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