ABSTRACTTwo grass sUages made in June and two made in September, all from the same field, were compared in a 16‐week winter‐feeding experiment with 12 Ayrshire cows. The silages were fedad lib. with a supplement of barley and groundnut eake. One of the sUages made in June and one made in September had been treated with 1/2 gallon of formie acid per ton of herbage when cut, whereas the other two silages were untreated. The contents of digestible organic matter in the silage DM made with and without the acid additive were 67.4 and 63.8%, respectively, in the June silage, and 66‐1 and 62.7% in the September silage. The intakes of silage and total DM were higher in the treatments containing the acid‐treated silages than in those containing the untreated silages. The mean daily milk yields from cows fed on the sUages made with and without the additive were 36.3 and 33.8 lb (16.5 and 15.3 kg), respectively, for silages made in June, and 35.4 and 34.1 lb (16.0 and 15.5 kg) for those made in September. The solids‐not‐fat (S.N.F.) contents of the milk averaged 8.60 and 8.50% respectively, from the silages with and without the additive. It is concluded that the silage with the formic‐acid additive was superior to the untreated silage as a feed f