Allelopathy of wheat following sorghum (milo)

Economics can upset planned crop rotations and the reduction in corn and sorghum prices has seemed to increase winter wheat plantings. As the grain sorghum harvest has begun in earnest, several farmers have been following the combines planting winter wheat. Seeing this reminded me of a question some time ago from an area wheat producer about the allelopathic effects of grain sorghum residue on wheat.

Allelopathy is defined as a biological phenomenon by which an organism produces one or more biochemicals that influence the growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial (positive allelopathy) or detrimental (negative allelopathy) effects.

Grain sorghum residue has been demonstrated to have negative allelopathic effects on a number of weed species, primarily broadleaf weeds. Unfortunately, sorghum as the previous crop has also been found to reduce wheat yields. One study compared several varieties of winter wheat planted on fallow, following tilled sorghum residue, planted no-till into sorghum residue, and no-tilled into millet residue. Millet residue is known to not have allelopathic effects on wheat, so was included as a close comparison to sorghum to test potential allelopathy. If regularly grown in tight crop rotations with wheat, wheat following millet can lead to weed problems, particularly cheatgrass, as well as root and crown rot diseases.

Emergence promptness was not significantly impacted by residue treatment for most varieties, but grain yields were consistently lower for wheat no-tilled into sorghum residue. Emergence was somewhat delayed in the wheat following tilled sorghum residue, and attributed to the allelopathic compounds in the residue incorporated into the soil being rapidly solubilized. The marked ability of wheat to compensate for differences in seedling development was believed to explain the lack of reduction on grain yield in wheat following the tilled sorghum residue.

For fields that are already planted to winter wheat after sorghum, tillage is obviously not an option, and likely wouldn’t have been considered for producers who prefer no-till practices. For future reference, the advantages of tillage to reduce the allelopathy of sorghum residue on wheat must be weighed against the soil and water conserving benefits of not tilling. Even one tillage operation can negatively affect the soil health and structure benefits that have been generated under no-till, and take years of no-till to return to the pre-tillage level. Little difference among wheat varieties for resistance to allelopathy by sorghum limits that as a management tool. One practice that is reported to have seen some level of success when planting wheat after sorghum is a moderate increase in nitrogen fertilizer, which could still be accomplished.

Another other risk of planting wheat into sorghum residue is Fusarium head blight or scab. Planting wheat into sorghum residue is not as great a risk as planting wheat after corn, but if wet, humid conditions prevail when the wheat crop is flowering, significant infection can occur. Producers who are planting wheat after sorghum would be advised to closely monitor weather conditions when their crop is flowering and shortly after, and be prepared to apply a foliar fungicide in a timely manner. Properly timed fungicide applications have been shown to reduce the incidence of scab by 60-70%.