Research conducted by Kansas State University shows that when it comes to winterkill and wheat yields, things are not always equal.

"Contrary to what some people might think, we found that a percentage of winterkill loss does not result in the same percentage yield loss," said Merle Witt, southwest area agronomist with K-State Research and Extension. "Also, growing conditions in early spring when tillering is occurring and during the grain-filling period when kernel numbers and kernel size are being determined affect the extent to which winterkill damage expresses itself. Thus, 25 percent winterkill damage in one year may not be as damaging as the same percent winterkill in another year."

Witt's study, which simulated winterkill in Kansas wheat, was conducted over a two-year period at the Southwest Research-Extension Center near Garden City. The site was a Ulysses silt loam soil in a wheat-fallow rotation. To reflect a range of winterkill levels, four treatments were used: 0 (check) percent winterkill, 25 percent, 50 percent and 75 percent winterkill.

The research was funded by National Crop Insurance Services, based in Overland Park, Kan.

In the study's first year, wheat plants were hoed out in February to the appropriate winterkill levels. In the second year, a beardless, spring wheat variety was mixed at planting with the winter wheat varieties so that when the spring wheat died during winter, the result was the desired levels of winterkill damage. Two winter wheat varieties, TAM 107 and Trego, were used.

The average yield for the check treatment (0 percent winterkill) in the first year was about 59 bushels per acre. The yield for the 25 percent winterkill treatment was 55.3 bushels per acre or about a 6.5 percent yield loss. With the 50 percent winterkill treatment, the yield was 50.7 bushels per acre or about a 14 percent yield loss. For the 75 percent winterkill treatment the yield was 46.8 bushels per acre or about a 21 percent yield loss.

Average yields in the second year of the study were lower because growing conditions during grain-filling were not as good, Witt said. The check or 0 percent winterkill treatment yielded 46 bushels per acre. The 25 percent winterkill treatment yielded 39 bushels per acre or close to a 15 percent yield loss. The average yield for the 50 percent winterkill treatment was 32.5 bushels per acre (29.5 percent yield loss), and the average yield for the 75 percent winterkill treatment was 18.5 bushels per acre (59 percent yield loss).

In both years the most severe winterkill treatment resulted in a delay in the heading date.

"This (delay) is a common occurrence and probably would be more pronounced in a field situation where there was winterkill in a pure stand of winter wheat, instead of the (killed) spring wheat of the spring-winter wheat mixture that was used in this experiment," Witt said. "Also, it is common to observe a reduction in test weights with delayed heading dates. The first year there was only a small reduction in test weights, while in the second year, with less conducive growing conditions, there was a 3.5-pound-per-bushel reduction in test weight from the 0 percent winterkill treatment to the 75 percent winterkill treatment.

"The study gave us a range of expected yield losses over a range of winterkill damage. A limitation of the study, however, is the fact that winterkill damage was distributed uniformly over the study area and that doesn't normally occur in a field-wide situation," Witt said. "Generally, winterkill damage occurs on terraces and elevated areas of the field where soils tend to be drier, and it can be large areas of the field with little chance for the surviving plants to compensate."

For more information on the study, interested persons can check with a local K-State Research and Extension office for the Field Day 2002 Southwest Research-Extension Center, Report of Progress 895, Page 65, and the Field Day 2003 Southwest Research-Extension Center, Report of Progress 910, Pages 37-38.