Even though some of the toughest weed issues may be in soybeans -- such as waterhemp and giant ragweed (as discussed in previous issues of the Agronomy Update) -- weeds in corn are also becoming resistant to some herbicides, especially glyphosate.

"Studies have shown that weeds can start to affect yield even when the corn is only 2-4 inches [tall]," said Peterson. Products such as Acuron®, Lumax®, and Staunch® can be a great start to clean fields.

A fairly new tool, said Peterson, is the Herbicide Classification Chart, which provides information on how to get more than one mode of action when tank mixing -- the best way to combat resistance. The chart (a cut-out of which is shown above) shows, for instance, that Acuron has four active ingredients and three modes of action. Every product label includes this same information; the chart combines info from many labels.

Always consult the label of any product being considered to determine if it will cover weed pressures throughout the season. "Using a full-labeled rate of a good pre will give you the longest residual, and the best defense on resistance," said Peterson.

Even with a good pre-emerge herbicide program, today's weed battles in corn will require a post application. "Glyphosate used in the Roundup Ready crop system is the most popular," said Peterson, "but it only has one mode of action." Adding an HPPD product such as Impact®, or even adding atrazine, will boost weed control and combat resistance.

Liberty®, a Bayer product, is another good post product gaining more attention, but it also has only one mode of action, Peterson noted. Check labels for proper tank mixes and check the mode of action classification to avoid doubling up in the same categories.

"Remember," said Peterson, "post-applied products are aimed at 4-inch (or less) weeds, and timing is very important." 

Your Federated Agronomist can help determine the best options to get started with clean fields and keep them clean to produce top yields and fight resistance in future crops.