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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 less than .01) with the MP diet. Nitrogen retention and serum urea-N values were lower for the ewes nursing T lambs. Plasma beta-hydroxybutyrate and serum glucose values were lower (P less than .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 less than .01) with time throughout lactation. Triiodothyronine (T3) concentrations were higher (P less than .01) in ewes nursing T lambs. Thyroxine (T4) was greater (P less than .04) with the LP diet and greater (P less than .01) for ewes nursing T lambs. The T4:T3 ratio was lower (P less than .02) in the ewes consuming LP. The portion of the ewe's retained N used for milk synthesis was lower (P less than .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.

Changes in the serum amino acids of ewes obtained with regulated protein intakes may be of value for indicating the status of protein metabolism during gestation. Thirty crossbred ewes in midgestation were assigned in equal numbers to three treatments of 12%, 9% and 7% crude protein, isocaloric diets fed al libitum. Serum amino acids were determined by reverse phase, high performance liquid chromatography. Several consistent amino acid changes were related to protein intake over the last half of gestation. Glutamine plus alanine were increased and the branched chain amino acids (isoleucine, valine and leucine) were reduced. These responses suggest that tissues, such as muscle, undergo catabolism during late gestation as a result of low protein intake to release glutamine and alanine for gluconeogenesis. Serum Leu was reduced by a decreased level of dietary protein indicating inadequate protein status. Lowering protein intake also induced a consistent increase in serine through the last half of gestation. Increasing serine values may be the result of induced synthesis by liver in response to the low protein intake. This possibility suggests that the protein status of gestating ewes can be evaluated by following serum amino acid changes and the release of amino acids from specific tissues such as liver and muscle can be monitored through changes in the circulating amino acid values.