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

Seventy 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 p ostpubertal period on metabolic hormone concentrations, growth and milk pro duction. In experiment 1, treatment 1 and 2 heifers were given food to achi eve a live-weight gain of 0.70 and 0.95 kg/day from 3 to 10 months of age o n a grass silage based diet, while treatment 3 heifers were given food to a chieve 0.95 kg/day on a barley straw/ concentrate diet. During the pre-pube rtal period, heifers reared on treatment 1 had significantly higher growth hormone (GH) concentrations (ng/ml per 1 h) than heifers reared on treatmen t 2 (P < 0.01) and had significantly lower insulin concentrations than heif ers reared on treatment 3 (P < 0.01). Heifers reared on treatment 2 had sig nificantly lower insulin-like growth factor 1 (IGF-1) concentrations than t hose reared on treatment 3 (P < 0.01). At 10 months of age heifers reared o n treatment 2 were of lower condition score (P < 0.01) than those on treatm ent 2 and had a smaller heart girth diameter (P < 0.01) than those on treat ments 2 and 3. During the first lactation, milk yield and composition produ ced by the heifers was not significantly affected by treatment. In experime nt 2, treatment A heifers were given,from 14 to 24 months of age, a low pla ne of nutrition to allow a live-weight gain of 0.65 kg/day on a grass silag e and grass based diet during the winter and summer periods respectively. T reatment B heifers were kept on a high plane of nutrition to allow a live-w eight gain of 0.90 kg/day on the same forage along with concentrate supplem entation. During the rearing period, GH and IGF-1 concentrations were not s ignificantly affected by treatment. Treatment A heifers weighed less before calving (P < 0.05), had a lower condition score (P < 0.01), and had a smal ler heart girth diameter (P < 0.01) than those on treatment B. During the f irst 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, ho wever, by 20 weeks of lactation these effects had disappeared. Milk yield a nd composition during the first lactation were not significantly affected b y 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 po st-pubertal period may not be required.