Roy G. Stout
Florida Crop and Livestock
Department of Agricult ciEconomics
Florida Agricultural Ex ir(riinCStation .';
Report No. 62-2
SIZE OF FRUIT AND DROPPAGE RATES INFLUENCE
TOTAL CITRUS PRODUCTION
Data reported in this report were collected primarily for use in
citrus crop forecasting. These data collections were made possible from
funds provided on a matching basis by the Growers Administrative
Committee and the Agricultural Marketing Service, United States
Department of Agriculture,
Many thanks are extended to the Florida Crop and Livestock
Reporting Service for their assistance and use of data in preparation
of this report. Particularly appreciation is extended to Mr. J. C.
Townsend, Jr., Statistician-ln-Charge and to Mr. Paul E. Shuler,
Citrus Statistician and Mr. J. VV. Todd, Survey Statistician.
INTRODUCTION.... .. ....... * .* 1
GROWTH RATES. .............. . ... 2
Average Monthly Growth Rates 3
October to Maturity Date Growth Rates 3
DROPPAGE RATES . ....... .. . 10
Annual Droppage Rates 11
Monthly Droppage Rates 12
EFFECTS ON TOTAL PRODUCTION .. ... .. ... 13
Number of Fruit per Box 14
Change in Total Production 14
SUMMARY.. ....... .. ... 18
SIZE OF FRUIT AND DROPPAGE RATES INFLUENCE
TOTAL CITRUS PRODUCTION
Roy G. Stout1
The size of the citrus crop is influenced by many factors such as climatic
conditions, soil type, and cultural practices. In direct relation to these pro-
ductive factors and the size of the citrus crop is the number of fruit set on the
tree, the number of fruit that drops off during the growing season, and the size
of the fruit at picking. For the past several seasons the United States Department
of Agriculture has collected information on growth rates and droppage rates in
connection with forecasting the citrus crop.
These growth and droppage rates and their influence on the total citrus
crop are summarized in this report. All data presented herein relates to esti-
mated averages for the State, Individual growers are cautioned that their groves
may or may not behave in a manner similar to the State averages.
Information on growth rates and droppage rates was collected from approxi-
mately 445 groves throughout the citrus producing counties. The sample in 1960-61
consisted of 135 early and midse.ason groves, 125 Valencia groves, 78 seedy grape-
fruit groves, and 117 seedless grapefruit groves.
Assistant Agricultural Economist, Department of Agricultural Economics.
For collecting growth rate data, 2 trees per grove were randomly selected
and a spot on a limb was tagged. The 10 nearest fruit to this spot was measured
each month. When a sample tree was picked of fruit the next grove on the route
of like variety was substituted. If one of the fruit that was being measured dropped
off, the next closest one to the tagged spot was measured as a substitute.
A limb on each of the 2 trees with a cross-sectional size of about 2 percent
of the cross-sectional size of the trunk was selected and tagged for collecting
droppage data. The number of fruit on this limb was counted as of September 1 and
each month thereafter. The droppage rate each month was determined by the
difference in the number of fruit on the limb for the current month and the previous
month. When a tree was picked, no replacement was added to the sample.
Each individual fruit was measured by the use of specially built calipers
which measured the circumference of the fruit in inches, These measurements
were converted to volume of fruit in cubic inches. The size of the fruit was
converted to volume basis in order to determine the number of fruit required to
equal a 90 pound box equivalent. Consequently this report considers volume as
the unit of size measurement. These data have been obtained monthly from
September to the end of the season since the 1954-55 production year.
Average Monthly Growth Rates
Oranges.--The 7 year average size per fruit of early and midseason and
Valencia oranges by months is shown in Figure 1. The average size of early and
midseason oranges in September was nearly 7 cubic inches. The average size at
the date of maturity of January 1 was 11.6 cubic inches. Valencia oranges
average larger in size than the earlier varieties; perhaps this is due to the longer
growing season. The average size of Valencias in September was about 6.4 cubic
inches whereas the average was about 13 cubic inches at the date of maturity of
Tangerines.--The 7 year average growth of tangerines from September to
December was about 1.4 cubic inches each month (Figure 2). The average size
in September was 2.6 cubic inches and 6.7 inches in December. Growth from
December to January was a very small amount.
