EFFECT OF PLANE OF NUTRITION BETWEEN 6 AND 16 MONTHS OF AGE ON BODY-COMPOSITION, PLASMA-HORMONE CONCENTRATIONS AND 1ST-LACTATION MILK-PRODUCTION IN HOLSTEIN HEIFERS

In a completely randomized design, 47 dairy heifers (6-8 mo of age) were assigned to a low (L), medium (M) or high (H) plane of nutrition to determine its effect on body composition (Slaughter Group, n = 23), subsequent first lactation (260 d) milk production (Production Group, n = 24) and blood somatotropin and insulin concentrations (n = 12 from each Slaughter and Production group). Actual daily gains between 6 and 16 mo of age were 611 g (L), 737 g (M) and 903 g (H). The proportion of carcass fat in heifers sacrificed at 16 mo increased while the proportion of lean and bone tissue decreased with increasing plane of nutrition. Concentrations of somatotropin measured at 9 and 14 mo of age decreased but insulin did not differ (9 mo) or tended (P < 0.10) to increase (14 mo) with increasing feeding level. Insulin concentrations at 9 and 14 mo of age were correlated positively with mammary crude protein and dry fat-free tissue at slaughter (9 mo, P < 0.05; 14 mo, P < 0.07), implying that insulin may play a role in mammogenesis in vivo. Plane of nutrition during the rearing period did not significantly affect production and composition of milk in the first lactation. It is speculated that this may be due to the fact that the heifers weighed approximately 200 kg at the start of the experiment, because recent research has demonstrated that the critical weight range during which mammary gland development is affected by plane of nutrition is between 90 and 200 kg.