Forty British Friesian calves aged approximately 90 days, mean initial live weight 107 kg, were divided into five groups and the animals in each group offered, ad libitum for 84 days, one of five diets based on maize silage. The basal diet of maize silage and 20 g urea/kg dry matter (diet 1) was supplemented with fish meal at 47 g/kg dry matter (diet 2) or with legume silage supplements at 310 g silage dry matter/kg dry matter. The silage supplements were red clover ensiled with formic acid (diet 3), red clover ensiled with formic acid plus formaldehyde at 30 g/kg crude protein (diet 4), or sainfoin ensiled with formic acid (diet 5). Diets 1 to 5 contained 23·7, 28·8, 28·3, 28·3, and 27·1 g total nitrogen/kg dry matter respectively.
The proportion of the total nitrogen insoluble in water, an indicator of the protein in the diet that was likely to escape degradation in the rumen, was 273, 356, 377, 413 and 339 g/kg for diets 1 to 5 respectively. Increasing the proportion of insoluble nitrogen in the diet by including fish meal or formaldehyde-treated red clover silage improved rates of live-weight gain and efficiency of feed conversion. Mean daily intakes of 15·2, 16·2, 16·2, 15·4 and 15·8 g digestible organic matter/kg live weight produced daily live-weight gains of 0·79, 1·05, 0·88, 0·93 and 0·83 kg/head by calves on diets 1 to 5 respectively. Diet 2 produced live-weight gains and gains per kg of digestible organic matter intake that were greater than those for diets 1, 3 or 5 (P<0·05), but not significantly greater than those for diet 4 (P<0·05). Calf live-weight gains were similar and satisfactory on the basal diet (maize silage and urea) and the diets with formic acid-preserved silage of red clover or sainfoin, but poorer than on the diet with 47 g fish meal/kg diet dry matter (P<0·05).