The effects of plane of nutrition and diet type on metabolic hormone concentrations, growth and milk production in high genetic merit dairy herd replacements

Abstract

AbstractSeventy high genetic merit Holstein heifers were used in two experiments to investigate (1) the effects of plane of nutrition and diet type during the pre-pubertal period and (2) the effects of plane of nutrition during the post-pubertal period on metabolic hormone concentrations, growth and milk production. In experiment 1, treatment 1 and 2 heifers were given food to achieve a live-weight gain of 0·70 and 0·95 kg/day from 3 to 10 months of age on a grass silage based diet, while treatment 3 heifers were given food to achieve 0·95 kg/day on a barley straw/concentrate diet. During the pre-pubertal period, heifers reared on treatment 1 had significantly higher growth hormone(GH) concentrations (ng/ml per 1 h) than heifers reared on treatment 2 (P < 0·01) and had significantly lower insulin concentrations than heifers reared on treatment 3 (P < 0·01). Heifers reared on treatment 1 had significantly lower insulin-like growth factor 1 (IGF-1) concentrations than those reared on treatment 3 (P < 0·01). At 10 months of age heifers reared on treatment 1 were of lower condition score (P < 0·01) than those on treatment 2 and had a smaller heart girth diameter (P < 0·01) than those on treatments 2 and 3. During the first lactation, milk yield and composition produced by the heifers was not significantly affected by treatment. In experiment 2, treatment A heifers were given, from 14 to 24 months of age, a low plane of nutrition to allow a live-weight gain of 0·65 kg/day on a grass silage and grass based diet during the winter and summer periods respectively. Treatment B heifers were kept on a high plane of nutrition to allow a live-weight gain of 0·90 kg/day on the same forage along with concentrate supplementation. During the rearing period, GH and IGF-1 concentrations were not significantly affected by treatment. Treatment A heifers weighed less before calving (P < 0·05), had a lower condition score (P < 0·01), and had a smaller heart girth diameter (P < 0·01) than those on treatment B. During the first 10 weeks of lactation, heifers on treatment A had a higher silage dry matter intake and lost less weight (P < 0·05) than those on treatment B, however, by 20 weeks of lactation these effects had disappeared. Milk yield and composition during the first lactation were not significantly affected by treatment. Overall, the findings of experiments 1 and 2 did not show any beneficial effects of higher weights at first calving in high genetic merit Holsteins and therefore indicate that accelerated growth in the pre- or post-pubertal period may not be required.