Estimating Wheat Grain Yield Potential

July 2017


Wheat grain yield is the product of heads per square foot, seeds per head and seed weight. Using estimates of these parameters, farmers can derive an estimated grain yield potential. These estimates are generally more accurate as the season progresses. An estimate of the number of heads per square foot in the late spring, for example, will be much more accurate than one made in the fall. It is also important to note that any estimate is an estimate, and nothing more, until the crop is harvested.

Heads per Square Foot
Heads per square foot is dictated by seeding rate, tillering and tiller survival. Most wheat seed in Oklahoma has between 13,000 and 16,000 seeds per pound. At a 60 pounds per acre seeding rate, this will deliver 18 seeds to 22 seeds per square foot. Wheat needs 60 heads to 70 heads per square foot at harvest for full yield potential. October-sown wheat will generally produce two to three good tillers per plant and will reach the 60 heads per square foot target when sown at 60 pounds per acre. November-sown wheat may have only one to two good tillers per plant and will, therefore, fall short of the 60 heads per square foot target when sown at 60 pounds per acre.


Potential heads per square foot can be estimated in field by counting tillers in one linear foot of row and using Equation 1 to calculate the number of tillers per square foot. These calculations have been performed for various row spacings and tiller counts in Table 1. It is important to count only tillers with at least three unfolded leaves. It is also important to keep in mind that these are tillers that can produce grain heads; environmental conditions could still cause the plant to abort some of these tillers. The closer to harvest that these counts are made, the more accurate the estimate will be.

Equation 1

tillers per sq. ft. =

Table 1. Tillers per square foot based on tiller counts per linear foot of row at various row spacings.   
Tillers per foot of row Row Spacing in Inches
67.59
---------tillers/ft2----------
10201613
12241916
14282219
16322621
18362924
20403227
22443529
24483832
26524235
28564537
30604840
32645143
34685445
36725848
38766151
40806453
42846756
44887059
46927461
48967764
501008067

Seeds per Head
Seeds per head is simply the product of the number of seeds per spikelet and the total number of spikelets per head. Oklahoma wheat averages between 2 grains and 2.5 grains per spikelet and from 8 spikelets to 12 spikelets per head for a total of 20 grains to 30 grains per head. This is the “typical” range and actual grains per head can deviate greatly from this range. Factors such as variety, emergence time, grazing, tiller population density, fertility, foliar disease, insect infestations, weed control and moisture availability will affect the final number of seeds per head.

If a yield estimate is being calculated during or after grain fill, seeds per head can be estimated very precisely by counting seeds in random head samples from across the field. Estimates of seeds per head earlier in the year will be less reliable, as they rely upon several assumptions that may or may not prove to be correct as the crop year progresses. It is also important not to count seeds that are shriveled or light, as these will likely not make it to the combine tank and will not contribute much to yield if they do.

Seed Weight
The list of factors that can influence seeds per head also influence seed weight. Within a given environment, however, variety probably has the largest effect on seed size. Even though seed size influences final grain yield, it is important to note that small-seeded varieties frequently compensate by producing more heads and more seeds per head than larger-seeded varieties. Given these caveats, most hard red winter wheat produced in Oklahoma will have between 13,000 and 16,000 seeds per pound. It is also important to note that seeds per pound will generally be larger in noncleaned wheat because small seeds are removed during the cleaning process.

Calculating the estimate
Once estimates of the number of heads per square foot, seeds per head and seed weight have been established, they can be combined using Equation 2 to produce an estimated yield potential. The ‘726’ value in Equation 2 is a constant that results from simplifying the equation. The results of Equation 2 using several possible combinations have been calculated in Table 2.

Equation 2
estimated grain yield (bu/A) =

Table 2. Estimated yield potential for seeds per pound, seeds per head, and heads per square foot combinations likely to be found in Oklahoma wheat fields.               
14,000 seeds per pound 15,000 seeds per pound 16,000 seeds per pound
 Seeds per head Seeds per head Seeds per head
Heads per square foot202224262820222426282022242628
grain yield estimate (bu/A) grain yield estimate (bu/A)grain yield estimate (bu/A)
20212325272919212325271820222425
22232527303221232628302022242628
24252730323523262830332224262830
26273032353825283033352426283133
28293235384127303335382528303336
30313437404429323538412730333538
32333740434631343740432932353841
34353942464933363943463134374043
36374145495235384245493336394246
38394347515537404448513438414548
40414650545839434650543640444751
42444852576141454953573842465053
44465055596443475155604044485256
46485257626745495358624246505458
48505560657046515660654448525761
50525762677348535863684550545964
52545965707650556065704752576166
54566267737852576368734954596469
56586470768154606570765156616671
58606672788456626773795358636874
60626875818758647076815460657176
62647177849060667278845662687379
64667380869362687481875864707681
66687582899664707783896066727884
68717885929966727986926268748086
707380879410268758188956470768389

Jeff Edwards
Small Grains Extension Specialist

 

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