Liz Stahl

Liz Stahl, Extension Educator, presents at the University of Minnesota Winter Crop Days in Waseca, discussing how to use grazed cover crops in an operation while controlling weeds.

WASECA, Minn. – Cover crops and livestock production really go hand in hand. While there may not be enough time in the growing season for two cash crops, there are good opportunities to raise forage for cattle, sheep or other livestock. However, feeding a cash crop adds challenges to a weed management program.

“When you are planting cover crops, one of the great ways to really help them pencil out economically is if you are going to graze them,” said Liz Stahl, University of Minnesota Extension educator, during a recent phone interview. “If you are going to use it for a feed or forage again, you have to follow what it says on the herbicide label for rotational restrictions and that can be really limiting.”

A rotational restriction is the time interval from applying a herbicide to when you can plant a crop that will be intended to be harvested and used as feed or grazing.

For example, if a grower was planning to use Callisto herbicide on a field and was planning to plant annual ryegrass for forage, they would have to wait 18 months from the herbicide application before they could plant the ryegrass.

“That is probably one of the main limiting factors when you look at what species you can plant or what herbicides you can use,” said Stahl. “But it’s one you really want to pay attention to because you do not want to end up with potentially illegal residues in your cover crop.”

Each herbicide has their own rotational restriction listed on the label. The restriction can vary between species of cover crop and application rate.

Zidua, for example, at a rate of 2.0 ounces per acre, has a rotational restriction of one month for wheat, 12 months for canola or rapeseed, and 11 months for small grains, other than wheat.

Part of the issue is that not all cover crops have been tested with a given herbicide, so many crops fall in the other crop category. On Zidua, the other crops category has a rotational restriction of 18 months.

“When these herbicides get registered, it takes a lot of money to register a product, it takes a long time and a lot of research that is involved,” she said. “It is research by the chemical companies because they're the ones that are bringing those products through the labeling process, so they're going to invest in the crops where they have a market.”

That means research on residual levels on the herbicide in other cover crops does not always get done. The label will always err on the side of caution.

Some of the non-residual herbicides have much shorter rotational restrictions. Glyphosate, for example, is zero months for annual ryegrass. Dicamba is only 22 days per 8 ounces applied (west of the Mississippi) for ryegrass and 2,4-D is 1 or 3 months, depending on soil temperatures and tillage methods.

On the other hand, using non-residual herbicides could impact your weed control.

“When applying glyphosate to something like waterhemp that emerges over a long period of time, you are only going to control what's up, if it's not resistant to glyphosate,” said Stahl. “We know we have a lot of weed populations with waterhemp that are resistant to glyphosate.”

There are options for growers who want to graze cover crops and still control their weeds.

One management decision is field selection. If there is a field that has a lot of weed problems, especially resistant weeds, perhaps that field is not yet a candidate for grazing.

Cover crops can still be used. If there is no intent to graze, the rotational restriction does not apply. Therefore, a grower could spend a couple years using a combination of herbicide control and cover crops to reduce weed populations and getting ready to be a field to graze in the future.

The time the cover crop gets planted is a big factor.

“Shifting away from early inter-seeding, can give you more options,” she said. “You could get a residual herbicide down early on in the season and pick one that the cover crop you want to plant is more tolerant to.”

Instead of planting early on, for example in a V5 timeframe in corn, wait until closer to the end of the growing season, when corn or soybean plants have reached maturity and are starting to die off. Then plant a fast-growing variety to ensure adequate forage production for livestock.

“Later seeded cover crops can give you a better shot at establishment, if you are using a residual herbicide in the system and cover crops like cereal rye have been more tolerant,” said Stahl.

Cover crops can work well in a grazing and cropping system, there are just more factors producers need to be aware of.