizer
efficiency by placing top-dress fertilizer
applications and incorporating fertilizer.

In evaluating several nitrogen application
strategies in northwestern Mexico, Ortiz-
Monasterio et al. (1994b) found that delaying the
application of different rates of nitrogen until
first node formation resulted in increased yield
or the same yield as when all the nitrogen was
applied at planting (Figure 3). In addition,
delaying all nitrogen application until first node
resulted in a dramatic increase in the flour
protein concentration (Figure 4). It was also
found that at 225 kg N/ha there was further
improvement in nitrogen use efficiency if one-


0 75 150 225 300
Nitrogen (kg/ha)

Figure 3. Response of grain yield to different
rates and timing of nitrogen application.











third of the nitrogen was applied at planting
and two-thirds was applied before first node.
This improvement was reflected in a higher
yield (Figure 3) and a higher nitrogen recovery
by the crop (Figure 5). Table 5 shows that these
relationships hold for a number of genotypes of
bread and durum wheat at a level of 150 kg
N/ha. Although these results were obtained on
soils with high initial soil nitrogen, they do
show that in some situations nitrogen use
efficiency can be increased by delaying the
application of some nitrogen to just before first
node formation.


.S 10
2
9
o
LL 8

7


75 150 225
Nitrogen (kg/ha)


Figure 4. Response of flour protein to different
rates and timing of nitrogen application.


Fischer, Howe, and Ibrahim (1993) conducted
experiments in Australia to determine how
long a single supplemental application of
nitrogen to the wheat crop could be delayed
and still elicit a full grain yield response. A
significant yield loss did not occur until
nitrogen was applied just after first node
initiation (just after the onset of stem
elongation). Nitrogen recovery ranged from
44% to 77% and did not decline until nitrogen
was applied after stem elongation. In fact,


70

| 65'

860

| 55
.m
2 50

I 45
< 40


150 225
Nitrogen (kg/ha)


Figure 5. Response of percent apparent
fertilizer recovery to different rates and
timing of nitrogen application.


Table 5. Comparison of the application of 150 kg N/ha at planting and first node on grain
yield and grain protein percentage for various wheat genotypes

Grain yield (kg/ha) Grain protein (%)

Genotype Basal First node Basal First node

Rayon 89 (BW) 6,722 6,712 12.36 12.27
Oasis 86 (BW) 6,011 6,634 10.73 11.86
Weaver (BW) 5,830 6,413 11.49 12.26
Opata 85 (BW) 6,089 6,661 11.50 12.21
Bacanora 88 (BW) 6,577 6,504 10.74 12.04
Baviacora 92 (BW) 6,322 6,594 10.39 11.21
Aconchi 89 (DW) 5,447 6,719 11.73 12.68
Altar 84 (DW) 6,215 6,841 11.49 11.97

Mean 6,152a 6,635b 11.30a 12.06b

Note: Means followed by different letters are significantly different using the LSD test at 0.05 probability. LSD values are
379 kg and 0.72% for the grain yield and protein percent, respectively. Based on data from CIMMYT, Mexico.











nitrogen recovery increased slightly when
nitrogen was applied later (up to stem
elongation) compared to when it was applied
earlier.

Chinese scientists in the Yangtse River Valley
also recommend delaying nitrogen
applications for higher yields. They have
developed a system called "green-yellow-
green," in which some nitrogen is applied
early, but the crop is then given a nitrogen
stress during the main vegetative phase. At the
first node, the rest of the nitrogen is applied.
This results in less luxury biomass and
stronger stems, but no loss of yield.

Fertilizer timing is an important factor in zero-
tillage systems, in which fertilizer cannot be
placed at planting. Data from Pakistan (Aslam
et al. 1993a) indicate that if nitrogen cannot be
placed it is better to delay its use until later and
apply it as a top-dressing (Figure 6). Nitrogen
applied in the zero-tillage plots was taken up
20% less efficiently than in the traditionally
planted plots, where the basal nitrogen was
incorporated.


4,500

4 4,000

3,500

3,000

I 2,500'

2,000


Y = 2,574 + 21.14N 0.053N2
--------------------- ------------ -
Conventional

Y= 2,350 + 15.31N- 0.034N2
---- ---- -----------------------------------
Zero tillage



0 35 70 105 140 170 210


Nitrogen (kg/ha)

Figure 6. Nitrogen response for wheat under
conventional and zero tillage from seven
experiments in farmers' fields, Punjab,
Pakistan.


Planting Date and Crop
Establishment

It is well known that substantial increases can
be realized in wheat yields in the Indo-
Gangetic Plains if wheat is planted on time and
plant stands are good. Figures 7 and 8, which
are based on data from the Indian Punjab,
show responses of wheat to different dates of
planting which are typical for many other areas
of the world. Each figure shows that there is an
optimum date for planting, which is followed
by an almost linear decline in yield after that
date. There are differences between varieties:


5,500
5,000
4,500
" 4,000
S3,500
S3,000
2,500
2,t00


25 Oct 5 Nov 15 Nov 25 Nov 5 Dec 15 Dec 25 Dec
Planting date

Figure 7. Effect of planting date on wheat
yield, by variety, Punjab, India.


