A news release last week from the Weed Science Society of America covered the results of a study in Arkansas cotton fields to determine the effect over time of releasing 20,000 glyphosate-resistant Palmer amaranth seeds in a patch, within a one-square-mile area. Keep in mind that 20,000 seeds may represent only 2% of the possible seed from one plant, so this would appear to be an underestimation of future problems. However, glyphosate was the only herbicide used for several years after the introduction of seed, evidently to allow determination of the “worst case scenario”. So here’s what happened. In the first growing season, a separate patch of Palmer amaranth emerged 375 feet from the original location. In the second year, resistant plants expanded to reach field boundaries and infested 20% of the entire area, resulting in decreased cotton yield and interference with harvest. By the third growing season, Palmer amaranth had completely colonized the fields, making cotton harvest impossible.
The conclusion of the researchers was that the results show the need for a zero tolerance threshold on Palmer amaranth – prevention requires that not even a single plant be allowed to go to seed. Hopefully your first thought upon reading this is, “well I am using residual herbicides in addition to glyphosate, which should reduce the rate of increase following an initial Palmer amaranth introduction”. This is correct, and residual herbicides are a valuable tool in the management of Palmer amaranth. But it’s difficult to achieve 100% control of an established Palmer amaranth infestation with even the most effective program. The best approach is to not let it get established in the first place. The benefits of mid and late season scouting to find and remove Palmer plants before they produce seed cannot be overestimated.
On a related note, we were scouting some fields in southern Madison County last week to see if we could find emerged Palmer amaranth plants this early. We did find some, and if the concentration that we observed within a few square feet is any indication, it reinforces the principle that letting even one plant go to seed is a big mistake. Photos here - http://u.osu.edu/osuweeds.