EFFECT OF AMINO-ACID SUPPLEMENTATION ON WHOLE-BODY PROTEIN-TURNOVER IN HOLSTEIN STEERS

We used the [N-15]glycine single-dose urea end-product technique to me asure whole-body protein turnover-in six Holstein steers (250 +/- 18 k g). Steers were implanted with Revalor-S and continuously infused abom asally with water(4 L/d) or amino acids (AA; in 4 L/d water) In a cros sover experiment (two 14-d periods). The AA infusion contained the fol lowing (gld): lysine (5.3), methionine (3.3), threonine (3.2), tryptop han (1.0), histidine (2.1), and arginine (5.5). Steers were fed a diet containing 85% rolled corn, 10% prairie hay, and 1.1% urea (DM basis) at 2.16% of body weight. Nitrogen retention tended (P = .15) to incre ase with AA infusion, from 27.9 to 32.9 g N/d. Amino acid infusion num erically increased whole-body protein turnover from 168.6 to d, and pr otein degradation from 124.7 to 136.4 g N/d. Enhanced protein accretio n may have resulted fi-om a larger increase in protein synthesis than in degradation. The tendency fbr increased N retention is interpreted to suggest that the implanted, lightweight; Holstein steers fed a corn -urea diet in our study were able to respond to AA supplementation, su ggesting that at least one of the infused AA was Limiting in the basal diet. Protein turnover data suggest that cattle, like other animals, may increase protein synthesis and protein degradation in response to supplementation with limiting AA. The [N-15]glycine single-dose urea e nd-product technique for measuring whole-body protein turnover in catt le may be useful.