6,000
5,500
5,000 -- -- -----89/90-
4,500
4r5QQ ---- ---


S4,000
- 3,500
3,000 87/88
2,5000
2,500


',UUU


25 ct 5Nov 15Nov 25Nov 5 Dec 15 Dec 25 Dec
Planting date


Figure 8. Effect of planting date on wheat
yield, 1987-92.


PBW34










some genotypes are more stable over a range of
planting dates than others (Figure 7), and the
shape of the curve also varies over years
(Figure 8). Declines of 0.7-1.5% per day of delay
in planting after the optimal date of planting
are common (Saunders 1990; Hobbs 1985;
Randhawa, Dhillon, and Singh 1981; Ortiz-
Monasterio et al. 1994a).

The theory behind the loss in yield at later
dates of planting can be related to the effect of
the PTQ. Fischer (1985) and Midmore,
Cartwright, and Fischer (1984) showed that
kernel number is associated with the PTQ over
the 30 days before anthesis. In Figure 7, the
optimum date of planting for the longer
maturing variety (PBW34) was 5 November;
the optimum date for the two shorter duration
varieties was 15 November. When these three
varieties were planted at these optimum dates,
they all reached anthesis at the same date
(Ortiz-Monasterio et al. 1994a), which
coincided with the time of year when the PTQ
for that location was highest. The highest yield
in this location was obtained when the PTQ
value at 20 days before heading and 10 days
after heading was maximized. Additionally,
higher temperatures close to the flowering and
grain filling periods of late-planted wheat
result in grain abortions and forced
development of underweight grains.

The efficiency of inputs such as nitrogen is also
affected by late planting. When planting is
delayed, nitrogen response curves are flatter;
wheat responds only to lower nitrogen levels.
In other words, late planting cannot be
overcome by raising the nitrogen dose.


Many factors can cause wheat to be planted late.
In the intensive, irrigated cropping systems of
the Asian Subcontinent, late harvest of the
previous crop and the long turnaround between
rice harvest and wheat planting are two of the
major causes of late wheat planting (Hobbs, Giri,
and Grace 1998). Excessive tillage, unfavorable
soil conditions, and poor power sources are
common reasons for long turnaround in South
Asia (Hobbs, Bronson, and Meisner 1996). Late
harvest of cotton and basmati rice commonly
delays wheat planting.

One solution to this problem is to introduce
reduced and zero-tillage options to farmers,
with the objectives of reducing turnaround time
and planting wheat closer to the optimum date.
Research over the past decade in the rice-wheat
areas of South Asia, on farmers' fields as well as
experiment stations, has identified several tillage
options to cope with this problem (Hobbs,
Bronson, and Meisner 1996; Hobbs, Giri, and
Grace 1998).1

1. Wheat is surfaced seeded onto unplowed
soil either before or just after the rice harvest.
The key to this system is maintaining proper
soil moisture at seeding and during initial
root extension. This system is particularly
relevant on finely textured, poorly drained
soils, where planting is delayed because of
excess moisture. It is also relevant for small-
scale farmers, since no equipment or power
source is needed for the operation.

2. Wheat is sown into unplowed soil using an
inverted-T coulter or double disk opener.
This practice is mainly used where four-


1 Aside from the tillage technologies described here, note that in areas where combines are used, such as parts of India
and Pakistan, some additional technical changes may be needed to foster the adoption of conservation tillage. Loose
straw left by the combine creates clogging problems with the drill described in (2), and special trash drills with disk
openers need to be developed. Another option would be to place a straw chopper in the combine, so the straw can be
chopped and distributed evenly on the soil. This would give the additional benefits of mulching and helping to
maintain good soil moisture.











wheel tractors are available. Local artisans
are producing the equipment and selling it
at prices within farmers' budgets, and the
practice is gaining popularity in India and
Pakistan in fields where rice stubble is not
too much of a problem.

3. Another option is to prepare the soil and
plant in one operation. This reduced-tillage
option utilizes a shallow rotovator ahead of
a seed drill, followed by a roller. A two- or
four-wheel tractor can power this
machinery. The introduction of two-wheel
Chinese tractors in Bangladesh, Nepal, and
eastern India will be particularly relevant
for small-scale farmers who are finding it
excessively expensive to continue keeping
bullocks for plowing. These two-wheel
tractors can also be hooked up to other
implements for threshing, pumping,
reaping, deep plowing, and transport.

