Irrigated corn yields are not immune to the deleterious effects of the extended heat wave Kansas is experiencing this year, says Kraig Roozeboom, K-State Research and Extension crop production specialist. There are several reasons for potential yield reductions in irrigated corn due to extreme heat, and all of them are somewhat related.

High evapotranspiration (ET) rates during the vegetative stage. When temperatures are unusually high and the relative humidity is low, evapotranspiration rates may be so high that most irrigation systems can’t keep up, Roozeboom says. Stress from high ET can reduce yields in the vegetative stage, even before the crop has silked.

“Potential ear size is being determined before tassel. Stress during the V8-V11 growth stages can reduce row numbers on the ear. Stress during V12-V17 stages – just prior to tasseling – can reduce the number of kernels per row,” he says.

Extreme heat during early grain fill. Research at Iowa State University has shown that extreme heat during grain fill reduces the rate and duration of grain fill, even if moisture is not a limiting factor, Roozeboom says. The ideal temperature for grain fill in corn is about 72° F.

Field studies and observations indicate a yield loss can occur at higher temperatures. At temperatures from about 80° to 95° F, the rate of grain fill starts to slow down, even if roots are kept at normal soil temperatures from the shading of the canopy. At temperatures of about 105-110° F, the rate of grain fill and duration of grain fill are both reduced, resulting in yield losses.

Stalk rot. Fusarium is the primary type of stalk rot on irrigated corn in Kansas, says Doug Jardine, K-State Research and Extension plant pathologist. Fusarium stalk rot incidence increases when corn is subjected to stresses such as drought and poor fertility. Fusarium stalk rot interferes with water and nutrient movement within the plant, and causes smaller ear size and stalk lodging.

Stalk lodging. Although stalk rot is often present in corn that has lodged, the disease may or may not be the main reason for the lodging, Jardine says.

“Any factor that causes carbohydrate depletion in the stalk can predispose the plants to stalk weakening and potential lodging. Heat stress during grain fill, nutrient imbalances, hybrid characteristics, reduced photosynthetic capacity due to loss of leaf area from foliar diseases and spider mite infestations are other factors that may be involved. With improved breeding for standability, plants infected with stalk rot may not lodge at all,” Jardine says.