Corn gluten meal has long been recognized as having herbicidal properties. This byproduct from the wet milling of corn (Zea mays L.) has been shown to possess some degree of pre-emergent effectiveness against various annual grasses and broadleaves, including large crabgrass (Digitaria sanguinalis (L.) Scop.) and buckhorn plantain (Plantago lanceolata L.). However, a lot of the preliminary work has been done in greenhouse trials, and field studies have been somewhat inconclusive in their results. The possibility of an organic herbicide with pre-emergent capabilities is pretty neat, so folks at Kansas State University decided to look at the efficacy of corn gluten meal for controlling dandelion (Taraxacum officinale G.H. Weber ex Wiggers) and smooth crabgrass (Digitaria ischaemum).
Rodney St. John, Ph.D., (who now works for Ryan Lawn & Tree) and research assistant Nadia DeMuro conducted a three-year field trial (on separate stands of tall fescue and Kentucky bluegrass) in which corn gluten meal was applied at rates of 10, 20 or 40 pounds of product/1,000 square feet. One of the benefits of corn gluten meal is that it contains a good amount of nitrogen (N) (~9%), so application of those rates of corn gluten meal also resulted in N application rates of 0.9, 1.8 and 3.6 pounds/1,000 square feet. So, is the weed control a result of herbicidal activity, or of increased competition from the turfgrass species, which is responding to that additional N? To try to answer this, the researchers also had separate plots that received N applications of urea or Milorganite (biosolid fertilizer) at the same N rates as supplied through the application of the corn gluten meal. These treatments were applied two ways: (1) spring and fall split applications of 0.9 and 1.8 pounds N/1,000 square feet to supply a total yearly N rate of 1.8 or 3.6 pounds N/1,000 square feet, and (2) a single spring application of N sources to supply 3.6 pounds N/1,000 square feet. There was also an unfertilized control. The data collected included turfgrass quality (1-9 scale), individual dandelion counts and percent crabgrass cover.
Nitrogen rate (0.9, 1.8 or 3.6 pounds N/1,000 square feet) — not N source (corn gluten meal, urea or Milorganite) — had the greatest effects on turfgrass quality. The highest rate of N (1.8 pounds N/1,000 square feet applied in both spring and fall) produced the highest-quality turfgrass, regardless of the source.
Control of dandelion and smooth crabgrass was also affected by N rate. In the Kentucky bluegrass, adding N at 3.6 pounds N/1,000 square feet (either all at once in spring, or split spring/fall) produced turfgrass with lower dandelion counts. In the final two years of the study, fertilizing the Kentucky bluegrass with the highest rate of N (3.6 pounds/1,000 square feet) resulted in less crabgrass than applying lower rates of N. Higher rates of N applied to tall fescue also resulted in lower amounts of crabgrass. N source did affect crabgrass levels in tall fescue (but never in Kentucky bluegrass). Crabgrass populations tended to be lower in plots treated with urea rather than corn gluten meal or Milorganite.
Turfgrass treated with corn gluten meal at 40 pounds of product/1,000 square feet (supplying 3.6 pounds N/1,000 square feet) was equal in quality to that fertilized with urea or Milorganite at 3.6 pounds N/1,000 square feet. In this three-year field study, corn gluten meal was no more effective than urea or Milorganite in reducing dandelions or smooth crabgrass, and it provided no additional benefit in weed control compared with products containing only N. The authors noted that no treatments reduced weed populations to an acceptable level, and that the corn gluten meal may not have been applied within a one-week window of crabgrass germination, which could have reduced effectiveness. Regardless, based on these findings, corn gluten meal does not appear to have much herbicidal activity in the field. The application of N created thick, healthy turf that reduced weeds, and it didn’t matter whether the N source was corn gluten meal, urea or Milorganite.
Source: St. John, R., and N. DeMuro. 2013. Efficacy of corn gluten meal for common dandelion and smooth crabgrass control compared to nitrogen fertilizers. Online. Applied Turfgrass Science doi:10.1094/ATS2013-0426-01RS
Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., editor-in chief for the American Society of Agronomy, and president-elect of the Crop Science Society of America. She is a 20-year member of GCSAA.