Efficiency of lysine utilization by growing steers1,2

Effect of ruminal ammonia supply on lysine utilization by growing steers1

“…The increases in N retention with Lys infusions indicated that steers were deficient in Lys in our research model, which was previously demonstrated by Batista et al (2015). By making a limiting AA (Lys) available for protein synthesis, whole-body protein deposition (N retention) was enhanced and less AA were diverted toward catabolism.…”

“…Considering that steers in our model were Lys deficient, the increases in plasma Lys with supplementation of 6 g/d Lys would contradict the prediction of Bergen (1979) that plasma AA should not increase until the supply of that specific AA exceeds the requirement for protein deposition, which Batista et al (2015) determined to be 9 g/d for maximal N retention. However, Batista et al (2015) reported increases in plasma Lys when 6 or 9 g/d of Lys was supplemented to steers under experimental conditions similar to our study. The increases in plasma Lys at supplementation amounts below the steers' requirement suggest that the prediction by Bergen (1979) is not applicable to plasma Lys in cattle.…”

“…By supplementing essential and nonessential AA (Table 2) above their suspected requirements, the confounding effects of other AA limiting protein deposition were avoided. The 6 g/d Lys treatment was designed to be less than the upper range of the linear response surface for N retention, which was established by Batista et al (2015) to be 9 g/d of supplemental Lys. This is important to ensure that responses to Lys supplementation in this study are within the linear response surface for N retention, which would allow estimates of Lys efficiency to be valid.…”

“…Because the ammonia loading could affect utilization of the basal dietary Lys as well as the supplemental Lys, both overall N retention and incremental efficiency would be expected to move in the same direction; the similarity in response supports the conclusion that the efficiency of Lys utilization for protein deposition was increased by greater ruminal ammonia loads. Batista et al (2015) observed that the incremental efficiency of Lys utilization was 40%. In that study, experimental conditions were essentially equivalent to our lowest urea treatment, which yielded an incremental efficiency of 51%.…”

“…Plasma urea was measured using the method of Marsh et al (1965). Plasma samples were prepared for AA analysis and analyzed as described by Batista et al (2015).…”