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Nitrogen Metabolism by Lactating Ewes and Their Lambs

U.S. Department of Agriculture, Beltsville, MD 20705

Abstract

Twenty multiparous ewes and their newborn lambs were assigned to a 2 x 2 factorial experiment in which ewes were fed ad libitum either a moderate (MP, 15%) or a low (LP, 10%) CP diet and nursed either twin (T) or single (S) lambs. Nitrogen (N) balance trials were conducted on both the ewes and lambs during wk 2, 4 and 8 of lactation. Nitrogen balance, N digested and the portion of digested N retained were greater (P < .01) with the MP diet. Nitrogen retention and serum urea-N values were lower for the ewes nursing T lambs. Plasma ²-hydroxybutyrate and serum glucose values were lower (P < .01) for ewes fed the LP diet, and certain plasma amino acids and the ketogenic amino acids were lower with the LP diet. Insulin increased (P < .01) with time throughout lactation. Triiodothyronine (T₃) concentrations were higher (P < .01) in ewes nursing T lambs. Thyroxine (T₄) was greater (P < .04) with the LP diet and greater (P < .01) for ewes nursing T lambs. The T₄:T₃ ratio was lower (P < .02) in the ewes consuming LP. The portion of the ewe's retained N used for milk synthesis was lower (P < .01) with the MP diet. Ewes fed LP and nursing T lambs utilized all of their retained N plus a portion of their body protein reserves for milk production by the 2nd wk of lactation. Body weights of creep-fed lambs were not changed by protein content of the ewe's diet.

¹ Ruminant Nutr. Lab., Livest. and Poult. Sci. Inst., Beltsville, MD 2070S.

² Statistical Consulting Analysis Team, ARS, Beltsville, MD 20705.

³ We gratefully acknowledge the technical assistance of Jill Bohrer and A. S. Kozak, Ruminant Nutr. Lab., Livest. and Poult. Sci. Inst., Beltsville, MD 20705.