9/21/2020

How Kernel Weight Varies by Hybrid in Iowa

Written by Ryan Van Roekel, PhD, Dennis Holland, Alex Woodall and Bill Long

Key Findings

Kernel weight is a key component of grain yield that can vary among hybrid families, and be affected by environmental conditions and management practices.
A 4-yr field study found that kernel weight can vary widely due to differences in growing conditions (from 52,000 to 120,000 kernels/bu) but that certain hybrid families tend to run consistently higher or lower than average.
These estimates for kernel weights by hybrid family can be useful for yield estimation, management decisions, and diagnosing yield results that differ from expectations.

The Challenge of Estimating Kernel Weight

Corn grain yield can be estimated in-field based on estimates of yield components: ears per acre, kernels per ear, and kernel weight.
The first two components are relatively straightforward to estimate - conducting several stand counts of 1/1000th of an acre can provide an estimate of ears per acre, and kernel counts can be used to estimate kernels per ear.
Furthermore, new technology has greatly improved the speed and accuracy of estimating ears per acre and kernel per ear:
- UAV imagery powered by Corteva Flight can provide field-wide stand counts.
- The Yield Estimator tool in the Pioneer Seeds app can quickly count kernels per ear.
- The Vegetation Index from satellite imagery in Granular Insights can be used to guide sampling according to field variability to get a better estimate of whole-field yield.
However, estimating the third yield component - kernel weight - remains challenging.
A common practice is to assume 90,000 kernels/bushel, but this practice often underestimates yield and does not account for differences among hybrids or environments.
While work is underway to develop a more reliable way to estimate kernel weights, Pioneer undertook research to characterize common hybrid families in local plots. The goal was to estimate how genetics influence kernels weights to provide more accurate yield estimates.
Additionally, knowing a hybrid’s expected kernel weight can help with understanding the yield impact of late-season management or environmental issues that may prevent a hybrid from reaching its normal kernel weight.

Our Research

Kernel weight data was collected from hybrid plots across Iowa in 2016-2019.
Kernel weights for each hybrid at a location were measured in one of two ways:
- A subsample of 100 random kernels, or more, was weighed and corrected to 15% moisture with the moisture data used to calculate the hybrid’s grain yield.
- Multiple stand/ear and kernel counts were performed prior to harvest to provide a reasonably accurate estimate of ears per acre and kernels per ear. This data was divided by the hybrid’s yield at 15% to determine kernels per bushel.
Both methods have limitations, but hybrid trends were consistent; thus, datasets were combined to increase the number of locations.
A location average kernel weight was calculated from the average of all hybrids in each plot location.
To account for environmental differences across locations, kernel weight for each hybrid within a location was calculated as a percentage of the location average. Those percentages were then averaged by hybrid family over all plot locations, as shown in Table 1.
The standardized kernels per bushel in Table 1 were calculated as the average kernel weight percentage divided by 80,000 kernels/bu to provide a reasonable estimate for kernels/bu by hybrid family. This figure is not the actual mean of the observed kernels/bu because the dataset is very unbalanced for locations between hybrids. As such caution should be used with these results.

Photo - Representative kernels from the tip, middle, and butt of an ear from hybrid families with above-average and below-average kernel weight in 2019.

Figure 1. Representative kernels from the tip, middle, and butt of an ear from hybrid families with above-average (P1197) and below-average (P1082) kernel weight in 2019. Photo courtesy of Bill Long.

Table 1. Kernel weight as a percentage and standardized kernels/bu by hybrid family.

Table - soil moisture effect on herbicide persistence

¹Calculated as hybrid kernels per bushel compared to the location average kernels per bushel, then averaged over all locations.
²Calculated as the kernel weight percentage applied to a “normal” value of 80,000 kernels per bushel, rounded to the nearest 500.
³Only hybrids with a minimum of 3 locations were included.

Results

Kernel weight (kernel/bu) was found to vary widely by hybrid and location.
The grand mean of all kernel weight observations was 77,426 kernels/bu but ranged from 52,192 to 119,602 kernels/bu. Grain yield averaged 232.4 bu/ac with a range from 152.9 to 297.3 bu/acre.
Individual hybrids also had a wide range in kernel weights between locations. For example, the P1197 family ranged from a high of 54,656 kernels/bu down to 92,704 kernels/bu. Across all locations, its kernel weight averaged 107.4% of the location average.

Key Points on Kernel Weight

With the wide variation in observed kernels weights between hybrids and locations, exercise caution when using the standardized kernels/bu shown in Table 1.
- Environmental and management factors can and will greatly influence a hybrid’s ability to maintain or extend its grain fill and express its full kernel weight potential.
- Often issues like disease pressure or nitrogen deficiencies can hinder late season plant health and limit a hybrid’s grain fill period and resulting kernel weight.
High kernel weights are not required for high yields.
- P1366 is an example of a hybrid family with average kernel weight that is capable of very high yields (up to 297 bu/acre in this study).
- P1366 tends to achieve high yields through kernel number (more rows around and/or ear length) vs hybrid families like P1197, which tends to have more average kernel numbers but high kernel weights.
Kernel weight is not correlated with test weight. Test weight is the weight of a volumetric bushel, while kernel weight is a measure of how many kernels are in a 56 lb bushel.
- An example of this distinction is the P1093 hybrid family, which has very high test weight with excellent grain quality but its high-density kernels tend to be smaller in physical size and thus weigh less per kernel.
- Contrarily, the P1197 hybrid family tends to have less dense, lower test weight grain but very large kernels that result in high kernel weights.
- The P0963 hybrid family tends to have both high test weight and high kernel weight.
When estimating yields, it is best to stick with an average kernel weight estimate of 80,000 kernels/bu for most hybrids.
- Consider using a lower kernels/bu (i.e. 70,000) for hybrid families like P0574, P0963 & P1197 and higher kernels/bu (i.e. 90,000) for hybrid families like P0919, P0950 & P1093.
- If late-season growing conditions are excellent, using a factor of 70,000 kernels/bu may be more appropriate.
- Conversely, if late-season conditions are poor, a factor of 90,000 kernels/bu might be more accurate.
- Be sure to get multiple, accurate estimates of kernels/ear and ears/acre to avoid overestimating yield.

Conclusions

Kernel weight is a key component of corn grain yield that varies greatly by hybrid and environment.
Having an idea of a hybrid’s normal kernel weight can be useful for more accurate yield estimates.
This knowledge also helps provide an understanding of how a hybrid makes its yield (kernel number vs kernel weight), which can be useful when making management decisions or when diagnosing yield results that differ from expectations.