By Micheal Owen, Iowa State University Department of Agronomy
Weeds are well underway for another year of reduced crop yields. Except for those producers that were able to take advantage of the one or two days available for field work in April, the opportunity to use a soil-applied residual herbicide for weed management in corn —in the manner that provides the most stewardship and economic benefits —has been lost. Now is the time to consider the next best opportunity to protect crop yields, as well as provide stewardship to mitigate the evolution of herbicide resistance in weeds for fields to be planted to corn.
In no-till fields, DO NOT wait to apply the traditional burndown herbicide treatment until after the corn has emerged. The philosophy of the delayed burndown approach is to reduce trips across the field; however, this approach usually guarantees considerable loss of crop yield potential and thus profit. Given the corn market now, this could be costly indeed. The best option now is to apply residual herbicides in combination with a burndown product immediately prior to, or immediately following planting, but prior to the emergence of the crop.
Early preplant herbicide opportunities still exist for soybeans, but given the weed growth that currently exists, a burndown product should be included with the soil-applied residual herbicide. The sooner a soybean herbicide is applied, the better the weed management and the greater the soybean profits.
Selection of herbicides must be done with an understanding of existing resistances to herbicides in weeds. Consider that all waterhemp in Iowa should be considered resistant to ALS inhibitors (Group 2) herbicides. Thus, any herbicide (single product or pre-package mixture) that is a Group 2 product will not provide any control or stewardship with regard to waterhemp. In Iowa, resistance to ALS, HPPD, triazine, PPO and glyphosate products exists in waterhemp populations and in most instances, the waterhemp populations have multiple herbicide resistances. The take-home message is make herbicide selection a well-thought and informed decision.