Data are being compiled in the Asian
Subcontinent on the establishment of wheat
after rice, under zero and reduced tillage. Table
6 shows some of the data from Nepal, in which
wheat grown under surface seeding and
reduced tillage with the Chinese drill is
compared with wheat grown under the
traditional system. With surface seeding and
reduced tillage, wheat yields and thousand-
grain weights are higher, and production costs
are lower. Data from Pakistan (Table 7) show


that zero tillage with an inverted-T coulter drill
will give better yields than traditional planting,
with greater benefits where planting is closer to
the optimum date. On average, more than one
ton of extra yield was obtained with zero tillage
compared to the farmers' practice, and planting
was 24 days earlier. Data are also being
compiled in India and Bangladesh on this
innovative, cost-reducing technology. What is
needed is more support from research and
extension directors to help farmers become
better acquainted with these options (and how
they perform in different wheat systems), as
well as better links with private sector


Table 7. Comparison of zero-tillage and
farmers' practice for establishing wheat
after rice in locations in the Pakistan Punjab
where the planting dates for the two
methods differed

Wheat yield (kg/ha)

Zero- Farmers' Days
Location tillage practice difference

Daska, site 2 3,143 3,209 10
Daska, site 1 3,842 2,735 13
Ahmed Nagar 4,308 3,526 20
Maujianwala 2,689 2,198 22
Mundir Sharif 4,245 2,660 33
Daska, site 3 3,838 3,420 44

Average 3,677 a 2,598 b 24

Source: (Aslam et al.1993).
Note: Means followed by the same letter do not differ
significantly at the 5% level using DMRT.


Table 6. Data from a wheat establishment trial following rice, Bhairahawa Agricultural Farm,
Nepal, 1993/94

Yield 1,000-grain Cost to plow Net benefit Extra days
Method (kg/ha) weight (Rs/ha) (Rs/ha) needed to plant

Surface seeding 2,775 a 46.11a 0 11,485 a 0
Chinese seed drill 2,831 a 45.43b 600 12,090 a 8
Farmers' practice 2,314 b 40.87c 2,300 8,065 b 15

Source: (Hobbs and Giri 1998).
Note: Figures followed by the same letter are not significantly different at 5% probability using DMRT.
a Number of extra days needed for land preparation before seeding compared to the surface seeding.










equipment manufacturers. Ultimately, only the
private sector is in a position to meet any
unfolding demand for new tillage and
establishment machinery for wheat systems.

Tillage and establishment practices are
important factors in the sustainability equation
for wheat systems in the Indo-Gangetic Plains.
Farmers use tillage to help control weeds,
develop a good seedbed, and control crop
residues. However, excessive tillage
unnecessarily consumes vast amounts of energy.
It also results in soil compaction, increases the
likelihood of erosion, fosters more rapid
decomposition of organic matter, and enhances
the germination of weed seeds.

Much information is being released on the long-
term benefits of reduced- and zero-tillage
systems. Although much of this research is
focused on developed countries, some of the
results may nevertheless contribute to the
design of research in developing countries. A
six-year trial in New Zealand resulted in
improved soil physical properties with direct
drilling compared to conventional cultivation
(Francis, Cameron, and Swift 1987). Zero tillage
resulted in better aggregate stability, more
earthworms, a more open and continuous
network of soil pores, more roots in the top 100
mm of soil, and the same yield at lower cost.
Guo Shaozheng et al. (1995), who studied zero
and minimum tillage in wheat for 25 years in an
area where soils are heavy and plant stands are
poor, report that conservation tillage helps to
preserve surface soil moisture, improve plant
stands, and improve soil structure. Conservation
tillage has reportedly been adopted over one
million hectares in Jiangsu Province, or 80% of
the total area where wheat is grown after rice.
However, Guo Shaozheng et al. (1995) note that
after three to four years, the soil must be deeply
plowed to improve soil physical and chemical
properties and weed control.


Many researchers report no yield sacrifice by
reducing tillage. Different results are reported
in regard to disease incidence. Herman (1990)
reported that zero-tillage plots had higher
antagonistic activity by rhizosphere flora
(measured by better observed growth), and less
incidence of Gaeumannomyces graminis, than in
conventionally tilled plots. Work in Australia
has shown higher levels of crown rot in zero-
tillage treatments of wheat. Crown rot was
associated with retention of the stubble
(Dodman and Wildermuth 1989). In the rice-
wheat systems of South Asia, opponents of zero
tillage cite problems of insects and weeds to
discourage the introduction of this technology.
However, Inayatullah et al. (1989) found that
rice stem borers were not a problem as first
hypothesized. When a crop of wheat is grown
with irrigation and fertilizer, stemborer
populations fall as the rice stubble decays.
Weeds such as Phalaris minor, a major problem
in rice-wheat systems, are also found to be
lower in zero-tillage plots than in
conventionally tilled plots because the soil is
disturbed less.

