Production characteristics of high-yielding dairy cows

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Abstract
Understanding the relationships between food intake, milk output and body condition in high-yielding dairy cows is crucial in determining suitable management strategies. During two winter feeding periods 38 and 37 cows were individually fed, to appetite, complete diets which on average contained 11·7 MJ metabolizable energy per kg dry matter and comprised grass silage, concentrate meal and brewers' grains (draff). The groups' mean 305-day yield was 7 240 kg (s.d. 1 281) with 42 g (s.d. 4·3) fat per kg. Regression analysis was carried out to describe dry-matter intake both for 26 weeks post calving and for four successive 6-week periods from calving. The final equations, which had a residual s.d. of 0·07 to 0·10 of the observed intake, included milk yield, cow size and a measure of body-condition change. The cows were divided into three groups (high, medium and low) on two criteria: (1) mean milk yield (MJ/day) during the first 26 weeks of lactation and (2) post-calving backfat index determined ultrasonically. Differences were found between milk-yield groups from gross efficiency (milk yield (MJ)/energy intake (MJ metabolizable energy)) (P < 0·001), mean metabolizable energy intake (MJ/day) (P < 0·01), dry-matter intake as a proportion of live weight (P < 0·05), and post calving live weight (kg) (P < 0·05). Differences were found between backfat-index groups for maximum backfat loss and loss to day 42 (P < 0·001); also for mean live weight during the 26 weeks and post calving live weight (P < 0·001), dry-matter intake as a proportion of live weight (P < 0·05) and lactation number (P < 0·05). Interactions were found between the milk yield groups and backfat groups for milk yield (P < 0·01) and gross efficiency (P < 0·05) with the fattest group containing the highest and lowest yields and efficiencies.