Quantification of mobilization of body nitrogen and protein requirements of dairy goats in early lactation

Abstract

We aimed to trace the trajectory of body nitrogen status and estimate the protein requirements of dairy goats slaughtered at fixed time intervals from parturition to 56 days of lactation. We used 51 multiparous Alpine dairy goats to determine the animals' body nitrogen. After parturition, three goats (baseline) were slaughtered to estimate the initial body composition of the animals that remained in the experiment. Forty-eight goats, initial body mass of 57.2 (standard deviation = 8.4 kg) and parity number of two to four, were assigned to a completely randomized design. The treatments were the eight subsequent weeks of lactation (7th, 14th, 21st, 28th, 35th, 42nd, 49th, and 56th days after parturition). Six goats were slaughtered per week in order to quantify the body's nitrogen. All goats were twice daily milked. Urine and feces were sampled from six goats to determine nitrogen excretion. We observed that the metabolizable protein for maintenance (MPm) was 3.8 g/BW3/4. The net protein for maintenance (NPm) and net protein for lactation (NPL) decreased by 24% and increased by 32.7%, respectively, with the lactation advance. The efficiency of using protein for lactation was estimated through the reciprocals 1/β2 from a multiple linear regression equation yielding 0.74. The empty body weight (EBW) (P < 0.0001), carcass (P = 0.0002), and non-carcass components (P < 0.0001) presented a linear decrease, whereas the mammary gland mobilization had curvilinear behavior (P = 0.0359). In conclusion, dairy goats at the beginning of lactation mobilize nitrogen from the carcass and non-carcass components similarly. In the first 56 days of lactation, dairy goats mobilized an average of 8.0 ± 0.2 g of EBW nitrogen/day. The net protein required for maintenance is 0.4 ± 0.2 g/BW3/4/day, and the net protein required for lactation decreases 0.5 ± 0.3 g/kg milk/day during the first eight weeks of lactation.