Agronomists and farmers need to collaborate in
the development of complementary practices -
soil fertility, water, and nutrient management
practices to facilitate widespread adoption of
reduced and zero tillage options across a range
of wheat systems. By reducing tillage, the
sustainability of food production can be
increased through less fuel consumption and
wear and tear on tractors and implements,
better input efficiency, and, in some cases,
higher yields because of more timely planting.


Water Management

Sustainable increases in wheat system
productivity depend on adequate levels of
water as well as adequate levels of nutrients.










Excess water or waterlogging can reduce
yields. In rice-wheat systems, the soil is
puddled for rice production, which results in
poor aggregate size, formation of a plow pan,
and reduced water percolation in the
subsequent wheat crop (Hobbs, Woodhead,
and Meisner 1993). Care must be taken,
especially in applying the first irrigation, to
minimize waterlogging. Chinese farmers in
Yangtse Province use intricate in-field drainage
ditches to avoid yield loss during wet winters,
when waterlogging is a problem.

Many studies have looked at the yield losses
associated with water stress (drought) at
different phenological stages. Crown root
initiation and anthesis are two stages at which
yield losses from water stress can be most
critical in wheat.

Water scarcity is certain to become a more
acute problem in agriculture in the future.
Competition for this valuable resource from
industry and domestic use in urban areas
already places a constraint on farmers (Hobbs
and Morris 1996). This trend means that water
must be applied efficiently, supplied on time
and in sufficient quantities. This is a major
problem in many areas of the Asian
Subcontinent. In the large irrigation canal
systems of Pakistan, where it is difficult to
release water at critical growth stages, farmers
apply too much water when it is available,
hoping that the plants are not stressed before
the next water release (Kijne and Bhatia 1994;
Kijne and Vander Velde 1990).

Careful water management is very important
for establishing the wheat crop under zero-
tillage systems. In fact, water substitutes for
tillage by lowering soil strength at the time of
root elongation. In the surface seeding practice
described earlier, it is essential for the soil to be


saturated at seeding and remain moist during
rooting. A light supplemental irrigation may be
needed at root elongation on coarser textured
soils or the first irrigation may be needed
earlier than crown root initiation. In the
mechanically planted zero-tillage and reduced-
tillage systems described earlier, soil moisture
should be higher than the level that is normally
found when wheat is planted into
conventionally plowed soil (Guo Shaozheng et
al. 1995). In fact, the crown root initiation
irrigation commonly applied in wheat
production is important because it reduces the
soil strength at a time when these roots are
trying to penetrate the soil. The same holds
true for the seminal roots in zero tillage.

In South Asia, most soils are irrigated by
flooding, a simple but not very efficient
practice. In northwestern Mexico, where water
is an exceptionally scarce resource, farmers
have shifted to a bed-and-furrow system for
planting wheat. Wheat and other crops are
planted on top of the bed and water is passed
down the furrow, which results in significant
savings in water and increases water use
efficiency. This system is also being researched
in the high production areas of India. A new
set of agronomic practices must be developed
for this system, including the proper bed size,
number of rows, fertilizer application,
irrigation, weed control, and variety selection.
This is presently being done with good results.
Table 8 shows some initial results from the
Indian Punjab, and Table 9 presents similar
results from Mexico for a comparison of wheat
varieties under conventional versus bed
planting at two seed rates and under high
management conditions. Choice of variety is
important: some varieties flourish in bed-
planting systems while others, inexplicably,
perform poorly.











The bed-and-furrow system is also being
researched one step further by following the
wheat crop with another upland crop without
tillage. This system, called ridge-tillage or
FIRBS (furrow irrigated, reduced-tillage bed
systems), is particularly appropriate for cotton,
maize, sorghum, and soybean systems in which
wheat follows these crops. Table 10 presents
wheat yields from a trial comparing five
tillage/ straw management systems in a bed-


planted, wheat-maize rotation. Also included
are nitrogen rates and timing of applications for
each tillage/straw management system. Wheat
yields for all reduced-tillage treatments are
significantly better than with conventional
tillage. In addition, yields for nitrogen
applications at the first node stage are as good
as or better than yields for similar nitrogen
applications at planting. Although much more
research is needed, especially on the


Table 8. Effect of bed size configurations on wheat grain yield, Punjab Agricultural
University, Ludhiana, India, 1994/95

Sowing method

75 cm beds 90 cm beds Mean
On the flat yield
Variety 25 cm row 2 rows 2+1 rows 3 rows 3+1 rows (kg/ha)

Wheat yield (kg/ha)
PBW 226 5,740 6,170 6,390 6,160 6,320 6,160a
WH 542 6,290 5,830 6,360 6,000 6,040 6,110a
CPAN 3004 6,020 5,530 6,140 5,630 5,600 5,780b
PBW 154 5,460 5,110 6,000 5,930 5,880 5,680b
HD 2329 5,770 4,660 6,190 5,580 5,810 5,600b
PBW 34 5,650 5,610 5,800 5,580 5,630 5,650b

Mean 5,820 5,490 6,150 5,810 5,880

Source: Unpublished data from S.S. Dhillon, Wheat Agronomist, Punjab Agricultural University.
Note: Means followed by the same letter do not differ significantly at the 5% level using DMRT.


