Low protein diets and rumen-protected conjugated linoleic acid increase nitrogen efficiency and reduce the environmental impact of double-muscled young Piemontese bulls

Effects of diets differing in crude protein (CP) concentrations, top-dressed or not with 80 g/d of rumen-protected conjugated linoleic acid (rpCLA), on N efficiency of double muscled (DBM) Piemontese bulls were studied using four experimental diets being: high CP, HP: high CP plus rpCLA, HPrpCLA: low CP, LP; low CP plus rpCLA, LPrpCLA. The HP diets (145 g/kg DM of CP) were similar those used commercially and the LP diets (108 g/kg DM of CP) were used to evaluate effects of lower dietary CP levels on intake, digestibility and body retention of nutrients, as well as N excretion. Forty-eight young bulls (237 +/- 24.4 kg body weight (BW)), housed in 12 pens were fed one of the four experimental diets. Bulls were slaughtered at 668 +/- .56.2 kg BW, after 332 days on their diet. Dry matter intake was measured daily on a pen basis and bulls were individually weighed monthly. Blood, rumen fluid and faecal samples were collected from each bull at 63,179 and 283 days. Empty body composition at the start and the end of the feeding period was estimated using equations developed on DBM bulls and from the composition of the 5th rib collected at slaughter. Nitrogen balance was computed using a mass balance approach. Lowering of the diet CP level reduced both blood urea N (P<0.001) and rumen ammonia N (P<0.001) concentrations, but did not influence organic matter digestibility or body retentions of protein or fat. Compared to HP, LP increased N efficiency (N retained/N consumed) from 0.17 to 0.23 (P<0.001), likely due to more extensive N recycling. Nitrogen efficiency was also influenced by a CPx rpCLA interaction (P=0.047), wherein bulls fed rpCLA used N more efficiently than bulls not receiving rpCLA when fed the LP diet, but had similar N efficiency when fed the HP diet. This suggests that CLA could exert some metabolic protein sparing effects, particularly under conditions of dietary CP shortage with respect to the bull requirements. The magnitude of the increase of N efficiency with LP compared to HP diets has a relevant impact on the DBM Piemontese bull production system, particularly where the load of N per unit land area is restricted by law. With respect to the conventional HP diet, use of LP without or with rpCLA would increase BW production per unit of N excreted by 31% and 43%, respectively. (C) 2012 Elsevier B.V. All rights reserved.