Warm-Season Grass in the A.M.: The Ultimate Low-Carb Pasture?
If you want to offer good-quality grazing without too much sugar, think warm-season grasses—like Bermuda and modified crabgrass—and early morning turnout. According to a new study, recently developed versions of these two grasses have up to 60% lower nonstructural carbohydrate (NSC) concentrations than typical cool-season grasses. Those rates are even lower before noon, dropping to less than 10% between midnight and 8 a.m.

“These warm-season grasses offer a kind of dual benefit of production during the summer and potentially lower nonstructural carbohydrates, so horse owners have to worry a little bit less when they’re making a choice about whether to graze their fat horse or not,” said Jennifer Weinert-Nelson, PhD, in the Department of Animal Sciences at Rutgers, The State University of New Jersey, in New Brunswick.

Nonstructural carbohydrates include starch and water-soluble carbohydrates such as simple sugars and fructans. Nutritionists recommend restricting NSC levels in obese horses and those with metabolic dysfunction, such as insulin dysregulation and equine metabolic syndrome (EMS), to about 10-12%, said Weinert-Nelson.

Most horse pastures in temperate areas of the U.S., including the Northeast where the study took place, have cool-season grasses such as orchardgrass and Kentucky bluegrass that grow back every year (perennial), she said. But these grasses are known to have relatively high NSC concentrations.

Warm-season grasses, such as Bermuda (also a perennial) and improved forage varieties of crabgrass (which must reseed every year, whether naturally or through planting), usually have lower NSC concentrations, she said. But these species generally struggle to grow in the Northeast. However, recently developed varieties of cultivated warm-season bermudagrass, which tolerates the cold better, and forage crabgrass, which grows upright rather than flat the way native crabgrass does, are being planted successfully in the mid-Atlantic states (Connecticut, Delaware, Maryland, New Jersey, New York, Pennsylvania, Virginia, and West Virginia).

60% Lower NSCs in Warm-Season Grasses

Curious how modified warm-season grasses compare to typical cool-season pasture grasses in nutritional content, Weinert-Nelson and her colleagues went to the fields with their clippers. For each of nine planted horse pastures at the Rutgers campus, they clipped 20 random patches of grass every four hours, over a 24-hour period, for three days.

Three of the pastures contained a mix of cold-season grasses: Inavale orchardgrass (Dactylis glomerata), tower tall fescue (Lolium arundinaceum), and argyle Kentucky bluegrass (Poa pratensis). Three had a commercially prepared cultivated crabgrass (Digitaria sanguinalis)—a straight-growing grass not to be confused with the common crabgrass weed. The other three had warm-season perennial bermudagrass (Cynodon dactylon) that had undergone genetic improvement and commercial cultivation to better endure cold weather.

To ensure the samples were as comparable as possible, the researchers didn’t clip the pastures on the same calendar day but, rather, when each grass had reached the same “boot stage” of maturity—when the seedhead swells, indicative of harvest time. Each pasture reached this stage at different times of the year: May, July, and September, respectively.

Overall, the scientists found that, at boot-stage maturity, the two warm-season grasses were much lower in NSC than the cool-season grasses, Weinert-Nelson said. In fact, their NSC concentration never rose above 12.6% for the improved-variety bermudagrass and 12.1% for the improved-variety crabgrass. In the early morning hours, between midnight and 8 a.m., concentrations never even reached 10%.

By contrast, the cool-season grass mix always had at least 15% NSC concentration, regardless of the time of day, she said. That rate topped 20% at the 8 p.m. reading.

More specifically, the researchers found that:

  • The cool-season grass mix had the highest digestible energy and highest overall NSC but lowest crude protein.
  • The improved-variety bermudagrass had the highest dry matter content, lowest digestible energy, lowest acid detergent fiber, and highest neutral detergent fiber, with an average NSC of 10.6%.
  • The improved-variety crabgrass had the lowest dry matter content, the highest starch, and an average NSC of 10.9%.

Although the crabgrass had the highest starch reading, this single NSC component remained relatively low (4% concentration) and was offset by an overall low concentration of the other NSC components, she explained.

NSCs Lowest Before 8 a.m., Highest Between 4 and 8 p.m.

As for daily changes in NSC content, differences were most pronounced in the cool-season grasses, Weinert-Nelson said. Regardless of the grass type, NSC was always much higher—about 30% on average across all varieties—in the afternoon and evening (especially from 4 to 8 p.m.) compared to early morning (from 4 to 8 a.m.).

“We did see some diurnal (daily) variation in those warm-season grasses,” she said. “But it wasn’t nearly as pronounced as what we saw in the cool season grasses.”

Digestible energy and dry matter were also lower in the early morning, whereas crude protein and fiber were at their highest.

The findings suggest horses needing lower NSC concentrations could benefit from grazing on warm-season grass pastures, said Weinert-Nelson. Even on cool-season grasses, they would likely do better in the early morning hours when NSC concentrations are lower.

The study also highlights the fact that NSC concentrations continue to rise through the evening, with the maximum rates recorded at 8 p.m., she added.

Further studies should compare nutrient qualities of the grasses in the same period, regardless of maturity stage, to investigate nutritional qualities in a practical pasture environment, Weinert-Nelson said.

“ Diurnal Variation in Forage Nutrient Composition of Mixed Cool-Season Grass, Crabgrass, and Bermudagrass Pastures,” was published in March in the Journal of Equine Veterinary Science.