Plant & Soil Health: Sorghum: Another Weed Control Tool

Pigweed species have become problem weeds for farmers today. Years ago it seemed all growers had to contend with was redroot pigweed, but today we have smooth, waterhemp and Palmer as well. Fortunately, a trained eye can tell each of these species apart. But, unfortunately, Palmer and waterhemp are resistant to some herbicides, are increasingly difficult to control and require a multi-prong approach to management.

And all the best weed control strategies include knocking out the seeds when they germinate and preventing escapes from producing seed. Control is provided by tillage or residual herbicides and they do a fairly good job as part of an overall weed management system. But nature also can provide a degree of control if growers planted grain sorghum in their rotation.

Producers in states south of Illinois grow grain sorghum to help combat glyphosate-tolerant pigweeds such as Palmer and waterhemp. As with corn, growers are able to use atrazine—which is very effective on pigweeds—on sorghum, taking out germinating seeds unless they are resistant to triazine herbicides. A year of corn followed by a year of sorghum with atrazine included in the weed control program both years will help clean up fields and reduce the weed seed bank.

Another benefit of planting grain sorghum is that it releases compounds that can impede germination of seeds, including pigweed species.

Adam Davies, an ecologist with the USDA based at the University of Illinois, has studied allelopathy in a number of species. He explained that sorghum and sorghum-sudan grass release sorgoleone (hydroquinone), a phenolic compound with allelopathic properties.

Davies explained that when grain sorghum and sorghum-sudan residues (roots and shoots) decay they release allelopathic chemicals that inhibit weed germination and, particularly, germination of pigweed species. Allelopathy happens when plants or decaying plant residue release chemicals that negatively affect other species growing in the vicinity.

The living seedlings and roots, and decaying shoots, leaves and roots secrete sorgoleone that suppresses many weeds. Sorgoleone is active at extremely low concentrations and comparable to some synthetic herbicides. As early as five days after germination, roots begin secreting this allelochemical, which persists for weeks and has visible effects on weed seedlings.

Sorgoleone from sorghum and sorghum-sudangrass hybrids suppresses numerous annual weeds such as velvetleaf, large crabgrass, barnyardgrass, green foxtail, the pigweed species and common ragweed.

While you may not plant sorghum yet and the market isn’t necessarily as accessible as corn, it could fit well into a rotation with corn and soybeans and improve your ability to control waterhemp and Palmer amaranth. Consider it another tool in the toolbox.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.