When do Broodmares at Pasture Need Supplemental Feed?

When do Broodmares at Pasture Need Supplemental Feed?

If mares are grazing the pastures down to below 5 cm and/or getting thin, it's probably time to start supplementing with grain.

Photo: Anne M. Eberhardt/The Horse

Researchers know that boosting a grazing, lactating mare's energy intake isn’t always necessary, even though she has very high energy needs. It all depends on how much good-quality grass is available. So how much is enough? That’s what a team of French researchers recently set out to determine.

In their pioneering experiment, the team found a “cutoff point” for determining when grass alone isn’t enough (at least in their experimental setting) to meet mares' nutrient needs. Mares strip-grazing with 1- to 4-month-old weanling foals and changing high-quality pasture strips every two days need 66 grams of dry material (grass) per kilogram of body weight per day. Essentially, mares need access to roughly 39 kilograms (or 85 pounds) of dry material each day to obtain all their energy from pasture, said Claire Collas, PhD, researcher at French National Institute for Agricultural Research (INRA) in Saint-Genès-Champanelle. Collas presented her research at the 2015 French Equine Research Day, held March 12 in Paris.

That’s not how much the mare will consume, though; it’s how much needs to be available to the mare, Collas said. This amount takes into consideration a certain extent of loss—primarily from grass trampling, manure, and ground-level grass that is essentially unavailable to the horse, she said.

Unfortunately, that figure isn’t exactly easy for the common breeder to measure. So Collas offered a fairly reliable rule of thumb in the conditions of her experiment: Check the remaining grass height after the two days of grazing. If the mares are leaving 5 centimeters (cm) of grass or less (about 2 inches or less), she said, it’s probably time to offer a new area or to supplement to ensure they’re consuming adequate energy.

In their field test, Collas and her colleagues evaluated 18 mares residing in three different-sized pastures during the summer. The pastures contained the same kind of grass at the same height, but some had smaller surface areas than others (and, hence, less grass per mare). The low-quantity pasture had 62 m² (667 ft²) per mare per day, the medium level had 88 m² (947 ft²), and the high-quantity pasture had 122 m² (1,313 ft²). Half of the mares in each field received complementary barley feed daily (60% of energy requirements for lactation) while the others received no complementary feed.

Interestingly, all the mares, regardless of group, spent less time grazing when the grass was about 5 cm, she said. And once they’d grazed down to about 3 cm (a little over an inch), they spent even less time grazing. Studies of their feces indicated that they were ingesting less on the low-level grass compared to the medium-level grass, and less on the medium-level grass than on the high grass.

In a previous study, Collas demonstrated that lactating mares on unlimited pasture grass of high quality needed no complementary feed. The nonsupplemented mares in that study ate more grass than supplemented mares, Collas had said.

However, this new study looked at the realistic situation of pastures without unlimited grass. While all the study mares got their energy and protein requirements when grass was high, only the barley supplement group got enough energy on the medium- and low-level grasses, Collas said.

The team said further work is needed in other conditions (type of pastures, grazing management, sward quality, etc.) to determine the conditions under which energy supplementation is required.

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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