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 7-day period between time of application and harvest. 

From 2011 to 2013 we 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.

Sorghum response to pre-harvest glyphosate treatments to sorghum

Table 1 summarizes the effect of the pre-harvest treatments on grain sorghum. Hutchinson data is not included in the table because environmental conditions in 2012 prevented grain sorghum harvest.  There were no treatment-by-environment interactions so the data below is averaged across the five field trials over the 2-year period. 

Table 1. Effect of pre-harvest glyphosate applications on grain sorghum (averaged across 5 locations, 2011 and 2012)

Glyphosate was applied to the sorghum crop when grain moisture was approximately 18% to 21%. The grain was harvested 7 to 10 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.

Wheat response to pre-harvest glyphosate treatments to sorghum

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

Overall, when glyphosate was applied to the sorghum pre-harvest, wheat yielded 6% to 7% more on average than wheat following untreated sorghum. That is equivalent to an average increase of about 3 bushels for the wheat crop.

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.

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 in 2011 to 2012 compared to 2012 to 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. 

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.