By Stan Wise

Late summer is a good time for producers to inspect their alfalfa fields and address any problem areas. If there are any bare spots or a stand reduction caused by heat stress, flooding, drought or cold temperatures the previous winter, producers can seed those areas now before the weather turns cold.

Soil health matters, even in the middle of an alfalfa field. Leaving those spots bare not only reduces forage production, but it also leaves the soil exposed to the weather and without any living roots to feed the soil biology which is important for nutrient cycling.

There’s a good chance, however, that seeding those areas with more alfalfa is a bad idea, according to South Dakota State University Extension Forage Field Specialist Sara Bauder.

“Alfalfa can experience autotoxicity. Put simply, it poisons itself after a certain period of growth,” Bauder said. “So, if the stand has been around 13 months or longer, we don’t advise that people replant alfalfa back into it because autotoxicity prevents it from taking off and growing.”

The chemicals – released by alfalfa roots into the soil – that cause autotoxicity don’t affect other plants, so producers have other options to fill in their alfalfa stand.

“So typically, we look at grasses, and a lot of times people just want to put in an annual because they don’t want that alfalfa stand there forever. Once in a while, somebody wants a stand to persist a long time,” Bauder said. “Then they’ll put in a perennial, so it depends on the grower and their interest and their needs.”

For annuals, Bauder recommended wheat, oats, barley, annual ryegrass or triticale. “They’re not going to persist, per se, from year to year because those are annuals, but they’re going to come up quick and grow quick,” Bauder said. “These are cool season plants that work well when planted after August 1 for late season harvest or early in the fall to cover soils next year.”

Some perennial options include orchardgrass, perennial ryegrass, and tall fescue. These perennials have the advantage of surviving for multiple years along with the alfalfa crop.

Meeting the threshold

The first step for interseeding is evaluating the field.

“[Producers] want to make sure that they’re at the threshold for interseeding. So, if you have a great big bare spot, that’s going to be kind of obvious,” Bauder said. “But if you’re not sure and the stand is not very good, we usually say if a field has less than 39 stems per square foot, that’s kind of the standard estimation of ‘OK, we have a problem here. We need to do some type of stand modification.’”

Bauder recommends using a no-till drill to interseed the grass because it provides good seed-to-soil contact while minimizing disturbance.

Interseeding alfalfa with grass species that will overwinter is a good way to ensure forage production next spring as well as improve soil health.

“It goes back to the basics of anytime we have soil that is uncovered, we need to put the armor back on that soil. So just having a seedling growing there is really valuable,” Bauder said. “And of course, that’s going to boost biology, cover your soil, help with the erosion, all of those things. And technically, if you’re interseeding a grass into alfalfa, you’re adding diversity as well.”

For more information about species selection and seeding rates, visit the Natural Resources Conservation Service Cover Crops in South Dakota page at www.nrcs.usda.gov/state-offices/south-dakota/cover-crops-in-south-dakota.

For more information about soil health land management practices or a free cover crop poster, contact the South Dakota Soil Health Coalition at 605-280-4190 or sdsoilhealth@gmail.com or visit www.sdsoilhealthcoalition.org.