Grapefruit.--Seedy grapefruit averaged considerably larger than seedless
grapefruit (Figure 3). September average volume for seedy grapefruit was nearly
28 cubic inches and seedless grapefruit averaged 21.5 cubic inches. March 1
average sizes were 43.6 cubic inches for seedy and 35.2 cubic inches for seedless
October to Maturity Date Growth Rates
The first forecast of the citrus crop is released on October 10. Consequently
the October 1 size of fruit is the last measurement obtained to estimate the size at
maturity. Tables 1 and 2 show the average fruit size for October 1 and the maturing
- IiI I I
Jan. Feb. Mar. Apr.
Figure 1 .--Growth Rote of Early and Midsoason Oranges,
September-January, and Valencias, September-April,
January, 1954-60 Average
I I I i l I I .
Sept. Oct. Nov. Dec. Jan. Fob. Mar. Apr.
Figure 3.--Growth Rates of Soedless and Soody Grapefruit, September-
March, 1954-60 Average
Table 1 .--Average Volume per Fruit of Oranges and Tangerines as of October 1
ard Maturing Date, 1954-55 to 1960-61
----------------------. Cubic Inches--------------------
Average 9.54 11.62
aAs of January 1.
As of April 1.
CAs of January 1.
Volume per Fruit of Grapefruit as of October 1
and Maturing Date, 1954-55 to 1960-61
-----. --------------- Cubic Inches---------.------ ------
aAs of February 1.
date for each of the past 7 seasons. The average size of the fruit has varied con-
siderably during the 7 years covered. The October size for early and midseason
oranges, for example, was only 8.06 cubic inches in 1958-59. The following year
the average size on the same date was 10.71 cubic inches. However, the increase
in size from October to harvest is more consistent. Relative to their annual average
sizes, the other kinds of citrus show more variation in comparison between years than
was true for early and midseason oranges.
Table 3 shows the relationship of the October size to the size at picking
time. The size of the fruit at harvest time is pretty well established by October.
Early and midseason oranges averaged an increase of 22 percent from October 1 to
January 1 for the 7.years. The smallest increase was 14 percent in the 1954-55
season and the largest increase was 34 percent in the 1958-59 season.
Table 3.--October 1 Average Volume of Fruit as a Percent of the Average Volume
at Maturity Date, 1954-55 to 1960-61
Oranges Grapefruit Tangerines
Early and Midseason Valencias Seedy Seedless
1954-55 114 139 121 123 148
1955-56 120 149 118 123 199
1956-57 130 168 128 137 174
1957-58 113 137 116 116 150
1958-59 134 178 137 135 175
1959-60 121 148 123 132 153
1960-61 125 160 131 127 192
Average 122 153 124 127 168
Valencia oranges at harvest date averaged 153 percent of the October
size -- an increase of 53 percent. In the case of Valencias, there appears to be
2 growth rate patterns that follow from October to harvest. This undoubtedly
is due to whether there has been an early or late bloom and fruit set. During the
7 years, the percentage growth from October until harvest varied from 37 percent
in the 1957-58 season to 78 percent in the 1958-59 season, Part of the small
increase in size during 1957-58 was undoubtedly due to the December freezes.
However, there were other low growth years as in 1954-55 when the percentage
growth was only 39 percent. If the data for Valoncias in Table 2 are divided into
2 groups (those years when the October size was under 8 cubic inches in one
group and over 8 cubic inches in the second group) the percentage growth rate is
very similar within each group. For the small group the average harvest size
was 169 percent of the October size. For the large sized group the average
harvest size was 144 percent of the October size.
Tangerine growth from October to January is considerable, For the
7 years the average increase from October to January was 68 percent, with a
low of 48 in 1954-55 and a high of 99 in 1955-56. Tangerine size, like Valencias,
in October is a factor in determining the percentage growth for the remainder of
the year. If the data for tangerines in Table 2 is divided into two groups (those
years when the October size was under 4.1 cubic inches in one group and those
larger in a second group) the percentage increase from October to January for the
small group was 85 percent and 51 percent for the large group.
Grapefruit sizes are fairly well established by October. The 7 year
average for seedy grapefruit showed an increase of 24 percent from October to
February. Seedless increase averaged 27 percent. The range for seedy was from
a low of 16 percent during the 1957-58 year (year of the freeze) to 37 percent
in 1958-59. The seedless grapefruit range was about the same -- from 16 per-
cent in 1957-58 to 37 percent in 1956-57.
