Herbicide restrictions and carryover risks for alfalfa

By Emily Hansen, U of M Extension

With the drought affecting most of Minnesota this summer, we will most likely see the effects in next year’s crops. This is especially important when looking at your crop rotation and what herbicides were applied over the summer. As the drought continues, the potential reduction in herbicide breakdown is a concern.

Keep in mind that microbial degradation is the primary method by which herbicides break down in the soil. Soil temperature and soil water are two of the most important factors in this process. Our current hot and dry conditions will slow down this process causing the potential
carryover risk. For many of our different crops, this can cause issues; for alfalfa, which we expect will last more than two years, this is a huge concern. This is why it will be important to check herbicide labels and plan the crop rotation accordingly.

There are seven factors that will determine the risk of carryover to different crops. They are: chemical half-life, rate of herbicide applied, application date, soil characteristics, rainfall, sensitivity of rotational crop, and the growing conditions following planting in the spring. Many of these factors are completely uncontrollable. What can be done to minimize carryover risk is to pay attention to the sensitivity of the crop to previous herbicides applied. Rotation to a different crop that is less susceptible will help minimize potential risk. To help determine this risk for alfalfa, you can look at many of the crop rotational replant intervals listed on the labels of any herbicides previously used. While this isn’t always perfect, it can be a good indication and will provide valuable information that can be used to decide whether to plant alfalfa.

Here are just a few herbicides and their replant/rotational intervals:

• Atrazine – there are no restrictions.

• Callisto – 10 months.

• Dual Magnum – 4 months.

• Laudis – 10 months.

• Outlook – 11 months.

• Resicore – 10.5 months.

• Surestart/TripleFlex – Spring after application and when annual rainfall and/or irrigation is less than 15 inches on soils with less than 2% organic matter, this crop should not be planted until 18 months after treatment.

• Warrant – Can rotate the next spring following the previous year’s application.

If a producer is dealing with
carryover issues significant enough to warrant replanting, rotating to a different, less sensitive crop for a year may be advised. It may also be recommended to replant alfalfa in the late part of summer. Ideal seeding would be between Aug. 1-15.
Keep in mind the above
carryover risk factors when determining if it is safe to replant alfalfa. If the alfalfa stand is not totally lost, consider other forage options such as seeding different grass species into the alfalfa stand. Another option is to reseed alfalfa in the spring, only if the alfalfa stand was established the year before. If not, then alfalfa is not an option.

Sources: https://crops.extension.iastate.edu/cropnews/2012/08/carryover-concerns-2013.

https://blog-crop-news.extension.umn.edu/2020/04/evaluatealfalfa-stands-this-spring.html.

https://blog-crop-news.extension.umn.edu/2021/07/summer-reseeding-or-seeding-of-alfalfa.html.

If you have questions on the above information, please feel free to email me at ndrewitz@umn.edu, or call (608) 515-4414. To receive future events, educational programming, and agronomic updates by email, signup at http://z.umn.edu/tricountysignup.