Sorghum Grain Yield Reductions Caused by Duration and Timing of Freezing Temperatures

Management practices, as well as below‐normal temperatures that occur late in the growing‐season, can delay grain sorghum [ Sorghum bicolor (L.) Moench] maturity in the northern Great Plains. Delaying maturity increases the probability of a freeze occurring before the crop reaches physiological maturity. Studies were conducted during 1988 and 1989 using greenhouse‐ and field‐grown plants in Kansas to determine caryopsis weight and test weight reductions from exposure to subfreezing air temperatures. Treatments of 0, −2, and −4°C air temperatures for 2, 4, and 8 h were conducted in a growth chamber when developing grain had attained approximately one‐half of its maximum weight. A second experiment studied the effects of exposure to −2°C for 4 h at different stages of grainfill. Caryopsis weights were progressively reduced as air temperature was reduced from 0 to −4°C. Effects of exposure time to subfreezing air temperatures on caryopsis weight were not consistent from year to year. Air temperatures of −2°C or lower reduced test weight of grain from field‐grown plants, whereas only exposure to −4°C reduced test weight of grain from greenhouse‐grown plants. Exposure to −2°C for 4 h reduced caryopsis weights 81% at 200, 57% at 300, 25% at 450, and 3% at 600 growing degree days (GDD, base temperature 5.7°C) after anthesis (maturity = 850 GDD). Test weight response was similar to caryopsis weight response with test weights from plants exposed to freezing temperatures at 200 and 300 GDD after anthesis lower than at other treatment times or the control. These results indicate that minimum air temperature is the primary factor in determining caryopsis weight and test weight when grain sorghum is exposed to subfreezing air temperatures.