Protein Reserves in the Lactating Dairy Cow

Abstract

Protein reserves were measured in a N depletion-repletion experiment with 15 lactating Holstein cows. At 2-3 wk postpartum, cows were fed 14 kg/day of a ration containing 9% crude protein (dry matter). At the end of depletion (8-16 wk), animals were assigned randomly to rations containing 14, 18, or 22% crude protein. The amount of N repleted was measured by N balance. The animals were fed a complete ration of 60% corn silage and 40% concentrate dry matter. Intake was greater for the 14 and 18% protein groups during repletion. Milk production was greater for the 18% group, 26.5 kg/day compared with 22.8 and 21.9 kg/day for the 14 and 22% protein groups. N balance was affected by protein content of the ration. Means were .24, .43, and .60 g N/day per kg body wt.75. Urinary N increased with crude protein; fecal N was not affected. Milk N output was greater for the 18% group. Apparent means of digestibilities for dry matter and N were 62.0 and 63.0 for the 14% group, 64.3 and 71.7 for the 18% group, and 67.1 and 75.1 for the 22% group. The groups were repleted for a period of 12.4, 9.6 and 7.4 wk and accumulated 19.7, 29.2 and 31.7 g N/kg body wt.75. There was a greater amount of N repleted by the medium and high groups compared with the low-protein group. On a live weight basis, the treatment groups repleted an estimated 16.7, 25.1 and 27.1% of total body N.