The effect of feed form and crude protein content on nitrogen digestibility in broilers

Nitrogen (N) excreted by poultry is converted to ammonia (NH3) which presents an environmental risk and a health risk to the farmer and animals. A reduction in dietary crude protein (CP) can effectively reduce N excretion in broilers provided that amino acid (AA) requirements are met. Feed form influences gastrointestinal tract development, feed and water intake and therefore the N and moisture level of the excreta. The aim of this study was to investigate the extent to which feed form and CP content influence N digestibility broilers. In total, 72 30-day-old Ross 308 male birds were assigned to 36 digestibility cages. Birds received diets with 19.5% CP (control), a 1.5 and 3.0% reduction in CP, while digestible AA:Lysine ratios were maintained. Each treatment was fed in mash and pellet form, which resulted in a 3×2 (CP level x feed form) experimental design with 6 treatments, each with 6 replicates. The study consisted of an adaptation and excreta collection period of 4 days each. Excreta was analysed for dry matter (DM), total N, total ammonium N (TAN) and uric acid (UA). Feed and water intake were also recorded. After the collection period, treatment diets containing 0.4% titaniumoxide (TiO) as an indigestible marker were fed for 3 days. To calculate apparent ileal N digestibility (AID), ileal digesta samples were analysed for total N and TiO. Data were analysed with linear mixed models using R 3.5.1 for Windows. Results were expressed as lsmeans and SE of those means. Pellets led to a higher feed (P<0.001) and water intake (P=0.018) compared to mash, while water to feed intake ratio was higher for control than for reduced CP treatments (P=0.003). Excreta moisture content did not differ between treatments, while CP (P<0.001) and UA (P<0.001) decreased with decreasing dietary CP. Faecal TAN was higher for pellets than for mash (P=0.002), while for UA the opposite was observed (P=0.001). Faecal CP digestibility (corrected for UA) was higher for mash than for pellets (P=0.006). Compared to the control, AID decreased with the 1.5 and 3.0% reduced CP treatments (P<0.001). N retention increased with a 3.0% CP reduction and was higher for mash than pellets (P=0.034). Although mash diets resulted in better N digestibility and retention compared to pellets, these results suggest that a dietary CP reduction within both mash and pelleted diets increased N retention and decreased excreta UA in broilers, resulting in lower levels of NH3 emitted from broiler litter.