Table 9. Grain yield (kg/ha at 12% moisture) for conventional versus bed planting at high
and low seed rates, CIANO Station, Sonora, northwestern Mexico, 1993/94

90 cm beds, 90 cm beds,
Conventional 3 rows/bed 2 rows/bed Genotype
Genotype (120 kg/ha seed) (100 kg/ha seed) (50 kg/ha seed) mean

Siete Cerros 8,273 8,281 7,756 8,103
Yecora 70 8,177 7,688 7,434 7,766
Ciano 79 8,059 7,805 7,993 7,952
Seri 82 9,671 9,393 8,948 9,337
Oasis 86 8,749 8,676 8,782 9,069
Super Kauz 88 9,763 8,644 8,581 8,996
Baviacora 92 9,767 9,796 9,699 9,754
Weaver "S" 9,741 9,391 9,205 9,446

Method mean 9,150b 8,709 a 8,550 a 8,803

Note: Genotype x planting method interaction was not significant.











development of appropriate machinery for
making and planting beds, this bed-planting
system would appear to have several main
advantages:

* It improves water distribution and
efficiency.
* It improves fertilizer efficiency by enabling
farmers to place the pre-plant fertilizer
applications below the bed. Top-dress
nitrogen can also be placed in the furrows
or on the bed and incorporated before
irrigation.
* It provides an alternative for weed control,
because the furrows can be cultivated. This
will be discussed in more detail later.
* It helps reduce lodging because the wheat
plants are not exposed to soft soil
conditions after irrigation, and more light
can penetrate the canopy, resulting in
stronger plants.
* It can potentially allow dramatic reductions
in seed rates (Table 9).


With regard to sustainability, the FIRBS system
combines the benefits of reduced tillage with
the benefits of water efficiency. By reducing
the tillage, farmers save on production costs,
which leads to higher profits or cheaper food
production. Savings in fuel and equipment
would be substantial. Better irrigation systems
would reduce groundwater pollution and the
problems of salinity and waterlogging, which
are often associated with poor water
management. In parts of northwestern India,
where water tables are falling at significant
rates, substitution of upland crops such as
soybeans or maize, grown on permanent bed
systems with wheat in place of rice, may help
slow this natural resource problem.



Lodging

Cereal genotypes grown by farmers before the
identification of the Norin 10 dwarfing genes
(Rhtl, Rht2) were prone to lodging at low yield


Table 10. Wheat yield (kg/ha) averaged over four years for tillage/straw management and
nitrogen management treatments for a bed-planted wheat (W) and maize (M) rotation

Tillagelstraw management

Conventional Reduced Reduced Reduced Reduced
Nitrogen (W-incorporate, (W-bum, (W-partial, (W-retain, (W-retain, Nitrogen
(kg/ha) M-incorporate) M-bum) M-remove) M-remove) M-retain) mean

0 N 3,105 3,188 3,147 3,834 3,332 3,321 a
75 N basal 4,024 4,650 4,572 4,455 4,914 4,523 b
150 N basal 5,435 5,809 5,475 5,626 5,742 5,615 c
225 N basal 5,910 6,393 6,007 6,063 6,480 6,171 d
300 N basal 6,212 6,700 6,520 5,949 6,660 6,408 e
150 N 1st node 5,876 5,988 5,687 5,917 6,023 5,898 d
300 N 1st node 6,356 6,342 6,453 6,167 6,731 6,410 e

Tillage/straw
management mean 5,274a 5,581c 5,409 ab 5,430 b 5,697c

Note: Means in rows or columns followed by the same letter are not significant by LSD (0.05) for interaction= 408
kg/ha. Reduced tillage consists simply of forming the bed again after each crop in the rotation. No soil inversion
is practiced. For straw management, "incorporate" burying all the straw; "partial" removing the straw cut by
the combine but leaving the standing wheat stubble; "remove" complete straw removal; and "retain"
chopping and leaving all straw in place on the surface. "1st node" first node stage of development.










levels. With the incorporation of these dwarfing
genes into wheat and rice in the 1960s, the
resulting modern varieties possessed not only
higher yield potential but also good lodging
resistance (the latter resulted from shorter and
stiffer straw) (Fischer and Wall 1976; Pinthus
1973; Vogel, Allan, and Peterson 1963; De
Datta, Tauro, and Balaoingl968). The effect of
lodging in older, traditional varieties was
studied using artificial means, and a yield loss
in the range of 30-40% was reported when
lodging occurred close to heading and
somewhat less when it occurred later
(Pinthus 1973).

