From 2014 through 2018, a study was conducted at Garden City, Tribune, and Colby to evaluate wheat yield response to different varieties and seeding rates.
The objective of this study was to address the following questions:
Popular varieties representing a range of tillering potential were selected and seeding rates were selected to represent the range of rates known to be in use by producers. Four wheat varieties (TAM111 in 2016 and 2017, TAM114 in 2017 and 2018, Byrd, T158, and Winterhawk) were seeded at five seeding rates (30, 45, 60, 75, and 90 lbs/ac) at Garden City, Tribune, and Colby into no-till or reduced-till fallow in a wheat-sorghum-fallow rotation. Data were collected from 960 individual plots across 14 site-years (locations x number of years) throughout the course of the study. The 2014 study was preliminary, subsequently we chose to evaluate a wider range of seeding rates. For the purposes of evaluating seeding rate response curves, only data from 2015-2018 is reported in this article.
Discussion of results
The effect of variety and seeding rate on grain yield was examined for each site-year, along with the interaction of both variables. A significant variety x seeding rate interaction means that the optimal seeding rate depended on the variety. As expected, variety selection was important as it significantly affected grain yield in all 14 site-years. Similarly, yields responded to changes in seeding rate in 13 of 14 site-years (over a wide range of seeding rates we would typically expect a yield response). However, optimal seeding rate depended on the variety used in just two site-years.
These two site-years (Tribune 2015 and Garden City 2015) were during stripe-rust outbreaks and the plots were unable to be sprayed with fungicide. At these two site-years, higher seeding rates of the stripe rust-susceptible varieties were able to partially compensate for the effects of the rust, resulting in different yield responses to seeding rate. In summary, varieties responded similarly to seeding rate in 12 of the 14 site-years.
Effect of location on yield
While location affected the overall yield level, with yields increasing in the order of Garden City < Tribune < Colby, location did not affect the overall yield response to seeding rate. As shown in Figure 1, the seeding rate response curve is similarly shaped for all locations when averaged across years and varieties.
Figure 1. Seeding rate response by location.
In Figure 1, data points within a location that have the same letter are not statistically different. For example, at Garden City there was no difference between the 60, 75, or 90 lb/ac rates, while all three of those rates were higher yielding than the 45 lb/ac rate, which was higher yielding than the 30 lb/ac rate. At Tribune and Colby, there was no significant difference in grain yield between the 60 and 75 lb/ac rates, however the 90 lb/ac rate was significantly higher than the 60 lb/ac rate. With location and variety selection not playing a significant role in optimal seeding rate, all data were then combined to look at the overall response to seeding rate (Figure 2.)
Figure 2. Effect of seeding rate on grain yield, averaged across varieties and site-years.
When the response to seeding rate was evaluated (Figure 2), grain yield significantly increased with increasing seeding rate up through the 75 lb/ac seeding rate. Yield between the 90 and 75 lb/ac rate were not significantly different. When translated into a seeds/ac basis, these seeding rates would have been 452,000, 678,000, 903,000, 1.13 million, and 1.36 million on average.
Important points to keep in mind
Summary of key results
Producers are often worried about having stands that are too thick, thus an excessive use of soil water in the fall. This is a very valid concern. However, one must also be aware of the two-edged sword. If good growing conditions occur in the spring, there are physical limits to how many kernels per head can be set and maximum kernel weight. If there is a shortage of heads/acre due to an insufficient stand and/or lack of fall tillering, yield will be left on the table in a good year.
Note: Expenses for this study at Colby were funded by the Cover Your Acres Winter Conference.
Lucas Haag, Northwest Area Agronomist, Colby
Alan Schlegel, Agronomist-in-Charge, Southwest Research-Extension Center – Tribune
John Holman, Cropping Systems Agronomist, Southwest Research-Extension Center – Garden City