Timing, Sugarcane Aphids and other Harvest Aid Considerations for Sorghum

Many growers in the Mid-South 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 stunted 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 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.

Turning a head sideways allows you to better observe sorghum 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 harvest sample is likely to be somewhere in the mid-20’s 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 field grain drying rate 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 ultimately contaminate your 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. Grain sorghum grown in the Mid-South 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 reduce combine efficiency or sample quality, if not treated. Sorghum is a perennial plant, so it will not naturally senesce after physiological maturity like annual crops, such as wheat, corn, and soybean. Thus, you may also wish to apply an appropriate herbicide to kill the sorghum to conserve soil moisture and assist preparation for planting a subsequent small grains crop in the fall.

Be aware that harvest aid application will deteriorate plant integrity and stability. This may promote sorghum stalk lodging, particularly if harvest is delayed longer than normal following harvest aid application. Thus, when environmental factors which control grain drying rate are not desirable (such as persistent rainfall, moderate to cool temperatures, and high humidity) and delay harvest, the use of a harvest aid can jeopardize crop condition. Therefore, be very cautious about applying harvest aids to more acreage than what you are ready to, and can quickly harvest. Also, sorghum weakened by recent heat or drought, Sugarcane aphids or other pests, sorghum grown at high plant population, and/or exposed to other stressful factors 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.

Products approved for pre-harvest use on sorghum are glyphosate, Aim, and sodium chlorate. Glyphosate (Roundup Powermax) is labeled at 22 to 44 ounces/acre. Glyphosate requires a 7-day pre-harvest interval when applied as a harvest aid in grain sorghum, but more time may be required for full desiccation. Glyphosate alone at 32 to 40 ounces per acre is the best choice of harvest aid if the field is relatively clean and the primary goal is to kill the sorghum plants. Aim is labeled at 1 ounce/acre and requires at least a 3-day pre-harvest interval when applied as a harvest aid in grain sorghum. Aim at 1 ounce/acre should be combined with glyphosate at 32 to 40 ounces/acre if morningglory species or other broadleaf weeds and vines are present and require desiccation to facilitate efficient harvest. Sodium chlorate should be applied at 6 pounds active ingredient/acre 7 to 10 days prior to harvest, and its activity is best when weather is hot and dry. Sodium chlorate will only desiccate green material that it contacts, so high water volume during application is important.

Harvest Aids and Sugarcane Aphids (Angus Catchot):

Sugarcane aphids can be a big problem for producers if they move to the head prior to harvest. The honeydew they secrete is a sugary compound that will gum up combine headers. There are some situations where you may choose to control sugarcane aphids prior to harvest to prevent this or the risk of this occurring. Currently we have two options for sugarcane aphid control in grain sorghum: Transform  at .75 – 1.5 oz. and Sivanto at 4 – 7 oz. Transform has a 14 day PHI and Sivanto has a 21 day PHI. Because of the shortened time interval for Transform, most producers have opted to use this product if co-applying with a harvest aid.

Are there advantages of co-applying an aphid material with harvest aids and when do you do it? Because this is only the second year we have dealt with this pest, we do not have a lot of research on exactly when this application makes sense and does not. This is what we know….

– Transform works well with Roundup and Chlorate

– If you kill the plant, you will not get further aphid build up

– When you kill the plant with Roundup or Chlorate, the leaves die first and what aphids are there, quickly move off dying tissue and often up the neck to the head

At this time it is common to find 25-200 sugarcane aphids on flag leaves with lower numbers in the bottom canopy on sorghum that is near or at black layer. We have struggled with what is best to do in this situation. At this time my “gut feeling” is that if they are increasing on flag leaf you may consider control to avert all risk. I have been told over the last couple of years several times by growers that the risk of clogging the throat of a combine header is one not worth taking. If there are aphids present but in very low numbers across the field and especially low numbers on flag leaf, I would probably ride them out and not treat.

The photos below show sugarcane aphids moving up the neck to the head and harvest equipment in TX covered with sugarcane aphids