Early season weed control is vital to both future yields and profitability, because early weed flushes compete intensely with corn for both nitrogen (N) and water. Dense weeds can also shade soils and make them cooler so that corn grows more slowly.
So, when does early season weed control need to be done before it’s too late to stop yield loss? According to weed research, conducted across the Midwest including South Dakota, given that you start with a clean field, the most competitive weeds in corn will be about 3-4 in. high when corn reaches the V3-V4 growth stage. If you don’t remove those 3-4-in. weeds promptly, you’ll be losing about 3 bu./acre for every day you delay. Minnesota studies over three years show corn lost between 12-13 bu./acre within the first week and 27-29 bu./acre within the second week if weeds were allowed to remain in the field after they reached 4 in. in height.
A big yield loss early in the season could mean the difference between making or losing money, Depending on soil moisture and fertility levels, waiting to control weeds until corn reaches the V3-V4 growth stage can push you over the economic threshold for profitability. At about 4-6-in.-tall corn and weeds, is when producers typically pass the breakeven mark and start losing money to lost yields from weed pressure after factoring in the cost of the herbicide application.
Especially in corn, profitable weed control is all about timing to ensure successful weed control. University Weed Scientists and Extension Specialists recommend the following five tips to help guide farmers towards more profitable corn weed management:
A clean field at planting is essential for starting the corn crop off right. This can be achieved by using tillage, herbicides or some combination of the two.
SDSU Extension Weed Science Project recommends using a burn-down with residual chemistry that is targeted to the specific weed spectrum for each field. Use of a soil-residual herbicide will help to both start the crop off clean and to manage the field for any potential glyphosate-resistant weeds, such as waterhemp and giant ragweed, or to reduce the potential development of these and other herbicide-resistant weed biotypes.
A total post-emergence program is the most risky weed-control system, because the timing of a post-emergence herbicide application is almost completely up to Mother Nature, and no one can control the weather. Instead, try using an integrated program with some soil-residual products. Also, farmers could consider a split application of an early pre-plant treatment followed either by a pre-emergence or a post-emergence treatment to provide more consistent weed control than a single, early pre-plant application.