Can dietary manipulations improve the productivity of pigs with lower environmental and economic cost? A global meta-analysis

Inappropriate management of pig manure contributes considerably to pollution of waterbodies by nitrogen (N) and phosphorus (P), and to air pollution by ammonia (NH₃) and hydrogen sulfide (H₂S) emissions. Dietary manipulation is recognized as a possible pollution mitigation measure, but it may affect pig growth and thereby production costs. Here we present a global meta-analysis of the effects of dietary manipulation on nutrient (N and P) excretion, gaseous (NH₃ and H₂S) emissions from manure, and growth performance of pigs, using data from 245 published studies. Four groups of dietary manipulation were distinguished, namely i) lowering dietary crude protein (CP) content, (ii) supplementing exogenous enzymes, (iii) supplementing fermented feed ingredients, and (iv) supplementing other additives (e.g. fermentable carbohydrates, acidifying agent/salts and probiotics) in feed. In addition, the cost-effectiveness of dietary manipulations was evaluated, expressed as US $ per kg N excretion abated. Results show that lowering CP content significantly reduced both total N excretion (28.5%) and NH₃ emissions (34.4%). Addition of protease reduced N excretion (18.2%) but did not affect NH₃ emissions. Supplementing other additives simultaneously reduced NH₃ emissions (21.5%) and H₂S emissions (23.2%). Adding phytase to feed significantly decreased total P excretion by 31.4%. Diets with fermented feed ingredients tended to decrease N excretion and emissions, but this effect was not statistically significant. All dietary manipulations significantly improved the growth performance regarding the weight gain and feed efficiency, except for lowering CP content. But lowering dietary CP content within a moderate level in combination with adding additional amino acids did not impair pig growth. The cost-effectiveness analysis indicated that various diary manipulation measures were economically beneficial to farmers through improved feed-to-meat conversion efficiency. Our results can support to the design of proper dietary formulations so as to simultaneously reduce N and P excretion and associated emissions, meanwhile enhance the growth performance of pigs with lower economic cost.