Enteric methane produced by finisher pigs is affected by dietary crude protein content of barley grain based, but not by corn based, diets

Reducing the crude protein (CP) content in pig diets changes its dietary C and fiber content and fiber composition, and may affect CH4 and CO2 produced by the pigs. The objective was to study effects of decreasing CP concentration on CH4 production, C retention efficiency and performance by finisher pigs. Four diets were formulated being: conventional (HP) and CP reduced, amino acid supplemented diets (MP), based on either corn grain/soybean meal (CS) or barley grain (BB). Twelve female pigs were assigned to each of the base ingredients (i.e., CS or BB), and a crossover design with two periods was used to evaluate effects of CP reduced diets. To extend the range of dietary CP studied, the BB-HP diet was also tested against a low CP diet (i.e., BB-LP) based on barley grain plus free amino acids, in a crossover design with 2 periods using 12 additional female pigs. All diets were formulated to achieve equal intake of metabolizable energy and true available amino acids. The CP contents were 193 g/kg (BB-HP), 160 g/kg (BB-MP), 120 g/kg (BB-LP), 198 g/kg (CS-HP) and 178 g/kg (CS-MP). An N balance was completed over 7 days, and CO2, CH4, heat production and O2 consumption were measured in all pigs for 4 h using an open circuit respiration system. Eight pigs were also subjected to 24 h respiration measurements to derive the 4:24 h ratio for gas exchange. The CP level of the diet did not affect daily gain (i.e., 780 g/day; SE 15.9) or protein deposition. Carbon, lipid and energy retention increased (P<0.05) when dietary CP was reduced, while heat production increased (P=0.007). Dietary net energy increased (P=0.01) when dietary CP was reduced. Lowering dietary CP concentration reduced enteric CH4 (P=0.03), but did not affect expired CO2 (P=0.19). Corn grain based diets had lower expired CO2 (P=0.02) but similar CH4 (P=0.11) than BB diets. There was no effect of intake on CO2 and CH4 production. The CH4 production was more closely related to dietary CP than fiber content, and increased by 1.2 g/day (SE 0.36; P=0.02) per 10 g/kg increase in dietary CP content. Carbon and N excretion increased (P=0.001) by 56.6 g/day (SE 14.1) and 2.85 g/day (SE 0.44) per 10 g/kg increase in dietary CP, respectively. Dietary CP reduction is a realistic option to reduce environmental impacts of pig production without reducing animal performance. rticle is part of the special issue entitled: Greenhouse Gases in Animal Agriculture – Finding a Balance between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors; K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson.