Methods for Calculating Corn Yield
September 20, 2001 8(25):159160
Joe Lauer, Corn Agronomist
Predicting corn yield prior to harvest is often useful for yield monitor calibration,
and for making feed supply and marketing decisions. The BEST and most accurate method
for estimating yield, other than weighing harvested grain from the entire field,
is to harvest and weigh representative samples from a plot area after plants have
reached physiological maturity. Below are a number of methods for calculating corn
grain yields listed in order of decreasing accuracy. To properly calculate yield
you must determine grain moisture, harvested area and grain weight.
Determining Moisture
Remember, corn yields are standardized to 15.5% moisture and 56 pounds per bushel.
Obtain a grain moisture sample by removing several rows of corn kernels the full
length of 10 randomly selected ears from each row sampled and thoroughly mix the
grain. Place grain in moisture proof container to avoid moisture loss. Establish
moisture content with an accurate moisture determination system.
Determining Harvested Area
Machine Harvest
Measure the total row length of the area harvested and multiply the average row
width. Measure length of row with a measuring wheel when row lengths are greater
than 100 feet. For row lengths less than 100 feet use a steel tape. Be sure to measure
the width of the strip at several places to account for the "guess" rows.
The measured area must include half the distance between the first and last rows
harvested and the ones next to them not harvested in the area.
For example, if 2000 feet are harvested "down and back" with a sixrow
corn head, and the average row width is 30 inches (2.5 feet) the calculations are:
(6 rows) x (2 passes) x (2000 feet) x (2.5 feet) divided by 43560 feet^{2}
per acre = 1.38 acre
Hand Harvest
Yield should be determined at 5 to 10 sites in the field and the average reported.
In a 1/1000^{th} acre area, collect and count all harvestable ears. Table
1 gives row length equal to 1/1000^{th} acre for several row widths. A larger
area of 1/100^{th} acre is preferable and can be obtained by harvesting
10 rows.
Table 1. Row length equivalent to 1/1000th acre at various row widths.

Row width

Length for 1/1000th acre

inches

feet

7

74.8

15

34.8

20

26.1

28

18.7

30

17.4

32

16.3

34

15.4

36

14.5

38

13.8

40

13.0

Determining Grain Weight (Shelling Percentage)
Weigh 10 randomly selected ears. Shell ears and weigh grain. Calculate shelling
percentage using (grain weight / ear weight) x 100. Shelling percentages of normal
ears usually average about 80% when fields are ready for combine harvest (20 to
25% grain moisture).
Method for Calculating Machine Harvested Corn
For grain corn, first determine acreage harvested as described above. Calculate
pounds of dry matter (1grain moisture %) and convert to 15.5% moisture. Finally
divide acreage and test weight of 56 pounds per bushel.
For example, if the total grain weight from 1.38 acre is 15,000 pounds at 22 % kernel
moisture (10.22=0.78), the yield is calculated as follows:
15,000 pounds x 0.78 = 11,700 pounds dry matter
11,700 ÷ 0.845 = 13,846 pounds at 15.5% moisture
13,846 ÷ 56 = 247 bushels at 15.5% moisture
247 bushels ÷ 1.38 = 179 bushels per acre at 15.5% moisture
For ear corn, determine acreage harvested, total ear weight, and kernel moisture.
Use Table 2 to find the pounds of ear corn required for a bushel of 15.5% shelled
corn.
For our example above, suppose we harvested 18,820 pounds of ears at 22% kernel
moisture (use Table 2) from 1.38 acre with a corn picker, the calculations would
be:
18,820 ÷ 76.2 pounds ear corn at 22% moisture per shelled corn equivalent at 15.5%
moisture = 247 bushels at 15.5% moisture
247 bushels ÷ 1.38 acre = 179 bushels per acre at 15.5% moisture
Method for Shelled Corn
This method is similar to methods for machine harvest. Take special care in measuring
area.
1. Weigh grain from 1/1000^{th} acre and measure total grain weight and
moisture.
2. Calculate percent dry matter = (1 – grain moisture %)
3. Multiply grain weight x percent dry matter x 1000 = Pounds of dry matter per
acre
4. Now dry matter per acre must be converted to 15.5% moisture (1  0.155 = 0.845)
and a test weight of 56 pounds per bushel. The calculations are:
Pounds of dry matter per acre ÷ 0.845 = Pounds of grain at 15.5% moisture
Pounds of grain at 15.5% moisture ÷ 56 = Bushels of per acre at 15.5% moisture
For example, suppose we harvest 10 pounds of grain from 1/1000^{th} acre
at 22% moisture.
10 X 0.78 X 1000 = 7800 pounds of dry mater per acre
7800 ÷ 0.845 = 9231 pounds per acre at 15.5% moisture
9231 ÷ 56 = 165 bushels per acre at 15.5% moisture
A short cut to these calculations
is to harvest 1/1000^{th} of an acre of corn, determine total weight and
moisture content, convert total weight to total dry matter content and then multiply
by 21.13. ÷ 87.04) x 1,000] = 158 bushels per acre
Table 2. Pounds of corn required to equal one bushel of
Number 2 shelled corn (at 15.5% moisture) when corn is harvested at various moisture
levels. Derived from Purdue University AES Circular 472.

