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Leucine and protein metabolism in the lactating dairy cow mammary gland: responses to supplemental dietary crude protein intake

Published online by Cambridge University Press:  01 June 2009

Brian J. Bequette ,
John A. Metcalf ,
Diane Wray-Cahen ,
F. R. Colette Backwell ,
John D. Sutton ,
Michael A. Lomax ,
John C. Macrae  and
Gerald E. Lobley
Brian J. Bequette
Affiliation:
Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB, UK
John A. Metcalf
Affiliation:
Department of Biochemistry and Physiology, University of Reading, Reading RG6 2AJ, UK
Diane Wray-Cahen
Affiliation:
Department of Biochemistry and Physiology, University of Reading, Reading RG6 2AJ, UK
F. R. Colette Backwell
Affiliation:
Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB, UK
John D. Sutton
Affiliation:
Department of Agriculture, University of Reading, Reading RG6 2AT, UK
Michael A. Lomax
Affiliation:
Department of Biochemistry and Physiology, University of Reading, Reading RG6 2AJ, UK
John C. Macrae
Affiliation:
Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB, UK
Gerald E. Lobley
Affiliation:
Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB, UK
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Summary

Mammary gland protein metabolism, determined by an arteriovenous difference technique, was monitored in four Holstein-Friesian dairy cows in response to supplemental dietary protein (provided as rumen-protected soyabean meal) during late lactation (weeks 24–30). Each cow was offered two isoenergetic diets composed of grass silage (170 g crude protein/kg dry matter) plus either a low (108 g/kg) or medium (151 g/kg) crude protein concentrate in a single crossover design involving two 21 d periods. On day 21, arteriovenous measurements across the mammary gland were made during a 13 h continuous i.v. infusion of [1-13C]leucine and with frequent (2 hourly) milk sampling during the final 6 h. Although total milk yield was slightly increased (+1 kg/d) by protein supplementation, milk protein yield was not significantly affected. Whole body protein flux (protein synthesis plus oxidation) was not significantly affected by supplementation. Total mammary gland protein synthesis (milk plus non-milk protein) was also not affected by supplementation but on both diets gland synthesis was always greater (by 20–59%) than milk protein output. The fractional oxidation rate of leucine by the mammary gland was significantly increased by protein supplementation (0·047 v. 0·136). Although the enrichment of leucine in secreted milk protein continued to increase, the final value (at 13 h) was 0·94 of the arterial plasma free leucine plateau value (not significantly different), suggesting almost exclusive use of plasma free leucine for milk protein synthesis. Based on current feeding schemes for dairy cattle, a fixed proportion (0·65–0·75) of the additional protein intake (+490 g/d) should have been partitioned into milk protein. Instead, leucine oxidation by the mammary gland was increased. Whether oxidation of other amino acids was also enhanced is unknown but if amino acid oxidation and the ‘additional’ non-milk protein synthesis occurring in the gland are not crucial to milk synthesis, then by reducing such activities improvements in the efficiency of converting absorbed amino acid into milk protein can be achieved.

Type
Original Articles
Information
Journal of Dairy Research , Volume 63 , Issue 2 , May 1996 , pp. 209 - 222
Copyright
Copyright © Proprietors of Journal of Dairy Research 1996

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