Similar artificial lodging studies were
conducted on modern, shorter varieties in the
early 1970s in Mexico (Fischer and Stapper
1987). Culm lodging to an almost horizontal
position caused grain yield to be reduced by 7-
35%. Lodging losses were greater when they
occurred in the first 20 days after anthesis and
were significantly less when lodging occurred
before anthesis or more than 20 days after
anthesis. The greater the angle of bending of
the culm, the greater the yield loss. Fischer and
Stapper (1987) suggested that plants were able
to right themselves by node bending when
lodging occurred before anthesis. They also
showed that kernel number per unit area was
reduced by early lodging and kernel weight by
later lodging together with a small increase in
grain nitrogen percentage. Fischer and Stapper
also pointed out that the percentage of
sprouted grain could increase with lodging and
suggested that even more yield loss would
occur where combine harvesting was done.
These researchers also reported results of trials
with natural lodging in which grain yield was
reduced up to 37%. However, their results were
difficult to interpret because lodging in any one
plot could occur at different growth stages.


In Australia, lodging experiments produced
similar results (Stapper and Fischer 1990c); yield
reductions from lodging were as high as 45%.
Stapper and Fischer concluded that high yields
under irrigation could be achieved consistently
and efficiently only with genotypes that resist
lodging (because of their short, stiff stems) or
avoid it (by maturing early). Interestingly, even
double dwarf genotypes lodged at high yield
levels. This suggests that lodging is a major
problem for wheat produced under irrigation,
especially at yield levels higher than 5.5 t/ha.
Researchers in the Indian Punjab have trouble
breaking the 5.5-6.0 t/ha yield barrier in their
trials, even though the yield potential of the
varieties in those trials should be at least 8 t/ha.
Two constraints related to lodging may be
limiting yields in India.

First, researchers use no more than 150-180 kg
of nitrogen in their maximum yield trials. Past
experiments have shown no response above this
nitrogen level, but as explained earlier, this
nitrogen level is insufficient to produce yields
above 5.5 t/ha under average conditions.
However, at higher nitrogen levels wheat plants
probably lodge when the recommended
irrigation is given. Second, researchers tend to
avoid the last irrigation (at grain filling) because
it results in lodging. This practice may have
resulted in water stress at this critical stage and
thus in reduced yield. Thus the combination of
limiting nitrogen and water could explain the
inability to break the 5.5-6.0 t/ha yield barrier
encountered in experimental plots.

It would be appropriate to investigate this
hypothesis by conducting an experiment in the
Punjab with various genotypes with and
without physical support (netting) to prevent
lodging, making sure that nitrogen and water
are not limiting and that there are no
compaction layers or biotic stresses. (Similar










experiments have been conducted in
northwestern Mexico.) If results show that
lodging is a major factor limiting yields,
especially at the higher yield levels (greater
than 5.5 t/ha), then future research should
give greater emphasis to this issue.

Lodging risk increases with increased dry
weight at anthesis and for taller crops; both
traits are associated with the duration from
sowing to anthesis (Stapper and Fischer 1990a,
1990b). Other traits, however, are associated
with resistance to lodging, including height,
stem stiffness (biochemical composition),
angle of roots, and shoot density. Studies are
underway in Mexico to identify genotypes
with these traits and to incorporate them into
new varieties. Note that some taller varieties
with a single dwarfing gene, such as Baviacora
92, are much taller than varieties with double
dwarfing genes yet lodge less. It is thought
that their root systems, which spread out
more, are responsible for this difference.

Several management strategies can also be
used to decrease the effect of lodging and raise
yields. First, as mentioned earlier, the timing
of irrigation is crucial. This poses a dilemma,
as water must be supplied at the critical stages
of flowering and grain filling to obtain a good
yield, but lodging often accompanies
irrigation at these growth stages. When wheat
is grown on the flat, irrigation will create a wet
condition around the roots, where soil
strength is not sufficient to support the plant.
Growing wheat on beds (discussed earlier)
instead of on the flat is one way to adjust
irrigation and to prevent the wet soil surface
that will lead to lodging, especially under
windy conditions. Another advantage of the
beds is that they drain faster, therefore leaving
the crop vulnerable to lodging for a shorter
period of time.


Second, nitrogen timing can be adjusted to
reduce lodging. By delaying the nitrogen
application until just before first node,
excessive foliage is reduced, stems are less
etiolated and stronger, and lodging should be
less (Table 11).

A third management strategy, developed by
Chinese researchers, is to plant wheat in a
skipped row configuration for yields above 5.5
t/ha (Guo Shaozheng, pers. comm.). In this
system, two paired rows are planted and the
third row is skipped, resulting in a
configuration similar to ridge-and-furrow
planting. Of course, farmers must then ensure
that nitrogen and water are not limiting. The
theory is that more light enters the canopy,
resulting in stronger plants.

