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Allelopathy--Grain Sorghum

Grain sorghum is not a major crop acreage-wise in Mississippi, with only an estimated 46,000 harvested acres in 2012. However, it is recognized as being superior to corn in both drought and heat tolerance (Crop Management–accessible with subscription). Therefore, it has the potential to become a significant rotation partner with soybeans in a dryland production system.

In a previous article, I stated that corn allelopathy arguably has received less attention than has allelopathy of other crops. Conversely, sorghum allelopathy has received considerable attention. Sorghum’s allelopathic properties are more pronounced than those of most other plants that have been studied.

Research findings are presented here to support grain sorghum’s allelopathic effect and the potential usefulness of that property in cropping systems.

A set of publications from the USDA-ARS Natural Products Utilization Research Unit in Oxford, MS provides results from a continuing investigation of grain sorghum’s allelopathy. Scientists responsible for these publications are F. Dayan, I. Alsaadawi, A. Rimando, Z. Pan, S. Baerson, A. Bimsing, S. Duke, L. Pratt, D. Cook, and I. Kagan.

●          Sorgoleone with its lipid resorcinol analogue is a potent phytotoxin produced by grain sorghum root hairs, and likely accounts for much of the allelopathy attributed to sorghum. Sorgoleone is one of the most studied allelochemicals.

●          Increased production of sorgoleone by sorghum roots was measured when a crude extract from velvetleaf roots was added to the growth medium.

●          Sorgoleone can be absorbed through the hypocotyl and cotyledonary tissues of germinating seedlings, thus leading to the possibility that its effects are the result of photosynthesis inhibition in young seedlings.

●          Research results suggest that sorghum roots have the capacity to continuously exude sorgoleone into the soil. As this phytotoxic exudate is released directly into the soil, its action is similar to that of a soil-applied herbicide.

●          The potential for continuous exudation of sorgoleone into the soil may sustain its presence and subsequent activity in soil over a much longer period than that of an applied herbicide.

●          Sorgoleone is persistent in soil, but it is mineralized by microorganisms over time.

●          Scientists at the above ARS Unit are developing information needed to genetically increase production of sorgoleone in sorghum.

Einhellig and Rasmussen reported in the Journal of Chemical Ecology (Vol. 15, 1989) that a grain sorghum crop reduces weediness in the following crop year compared to corn and soybeans. This effect was primarily on broadleaf weeds, and involved both delayed emergence and growth inhibition of weeds during the growing season. They concluded that allelopathic conditions from growing grain sorghum must be considered as a major factor in weed inhibition, and that this has potential applications in production agriculture.

Roth, Shroyer, and Paulsen (Agronomy Journal, Vol. 92, 2000) reported that wheat yields following grain sorghum were reduced by 15 and 30% compared to fallow when the sorghum residue had been tilled or left on the soil surface with no tillage, respectively. Their results suggest that tillage of the sorghum stover abated but did not completely offset the effect of the allelopathic compounds in the sorghum stover. They found no differences in tolerance to sorghum residue among wheat hybrids.

In summary, there is strong research evidence that grain sorghum has an allelopathic effect on plants of a following crop, and on weed plants that may appear in a growing sorghum crop.

It is anticipated that grain sorghum’s allelopathic effect will be the first to be exploited for use in production agriculture. However, the application of this trait has yet to be realized.

P.S. In my years of research, I had the opportunity to use grain sorghum in several studies. I can confirm that the allelopathic activity of sorghum residue against weeds was visibly evident in comparison to that of corn and soybeans. Certainly, this property of grain sorghum needs further attention so that its potential for use in weed control and curtailing the use of synthetic herbicides can be determined and hopefully become a reality.

Composed by Larry G. Heatherly, Nov. 2012, larryheatherly@bellsouth.net