Sorghum and Heat Stress
By Brent Bean, Sorghum Checkoff Agronomist
Not only does grain sorghum withstand periods of drought, it also has an ability to tolerate high temperatures better than other crops. Successful pollination is critical for any grain crop, and heat stress during this time can be devastating to final grain set. Unlike some of the other grain crops, grain sorghum has perfect flowers, with the male and female parts located together. Each sorghum head is considered self-pollinating, with pollen only having to travel a short distance for successful fertilization to occur, making it less susceptible to heat damage. Grain sorghum also produces an abundance of pollen, estimated at 45 million pollen grains per panicle. Plus, sorghum avoids heat during the pollination process by releasing pollen from its anthers in the early morning hours when the temperature is typically at its coolest.
Although sorghum has these mechanisms for increasing its heat tolerance, yield can be reduced under extreme heat conditions. The most sensitive time for heat stress to occur is from head emegence through flowering. This period lasts approximately 15 days.
A Kansas State University growth chamber study showed grain set was reduced approximately 50 percent when heat stress was imposed for several hours with a maximum daily temperature of 97 degrees and a minimum night temperature of 79 degrees for five days during this 15-day critical growth stage. Very little loss in grain set was noted when stress was applied outside of this 15-day window. In the field, yield losses as high as 40 percent were recorded when the maximum average daily temperature was 109 degrees over a 28-day period. Of course, this temperature is extreme and would not be expected to occur under most conditions.
Most agronomists recommend planting dates that avoid heat stress during this critical 15-day growth stage. Growers should take into account their historical weather patterns and a hybrid’s days-toflowering rating. For instance, if a hybrid reaches 50 percent bloom in 66 days, the critical growth stage of sorghum will begin approximately 50 days after emergence.With this timing in mind, the planting date should be adjusted to avoid the potential of heat stress-reducing yield.