There is a reason Marta Moura Kohmann included risk of herbicide carryover when asked to address key issues for alfalfa producers today. Kohmann, Extension specialist in forage systems agroecology at the University of Wisconsin, spoke to forage producers at the 2024 World Dairy Expo.

“We’ve seen more cases of herbicide injury to new stands of alfalfa recently,” Kohmann said. “We suspect it is partly because farmers are battling resistant weeds, especially waterhemp. It can germinate later than many other weeds. As a result, applicators apply more residual herbicides, often later in the spring season.”

How damaging can herbicide carryover become? Kohmann said work by researchers in Michigan showed that when alfalfa was seeded after corn where atrazine was applied at recommended rates, density was reduced 50%. Production dropped 25%.

However, work by Mark Renz, Extension specialist in weed science at the University of Wisconsin, indicates that visual, aboveground symptoms aren’t always so dramatic. Sometimes, belowground damage to the root mass can be even more harmful.

Even stands that appear normal may suffer. The only way to know is to compare a suspect stand to a stand where herbicide wasn’t applied. That takes forward planning and leaving an untreated area for comparison.

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Work is underway at two locations in Wisconsin to learn more about potential impact of herbicide carryover, Kohmann said. Corn was sprayed normally in 2023, and soil was collected for bioassays in November. Alfalfa was seeded in spring 2024.

Renz conducted bioassays in his lab. They consist of placing soil collected from plots into individual pots and growing typical alfalfa varieties. Observations are made after 30 and 50 days. Biomass is measured both aboveground and belowground.

“At one site, an interaction with a soilborne disease made carryover issues worse,” Kohmann said.

Minimizing herbicide injury

Dry falls, like those in 2023 and 2024, increased risk for carryover. Renz found that soil moisture and soil temperature over extended periods impact how quickly a herbicide degrades in the soil. The drier the environment, the more slowly active ingredients break down. The cooler the weather over time, the slower the breakdown.

Soil type, percent organic matter, herbicide rate applied and type of active ingredient factor into breakdown rates. As an example, Renz notes that for one specific active ingredient, breakdown is four times faster at 80 degrees F vs. 50 degrees F.

Here are steps you can take if you plan to establish new alfalfa stands soon:

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Respect the label. It’s the law, but it’s also the best information you have about breakdown of active ingredients, Kohmann said. Be sure to read every footnote. Sometimes companies pack important details about herbicide carryover into footnotes. For example, they may explain how rainfall patterns could affect carryover.

Consider a bioassay. It can be a useful tool, especially if you’re unsure about whether herbicide injury could impact your new stand.

Calculate carefully. Labels for about half the typical herbicides applied for corn call for a 10- to 10.5-month plant-back restriction to alfalfa. If the application was made in early June, that means mid- to late April in many cases. One option is to plant alfalfa a bit later in the spring than you normally would to move beyond that window.

Use resources. For a quick summary of herbicides of concern for carryover in Wisconsin and surrounding areas, see Alfalfa Herbicide Rotation Restrictions.