With so many glyphosate-resistant weeds, starting with clean fields is the best strategy.

In spring, apply a preemergence herbicide followed by one or more postemergence herbicides. In Minnesota, we use a lot of protoporphyrinogen oxidase herbicides preemergence. PPOs are very effective if used at higher rates and layered with another preemergence chemical across the field. They should be used sparingly during the season, however, because this is one of the few chemistries that currently doesn’t have much weed resistance. Even if you are planting some of the new dicamba- and 2,4-D-resistant soybean technologies, you cannot ease up on your weed management program.

Kyle Reiner

Use multiple modes of action
Employ multiple modes of action and layer residuals of those preemergence herbicides. Then, once the crop has emerged, come back with a postemergence tankmix that contains some residual. If you have glyphosate resistance in your fields, you’re probably going to be using up to three or even four modes of action to successfully control weeds. Adding an adjuvant in the spray tank can help increase deposition and improve drift control to keep your herbicide applications on target.

Get them while they’re small
Spray early and continue to control weeds when they are at the labeled height or below. Letting weeds stick out over the top of the canopy of the crop and expecting our chemistries to be Superman and eliminate all of them will not bode well for the future.

To know what types of weeds you have in your fields takes good scouting, and working with your local agronomist to understand what chemistries work on what types of weeds. For example, applying a PPO herbicide when a weed is 3 to 4 inches tall or less is critical to the success of your crop.

Tall waterhemp, giant ragweed and common ragweed were some of the primary weeds Minnesota farmers battled in corn and soybean fields in 2016. Even though commodity markets are not where they were, don’t cut back on the weed control program you have in place. Use between three-quarters of the full rate and the full rate of preemergence herbicide as indicated on the label, and follow up with an effective broadleaf herbicide in the tank that mixes well with a postemergence glyphosate application for optimal weed control.

Prepare for new challenges
Last fall, Palmer amaranth was detected in Minnesota for the first time. This invasive species, which is on the Minnesota noxious weed list as an “eradicate” weed, has long been a thorn in the side of farmers in the Southern U.S. It can grow 2 to 3 inches per day, and a single female plant can produce 100,000 to 500,000 seeds. According to Minnesota statutes, landowners must attempt to eradicate any Palmer amaranth found by destroying all above- and belowground parts of the plant. You should also report any Palmer amaranth you see to the Minnesota Department of Agriculture.

Effective preemergence herbicide applications need to be followed up by timely and effective postemergence herbicides when Palmer amaranth is less than 3 inches tall. In areas where larger, thicker patches are found, weeds need to be physically pulled out and removed from the field. Palmer amaranth is a prolific, tall plant with a large stalk that will not feed through a sickle section very well, and you would not want to distribute seeds after running the weed through the combine. Palmer amaranth will change the way we manage all crops, and farmers need to be aware of it and deal with it promptly.

Reiner is a master agronomy adviser with WinField United in southwestern Minnesota. Contact him at [email protected].