Percent moisture
in corn

Pounds of shelled corn
needed to equal one bushel

Pounds of ear corn
needed to equal one bushel

%

pounds

pounds

11.0

53.17

63.3

12.0

53.77

64.2

13.0

54.39

65.2

14.0

55.02

66.2

15.0

55.67

67.3

15.5

56.00

67.8

16.0

56.33

68.4

17.0

57.01

69.6

18.0

57.71

70.8

19.0

58.42

72.1

20.0

59.15

73.4

21.0

59.90

74.8

22.0

60.67

76.2

23.0

61.45

77.7

24.0

62.26

79.2

25.0

63.09

80.7

26.0

63.95

82.2

27.0

64.82

83.7

28.0

65.72

85.2

29.0

66.65

86.7

30.0

67.60

88.2

31.0

68.58

89.9

32.0

69.59

91.7

33.0

70.63

93.6

34.0

71.70

95.6

35.0

72.80

97.7

36.0

73.94

99.9

Method using Corn Ear Weight
The ear weight method can only be used after the grain is physiologically mature
(black layer), which occurs at about 3035% grain moisture. Since this method is
based on actual ear weight, it should be somewhat more accurate than other methods
listed below. However, there still is a fudge factor in the formula to account for
average shelling percentage.
Sample several sites in the field. At each site, measure off a length of row equal
to 1/1000^{th} acre. Count the number of harvestable ears in the 1/1000^{th}
acre. Weigh every fifth ear and calculate the average ear weight (pounds) for the
site. Hand shell the same ears, mix the grain well, and determine average percent
grain moisture with a portable moisture tester.
Calculate estimated grain yield using the ear weight method as follows:
1. Multiply ear number by average ear weight.
2. Multiply average grain moisture by 1.411.
3. Add 46.2 to the result in step 2.
4. Divide the result from step 1 by the result from step 3.
5. Multiply the result from step 4 by 1,000.
For example, you evaluate a field with 30inch rows and count 24 ears (per 17 ft.
5 in. section). Sampling every fifth ear resulted in an average ear weight of ½
pound. The average grain moisture was 30 percent. Estimated yield would be:
[(24 x 0.5) / ((1.411 x 30) + 46.2)] x 1,000 = 135 bushels per acre.
Method using Corn Ear Length (Hicks, MN)
This method is less accurate than others described above due to the "fudge"
factors used in its calculation, but it is a relatively quick and easy way to get
an idea of grain yield.
1. Determine row width
2. Measure 30 feet of row length
3. Count the number of ears on two adjacent rows and determine an average
4. Find the yield at the intersection of row width and average ear number in Table
3.
5. Husk ears from 10 consecutive plants and determine the average length of ear
with kernels. Yields in Table 3 assume halfpound dry ears 7.5 inches long. Use
Table 4 to adjust the yield if ear length is shorter or longer (multiply the yield
from Table 1 by the appropriate factor in Table 4).
6. When the number of ears on 30 feet of row are not included in Table 3, you can
estimate the value. For example, suppose you have 30 ears and a 30inch row spacing.
Extrapolate between the yields given for 29 and 31 ears to arrive at 124 bu/A. Another
alternative is to determine the value of 15 ears, which is 62 bu/A and double it
to obtain an estimate of 124 bu/A.
Table 3. Corn grain yields for various numbers of ears in 30 feet of row and various
row spacings. Values are based on an average ear dry weight of 0.5 pounds.


