When is it safe to Apply a Grain Sorghum Harvest Aid and what to Expect?

Erick Larson, State Extension Specialist - Grain Crops
By Erick Larson, State Extension Specialist - Grain Crops August 14, 2014 08:45

When is it safe to Apply a Grain Sorghum Harvest Aid and what to Expect?

Many growers in the Mid-South apply a herbicide shortly prior to sorghum harvest to facilitate combine efficiency and suitable grain moisture. This season, this issue is even more critically important because of severe harvest problems potentially created by White Sugarcane aphid infestation. This article will discuss appropriate timing and potential outcome of such harvest aid applications.

Sorghum head not yet matureIf a “harvest aid” product is applied prematurely or before physiological maturity you will sacrifice grain yield because you will hasten seed fill. In fact, about 25% of sorghum kernel seed weight is filled during the last 10-15 days prior to physiological maturity. You will also sacrifice test weight if premature harvest aid application hastens seed fill. Thus, it is important to fully scout your sorghum field and be able to properly identify mature kernels before making a harvest aid application. Sorghum kernels change color and accumulate hard starch much the same as corn kernels mature. Kernels will mature first at the top of the head and progress down, so focus scouting on the kernels at the base of the heads. If you can quickly see a considerable amount of green kernels, rather than the burnt orange / brown color of mature kernels, you need to give the crop some more time to fully mature.

Sequence of maturing sorghum kernels

You can evaluate maturity more closely by examining whether hard starch has formed inside the kernel. Pinch a kernel between your fingernails and if you easily penetrate soft dough at the base of a kernel, it is not mature.  Hard starch forms first at the kernel crown, and progressively moves toward the base where it develops a black layer, similar to corn.  Sorghum grain moisture at physiological maturity will be about 30% moisture.

This “black layer” is an abscission layer that effectively cuts off moisture transfer between the green plant and grain in the head.  Therefore, desiccating or killing sorghum vegetation with a harvest aid after maturity has little or no effect on grain moisture or field drying rate.  Grain drying rate is nearly entirely dependent upon environmental conditions. Thus, sorghum grain moisture improvements may not be realized, unless your field has some late developing heads with green kernels present, which may ultimately contaminate the grain sample. In this case, it may be more prudent to sacrifice those green heads, rather than subject the bulk of your crop to weathering for an extended time. For example, grain sorghum grown in the Mid-South is quite vulnerable to kernel sprouting, if it endures frequent showers and high humidity after kernels are mature and ready to harvest.

Of course, harvest aids may be useful to control excessive weed growth and minimize green vegetation which may reduce combine efficiency or grain quality. Sorghum is a perennial plant, so it won’t naturally senesce after reaching physiological maturity like annual crops, such as wheat, corn and soybeans. Thus, you may also wish to apply an appropriate herbicide to kill the sorghum, to assist preparation for planting small grains following in the fall.

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Erick Larson, State Extension Specialist - Grain Crops
By Erick Larson, State Extension Specialist - Grain Crops August 14, 2014 08:45
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