Grain sorghum acreage is up an estimated five times more than last year in North Carolina and interest in growing the crop in other Southeastern states is increasing.

Some of the additional acreage is targeted for a double-crop with wheat, but that may not be such a good idea says North Carolina State University Grain Specialist Randy Weisz.

“Sorghum leaves a chemical in the soil that can hurt wheat. Little is known about it, and tests have never been done in this part of the country. So we have little data to go on,” Weisz says.

Grain sorghum contains a number of compounds that suppress other plant species, an effect known as allelopathy. These toxic compounds, which include several phenolic acids; cyanogenic glycosides; and a hydroquionone, sorgoleone, occur in both the roots and the shoots of sorghum.

The yield reduction caused by these compounds in a number of crops also is called allelopathy.

After sorghum is harvested for grain, the compounds may leach from the residue into the soil and affect the germination, growth, and yield of the following crop, including wheat.

The problem is most severe inno-till wheat following sorghum. Some reports have shown up to a 25 percent yield reduction when no-till wheat follows sorghum. Some reports have shown less, Weisz says. 

In no-till systems the residue acts as a soil blanket, so temperatures will increase more slowly compared to tilled soil. This could slow seedling emergence and reduce the final plant population.

Planting the seed deeper than the recommended depth could also slow emergence or reduce germination. When planting early into no-till soils, a seed treatment (fungicide and insecticide) and starter fertilizer should reduce the risk of seedling loss.

Tillage helps. Yield reductions in tilled wheat following sorghum have ranged up to 10 percent.

Bottom line

The bottom line in the Southeast is that there is no definitive answer as to how much wheat yield may be lost, if any, when planting in a double-crop with grain sorghum.

“We simply don’t know how big a problem wheat double-cropped with sorghum may be, but growers who want to plant wheat following sorghum should know that this might be a problem for them,” Weisz says.

Production recommendations almost always come after several years of intensive university research, but there has not been much interest in grain sorghum in recent years in the Upper Southeast, therefore little research of any kind has been done on the crop.

“Because very little research has been done in this area, it is difficult to make recommendations to assist folks who want to do this. But, here are several suggestions that may help,” Weisz says.

• Use glyphosate to kill the sorghum prior to harvest. If the sorghum is left alive and starts to regrow after harvest, the new roots will continue to exude the toxic compound.

• Use tillage to incorporate sorghum residues and hasten their decomposition.

• Delay wheat planting. This is tricky. Delaying wheat planting can in-and-of-itself reduce wheat yield, but it may also help to allow the toxic compounds to decompose.

• Make sure the wheat is treated to high pre-plant fertility levels. Make sure pre-plant N, P, K, and S are at or above recommended levels.

• If planting wheat after tillage, check stand establishment and watch early tillering. The problems are most likely to show up early in the season and look like either a poor stand, or a good stand that starts to go backwards. Early nitrogen in February may help.

• If planting wheat no-till, watch the wheat plants both early for stand establishment, tillering (and need for February nitrogen), and also in the spring. Research has shown the problem may not start in no-till wheat until the spring when plants may begin to turn yellow and abort tillers.

Allelopathy also might be reduced by proper management of the sorghum residue. After the toxic compounds leach from the residue, they are degraded into harmless chemicals by microorganisms in the soil.

Tillage of the residue undoubtedly affects leaching and degradation of the allelopathic compounds, but the most effective methods have not been determined.

Research in the Midwest indicates that growers may get some idea of what level of yield loss they can expect based on when and how grain sorghum was planted.

Correct hybrid is essential

Selecting the right grain sorghum hybrid is essential for producers who are hoping to plant wheat after sorghum harvest in the fall. Growers, who planted sorghum on time, or even early, and harvested the crop on time or early, may see less yield loss in double-cropped wheat.

Growers who planted sorghum based solely on yield potential may expect more problems with the following wheat crop.

Hybrids with early vigor, good dry-down traits, insect and disease resistance and some drought tolerance may be an advantage when planting wheat behind sorghum.

Earlier maturing sorghum hybrids will provide the greatest potential for an early harvest. With the right genetics and management practices, many early and medium-early hybrids can produce yields that compete with the longer-season hybrids in the Midwest.

When considering planting wheat behind grain sorghum, it may be beneficial to take a close look at the growth characteristics of the variety they planted.

When considering planting wheat in the fall after sorghum harvest, wider rows in the preceding sorghum crop may be beneficial. Sorghum planted into narrow-rows tends to produce more biomass, but not necessarily yield. The increase in residue could create a better environment for toxins from the sorghum plant and could exacerbate yield problems associated with a soybean and wheat double-crop.

On the negative side, narrow-rows have historically meant higher sorghum yields, probably because of better shading out of weeds early in the growing season.

In fields with a history of weed problems, growers may need to carefully evaluate the trade-off between yield loss from competition with weeds in wider row spacing versus greater potential for yield loss from toxins from sorghum in following wheat crops.

North Carolina growers alone are expected to plant nearly a million acres of wheat this fall. Though only about 50,000 acres of grain sorghum was harvested this year, that number is expected to increase in the next few years and growers planning to plant it and double-crop it with wheat need to be aware of potential yield drag in such a cropping system.

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