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 193g/kg (BB-HP), 160g/kg (BB-MP), 120g/kg (BB-LP), 198g/kg (CS-HP) and 178g/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 4h using an open circuit respiration system. Eight pigs were also subjected to 24h respiration measurements to derive the 4:24h ratio for gas exchange. The CP level of the diet did not affect daily gain (i.e., 780g/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 1.2g/day (SE 0.36; P=0.02) per 10g/kg increase in dietary CP content. Carbon and N excretion increased (P=0.001) by 56.6g/day (SE 14.1) and 2.85g/day (SE 0.44) per 10g/kg increase in dietary CP, respectively. Dietary CP reduction is a realistic option to reduce environmental impacts of pig production without reducing animal performance. This article 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.