Statewide
“Do you regularly see grain sorghum suffer from heat stress, even if moisture is adequate? If so, about how hot is too hot and at what stage of growth is grain sorghum most susceptible?”
There are two critical times in sorghum growth and development which are particularly sensitive to heat: 1) about 30-35 days after planting, when sorghum’s growing point switches from vegetative to reproductive and thus begins the process of determining the number of potential spikelets and seeds per spikelet in each head, and 2) at flowering when pollen is sensitive during formation when hot conditions can reduce viability. Grain sorghum’s upper temperature when heat units are calculated is 100°F. This implies that additional heat does not meaningfully contribute to growth and development (or could in fact be detrimental). Grain sorghum handles these temperatures better than most other crops, but that doesn’t mean it isn’t stressed. Low soil moisture and low humidity make this potential stress worse.
Occasionally we see what is called ‘sun scald’ on leaves resulting from heat stress (see Figure 1), and this can occur in irrigated grain sorghum as well. An entire leaf may occasionally die but usually just part of a leaf. I do not believe we have ever seen significant yield reduction due to sun scald alone, but heat of course can stress a crop. In the record-breaking heat of 2011 some sorghum was damaged due to heat stress, even in irrigated fields. Seed set was less than normal due to damage to pollen (reducing viability) and stigmas (less receptivity). Night-time low temperatures that remain near 80 F can compound this problem.
Figure 1
