Effect of dietary energy on milk production and metabolic hormones in thin, primiparous beef heifers.

Thirty-six primiparous heifers were used to determine the effect of dietary energy on postpartum interval, milk production, serum concentrations of insulin, insulin-like growth factor I (IGF-I), growth hormone, and cerebrospinal fluid (CSF) concentrations of neuropeptide-Y (NPY). Low-quality hay was fed during the last trimester of pregnancy to achieve suboptimal calving weight (370 +/- 5 kg) and condition score (4.0 +/- .1). After calving, cows were allotted to one of four dietary treatments that differed in metabolizable energy. Experimental diets were fed at 2.5% of shrunk body weight and formulated to provide 1.8 (low), 2.1 (maintenance), 2.4 (maintenance high), or 2.7 (high) Mcal of ME/kg DM. Daily milk production was estimated at approximately 30, 60, and 90 d postpartum. Condition score change and weight change were defined as change from calving to d 90 postpartum. As energy intake increased, condition score change (P < .001), IGF-I (P < .001) and insulin (P < .01) increased and postpartum interval decreased (P = .04). No relationship existed between postpartum interval and CSF concentration of NPY (P > .1). Condition score change was positively associated with IGF-I and insulin (r = .71, P < .001; r = .38, P = .02, respectively) and negatively associated with GH (r = -.67, P < .001). Weight change and serum concentrations of GH did not differ (P > .10) among treatments. Increasing dietary energy intake was associated with a curvilinear increase in milk yield (P = .04) and percentage milk fat (P = .03) and a linear increase (P = .04) in energy available for milk production. Greater milk yield at d 30 was associated with a longer postpartum interval (r = .34, P < .05). In conclusion, a greater proportion of net energy is partitioned to maternal tissue realimentation when cows receive high compared to low-, maintenance-, or moderate-energy diets.