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

ABSTRACT 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.

Implications

Feeding high-energy diets postpartum reduces the negative effects of prepartum nutrient restriction but does not completely reverse those effects. Furthermore, increasing dietary energy intake increases net energy required for lactation. Consequently, moderate increases in energy intake through supplementation may be somewhat inefficient if the goal is to improve energy status of thin heifers. More research is necessary to better understand the relationships between nutrient intake and utilization, milk production, and reproduction in beef cattle, which collectively dictate our ability to optimize calf growth and reproductive efficiency while minimizing production costs.

Key Words: Beef Cows, Energy Balance, Milk Production, Postpartum Interval

© 2000, by the American Society of Animal Science. All rights reserved.

J. Anim. Sci. 2000. 78:530-538