PROTEIN RESERVES IN LACTATING DAIRY GOATS

Six primiparous Alpine does were confined to metabolism crates beginning 2 weeks postpartum. Initially, all were fed a 15.8% crude protein diet which met N.R.C. requirements for maintenance and lactation. Thereafter, subjects were fed an isocaloric diet containing 7.2% crude protein to deplete body protein, and then repleted with the higher protein diet. Nitrogen balances were determined by continuous collection of urine, feces and milk. Samples of these products were analyzed for nitrogen by the Kjeldahl method. Deuterium oxide (D2O) dilution was used to determine body composition. Nitrogen mobilization ranged from 0 to 10.8% of total body nitrogen. Urinary nitrogen output decreased 75% during the first 6 days of depletion and remained stable until repletion. Milk nitrogen decreased an average of only 31% during the depletion period with wide individual variation while fecal nitrogen declined 26%. Milk and fecal nitrogen declined gradually over the whole depletion period. The primary route of nitrogen conservation appeared to be an immediate decrease in urinary nitrogen, followed by a decline in milk production. Milk production capacities of tested does were related to the ability to mobilize protein or use tissue reserves. This has important implications for the study and management of protein nutrition in lactating ruminants. © 1990.