When is it Safe to Apply a Harvest Aid to Sorghum?

Erick Larson, State Extension Specialist - Grain Crops
By Erick Larson, State Extension Specialist - Grain Crops August 7, 2021 11:59 Updated

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Many growers in the Midsouth apply a herbicide shortly prior to sorghum harvest to facilitate combine efficiency.  Although it is possible to harvest sorghum without using a harvest aid, understanding the benefits of usage, along with adverse consequences, will help you implement a practical plan that will enhance your sorghum harvest.

A “harvest aid” product must not be applied prematurely or you will sacrifice considerable grain yield.  In fact, about 25% of sorghum kernel seed weight is filled during the last 10-15 days prior to physiological maturity. Sorghum heads naturally reach maturity over a much wider time period (10-14 days is common) compared to other grain crops, like corn and wheat, especially if environmental conditions hinder development during early vegetative stages. 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, so focus scouting on the kernels at the base of the heads. If you can clearly see a considerable amount of green kernels, rather than the burnt orange / brown color of mature kernels, you need to give the crop more time to fully mature.

Turning a sorghum head side-ways allows you to quickly evaluate kernel maturity.

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. However, because of the natural variance in sorghum kernel maturation found in a field, and because you should wait for the late kernels (at the base of the head) to attain maturity before applying a harvest aid, the actual grain moisture from a sample is likely to be near 20% at proper harvest aid timing.

The black layer is an abscission layer that forms at the base of each kernel that effectively cuts off moisture transfer from the green plant.  Therefore, desiccating or killing sorghum vegetation with a harvest aid after maturity has no effect on grain drying rate in the field and little effect on grain moisture. 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 contaminate your grain sample. In this case, it may be more prudent to sacrifice these green heads, rather than subject the bulk of your crop to potential weathering for an extended time. Grain sorghum grown in the Midsouth is also quite vulnerable to kernel sprouting, if it endures frequent showers and high humidity after kernels are mature and ready to harvest.

Progression of sorghum kernel maturity from hard dough (left) through physiological maturity or black layer (right).

Of course, harvest aids may be useful to desiccate excessive weed infestations and green vegetation, which may improve combine efficiency or sample quality. Sorghum is a perennial plant, so it will not naturally senesce after physiological maturity like annual crops, such as wheat or corn. Thus, you could also apply an appropriate herbicide to kill the sorghum to aid preparation for planting a subsequent small grains crop, as this will likely conserve soil moisture.

Be aware that harvest aid application will likely deteriorate plant integrity and stability. This may promote sorghum stalk lodging, particularly if harvest is delayed longer than normal following harvest aid application. For example, if rainy weather, cool temperatures or high humidity delay harvest, the use of a harvest aid can jeopardize crop condition. Therefore, be cautious about applying harvest aids to more acreage than what you can quickly harvest. Also, sorghum weakened by drought, Sugarcane aphids, high plant population or other types of stress are much more vulnerable to lodging, compared to healthy sorghum.

The drydown rate of sorghum grain can vary considerably depending upon environmental conditions. For example, drydown rate during mid-August will be much faster than what you would expect in October, due to lower air temperatures later in the fall. Realistic drying rate expectations under the most favorable, hot, dry conditions are around 0.75-1.0+% moisture loss per day. Conversely, rates in the late fall will be minimal.

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Erick Larson, State Extension Specialist - Grain Crops
By Erick Larson, State Extension Specialist - Grain Crops August 7, 2021 11:59 Updated
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