1. The primary growth of a tetraploid red clover was cut from 17 to 19 June and ensiled with formic acid at 3·81/t fresh crop (F), equal volumes of formic acid and formalin at 9 11/t (67 g formaldehyde per kg crude protein) (FF) or wilted and ensiled without additive (W). The silages were given ad libitum either alone or with supplements of dried grass or barley/fish meal at 6·7 g dry matter per kg live weight to 45 British Friesian male castrates, initially 3 months old and 104 kg live-weight.
2. The use of formaldehyde reduced the concentration of fermentation acids and of ammonia-nitrogen in the silage. Wilting the crop increased silage dry-matter content by 74g/kg fresh weight but this silage had the highest concentration of fermentation acids and of ammonia-nitrogen.
3. Digestibilities of diets containing silage FF were lower than those of diets containing silages F and W except when silage FF was supplemented with dried grass (interaction P <0·05).
4. The intake of silage W was on average higher than that of silages F and FF (P < 0·001). Supplements of barley/fish meal and dried grass depressed silage intake to a similar extent (P <0001). When silages were given as the sole feed, calves consuming silage F had higher live-weight gains than calves given silages FF and W (P <001). This effect was not apparent when silages were supplemented with barley/fish meal, but with a supplement of dried grass the calves given silage FF grew more slowly than those given silages F and W.
5. The results are discussed in relation to the possible effect of treatments on the supply of rumen-degradable protein and of amino acids to the animal.