Measurement of the Voluntary Dry Matter Intake (V.D.M.I.) is difficult in horses and a number of techniques have been tried to obtain estimates in grazing horses. These generally range from 1.5% to 3.1% of bodyweight. The average figure is generally accepted as being equivalent to 2.0% of bodyweight except for lactating mares who will consume 2.8% of their bodyweight as feed dry matter. A 300kg weanling would therefore consume about 6kg DM/Day whereas a 550kg wet mare may eat a little over 15kg DM/Day in total. Note these figures include hard feed as well as forage.
Factors affecting forage intake are not completely understood but sward length, maturity, sugar content, pasture species, herbage quality and grazing activity of other horses nearby may all have an impact. Restriction of grazing time may have a limited effect on total intake and ponies restricted to 2-3 hours grazing per day were able to consume close to their total calculated pasture intake once they had recognized that grazing time was limited. For this reason, use of a grazing muzzle or strip grazing may be more suitable when restricted pasture intake is the aim.
For hay, the intake rates are similar, averaging around 2.0% for grass hay and 2.4% for long stem Lucerne hay. Rates are the same when hay is chopped as chaff but do increase if the hay is pelleted or turned into cubes due to increased bit rate decreased chewing time (NRC 6th Ed 2007)