Thin cereal rye cover crop stand got you down? Don’t write it off just yet – cereal rye stands that struggle to produce a bountiful biomass could still provide farmers with good weed suppression benefits, according to new research from Texas A&M.
Cereal rye’s biomass can prevent weeds from germinating and block sunlight from reaching the soil, even at lower-than-optimal biomass levels, according to Texas A&M graduate student Gustavo Silva and his advisor, weed scientist Dr. Muthukumar Bagavathiannan. And in the meantime, cereal rye roots release weed-suppressing chemical compounds underground.
Those effects can build on each other: Bigger biomass results in a thicker aboveground barrier and more shade, and larger belowground roots.
Biomass Blankets the Soil and Robs Weeds of Sunlight
Cereal rye has already earned a name for itself as one of the most popular cover crops to use in row crop productions according to the USDA – in part because of its weed-suppressing power. And most of that power comes aboveground.
Specifically, terminated cereal rye forms a thick mat of residue over the soil. Silva found that this mat alone could suppress 82% of weeds with just 3,000 pounds per acre of biomass when compared to a no-treatment plot. Small-seeded weed species are often too weak to break through that blanket and emerge into the crop above. Large-seeded weeds such as sicklepod, though, are capable of breaking through thick biomass just like your soybeans or corn crop.
Silva notes that living and terminated cereal rye also shades the soil, keeping soil temperatures cool and preventing germination in weed seeds that need sunlight to germinate. In his research, Silva found that shade could reduce weed emergence by 50% compared to a no-treatment plot.
Underground Weed Control
But cereal rye’s weed-stopping mechanics don’t stop at the soil, Silva found.
“[Weed suppression] isn’t just about biomass production,” Silva says.
Specifically, cereal rye and other grasses and small grains release acid compounds called benzoxazinoids through their roots. These compounds are plant defense mechanisms that act as insecticides, fungicides, and herbicides – a phenomenon called allelopathy.
“[Benzoxazinoids] are very good at suppressing weeds; they basically prevent root and shoot development,” Silva explains.
Silva chopped down the cereal rye’s aboveground biomass to see the acid compound’s weed-suppressing effect, and to make sure that the rye’s biomass wasn’t throwing off the belowground research by blanketing and shading the soil. He found that the cereal rye’s root excretions reduced weed seedling emergence by 39% compared to a no-treatment plot. Previous USDA-ARS research has found that these excretions will remain in the soil for up to two weeks after cereal rye termination.
Previous research by Silva found that cereal rye will even reduce tough weeds like johnsongrass belowground by suppressing their rhizomes. He is now investigating if allelopathy is the cause of reduced johnsongrass rhizomes.
Silva also examined whether cereal rye’s known tendency to absorb soil nitrogen could contribute to weed suppression, but he didn’t find a link between the two factors. This could be because Texas’ soils are naturally nutrient deficient, and weeds are likely adapted to the low amounts of nutrients, Silva says.
Ultimately, cereal rye’s above and belowground weed suppression tactics compound on each other, Silva reports. While cereal rye’s weed suppression doesn’t entirely hinge on having the recommended biomass, having more biomass will result in a thicker soil blanket, more soil shade, and more roots that release weed-suppressing compounds.
“I think this can be encouraging for farmers who don’t have a lot of biomass production,” Silva notes. All of his findings come from cereal rye with just 3,000 pounds per acre of biomass – roughly half of what is typically recommended for weed suppression.
But Silva warns that different regions and soil types could experience slightly different results. This research was also conducted without any crops present, which could also influence cereal rye’s weed suppression mechanics as the growing season progresses.
Silva’s future research will examine how different weed species respond to each of cereal rye’s weed-suppressing mechanisms.
Explore GROW’s website for information on cover crop management and termination, and to learn more about recent cereal rye research!
Article by Amy Sullivan, GROW; header photo by Claudio Rubione, GROW; feature photo by Cynthia Sias, Virginia Tech
