Calculating Yield

Wheat yields are a function of a combination of factors

• Plant population: which affects tillering and number of heads
• Head size: the number of spikelets per head
• Number of kernels per spikelet
• Size of the kernels.
• The amount of nitrogen available to the plant through Feekes stage 6 is critical to determining the number of tillers formed, unless water or some other factor becomes limiting. Feekes stage 6 is when the first node of the stem becomes visible.
• The number of tillers that will form a head is determined by Feekes stage 6.
• The factors that affect head size are most significant from mid to late tillering, which corresponds with Feekes stage 3 for winter wheat (this occurs by the time the main stem has five or six leaves in spring wheat).
• Number of kernels per spikelet is determined at late jointing - Feekes stages 5 to 6, while the maturing process occurs during the period between boot and early heading (stages 10 to 10.5).
• Management practices starting at stage 8 can have a significant effect on kernel size. The key is to keep the flag leaf healthy as long as possible and to provide enough water and nutrients for maximum grain fill.

—Jim Bauder, Montana State University
extension soil and water quality specialist

Formulas for Estimating Wheat Yields

1) Bushels = (Heads) x (spikelets) x (2.3) (divided by row space) (per foot) (per head) (in inches) x 0.48 per acre

2) Bushels per acre of hard red spring wheat = kernels per head x heads in 3 ft x 0.0319

3) Bushels per acre of durum = kernels per head x heads in 3 ft x 0.0377

4) Bushels per acre of barley = kernels per head x heads in 3 ft x 0.0389

Formulas 2, 3, and 4 are for 7”  row spacing and three foot of row (For 6” spacing, multiply final answers above by factor of 1.17)

Examples above are based on three foot row sample.

Estimating Corn Yield
There are several techniques for estimating corn grain yield prior to harvest. This version was developed by the Ag. Engineering Department at the University of Illinois and is the one most commonly used. A numerical constant for kernel weight is figured into the equation in order to calculate grain yield. Since weight per kernel will vary depending on hybrid and environment, the yield equation should only be used to estimate relative grain yield. For example, yield will be overestimated in a year with poor grain fill conditions, while it will be underestimated in a year with good grain fill conditions.

Step1. Count the number of harvestable ears per 1/1000th acre. (See table below).

Step 2. Count the number of kernel rows per ear on every fifth ear. Calculate the average.

Step 3. Count the number of kernels per row on each of the same ears, but do not count kernels on either the butt or tip that are less than half size. Calculate the average.

Step 4. Yield (bushels per acre) equals:

(ear #) X (avg. row #) X(kernel #)

90

Estimated % corn grain yield loss due to defoliation at various growth stages¹

	% Leaf Defoliation
Growth Stage2	10	15	20	25	30	35	40	45	50	55	60	65	70	75	80	85	90	95	100
	% Yield loss
7 leaf	0	0	0	0	0	0	1	1	2	3	4	4	5	5	6	7	8	9	9
8 leaf	0	0	0	0	0	1	1	2	3	4	5	5	6	6	7	8	9	10	11
9 leaf	0	0	0	1	1	2	2	3	4	5	6	6	7	7	9	10	11	12	13
10 leaf	0	0	0	1	2	3	4	5	6	7	8	8	9	9	11	13	14	15	16
11 leaf	0	0	1	1	2	3	5	6	7	8	9	10	11	12	14	16	18	20	22
12 leaf	0	0	1	2	3	4	5	7	9	10	11	13	15	16	18	20	23	26	28
13 leaf	0	1	1	2	3	4	6	8	10	11	13	15	17	19	22	25	28	31	34
14 leaf	0	1	2	3	4	6	8	10	13	15	17	20	22	25	28	32	36	40	44
15 leaf	1	1	2	3	5	7	9	12	15	17	20	23	26	30	34	38	42	46	51
16 leaf	1	2	3	4	6	8	11	14	18	20	23	27	31	36	40	44	49	55	61
17 leaf	2	3	4	5	7	9	13	17	21	24	28	32	37	43	48	53	59	65	72
18 leaf	2	3	5	7	9	11	15	19	24	28	33	38	44	50	56	62	69	76	84
19-21 leaf	3	4	6	8	11	14	18	22	27	32	38	43	51	57	64	71	79	87	96
Tassel	3	5	7	9	13	17	21	26	31	36	42	48	55	62	68	75	83	91	100
Silked	3	5	7	9	12	16	20	24	29	34	39	45	51	58	65	72	80	88	97
Silks brown	2	4	6	8	11	15	18	22	27	31	36	41	47	54	60	66	74	81	90
Pre-blister	2	3	5	7	10	13	16	20	24	28	32	37	43	49	54	60	66	73	81
Blister	2	3	5	7	10	13	16	19	22	26	30	34	39	45	50	55	60	66	73
Early milk	2	3	4	6	8	11	14	17	20	24	28	32	36	41	45	50	55	60	66
Milk	1	2	3	5	7	9	12	15	18	21	24	28	32	37	41	45	49	54	59
Late milk	1	2	3	4	6	8	10	12	15	18	21	24	28	32	35	38	42	46	50
Soft dough	1	1	2	2	4	6	8	10	12	14	17	20	23	26	29	32	35	38	41
Early dent	0	0	1	1	2	3	5	7	9	11	13	15	18	21	23	25	27	29	32
Dent	0	0	0	1	2	3	4	6	7	8	10	12	14	15	17	19	20	21	23
Late dent	0	0	0	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
Nearly mature	0	0	0	0	0	0	0	0	1	2	3	4	5	5	6	6	7	7	8
Mature	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0

¹Adapted from the National Crop Insurance Association’s “Corn Loss Instruction” (Rev. 1984).
²As determined by counting fully expanded leaves (i.e., those with 40-50% of leaf exposed from whorl and whose tip points below the horizontal.)

Estimating Soybean Yield
Soybean yield estimates are most accurate within three weeks of maturity, but are still only estimates. Assume 2.3 bean per pod.

• 1. Determine the number of feet of row needed to make 1/1000 of an acre (See table below).
• 2. Count the number of plants in ten (10) different randomly selected sample areas. Calculate the average.
• 3. Count the number of pods per plant on ten (10) randomly selected sample areas. Calculate the average.
• 4. Calculate pods/acre by multiplying plant population by pods/plant.
• 5. Calculate seeds/acre by multiplying pods per acre by an estimate of 2.3 seeds/pod.
• 6. Calculate pounds/acre by dividing seeds/acre by an estimate of 3000 seeds/pound.
• 7. Estimate yield by dividing pounds/acre by 60 pounds/bu.

Length Of Row Equal To 1/1000th Acre|An accurate estimate of plant population per acre can be obtained by counting the number of plants in a length of row equal to 1/1000 of an acre. Make at least three counts in separate sections of the field, calculate the average of these samples, then multiply this number by one thousand (1,000).

Row Width Length of Single Row to Equal 1/1000 of an acre (inches) (feet) (inches) 6 87 1 7 74 8 8 65 4 10 52 3 15 34 10 20 26 2 28 18 8 30 17 5 32 16 4 36 14 6 Source: NDSU