Fourth, various growth hormones that reduce
plant height can be used to diminish lodging.
These are commonly used in Europe, where
some of the highest yielding commercial wheat
crops are produced. However, some studies
have shown that growth regulators do not
always work. Fischer and Stapper (1987)
reported that growth regulator was applied at


Table 11. Strategies for applying 225 units
of nitrogen fertilizer and resulting effect on
lodging


Initial soil nitrogen level


Nitrogen Low-
treatmenta Low medium Medium High Mean

225/0/0 5.0b 5.0 5.0 5.0 5.0
0/225/0 1.6 4.0 2.3 3.6 2.9
0/0/225 1.0 1.3 1.0 1.3 1.1
75/150/0 5.0 4.6 5.0 5.0 5.0
75/75/75 4.3 5.0 5.0 5.0 5.0

a The three numbers refer to the rate of N applied at three
different times: planting, first irrigation (close to first
node), and second irrigation (early booting).
b Lodging values are on a scale of 1 to 5, in which 1 0%
lodging and 5 = 100% lodging.











the correct stage and that it reduced plant
height but did not sufficiently reduce lodging
to give a yield advantage. However, in this
experiment, lodging commenced before the
first application of the regulator. Figure 9
shows data from a trial in Mexico in which two
varieties were sprayed with Ethephon, a plant
growth regulator. Lodging was significantly
reduced by this treatment. More work is
needed on this subject.

Finally, vigorous rooting should also help
reduce lodging. Use of organic manures and
removal of any physical root barriers should
promote good rooting.



Weed Control

Any organism that competes with wheat for
light, water, or nutrients will reduce wheat
yields. Weeds are a good example of this
relationship: without proper control, broadleaf
and grassy weeds can significantly limit wheat
yields. In the irrigated, high-yielding wheat
areas of northwestern Mexico and the Asian
Subcontinent, it is the two grassy weeds P.
minor and Avenafatua that cause major yield
losses.


h ll1111r


-r---
7,000
] 6,000
a 5,000
7 4,000
S3,000
S2,000
1,000
n nnn


S- Ethephon


+ Ethephon


u,uuu
Altar 84 durum wheat Rayon 89 bread wheat

Figure 9. Effect of Ethephon on yield of two
wheat varieties.


As observed earlier, several crop management
options, including rotations, alternative tillage
strategies, and bed-planting systems, offer
potential for controlling weeds and improving
wheat yields sustainably. Herbicides are
another weed control option, but greater
attention must be given to alternative control
methods and to ensuring that chemicals are
used properly to reduce health risks and
environmental damage. Herbicides are less
effective if improperly applied for instance,
at the incorrect time and dose, or without
appropriate adjuvants. This is a common
problem in the Asian Subcontinent, where
many farmers lack the correct spraying
equipment to apply chemicals uniformly and
also lack knowledge of proper doses and
application methods. In fact, many farmers in
India and Pakistan apply Isoproturon, a good
grassy weed herbicide, by broadcasting it with
sand or urea.

Improper herbicide use has probably
contributed to the herbicide resistance that is
appearing in P. minor and A. fatua in Mexico
and India (Malik 1996; Malik and Singh 1995).
The use of new herbicides or a mixture of
herbicides is one alternative and will remain a
part of the weed control strategy, but other
control methods are needed because weeds are
likely to develop resistance to these new
herbicides over time.

In integrated weed management, the use of
chemicals, rotations, cultivation, and other
management practices such as bed planting are
all part of the weed control package. A few
examples follow.

* As discussed previously, one advantage of
growing wheat on beds in a ridge-and-
furrow system is that farmers can cultivate
between the furrows. When combined with
nitrogen top-dressing, this system is even










more efficient. Data also suggest that
growing wheat this way creates a drier soil
surface next to the stems and reduces P.
minor growth, since P. minor prefers a
moister soil. This system is being examined
in Mexico and India as an important
alternative for control of herbicide-resistant
P. minor.
* Data from South Asia suggest that the
populations of P. minor in zero-tilled plots
are smaller than in traditionally established
wheat (Majid et al. 1988; Aslam et al.
1993b). It is hypothesized that zero tillage
reduces disturbance of the soil and that
fewer weed seeds are exposed for
germination, and studies have been
initiated to exploit this possibility for
controlling grassy weeds. It is proposed to
irrigate fields after the rice harvest and
allow the first flush of resistant P. minor to
germinate. This flush is controlled with a
nonselective herbicide, glyphosate
(Roundup). The fields are then planted by
zero tillage to reduce weed germination. If
required, selective chemicals can be used to
control later flushes.
* Rotations form a major part of farmers'
weed control strategies. In northwestern
India, farmers commonly grow sugarcane
and its ratoons for two to three years before
returning to wheat (Hobbs et al. 1991, 1992;
Fujisaka, Harrington, and Hobbsl994).
Break crops such as sunflower and Brassica
spp. are used more and more by farmers in
India and Pakistan during the wheat
season. If the sunflower is grown on beds
or in rows, intercultivation can be done,
with even better weed elimination.

