Pre-Harvest Glyphosate Treatment on Sorghum in a No-Till System

 

Producers who would like to facilitate a timely sorghum harvest may be thinking of applying glyphosate as a desiccant. Will this affect standability or yield of the sorghum crop?

 

The answer to the question about standability is “yes,” applying glyphosate as a desiccant to sorghum can affect the stalk quality and standability of sorghum in some cases. Unlike corn, grain sorghum is a perennial plant and remains alive until it is killed by a hard freeze. Killing the plants before a freeze can affect the integrity of the stalks. For that reason, inspect sorghum field for existing stalk issues prior to applying the glyphosate. If stalk rots are present, applying glyphosate may increase the chance of plant lodging if not harvested in a timely manner.

 

Conditions this year, especially in south central Kansas, have been favorable for the development of Fusarium stalk rot. Although we have not received any samples to date, checking for stalk rot prior to desiccant usage might be particularly important this year. We have also had reports of grain mold development on the heads in the Reno County area due to recent wet weather. If conditions stay humid and wet, grain molds will continue to develop in the field and therefore, early harvest could reduce grain losses or dockage at the elevator for poor quality grain.

 

The answer to the second question about the effect of a desiccant on sorghum yields is not as straightforward. It depends of the timing of the desiccant application.

 

Most glyphosate labels require that applications be made to the sorghum crop when grain moisture is at 30% or less to minimize any possible yield reductions. In addition, there is a seven-day period between time of application and harvest.

 

 

If glyphosate is applied at the correct time, K-State research in 2011 and 2012 by former Agronomy graduate student Josh Jennings found that using a desiccant did not affect sorghum yields.

 

From 2011 to 2013, he established six field trials to test the effect of pre-harvest glyphosate treatments on sorghum. In 2011 to 2012, field trials were conducted at Belleville, Manhattan, and Ottawa. In 2012 to 2013, field trials were located in Belleville, Manhattan, and Hutchinson (yield not collected in 2012).

 

Table 1 summarizes the effect of the pre-harvest treatments on grain sorghum. The response was similar in all harvested experiments, so the data below is averaged across the five field trials over the two-year period.

 

 

 

 

Glyphosate was applied to the sorghum crop when grain moisture was approximately 18-22%. Grain harvest occurred 8-11 days following the application. Average yield reduction to the sorghum crop when sprayed with glyphosate was about 1 bushel or roughly 1% less than untreated.

 

Another factor is whether the presence of aphids, headworms, or other insect pests in the head should make any difference in the decision to use desiccants. There is no research on this, but by the time a desiccant is applied, the grain fill period is complete and these insects can really do no more damage than they have already done. As a result, the presence of insects at this late stage of development should not play any role in the decision of whether to use a desiccant.

 

 

In addition to getting the sorghum crop ready for harvest earlier than normal, desiccants can be helpful in cropping systems where wheat is planted directly after sorghum harvest. Killing the sorghum plants early can help save soil moisture for the wheat crop.

 

The research mentioned above also tested the effect of using a sorghum desiccant on the yield of wheat planted directly after sorghum harvest. Wheat yield responses varied across field trials over both years, so the data in Table 2 includes wheat yields within each field trial over both years of the experiment.

 

 

 

Averaged over all three locations in 2011-2012, when glyphosate was applied to the sorghum pre-harvest, wheat yielded 12-13% more on average than wheat following untreated sorghum. This is equivalent to an average increase of about 5-6 bushels/acre. Averaged over all three locations in 2012-2013, wheat yields following grain sorghum treated with pre-harvest glyphosate were increased by only 1%, or less than a bushel.

 

In 2011, applications of glyphosate, on average, were applied 22 days earlier than glyphosate treatments in 2012. The first freeze date was also 12 days later in 2011 than in 2012. As a result, the pre-harvest applications of glyphosate were applied, on average, 38 days prior to the first freeze in 2011 and only 6 days prior to the first freeze in 2012. A hard freeze soon after a pre-harvest glyphosate application to sorghum essentially negated the effect of the glyphosate application.

 

 

The use of glyphosate as a pre-harvest desiccant on grain sorghum will reduce the moisture level of grain sorghum and may allow producers to harvest the crop earlier than normal. However, care must be taken to ensure the crop is harvested in a timely manner. If not, the desiccant could increase lodging potential. If applied at the proper time (after physiological maturity - formation of black layer at the bottom of the sorghum grains), a desiccant will probably have little or no effect on sorghum yields.

 

Applications of glyphosate to grain sorghum prior to fall harvest can also help improve the performance of the following wheat crop if applied early enough in the late summer/early fall. Wheat yields following glyphosate-treated grain sorghum, on average, were 6% greater in 2011-2012 compared to 2012-2013 when glyphosate treatments were made at least 38 days prior to the first freeze date. When pre-harvest glyphosate is applied to the grain sorghum crop later than that, response of wheat yields following treated sorghum may be minimal.