The amount of fruit that drops off a tree even after the fruit has passed
the halfway mark in the length of the growth period is considerable. For many
trees, half of the fruit that was on the tree as of September 1 dropped off before
the tree was picked. This rate of droppage has caused considerable alarm to many
people in the industry, particularly those who have been directly affected. Again
it should be pointed out that the droppage rate presented represents only averages
for the State sample and individual groves may show considerable variation from
these averages. It is anticipated that a later publication will include a study of
the variation in droppage rates and growth rates.
Information on droppage rates for the 1959-60 and 1960-61 season was
obtained according to the methods described in the Introduction. For the 3 previous
seasons, droppago data were obtained by clearing the ground under the sample
trees in September and returning each month to count the fruit on the ground. An
average drop per tree was computed for the sample. This was used to adjust the
average number of fruit on the tree as estimated in the fruit count survey. Mechanics
of carrying out the field work plus the advantage of having complete data, that is,
number of fruit on the sample limb and the droppage from these limbs, made it
desirable to change to the tagged limb sample.
Annual Droppage Rates
The estimated percentage of the crop that was on the tree as of September 1
that dropped off before the end of the harvest season is shown in Table 4.
Valencia oranges dropped considerably more on a percentage basis than any
other kind of fruit. Again the longer growing season contributes greatly to
this increased droppage. A higher percentage of seedy grapefruit dropped off
the tree than of the seedless variety.
Table 4.--Estimated Percent of Total Crop on Tree as of September 1 that Dropped
During the Seasons 1956-57 to 1960-61
10.3 19.0 14.6 11.1
16.3 40.4 25.6 18.0
12.4 24.7 16.6 16.2
16.6 33.7 12.3 10.4
23.9 41.9 31.9 27.0
15.9 31.9 20.2 15.1
Early and midseason oranges dropped off at an average rate of about 16 per-
cent; Valencias, 32 percent; seedy grapefruit, 20 percent; and seedless grape-
fruit, 15 percent. The 1957-58 freeze in December undoubtedly affected Valencias
and grapefruit the most since much of the early varieties crop was already
harvested. Likewise the September hurricane affected the 1960-61 droppage
even though droppage during the hurricane was estimated as 10 percent and has
been subtracted out of the data in Table 4.
Monthly Droppage Rates
The month-to-month variation in droppage rates is rather variable, con-
sequently it is rather difficult to estimate droppage rates. However, the 5 year
monthly average droppage rates indicate a significant droppage each month
(Table 5). The September droppage for oranges was slightly over 4 percent.
This reduced slightly, percentagewise, as the season advanced to about 3 per-
cent in January and February. Grapefruit averaged about 3 percent droppage
in September and about 2.5 percent in February and March.
In any one year weather conditions, as the freeze in 1957-58 and the
hurricane in 1960-61, can cause large droppage in any one month. In addition
to this unusual and occasional weather effect, there are other factors that cause
some droppage to occur every month of the season.
Table 5.--Average Percent of Fruit on Tree as of the First Day of the Month that
Dropped During the Month, 5 Year Averagea
Kind of Fruit an
Sept. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May
-....--------------.- -- Percent-- ---------------..
Early and midseason
oranges 4.1 3.7 3.1 2.4 2.9 3.2
Valencias 4.2 4.9 4.6 2.7 3.1 3.2 2.9 3.4 2.0
Seedy grapefruit 3.8 2.5 3.7 3.1 3.1 2.8 2.6
Seedless grapefruit 3.1 1.9 2.6 2,2 2.2 2.5 2.6
aDroppage during 1957-58 after the freeze was not included in computing
these monthly averages.
bMonthly figures are based on the crop remaining on the tree, therefore
the total of the monthly percentage drops is greater than the percent of the total
crop that dropped off the trees.
EFFECTS ON TOTAL PRODUCTION
Droppago rates and growth rates both have a considerable effect on the
size of the total citrus crop. Droppage rates require no explanation as to their
effect on the size of the crop harvest, because if an orange drops off the tree then
it is entirely eliminated from the harvest potential. However, variations in
growth rates are not so straightforward. Variations in the size of the individual
fruit cause variations in the number of fruit required to make a box. That is the
number of fruit required to fill a standard sized box is dependent on the size of
the individual fruit.
