Rotational Grazing Systems and the Equine Intestinal Microbiome

The exquisitely sensitive intestinal microbiome of the horse can be thrown off balance by various stressors, dietary changes, and medications. A change in diet between cool- and warm-season grasses, however, does not appear to alter the intestinal microbiome, which is good news for farms that use rotational grazing strategies.

Rotational grazing systems are particularly beneficial in temperate regions where the spring and fall are cool and the summers hot. Cool-season grasses are most productive during the spring, early summer, and fall but produce poorly in the summer, leaving little forage for horses managed entirely on cool-season pastures. In contrast, because warm-season grasses have different photosynthetic mechanisms, these grasses are more vigorous during the summer when cool-season grasses struggle to produce.

Examples of cool-season grasses include fescue, timothy, orchardgrass, Kentucky bluegrass, cocksfoot, ryegrass, brome, and various species of Phalaris. Warm-season grasses include Bermuda grass and crabgrass.

“Integrated rotational grazing systems that use both cool- and warm-season grasses help extend the grazing season and reduce the need for supplemental forages during the hot, dry summer months,” said Catherine Whitehouse, M.S., a Kentucky Equine Research nutrition advisor.

While pasture rotation seems advantageous, a horse’s intestinal microbiome may not respond favorably as changes in forage can upset the delicate balance of microorganisms in the hindgut. To determine the effect of alternating between cool- and warm-season forages, researchers evaluated intestinal microbiomes using standard fecal-testing protocols.^*

Eight healthy adult Standardbred mares were included in the study. Each horse was allowed to graze from early June until November in New Jersey. Horses grazed on a cool-season pasture until warm-season pastures reached sufficient sward height (6 in or 15.2 cm). When the forage was approximately 3 in (7.6 cm), horses were moved to a new section of pasture to graze. Fecal samples were collected manually from the rectum of all horses on days 0, 2, 4, and 6 after transitioning between the cool- and warm-season pastures.

“Regardless of whether the horses were transitioned from cool to warm grasses or warm to cool grasses, the fecal microbiota were relatively stable. Compared to other feed transitions with cereal grains, the horses in this study appeared to adapt well to the subtle dietary changes in this integrated rotation system,” Whitehouse explained.

In addition to increased pasture yield from early spring to late fall, access to quality pasture decreases the risk of colic, increases movement, improves respiratory health, and negates behavioral issues frequently associated with stalling, such as the development of stereotypies.

Whitehouse added, “Horses grazing pasture during the spring and fall when cool-season grasses are flourishing can benefit from supplementation with a hindgut buffer to help stabilize the intestinal environment. The potential for large intakes of rapidly fermentable fiber during these grazing periods can lead to excess gas production, a drop in hindgut pH, and loose manure. Other important management strategies to help minimize digestive disturbance include controlling rate of intake and access to lush pasture by use of a grazing muzzle or a drylot.”

 ^*Weinert-Nelson, J.R., A.S. Biddle, and C.A. Williams. 2022. Fecal microbiome of horses transitioning between warm-season and cool-season grass pasture within integrated rotational grazing systems. Animal Microbiome 4(1):41.