By Harold Watters and Greg LaBarge, Ohio State University Extension
When maximizing yield is the goal, controlling early season weeds is critical. Managing weed competition is the one pest management decision we make each year in every field. Weeds compete with the crop for available water, nutrients, and light starting at crop emergence. The first bolded statement in the Ohio, Indiana and Illinois Weed Guide is “Years of research have shown that good weed control within the first 4 to 6 weeks after crops are planted is critical in order to avoid a yield reduction from weeds.” Sound advice.
Some work on soybean out of Michigan shows how weeds present at emergence cause season long yield loss even when removed. Their work showed for each day burndown herbicide application is delayed after soybean planting, a quarter bushel per acre per day yield loss accumulates. By delaying burn down treatment until a soybean plant’s unifoliate stage, the unrecoverable yield loss will total 8%. In economic terms, based on 2021 soybean prices, that is $48 per acre.
Until we get canopy closure, where the crop can shade out weed emergence and growth, managing weed size is also important with postemergence applications. You may have seen Figure 1 at past Extension meetings. It is a great illustration of how yield loss based on weed size at post- application can add up. Keep in mind that weeds grow on average 2 to 4 inches every 3.5 days.
Up to this point we have been using soybean as the example, so what about corn? Corn is less competitive with weeds than soybeans with 2- to 4-inch-tall weeds being the point where yield loss starts to accumulate, with an average yield loss of 0.9 bushel per day.
A final thought on weed size at post emergent application is know your target weed and selected herbicide program. Species such as waterhemp or Palmer amaranth are ideally controlled at 2 inches with good control up to 4 inches, past that point control declines. Weed size with glufosinate herbicide should not exceed 6 inches. Keeping weeds small, non-competitive with the crop in the first 6 weeks after planting is crucial to maximum yields!
Harold Watters and Greg LaBarge, OSU Extension Agronomy Field Specialists, are rarely in their offices, so they can best be reached at email@example.com or by phone at 937 565-6064. Greg LaBarge is available at firstname.lastname@example.org.