All of these weed control practices have
implications for the sustainability of wheat
production. The example of herbicide resistant
weeds in rice-wheat systems of South Asia
highlights the risks of depending on chemical


control strategies. A sustainable system would
be based on an integrated approach to
controlling weeds or other biotic problems.
When a bed-planting system is used for
planting wheat, weeds can be removed from
furrows by intercultivation. The use of stubble
as mulch can also suppress weed growth.
Where noxious weeds are present, low doses of
herbicide, combined with some cultivation, can
be effective. Rotations can also help reduce
weed populations; the introduction of
sugarcane into rice-wheat areas of western
Uttar Pradesh is a good example of using
rotations to control P. minor. Rotations also
provide other benefits that contribute to
sustainably improving wheat yields, such as
improvement of soil physical properties,
breaking of disease and insect pest cycles, and
improvement in soil fertility. Legumes are
often cited as a means of improving soil
fertility, especially when they are grown as
fodders or green manures. All of these issues
must be considered in developing a strategy to
increase yields and meet the growing demand
for wheat.


Conclusion

As this paper has attempted to demonstrate,
the potential for achieving sustainable
increases in wheat yields throughout the world
is still considerable. Food security will depend
not only on our ability to improve yield
growth, but also on our ability to improve this
yield growth in such a way that food prices
remain stable and the natural resource base
remains unharmed. Agronomy and crop
management research hold some of the most
exciting opportunities for sustainably
improving wheat system productivity in areas
such as the Indo-Gangetic Plains. Breeders and
pathologists play an important role by
providing genotypes that have high yield











potential and resistance to biotic and abiotic
stresses, including lodging, and that use water,
nutrients, and other resources more efficiently.
Agronomists contribute by developing
strategies for farmers to exploit the yield
potential in these genotypes in ways that are not
detrimental to the natural resource base. This
paper has given examples of several promising
strategies, especially tillage and nutrient
management practices, whose adoption may
make the difference between food security and
food scarcity in the years to come.

In the next two or three decades, it is imperative
that scientists from various disciplines continue
working on farmer-identified problems at
specific sites to refine technologies that
sustainably increase yields. It is essential for
these researchers to work closely with farmers
and extension personnel, both in the public and
private sectors as well as in non-governmental
organizations, to ensure that the newly
developed technology is relevant, available, and
that farmers can quickly accept and use it to
increase food production.



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New Papers from the Natural Resources Group


Paper Series

96-01 Meeting South Asia's Future Food Requirements from Rice-Wheat Cropping Systems: Priority Issues
Facing Researchers in the Post-Green Revolution Era
P. Hobbs and M. Morris

96-02 Soil Fertility Management Research for the Maize Cropping Systems of Smallholders in Southern
Africa: A Review
J.D.T. Kumwenda, S.R. Waddington, S.S. Snapp, R.B. Jones, and M.J. Blackie

96-03 Genetic Diversity and Maize Seed Management in a Traditional Mexican Community: Implications
for In Situ Conservation of Maize
D. Louette and M. Smale

96-04 Indicators of Wheat Genetic Diversity and Germplasm Use in the People's Republic of China
N. Yang and M. Smale

96-05 Low Use of Fertilizers and Low Productivity in Sub-Saharan Africa
W. Mwangi

96-06 Es Intensificaci6n de sistemas de agriculture tropical mediante leguminosas de cobertura: Un marco
conceptual
D. Buckles and H. Barreto

96-07 Intensifying Maize-based Cropping Systems in the Sierra de Santa Marta, Veracruz
D. Buckles and 0. Erenstein

96-08 In Situ Conservation of Crops and Their Relatives: A Review of Current Status and Prospects for
Wheat and Maize
G.J. Dempsey

97-01 The Adoption of Conservation Tillage in a Hillside Maize Production System in Motozintla, Chiapas
O. Erenstein and P. C.id. n.i-liii ,..: (Also available in Spanish)

97-02 Farmer Assessment of Velvetbean as a Green Manure in Veracruz, Mexico: Experimentation and
Expected Profits
Meredith J. Soule (Also available in Spanish)

98-01 Increasing Wheat Yields Sustainably through Agronomic Means
P.R. Hobbs, K.D. Sayre, and J.I. Ortiz-Monasterio

Reprint Series
96-01 Evaluating the Potential of Conservation Tillage in Maize-based Farming Systems in the Mexican
Tropics
O. Erenstein

97-01 Are Productivity-Enhancing, Resource-Conserving Technologies a Viable "Win-Win" Approach in
the Tropics? The Case of Conservation Tillage in Mexico

97-02 Conservation Tillage or Conservation of Residues? An Evaluation of Residue Management in Mexico
O. Erenstein (Also available in Spanish.)




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