Carbon mass balance in growing-finishing pigs

Carbon (C) is widely distributed in nature and forms more compounds than all other elements combined. However, when more C is released into the atmosphere as CO₂ and CH₄ than can be reabsorbed by plants the global ecosystem becomes imbalanced. Although pigs emit less CO₂ and CH₄ than ruminants, accurate accounting of the C mass balance and emissions in swine production systems is needed for developing feeding strategies to improve C use efficiency (CUE) and to accurately estimate the carbon footprint of pork supply chains. Improvement in CUE minimizes C losses into the environment, optimizes natural resource use, enhances food security, and improves the economic and environmental sustainability of pork production. Thermal combustion analysis is the most common method for determining total C concentrations in a wide variety of matrices. Analytical results of the C concentration in 80 feed ingredients were highly variable (0.02 to 77% C) within and among feedstuff classifications. There was a strong positive association between fecal C and gross energy (GE) excretion (r = 0.998; P < 0.001) and between urinary C and GE excretion (r = 0.934; P < 0.001), with C excretion corresponding to a daily loss of 10.55 kcal GE/g of C excreted in feces and a loss of 11.91 kcal GE/g of C excreted in urine. On a mass-balance basis, about 14% of dietary C intake is excreted in feces, 2% is excreted in urine, and 25% retained in the whole body as protein and fat. Depending on the method and assumptions used, exhaled C (i.e., CO₂ and CH₄) represents the greatest loss of C, representing 50% to 55% of dietary C intake. These results indicate that dietary CUE in growing-finishing pigs is relatively poor (about 25% of C intake) and may be further reduced in pigs subjected to heat stress, immune challenges, and other metabolic disruptions that occur under commercial production conditions.