Nitrogen and lysine utilization efficiencies, protein turnover, and blood urea concentrations in crossbred grower pigs at marginal dietary lysine concentration

Nitrogen utilization efficiency (NUE) and lysine utilization efficiency (LUE) are key indicators of sustainable pork production and vary depending on nutritional and non-nutritional factors. The objective was to study NUE and LUE together with concentrations of blood urea nitrogen (BUN) and other metabolites in growing pigs fed diets with marginal Lys concentrations at 11-13 wk (40.5 kg mean BW) and 14 to 16 wk (60.2 kg mean BW). The cereal grain-soybean meal-based diets contained 10.6 and 7.9 g Lys/kg DM in periods 1 and 2, respectively. Feed intake and BW were measured for 508 individually penned pigs, and blood samples were collected 5 h after morning feeding at weeks 13 and 16. A subgroup of 48 barrows was used in a nitrogen (N) metabolism trial at weeks 13 and 16. In this subgroup, the mean N retention of pigs (27.3 g N/d) and mean LUE (70%) were not different between the periods, but NUE was higher in period 1 (47%) than in period 2 (43%) (P < 0.001). After administration of a single dose of N-15 labeled glycine and measurement of N-15 recovery in urine, the calculated whole-body protein turnover did not differ between the periods. The rate of protein synthesis was positively correlated with NUE (P < 0.001), but protein degradation was not. Excretion of urea-N in urine accounted for 80% of the total urinary N and was positively correlated with BUN. The N retention of all 508 pigs was estimated using an equation that was derived from the N metabolism data. N retention was on average 31.4 g/d, equal in both periods, and higher in barrows than in gilts in period 2, but not in period 1 (P = 0.003). The calculated NUE was, on average, 47% and was lower in barrows than in gilts (P < 0.001) and higher in period 1 than in period 2 (P < 0.001). The calculated LUE was, on average, 71%, and was lower in barrows than in gilts in period 2, but not in period 1 (P < 0.001). The BUN concentration was higher in barrows than in gilts (P < 0.001) and higher in period 1 than in period 2 (P < 0.001). BUN concentration was negatively correlated with NUE in Periods 1 (r = -0.50) and 2 (r = -0.15) (P < 0.05). We concluded that the maximum LUE was in the range of 70-72% under the conditions of this study, and only small differences between the periods and sexes existed. Protein synthesis, rather than degradation, appears to affect NUE. BUN concentration may be useful for estimating NUE in a large group of animals fed a diet with a marginal Lys concentration. The mean utilization of lysine by growing pigs can reach 72% when fed diets containing cereal grains and soybean meal. Nitrogen utilization varied among pigs and was associated with blood urea concentration and whole-body protein synthesis. Lay Summary Increasing the utilization of protein and amino acids by pigs is an effective tool for improving the sustainability of pork production. This study showed that the utilization of lysine, the limiting amino acid in pigs, can reach 72% in growing pigs aged 11-16 wk when fed diets based on cereal grains and soybean meal. The average utilization of total nitrogen was 47%, with high variation among the pigs. Higher nitrogen utilization was associated with lower concentration of urea in the blood. This indicates that the blood urea concentration is an easy-to-determine proxy trait for nitrogen utilization in pigs when the concentration of lysine in the feed is low. Only small differences were observed between gilts and barrows during the investigation period. Nitrogen utilization increased when pigs had a higher rate of whole-body protein synthesis, which was associated with anabolic hormone concentrations in the blood.