Optimization of dairy heifer management decisions based on production conditions of Pennsylvania.

We used a dynamic programming model to determine optimum rearing decisions of dairy replacements. Heifers were described in the model by age, season, body weight, pregnancy state, and prepubertal growth rate. Prices and parameters were chosen to represent the dairy population of Pennsylvania. We calculated monthly costs and revenues from calf value, feed costs, veterinary costs, semen costs, carcass value, and full-grown heifer value. The model considered a stochastic variation in the onset of puberty, conception, involuntary disposal, and a seasonal variation in the prices of calves, heifers, and feed. Based on a critical prepubertal average daily gain of 0.9 kg/d and a maximum achievable postpubertal growth rate of 1.1 kg/d, the optimum practice resulted in an average age at first calving of 20.5 mo at a body weight of 563 kg. Discounted net returns equaled $107 per heifer per year. The optimum rearing practice was not sensitive to seasonal variation in prices. Nevertheless, the economic results per season of birth varied considerably; the highest income per heifer was obtained from heifers born in December ($142/yr), whereas those born in May yielded the lowest ($100/yr). Sensitivity analyses demonstrated a considerable influence of growth rate restrictions and variation in reproductive performance on both the optimal rearing practices as the expected net returns.