Nitrogen utilization in growing lambs fed oscillating dietary protein concentrations

The objective of this study was to determine the effects of oscillating dietary CP on N metabolism and microbial protein synthesis in growing lambs. Twenty Suffolk ram lambs (27.8 ± 2.3 kg initial BW) were used in a completely randomized design with a 24-d experimental period. The experimental treatments were a 95 g CP/kg DM (LOW; n = 4), 125 g CP/kg DM (MEDIUM; n = 4), 155 g CP/kg DM (HIGH; n = 4) or oscillating (OSC; LOW and HIGH diets oscillated on a 48-h basis; n = 8) CP diets. Lambs were adapted to experimental treatments for 16 d followed by 8 d of data collection. Nitrogen intake did not differ (P=0.50) between lambs fed MEDIUM (29.8 g/d) and OSC (28.6 g/d) diets. Increasing dietary CP concentrations linearly increased N retention (P<0.001), and urinary N (P<0.001) and fecal N (P=0.006) excretion. Retained N in lambs fed OSC (13.6 g/d) was higher (P=0.003) than in lambs fed MEDIUM (9.6 g/d). Urinary urea-N increased linearly (P<0.001) from LOW (2.3 g/d) to HIGH (6.5 g/d), and lambs fed MEDIUM excreted more (P=0.001) urea-N in urine than those fed OSC. Microbial non-ammonia N (NAN) supply (g microbial N/d) increased linearly (P<0.001) with N intake. When compared to those fed MEDIUM (7.9 g/d), lambs fed OSC (9.0 g/d) tended (P=0.09) to have a higher microbial NAN supply. In summary, these data indicate that feeding oscillating dietary CP concentrations improve N retention in ruminants, and the mechanism by which this occurs may involve, at least in part, an increase in microbial protein production.