Producers in Kansas are familiar with the effects of drought stress on summer row crops. But high-temperature stress can also affect crop development and yields, and this is not always associated with drought stress. Row crops grown under full irrigation sometimes have below average yields in years when temperatures are unusually hot during the sensitive stages of crop development. If crop yields are less than expected given adequate rainfall or irrigation, look at temperatures during flowering and grain fill, which can explain part of yield variability across years.

The following is a brief discussion of the impact of high temperatures on the major row crops grown in Kansas:

Corn

By early August, most of the corn in Kansas has been pollinated and is moving into grain filling. Within 10 to 14 days after pollination, a corn kernel can be aborted in response to drought stress combined with high temperatures. Once the kernels are at or beyond blister stage, the final kernel number won't change much but kernel weight can. After the blister stage, yield is determined by grain fill rate and duration. Research has indicated that 72 degrees F is the ideal temperature for grain fill in corn.

The rate of grain fill usually goes up with higher temperatures, meaning that more dry matter is deposited in the grain on a daily basis at warm temperatures than at cool temperatures. The problem is that the duration of grain fill typically is reduced at high temperatures. There are fewer days available to deposit dry matter in the grain. The balance of these two responses to high temperatures determines how much yield might be reduced, if any.

Sorghum

According to research conducted by Vara Prasad, Kansas State crop physiologist, and others, the two stages of grain sorghum reproductive development most sensitive to high-temperature stress are flowering and 10 days prior to flowering. In their research they used controlled environments to impose a day/night temperature regime of 104/86 degrees F for 10-day periods at various stages of plant development.

High-temperature stress in the pre-flowering and flowering stages caused maximum reduction in seed set, seed numbers and seed yields. Early seed filling periods were more sensitive to high-temperature stress than later periods. Seed yield losses during post-flowering stages were mainly due to decreases in seed size.

How are high temperatures reducing yields in sorghum? Lower seed yields were not the result of decreased leaf photosynthetic rates the rate of photosynthesis remained constant even under continuous exposure to high-temperature stress. This suggests that high-temperature stress reduced seed size by decreasing seed filling duration, without an increase in seed filling rate to help compensate.

Soybean

Exposure to heat stress during flowering results in pollen sterility and reduced seed set. Lower seed set under heat stress can be caused either by problems with pollen release or by decreased pollen viability or ovule function.

The impact of high-temperature stress will be different for determinate and indeterminate varieties. Indeterminate varieties (typically maturities of Group IV and below) develop flowers over a longer period of time. Plants that are stressed by heat can compensate and form new flowers and seed set later if environmental conditions improve. Also, a decrease in seed set and numbers can sometimes be partially offset by greater seed size.

In contrast, determinate varieties (typically maturities of Group V and above) flower over a shorter period of time. Stress during this period can have a great influence on reproductive development. High temperatures soon after seed-set cause abortion of embryos, leading to fewer seeds per pod.

Studies at the University of Florida have shown that reduced seed size in soybean is a result of decreased seed filling rate. In addition to the impact on seed number and size, heat stress can reduce grain or seed quality. Heat stress increased the percentage of shriveled seed and influenced seed composition. Oil concentration increased with increasing temperature, with an optimum at 77 to 82 degrees F, above which the oil concentration declined. Seed protein concentration of soybean was constant at temperatures between 60 and 77 degrees F, but increased at temperatures above 77 degrees F. Oil and protein concentration were inversely related to heat stress during seed fill.

Soybean plants grown at high day (95 degrees F) and high night (86 degrees F) temperatures produced seed with reduced germination and subsequent seedling vigor. Greater reductions in seed germination and seedling vigor were observed with longer duration of exposure to high temperatures, especially during seed fill and maturation.

For more information or assistance on this or other topics, please call the Extension Office at 272-3670, located at 501 S. Ninth St.