excerpt from NDSU Crop & Pest Report July 2, 2020
I received a phone call earlier last week from a grower wanting to estimate the yield of his winter wheat crop. The formula he was using was not working so I provided him an alternative, one that I use frequently. Estimating yield using a formula that is based on yield components can also give you insight into how environmental and management factors are influencing crop yield this season. The formula I like to use for predicting yield can be applied soon after the crop has headed. Most of the early planted crop is now in this stage. The three components that contribute to yield in wheat are the number of spikes per area, the number of kernels per spike and kernel weight. Kernel weight can vary by variety and the environment and cannot be accurately ascertained until maturity. This is one of the reasons that estimating yield is just that, an estimate with the potential of some level of variance from the actual. The formula described below uses a historic average for kernel weight to allow for an estimation prior to maturity and harvest. That value and other conversion factors are incorporated into the value “0.142” included in the numerator. Since small differences in the numbers used in this formula can results in large differences in the estimated yield, multiple samples taken from representative areas of the field will help improve the accuracy of the estimate.
Bushels/acre = (spikes per 3 ft of row X spikelets per spike X kernels per spikelet X 0.142
Row spacing in inches
Directions for obtaining the values for this formula:
1. Count the number of spikes in a three-foot length of row (taking an average of several counts improves the estimate). Do not count small heads that will not significantly contribute to yield. If the crop was planted with an air seeder in bands or as paired rows, count the spikes from the entire width of the band or both rows in the pair.
2. Count the number of spikelets in six or more randomly selected spikes. Omit the top and bottom spikelets as they contribute little to the overall yield. Spikelets are the primary branches on the spike.
3. The number of kernels per spikelet can be determined by hand threshing the head and counting the kernels and then dividing that number by the number of spikelets. Previous experience, however, has shown that 2.3 kernels per spikelet gives a more accurate yield estimate, unless the crop has been stressed, then 2.1 kernels per spikelet should be used.
4. If not known, determine the row spacing by measure the distance between several rows or bands of wheat. Most double disc drills are set at 6, 7, or 8 inch row spacings. The width of air seeded bands can vary. To determine the row spacing of the bands, measure the distance between the edge of one band to the same edge of the adjacent band.
Estimates from plots near Fargo this year (see Figure 1):
This was the earliest planted spring in our program (4/25/20).
(64 spikes in three foot row X 14.3 spikelets per spike X 2.3 kernels per spike X 0.142)/(7 inch row spacing) = 43 bu/acre
Comments on crop development in this plots:
Plant height was very short probably due to abnormally warm temperature during the period of when stem cells were developing.
Plants did not completely fill-in all inter-row space (see Figure 1). The number of spikes per three-foot row was about 50% fewer than we observed last year in same field. Low spike density can be attributed either poor tillering or an inadequate seeding rate. Though the hot weather during tillering probably took a toll on spike density (note the aborted tillers in Figure 2), the plant population in this area was also lower than optimum either because of too few seeds sown or poor emergence of the seeds that were sown. I definitely think we would have seen a yield bump by increasing the seeding rate at this location this year (or achieved better emergence). Spike size is sensitive to temperature during early reproductive development. Nevertheless, the spikelet numbers per spike this season were similar to what we observed in previous years, even though temperatures there were periods when the temperatures were much higher than higher than optimum during that phase of development.
Joel Ransom
Extension Agronomist, Cereal Crops
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Source: NDSU Crop & Pest Report