Row width (inches)

Number of ears in
30 feet of row

15

20

30

36

38


Bushels per acre

13

106

80

54

45

43

15

124

93

62

52

49

17

144

107

70

58

55

29

155

117

79

66

62

21

164

125

86

72

69

23

191

143

95

79

75

25

206

155

104

86

81

27

223

167

11

92

87

29

240

180

120

99

94

31

256

192

128

107

101

33

274

205

136

114

108

35

289

217

145

121

114

37

306

229

153

127

120

39

323

242

161

134

127

41

339

254

169

141

133

Table 4. Adjustment factors for estimating corn grain yield with differing average
ear lengths.

Average ear length

Factor

5.0

0.4

6.3

0.6

7.0

0.8

7.5

1.0

8.2

1.2

9.0

1.4

Method using Corn Yield Components (also referred to as the "slide rule"
or corn yield calculator)
The yield component method was developed by the Agricultural Engineering Department
at the University of Illinois. The principle advantage to this method is that it
can be used as early as the milk stage of kernel development. The yield component
method involves use of a numerical constant for kernel weight that is figured into
an equation in order to calculate grain yield. This numerical constant is sometimes
referred to as a "fudgefactor" since it is based on a predetermined average
kernel weight.
Since weight per kernel will vary depending on hybrid and environment, the yield
component method should be used only to estimate relative grain yields, i.e. "ballpark"
grain yields. When below normal rainfall occurs during grain fill (resulting in
low kernel weights), the yield component method will OVERESTIMATE yields. In a year
with good grain fill conditions (resulting in high kernel weights) the method will
underestimate grain yields.
Because it can be used at a relatively early stage of kernel development, the Yield
Component Method may be of greater assistance to farmers trying to make a decision
about whether to harvest their corn for grain or silage. If stress conditions, such
as drought, have resulted in poorly filled small ears, there may be mechanical difficulties
with sheller or picker efficiency that need to be considered. Corn yield "calculators"
that count kernel number roughly estimate yield and produce yield estimates that
are within 20 bushels per acre of actual yield.
Calculate estimated grain yield using the Yield Component Method as follows:
1. Count the number of harvestable ears in a length of row equivalent to 1/1000^{th}
acre.
2. On every fifth ear, count the number of kernel rows per ear and determine the
average. Try to use a system such as the 5^{th}, 9^{th}, and 13^{th}
ears from one end of the row.
3. On each of these ears count the number of kernels per row and determine the average.
(Do not count kernels on either the butt or tip of the ear that are less than half
the size of normal size kernels.)
4. Yield (bushels per acre) equals (ear number) x (average row number) x (average
kernel number) divided by 89.605* = bushels per acre
*or multiply by 0.01116
5. Repeat the procedure for at least four additional sites across the field.
For example, you are evaluate a field with 30inch rows and counted 24 ears (per
17' 5" = row section). Sampling every fifth ear resulted in an average row
number of 16 and an average number of kernels per row of 30. The estimated yield
for that site in the field would be (24 x 16 x 30) divided by 89.605 = 128 bushel
per acre.