Pasture Renovation to Address Invasive Exotic Grasses

Sunday, June 23, 2024

By Julia Gerlach

If you’ve been paying attention to South Dakota’s grasslands, you’re probably aware that one of the greatest threats to the health of the state’s prairies is the largely unchecked invasion of non-native (exotic) species such as smooth bromegrass, Kentucky bluegrass, crested wheatgrass, and cheatgrass.

For many of us, the distinction between grass species may be elusive. But for ranchers, healthy grasslands are very important for feeding livestock and providing habitat for wildlife. The main issue with exotics, of course, is that they can rapidly outcompete native species, which can quickly diminish biodiversity and alter feed quality and quantity.

Exotics aren’t all bad, of course. According to Dr. Cody Zilverberg, Consulting AgroEcologist at Dakota Lakes, the exotic species they’re worried about most are cool season grasses.

“At their best, they’re actually better than warm season grasses for digestibility and crude protein,” he says. “But at the end of June, their feed value goes down. Also, the seed heads become poky and can stick on the cattle’s eyes and tongues, and cows don’t like to eat them then.”

Native tallgrasses, on the other hand, will have good protein and digestibility in late June and will be preferred by the cattle at that time, he says. So, it’s important to protect the ecosystems that allow native grasses to thrive.

Hard Grazing of Exotics

And that is the main objective of a pasture renovation project that is in the works at Dakota Lakes Research Farm that began this spring.

Using 60 spayed heifers that were delivered on April 30 by Luke Perman of Lowry, SD, Zilverberg began with targeted grazing of the cool season exotics.

“We can target those cool season exotics by grazing them hard early in the season when they’re growing but the natives aren’t big enough to graze yet, or they’re there but last year’s residue is somewhat protecting them,” he says. “This should hurt the exotics.”

After about 10 days of grazing the exotics, Zilverberg moved the animals to a field of cover crops, allowing the pasture to rest.

Zilverberg is implementing different grazing schedules in different areas so he can study the effects of the varying management practices. So, they have some areas where the cool season exotics didn’t get grazed early.

He will then bring the animals back to the pastures in late June or July and graze all of them.

“In those fields that weren’t grazed early, the cool season grasses will be headed out and not palatable,” he says.

The goal in June and July will be to graze lighter.

“We’ll keep an eye on the warm season grasses. We want to leave about 4-8 inches of biomass so they can regrow, and then we’ll move the animals to the next paddock.”

Zilverberg says the outcomes will likely depend a lot on the weather conditions.

“If it’s hot and dry, the cool season grasses that were abused won’t regrow much,” he says. “If it stays cool and wet, they may regrow more. If we get the timing right, the cool season grasses will have competition from the warm season natives.”

Zilverberg says besides giving the warm season native grasses a better opportunity to be competitive, early grazing also inhibits the cool season exotics by cutting back somewhat on reseeding.

It’s not a perfect approach, though, as some species, such as smooth brome, reproduce by rhizome as well as from seed. In addition, cheat grass evades control because it’s shorter than the others and often gets missed by grazing cattle. Sometimes it gets pulled out of the ground because it has a weaker root system.

But again, Zilverberg says, the aim is to support the warm-season natives, not get rid of all cool season grasses.

“I would like to get rid of all exotics if possible and replace them with native grasses, whether they be cool season or warm season. But it’s not realistic to expect that we will ever eliminate the exotics. They will be here until the next ice age (or perhaps extreme warming). So, a reasonable goal is to manage them, reduce the amount of them, and create a more diverse plant assemblage,” Zilverberg says.

Plus, his goals require keeping a broader perspective.

“Ultimately, we still want production because we have animal objectives and we have vegetative objectives,” he says. “And using animals to control weeds is tricky because they can’t get everything all at once, the way an herbicide would.”

Study Extension

This renovation project is actually an extension of an earlier pasture renovation project that ran from 2017 to 2021, which focused on getting native tallgrasses established. That earlier project looked at different combinations of herbicide and seeding treatments where they either applied native grass seed, grass and forbs, or no seed at all, randomized with no herbicide applied, glyphosate applied, atrazine applied, or both glyphosate and atrazine applied.

They also had two stocking rates for the cattle: a low stocking rate and a high stocking rate.

Zilverberg explains the study and its results in this excellent video, but summarizes it by explaining that the only treatment that was deemed successful was where both glyphosate and grass seed were applied. Without the glyphosate the exotics still dominated the natives, and without the grass seed applied, there were no natives in the seed bank to draw upon. In addition, while there were two stocking rates, they found no difference in the results.

Zilverberg’s prediction for the current study is that after three years they’ll find little if any difference between the pastures that get early targeted grazing and those that don’t.

“This sort of thing takes time to change. I’m more confident about reducing exotics than increasing the tallgrasses,” he says. “But we’ll measure species and biomass again after five years and if we can reduce exotics by 25%, that would be great. We’ll never get rid of the exotics. It’s a matter of using them the best we can, and not letting them dominate the pastures.”

This pasture renovation project will be discussed at the upcoming Annual Field Day at Dakota Lakes Research Farm on June 27. Be sure to attend to see the progress for yourself!

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