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.