Number of Fruit Per Box
Figure 4 shows the relationship between average volume per fruit and
number of fruit per box for oranges. Assuming the average volume per fruit of
oranges is 15 cubic inches, it takes 165 oranges to fill a box; if the average
volume is 17 cubic inches the number of fruit per box is about 141, and if the
volume is 12 cubic inches the number of fruit per box is 204.
Figure 5 shows the relationship between volume per fruit and number of
grapefruit per box. If average volume is 60 cubic inches, it takes about 47 fruit
to fill a box. However, it requires 67 fruit to fill a box if the average volume
per fruit is 40 cubic inches.
Change in Total Production
The effects of an increase in one average size on tree production is
shown in Table 6. If grapefruit growth increased in volume from 37.4 to
42.0 cubic inches, the number of fruit per box decreases from 70 to 64 which
is about 8.5 percent. Assuming an average tree has 500 grapefruit, the number
of boxes per tree increases from 7.1 to 7.8 boxes. There are presently about
6.7 million grapefruit trees in Florida; consequently a change in average fruit
volume from 37.4 to 42.0 cubic inches per fruit would increase total production
by about 4.2 million boxes (assuming 500 fruit per tree).
120 140 160 180 200 220 240 260
Figure 4.-Relationship Between Fruit Size and Number of Fruit per Box
o 50 -
Figure 5.--Relationship Between Fruit Size and Number of Fruit per Box
Table 6.--Number of Fruit per Box and Number of Boxes by Two Sizes of Fruit
Volume Boxes per Tree Assuming
Kind No. Fruit per 500 Grapefruit and
per Box Fruit 1,000 Oranges per Tree
Grapefruit 70 37.4 7.1
64 42.0 7.8
Oranges 200 12.48 5.01
190 13.05 5.26
If the average volume per fruit of oranges increased from 12.48 to 13.05
cubic inches, the number of fruit per box decreases from 200 to 190, or about
4.6 percent in production. Assuming 1,000 oranges per tree, this would be an
increase from 5.01 to 5.26 boxes. There are presently about 26.5 million orange
trees in Florida, thus an increase in size from 12.48 to 13.05 would mean an
increase in total production of 6.6 million boxes.
According to the data obtained from the limb count survey conducted
in each August and September, the average number of fruit per tree for the past
5 years was slightly above 500 for grapefruit, 866 for Valencias, and 1185 for
early and midseason oranges (Table 7). If these figures are reduced by the
average percent droppage rates shown in Table 4, then the average number of
fruit harvested per tree was 402 for grapefruit, 590 for Valencias, and 997 for
early and midseason.
Table 7.--Estimated Average Number of Fruit per Tree as of September 1a
Season Oranges Grapefruit
Early and Midseason Valencias Seedy Seedless
1956-57 1,218 934 524 594
1957-58 1,380 1,056 524 543
1958-59 1,099 740 663 458
1959-60 1,005 796 321 453
1960-61b 1,221 805 607 508
Average 1,185 866 528 511
aFrom limb count survey.
Thus changes in the average size of the fruit could affect the total pro-
duction by as much as 8 to 10 million boxes from one year to the next, even if
the total number of fruit remains the same.
Data collected during the past 7 years indicate that the eventual size of
an individual fruit of early oranges and grapefruit is pretty well established by
October 1. The individual sizes of tangerines and Valencias vary more between
October size and maturity date size than early oranges and grapefruit. Average
growth rate of early and midseason oranges from October 1 to January 1 was
22 percent, Valencia oranges increased in size by an average of 53 percent from
October to April, tangerines increased by an average of 68 percent from October
to January, seedy grapefruit average increasing in size by 24 percent from
October to February, and seedless grapefruit increased in size by an average
of 27 percent.
Droppage of fruit off the tree after September is considerable and appears
to be on the increase. Valencia droppage is quite a bit higher than the other
kinds of citrus. The 5 year average droppage after September 1 for early and
midseason oranges and seedless grapefruit was about 15 percent; seedy grapefruit,
20 percent; and Valencia oranges, 32 percent.
Research work desperately needs to be conducted to determine the cause
of this heavy droppage. True droppage due to weather conditions cannot be
controlled, but there is considerable droppage due to the splitting of the orange.
If droppage due to this splitting alone could be controlled, citrus production
would be increased by a significant amount.
RGS: ms 7/61
Agr. Expt. Sta. -- 1,500