1. Experiment 1. Six treatment groups of one British Friesian and four South Devon × British Friesian bulls, initially 432 kg mean live weight and aged 491 days, were offered individually maize or grass silage ad libitum plus 0, 5 or 10g barley dry matter per kg live weight daily for 80 days. The silages had similar digestible dry matter and estimated metabolizable energy contents but the grass silage contained more ammonia and acetic, propionic and butyric acids. Mean values for groups receiving respectively maize and grass silage diets were for dry-matter intake 17·7, 20·3, 20·4 and 13·0, 16·6, 18·7 g/kg live weight and for live-weight gain 1·00, 1·32, 1·46 and 0·65, 0·98, 1·22kg/day. Significantly more maize than grass silage dry matter was eaten when the silages were given alone and dry-matter intakes, live-weight and carcass gains were greater for maize silage diets. Dry-matter intake, live-weight and carcass gains, efficiency of feed use and carcass quality significantly improved when barley was given.
2. Experiment 2. Six groups of five British Friesian bulls, initially 418 kg mean live weight and aged 474 days, were offered individually maize silage ad libitum with either urea or one of two quantities of aqueous ammonia mixed in at the time of feeding, plus 0 or 5 g barley dry matter per kg live weight daily for 90 days. The urea and ammonia-treated silages contained 125, 124 and 148 g crude protein per kg dry matter respectively, with pH values of 3·8, 3·9 and 4·3, and when given alone or with barley mean daily intakes (g dry matter per kg live weight) were 17·1, 18·6 for urea-treated silage diets, and 17·8, 18·8 and 16·9, 19·1 respectively for ammoniatreated silage diets. Live-weight gains were 0·69, 0·94, 0·63, 1·09, 0·64 and 1·07 kg/day. Ammonia treatment had no effect on intake or live-weight gain. Live-weight and carcass gains and carcass quality improved when barley was given.
3. The maize silage offered in Experiment 1 contained similar metabolizable energy but more starch than that in Experiment 2 and was used more efficiently for live-weight gain.