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Protein Reserves in the Lactating Dairy Cow¹

Animal Sciences Department, University of Kentucky, Lexington 40506

ABSTRACT

Protein reserves were measured in a nitrogen depletion-repletion experiment with 15 lactating Holstein cows. At 2 to 3 wk postpartum, cows were fed 14 kg/day of a ration containing 9% crude protein (dry matter). At the end of depletion (8 to 16 wk), animals were assigned randomly to rations containing 14, 18, or 22% crude protein. The amount of nitrogen repleted was measured by nitrogen 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. Nitrogen balance was affected by protein content of the ration. Means were .24, .43, and .60 g nitrogen per day per kg body weight⁷⁵. Urinary nitrogen increased with crude protein; however, fecal nitrogen was not affected. Milk nitrogen output was greater for the 18% group. Apparent means of digestibilities for dry matter and nitrogen 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 nitrogen per kg body weight⁷⁵. There was a greater amount of nitrogen 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 nitrogen.

¹ This manuscript (78-5-49) is published with the approval of the Director of the Kentucky Agricultural Experiment Station. Data are from the dissertation of the senior author, submitted to the Graduate School of the University of Kentucky in partial fulfillment of the requirements for the Ph.D. degree.

² Syntex Agribusiness, Inc., P. O. Box 1246, S.S.S., Springfield, MO 65805.

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