There are always questions about effective ways to manage grain sorghum to improve the performance of winter wheat planted into no-till sorghum residue following fall harvest. The technique most often asked about is applying glyphosate to the sorghum crop prior to harvest.

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. Also, there is a required seven-day period between time of application and harvest. 

K-State research results

In 2011 and 2012, former K-State agronomy graduate student Josh Jennings 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.

Glyphosate was applied to the sorghum crop when grain moisture was approximately 18-21%. 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.

For grain sorghum, the glyphosate pre-harvest treatments had no significant effect on yield, grain moisture at harvest, test weight, or seed size when averaged over both years and all locations.

The effect on wheat yields was more variable.

For the 2011-12 wheat crop, when the pre-harvest glyphosate treatments were applied to sorghum 38 days prior to the first freeze in the fall of 2011, the wheat following sorghum that received the pre-harvest glyphosate treatment yielded 12-13% more on average than wheat following untreated sorghum.

On the other hand, for the 2012-13 wheat crop, when the pre-harvest glyphosate treatments were applied to sorghum only 6 days prior to the first freeze in the fall of 2012, wheat yields were about the same when following either treated or untreated grain sorghum.

Summary

Applications of glyphosate to grain sorghum prior to fall harvest can 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 than wheat yields following untreated grain sorghum – but only when glyphosate treatments were made at least 38 days prior to the first freeze date. When pre-harvest glyphosate is applied closer to the time of the first freeze, response of wheat yields following treated sorghum may be minimal. 

It is important to follow the glyphosate label for application recommendations. Glyphosate applied at low rates or when temperatures are not adequate may reduce the effectiveness of the product. 

The sorghum field should also be inspected for stalk issues prior to applying the glyphosate. If stalk rots are present, applying glyphosate may increase the chance of plant lodging if it is not harvested in a timely manner.