Kansas State University scientists may be giving wheat growers reason to watch the weather more than they did before in October – and, for that matter, in May and June.

The scientists’ sweeping look at 55 years’ worth of historical wheat yield data showed that for every 1-inch increase in October precipitation, dryland wheat yield increased 4 to 5 bushels per acre. For every 1-degree (F) increase in fall (October or November) temperature, dryland and irrigated wheat yields both decreased about 1 bushel per acre.

In May, for every 1-degree increase in temperature, irrigated wheat yields increased 1 bushel per acre. In June, however, for every 1-degree increase in temperature, dryland and irrigated wheat yields both dropped about 1 bushel per acre.

“Our goal with this study was to determine the change in western Kansas wheat yields for the past 55 years, as influenced by precipitation and temperature,” said John Holman, Garden City-based K-State Research and Extension agronomist. He collaborated on the study with three other K-State agronomists: Tribune-based Alan Schlegel and Manhattan-based Curt Thompson and Jane Lingenfelser.

In the study, “Influence of Precipitation, Temperature, and 55 Years on Winter Wheat Yields in Western Kansas,” the researchers compiled data from four K-State Research and Extension experiment stations in western Kansas (Colby, Garden City, Hays and Tribune). They included dryland and irrigated wheat.

“Winter wheat is the most common crop in Kansas with 8.6 million acres grown in 2010,” Holman said. “However, the number of acres planted to wheat has steadily decreased since 1993, when 12.1 million acres of wheat were planted in the state.”

Several studies have evaluated wheat yield changes over time, he added, noting that wheat yields in the overall Great Plains have increased by close to 1 percent each year since 1959. About 50 percent of the increase is likely due to genetic improvements, but limited information has been available about the impacts of environment on crop yield over time. 

K-State’s research is the first evaluation of the effects of monthly temperature and precipitation for dryland and irrigated wheat.

Understanding weather effects

“One outcome of the study is that we now have an estimate of what an average freeze event costs us in yield—at least an 8 bushels per acre reduction,” Holman said, adding that average reduction is probably more than that. The experiment stations did not report very poor years with freeze damage.

“The results suggest that future wheat breeding and cropping systems research should work to improve stand establishment and minimize freeze injury,” he said.

Because the main effect of irrigation was significant for this study, the scientists analyzed the data separately for irrigated and non-irrigated experiments.

Holman acknowledged that the effect of monthly temperature and precipitation on wheat yield is not fully understood. The study indicated, however, that warm weather in fall (October-November), early spring (April), and June tend to reduce yields. Warm late-spring temperatures tend to increase yields.

That could be because warm fall temperatures cause more fall growth, he said. If the growth gets excessive, it can deplete soil moisture, increase susceptibility to freeze injury, increase insect and weed problems, and increase spring lodging – all of which can reduce yield potential.

“Warm early-spring (April) temperatures can cause wheat to break dormancy and initiate spring growth too early, which can result in freeze injury,” Holman said.

Warm late-spring (May) temperatures increase wheat growth and development, enabling the plants to initiate anthesis (flower opening) and maturation during a cooler period of the year. But, warm June temperatures increase leaf post-maturity senescence and reduce kernel fill, which ultimately can curb yield.  

Irrigated Versus Dryland

One of the key points that came out of the study is the average difference in yield between irrigated and dryland wheat across the time period – an 18 bushel-per-acre advantage for irrigation, Holman said.

“Irrigated wheat in western Kansas has increased about 0.5 bushel per acre per year,” the researcher said. Specifically, the yield increase in southwest Kansas dryland wheat was 0.3 bushel per acre per year and in northwest Kansas dryland, 0.8 bushel per acre per year.

“Another interesting thing that came out of this study is how dryland wheat in southwest Kansas differed from dryland wheat in the northwest part of the state, in terms of factors influencing yield and yield improvement over time,” Holman said.

The difference was largely caused by more freeze damage and worse stand establishment in the southwest, compared to the northwest. Dryland wheat in the southwest was affected more by October precipitation (stand establishment) and April temperature (spring freeze injury) than by crop yield improvements made over the past 55 years. Crop yield improvements included things such as new variety releases and improved agronomic practices, such as with fertilizers and pesticides.

Dryland wheat yields in northwest Kansas, however, were affected most by crop yield improvements (breeding and agronomic practices). Fall stand establishment was a secondary factor behind breeding and agronomics. Spring freeze did not affect dryland wheat yields in northwest Kansas. Behind fall precipitation, early spring precipitation was important for increasing yield. Precipitation that fell later in the growing season did not improve yield but could help improve test weight.

Irrigated wheat yields were only affected by temperature. For both dryland and irrigated wheat, warm fall and June temperatures reduced yield, while warm late-spring (May) temperatures increased yield. Irrigated wheat in southwest Kansas was not significantly affected by spring freeze damage, which implies that drought-stressed wheat may be more susceptible to spring freeze injury than wheat that is not stressed. 

Together, these results suggest northwest Kansas is a more favorable environment than southwest Kansas to grow dryland winter wheat, Holman said. There was no difference in irrigated wheat yields between the southwest and northwest.

Future dryland crop breeding and agronomic research needs to improve winter wheat establishment and improve the tolerance of wheat grown in southwest Kansas to spring freeze damage, he said. Both dryland and irrigated wheat will benefit from increased tolerance to warm fall and June temperatures.