The effect of protein content of the supplement for dairy cows with accessad libitumto high digestibility, wilted grass silage

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Abstract
Grass silage with a dry-matter (DM) content of 253 g/kg and containing 142 g crude protein and 670 g digestible organic matter/kg DM was offered ad libitum to 56 first- and second-lactation British Friesian cows during the first 75 days post-calving. In addition the animals received supplementary concentrates containing either 95 (T1), 137 (T2), 174 (T3) or 209 (T4) g crude protein per kg fresh weight, with extracted soya bean meal being used as the protein source. All concentrates were offered at equal levels across treatments, the amounts being 8 kg for first- and 10 kg per day for second-lactation animals. The silage DM intakes recorded over the period were 514, 524, 548 and 545 ± 19·9 kg and milk outputs were 1349, 1451, 1533 and 1628 ±40·2 kg for treatments Tl to T4 respectively. The rate of live-weight change and the milk fat, solids-not-fat, protein, lactose and ash contents of the milk were not affected significantly by any of the treatments. Diet digestibilities were determined using three animals per treatment. The digestibility coefficients were for DM 0·738, 0·746, 0·776 and 0·772 ± 0·0088 and nitrogen 0·597, 0·641, 0·707 and 0·727 ±0·0126 for treatments T1 to T4 respectively.