The response of spring-calving cows to a high level of protein in the supplement given with grass silage during early lactation

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Abstract
A randomized block experiment, using 30 British Friesian cows, was used to assess the effects of offering supplements containing 172 and 234 g crude protein per kg dry matter in addition to grass silage which had a very high digestible organic matter in the dry matter of 769 g/kg. The animals, which had a mean calving date of 1 February, were blocked according to calving date and lactation number and allocated at random within each block to the two treatments. The treatments were applied from day 8 of lactation until the animals went to pasture on 13 April and both supplements were offered at the same rate of 7·6 kg per day. The total yields of milk produced during the experimental period were 1626 and 1650 ±38·4 kg and the mean daily yields during the final week were 24·8 and 25·6 ± 0·58kg for the medium and high protein supplements respectively. The mean live weights for the two groups were 524 and 544 ± 5·5 kg, and body condition score 2·9 and 3·3 ± 0·34 for the medium and high protein supplements respectively at the end of the treatment period. No significant treatment effects on milk composition were recorded at any stage during the experiment. Analysis of blood samples taken during the final week of the experiment showed plasma urea levels, at 220 and 290± 18 mg/1 for the medium and high protein supplements respectively, to be significantly different.