Management Considerations For No-Till Wheat Following Sorghum Or Other Summer Crops

With fall harvest progressing at earlier than normal rates, favorable wheat prices, and many row-crop acres chopped for silage, producers may consider planting wheat back into freshly harvested summer crop residue, especially if they receive some essential precipitation.  While the current drought conditions offer significant challenges (see recent eUpdate article on planting wheat into dry soil), there are additional considerations when seeding wheat immediately after harvest of a summer crop.  Following are several key management considerations to keep in mind, a more detailed discussion on these can be found in K-State publication MF-2641, Managing No-Till Wheat Following a Summer Crop in Eastern Kansas.

Variety Selection:  If planting wheat after corn, adapted varieties with the best ratings for fusarium head blight (scab) should be used.  Due to moisture use by the previous summer crop, varieties with high levels of drought tolerance should be used.  As wheat after a summer crop is generally planted later, varieties with good ratings for winter hardiness and late fall tillering should be selected.  These ratings are available in K-State publication MF991, Wheat Variety Disease and Insect Ratings 2021.

Planting Date:  Wheat should be planted as soon as possible after harvest. Later planting dates shorten the window available for the formation of fall tillers, which are the more productive than spring-initiated tillers.

Seeding Rate:  Seeding rates should be increased with delayed planting or in challenging high-residue environments where adequate stand establishment may be difficult. As previously mentioned, delayed planting reduces the opportunity for the initiation of fall tillers, increasing seeding rates can overcome some of that reduction.

Harvest Considerations:  Evenly spreading crop residue at harvest is a key action that can positively affect the productivity of the subsequent wheat crop. Uneven distribution of the summer crop residue can make it difficult to maintain adequate depth of seed placement and can also result in uneven nutrient availability as areas with higher crop residue will likely immobilize more nitrogen.

Seeding Equipment and Seeding Depth:  Seeding to adequate depth in high-residue conditions is necessary to ensure healthy crown development, maintain access to sufficient soil moisture in drying conditions, and reduces the potential for winter injury. Producers should make sure that their drill is set to achieve adequate depth in the thickest layers of crop residue. Drills and air-seeders often need additional ballast (weight) added to the drill to maintain adequate downforce to cut through the residue and place seed at the target depth.  Producers should use ground speeds that maximize the performance of the seeding operation.  Often increasing ground speed aids in the flow of residue through the seeding tool, however increasing speed also increases the ballast and downforce requirements necessary to achieve the target seeding depth.

Nutrient Management:  In general, higher rates of fall applied nitrogen are necessary for wheat when planted after corn or sorghum.  This is especially true for after sorghum which generally his residue that is lower in nitrogen content, making it more likely to immobilize available nitrogen.  An additional 30 lb/ac of Nitrogen should be applied for wheat following grain sorghum or sunflowers as compared to other crops.  Wheat planted after soybeans should not have its nitrogen application lowered relative to continuous wheat as the organic nitrogen release from the soybean crop is likely to occur too late to significantly benefit the wheat crop.

Herbicide History:  Producers should check the label of all products used in the proceeding crop and their plant back restriction to wheat.  Precipitation received since application can play a large role in potential injury to the wheat crop by any herbicide residual.

Allelopathy:  The potential effect of allelopathy of sorghum residue on wheat stand establishment and yield is often difficult to separate from nutrient availability and other potential yield limiting factors.  Concerns over allelopathy should not discourage producers from no-till planting wheat after grain sorghum when the aforementioned production concerns are addressed.

Grain sorghum termination and desiccation with glyphosate

While drought has or will accelerate maturity in most of the state, some producers interested in facilitating 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.

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.

Sorghum response to pre-harvest glyphosate treatments

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.

Table 1. Effect of pre-harvest glyphosate applications on grain sorghum (averaged across five sites in 2011 and 2012).

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.

A potential question 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, grain filling is complete and yield reducing insect damage is unlikely.  The presence of insects at this late stage of development should not play any role in the decision of whether to use a desiccant.

Wheat response to pre-harvest glyphosate treatments to sorghum

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.

Table 2. Mean winter wheat yields following treated and untreated sorghum.

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.