Remember to Consider Pesticides When Trying Cover Crops

More women landowners are working with their tenants to try out cover crops on their fields to control erosion during the months when soil is otherwise bare. But pesticides used to control weeds in a corn-soybean rotation can have an effect on your cover crops, too. Here are some points to consider as you make your management decisions, provided by Liz Stahl of University of Minnesota Extension.Liz S DSA 2015

  1. Some weeds, like waterhemp or giant ragweed, are difficult to control to begin with, and some are developing resistance to common herbicides. That means farmers often choose herbicides that offer long-term, or “residual,” control. If the herbicide persists in your soil, it may hinder emergence or growth of your cover crop.
  2. The longer the delay between herbicide application and planting of cover crops, the less problem you should have.
  3. Some cover crops are more sensitive to herbicides than others. Tillage radish, for example, can be killed by broadleaf herbicides. Rye is tolerant to a wider range of weed-killers.
  4. Your decisions will differ depending on whether you plan to graze or harvest the cover crop, or are using it primarily for erosion control. If you want to plant a crop that will be harvested, or the crop will enter the food or feed chain, rotation or plant-back restrictions must be followed. “Remember, the herbicide label is the law,” says Liz.

Liz notes that herbicide companies have typically not done much testing of their products' effects on cover crops, and therefore you won't find much information on labels about rotations that include cover crops.

She adds that herbicide persistence depends on many factors, not just on the amount of rainfall your fields have received. The dominant mechanism for herbicide degradation in the soil is through microbial activity. Conditions favorable for microbial populations, such as warm temperatures, oxygen, good fertility and a medium soil pH, will favor herbicide breakdown. In contrast, flooded soil conditions where oxygen is lacking, will reduce microbial activity and herbicide degradation.

Climatic factors involved in the degradation or breakdown of herbicides include moisture, temperature, and sunlight. Generally, herbicide degradation rates increase with increases in temperature and moisture, while cool, dry conditions slow degradation.  56942c010d5ba.image

Finally, chemical properties of the herbicide affect the herbicide's persistence, including the herbicide's water solubility, soil adsorption, vapor pressure and susceptibility to chemical and microbial breakdown.

Talk to your tenant about the timing and type of herbicides being used on your cropland if you are using or planning to use cover crops. “Know as much as you can, and set yourself up for success,” Liz says.

More information is available at these links:

A Closer Look at Herbicide Rotation (University of Minnesota, blog post by Liz Stahl)

Herbicide Rotation Restrictions (University of Wisconsin; PDF, 2.4 MB)

Rotation Restrictions (Purdue University; PDF, 344 KB)

This article deals with establishing cover crops in the fall when you are not planning to use the cover crop for feed or forage (from Purdue University; PDF, 2.4 MB):  Click here.