Published July, 2013

Despite the fact that horses graze year-round, no pasture forage species actively grows all year. Pasture grasses vary from season to season and even from month to month. Each new season brings limitations and availabilities to pastures, and understanding these rhythms will help horse owners take advantage of forages year-round.

The most important difference to understand is cool-season vs. warm-season grasses. Cool-season grasses, such as orchardgrass, bluegrass, and tall fescue, grow during the cool spring and fall months, while warm-season grasses, such as Bermudagrass and crabgrass, grow during the summer months.

Cool-season grasses are most productive when the air temperature is 65-75° Fahrenheit. They respond well to high soil moisture, cooler temperatures, and short photoperiods, making them most productive in the fall and spring. As summer arrives and temperatures increase, growth will slow and cool-season grasses will enter dormancy, sometimes referred to as the “summer slump.”

Warm-season grasses become productive during the summer months before going dormant in the fall. Their growth rate is most efficient when the air temperature is 85-95° Fahrenheit. Warm-season grasses can tolerate low precipitation because they are more efficient at using water than cool-season grasses; warm-season plants require on average 50% less water than cool-season plants to produce a single unit of forage dry matter. Warm-season grasses are generally lower in protein than cool-season grasses, but tend to have a higher forage yield due greater efficiency with nitrogen use and photosynthesis. Some common warm-season grasses include Bermudagrass, crabgrass, and bahiagrass.

Depending on the region, warm-season grasses can provide high-quality grazing, or they can be a nuisance. Forage seasonality can provide challenges and opportunities that, with good planning and management, property owners can optimize for grazing. Each season it is important to understand what forage species you have, your operation's goals, and how to manage those forage species in your favor.

Common Warm-Season Grasses

Bermudagrass (Cynodon dactylon)
Bermudagrass is a sod-forming, warm-season perennial grass that is widely grown throughout the southern United States for both pasture and hay. It is well-adapted to a variety of soil types and conditions, but is most productive on fertile, well-drained soils. Bermudagrass is well-liked for its potential to produce high yields and, when managed properly, has a nutritional value comparable to that of orchardgrass or tall fescue. However, production depends on proper nitrogen and potassium fertilization. Due to its low-growing nature, Bermudagrass tolerates traffic well and can withstand intense grazing pressure. In a 120-day study at the University of Kentucky (UK) in 2007, 14 adult open mares were rotationally grazed on 4.9 acres of well-fertilized Bermudagrass and were able to maintain body condition throughout the summer. For more information on this study, see the UK publication "Bermudagrass: A Summer Forage in Kentucky."

Crabgrass (Digitaria spp.)
There are several crabgrass species, the most common being large crabgrass, smooth crabgrass, and southern crabgrass. While these species can vary in appearance, generally crabgrass has an oblong leaf shape and can have pubescence (hair) on the stem and leaf surface. Crabgrass is a bunch grass that has a tendency to quickly fill bare spots in pastures in late spring. Depending on the region, property managers consider crabgrass either a quality forage or a weed. In the Deep South crabgrass is managed and utilized as a highly nutritional forage. Further north, crabgrass has such a short growing season that overall productivity is low, making it an undesirable forage.

Bahaigrass
Bahiagrass is widely planted in Florida and other regions of the southeastern United States. It is well-liked for its low fertility and management requirements. However, it does not have high nutritional quality, so would not be suitable to maintain growing or lactating horses.

Other, Less-Desirable Warm-Season Grasses

There are several other types of warm-season grasses found in pastures that are not considered good forages for horse pastures. 

Millets
This leafy, rapidly growing annual forage has no risk of cyanide production. There are several types of millets, the most common ones being pearl millet and German (foxtail) millet. Pearl millet is very productive over a short growing season, and is identified by its seed head shape being a large cylindrical spike like a cattail. German millet is fine-stemmed but less productive than pearl millet. German millet is not recommended for horses due to a toxin that has been shown to cause kidney and joint issues.

Weedy foxtails
These rapid growing, clump-type annual grasses known for their seedheads that resemble a fox’s tail. There are a few types of weedy foxtails: giant foxtail, yellow foxtail, and green foxtail. Foxtails are safe for horses to eat; however, they have low nutritional value and palatability and are, therefore, not ideal forage for horse pastures. Another concern with grazing weedy foxtail is that mature seedheads produce long, pointed awns that have the potential to irritate horses’ mouths and cause infection. If grazing fields with foxtail, try to graze before seed heads emerge, or mow the seedheads before grazing. The best weedy foxtail control is to maintain good ground cover. Because they are annuals, shaded soil in the late spring and early summer can greatly reduce foxtail seed germination.

A Grass That’s Considered a Weed

Nimblewill (Muhlenbergia schreberi) is a wiry, upright growing warm-season perennial grass that looks similar to creeping bentgrass and Bermudagrass in structure. The leaves are short, thin, and flat, with veins visible on the upper surface. It can be troublesome in horse pastures for two reasons:

First, horses typically will not eat nimblewill, making it undesirable for pastures and classifying it as a weed. Second, nimblewill is very invasive, difficult to control, and hard to get rid of. It is extremely adaptable, thriving in the eastern and central United States, though it is most common north of the Bermudagrass belt. It has become an increasingly common weed in western Virginia, West Virginia, Kentucky, and surrounding states. Nimblewill spreads by stolons, which are aboveground stems that spread horizontally across the soil surface and produce new plants at beadlike nodes. (Bermudagrass differs in that it spreads through both stolons and rhizomes, or underground stems.) Chain dragging in the summer months is good practice for parasite control, but it can spread nimblewill and other weed seeds through a field. Mowing can also spread nimblewill through seeds if the mower operator does not clean it out between fields.

Because it is a grass, broadleaf weed herbicides do not have an effect on nimblewill, so complete removal and re-establishment will be necessary to eliminate it. Two glyphosate (Roundup) applications at the highest rate should be used to completely spray out nimblewill. Consider spot spraying when nimblewill is only found in small patches of an otherwise productive pasture, and always follow label recommendations and precautions. After spraying, reseed the field. Maintaining good ground cover will prevent nimblewill seeds from germinating and reinfesting the pasture. Refer to the article “When and How to Re-establish Horse Pastures” for more information on complete re-establishment.

Kelly Prince, an MS candidate, Krista Lea, MS, and Ray Smith, PhD, professor and forage extension specialist, all within the University of Kentucky Department of Plant and Soil Sciences, provided this information.