What happens when you rely strictly on post-emergence weed control in corn? After all, that's now a reality with both Roundup Ready and LibertyLink corn. Roundup Ready would be sprayed post with glyphosate, either as a Roundup formulation from Monsanto, or a generic. LibertyLink could be sprayed with Ignite, containing glufosinate. Bayer replaced Liberty with Ignite as the glufosinate product of choice for '09.
As it turns out, inquiring minds wanted to know the answer to these questions. They included weed scientists at Ohio State University, the Southern Illinois University near Carbondale and Purdue University. So weed scientists teamed up in a two-year study that gave them data on three locations each year.
The results were striking. "First of all, we think we grew pretty good corn for a group of weed scientists," quips Bill Johnson, Purdue University weed control specialist. Indeed, yields topped 200 bushels per acre in many of the plots where a residual herbicide was laid down before planting.
"More importantly, we believe it shows the value of controlling weeds early, and spraying weeds before they get too tall," he adds. "The residual herbicides helped control enough weeds early so that there was less competition, and less loss of yield potential, before we came back with the primary post treatment."
The researchers compared three residual treatments on Roundup Ready corn. They included atrazine at 1 quart per acre, Bicep at 1.4 quarts per acre and Lexar at 2.4 quarts per acre. Then they sprayed the plots at three different times: when corn was 12 inches tall, one week after corn was 12 inches tall, and 2 weeks after corn was 12 inches tall. At all three times for post applications, they applied 22 ounces per acre of Roundup OriginalMAX.
Even at 12-inch corn, there was nearly a 20-bushel per acre difference between corn which didn't receive a residual herbicide before planting and the most effective residual herbicide in the study. In each of the three timing applications, atrazine as a residual outperformed no residual. Bicep produced better yields than atrazine, and Lexar laid down as a residual herbicide produced led to the highest yields in each case.
The highest yield, 223 bushels per acre, came when glyphosate was applied over a Lexar plot on 12 inch corn. The longer the plots sat before the glyphosate was applied after the 12-inch corn stage, the more pressure on yields. The Lexar plot sprayed with glyphosate two weeks past the 12-inch corn stage, for example, yielded 211 bushels per acre.
What's striking, though, Johnson notes, is that by two weeks after the 12-inch stage, weeds competing with corn where no residual was applied had dropped yields to 181 bushels per acre. That's a 30-bushel per acre gap between using a residual herbicide, and not applying one at all.