Kraig Roozeboom
Crop Production and Cropping Systems Agronomist
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Grain sorghum development is a mixed bag this year. According to the September 9 Kansas Agricultural Statistics “Crop Progress and Condition” report, grain sorghum heading is 5% behind the 5-year average, but harvest is 3% ahead of the 5-year average. The combination of heat and drought experienced in most of Kansas this summer often delayed emergence, sped up vegetative development, but delayed heading. Within many fields, timing of emergence and heading later in the season were highly variable.

Dr. Vara Prasad and others from K-State Agronomy published a paper in Crop Science in 2008 that described work they did on the sensitivity of grain sorghum to high temperature stress during reproductive development. They found that ongoing high temperature stress after growing point differentiation (about 30 days after emergence) delayed heading and decreased height, seed set, seed number, seed yield, and seed size. They documented that sorghum was most sensitive to heat stress from about 10 days before and through pollination, causing maximum reductions in seed set and yield. Heat stress after seed set decreased yield as well, with the greatest reductions resulting from high temperatures during early stages of seed development.

At least at Hutchinson, the sorghum crop was subjected to very high temperatures during the critical period from 10 days before pollination through pollination (pollination occurs shortly after heading).

Statewide, sorghum heading started out ahead of average, largely due to relatively early planting and warm temperatures that caused the crop to move rapidly through the vegetative stages. However, as the drought intensified and high temperatures continued, sorghum heading slowed to the point of lagging behind the 5-year average.

In some cases, bloom may have been delayed to such an extent that the sorghum may not mature before being killed by frost. Sorghum in western Kansas may not reach maturity before frost if it had not bloomed by the end of August. If sorghum had not bloomed by early September in north central and northeast Kansas, the probability of maturing before frost is dropping to less than 50% based on historical weather patterns.

From here on, it’s simply a matter of getting enough heat units to finish maturing. There’s nothing producers can do to speed things along. Grain sorghum can continue grain fill any time temperatures are above freezing, although the rate of grain fill slows down considerably with cooler temperatures.

If frost occurs near the end of the grain filling period, most of the yield potential will be realized, but test weight will be lighter. If frost occurs well before the end of grain fill, yield can be reduced drastically.

See K-State Research and Extension publication C687, Grain Sorghum Production Handbook, for a detailed discussion and maps showing probabilities based on several different bloom dates:

http://www.ksre.ksu.edu/library/crpsl2/c687.pdf