Carbon footprint of beef cattle response to digestibility and crude protein ratios

Beef production is recognized source of greenhouse gas (GHG) emissions. However, information about the mitigation strategies from beef production systems are scarce. We aimed to assess the effect of varying the diet digestibility (DE) and crude protein levels in the beef cattle diets on the carbon footprint (CF). We estimated the GHG emissions using the model described by Cardoso et al. (2016). It was calculated the CF for 5 ratios of DE 50, 55, 60, 65 and 70% and 5 ratios of CP 6, 8, 10, 12 and 14% in dry matter basis. The model simulates emissions from all important sources of methane (CH₄), nitrous oxide (N₂O), and carbon dioxide (CO₂) emitted in a beef cattle production system of whole cycle. The emissions sources include the enteric fermentation, manure, cropland used in feed production, fuel combustion, electricity, machinery and fertilizer usage. The CF is the net exchange of all GHG in CO₂ equivalents units per kg of carcass weight. The marginal physical productivity (MPP) approach was used to determine the sensitivity of DE and CP inputs for CF. This approach shows the change value for an increase unit in one of the DE or CP inputs, and the other factors of production are assumed to be constant. The CF were 43.21, 35.67, 30.59, 26.90 and 24.08 kg CO₂eq kg^-1 carcass from the lower to the higher DE level. Comparing the lowest and the highest DE level, the contribution of CH₄ emissions to the CF decreased from 93.1% to 86.7% while the participation of N₂O doubled from 5.1% to 10.2% and the CO₂ increased from 1.8% to 3.1%. The variation in the CP concentration increased the CF, and values corresponded to 31.83, 34.10, 35.40, 36.72 and 38.04 kg CO₂eq kg^-1 carcass from the lower to the higher CP concentration. In these scenarios, the N₂O emissions increased from 4.3% to 17.4% of CF. The MPP were -0.94, 0.023 and -0.91 for CH₄, N₂O and CF when it was evaluated the %DE effect, and 0, 0.65 and 0.65 for CH₄, N₂O and CF when CP levels were simulated. The simulation study indicates that improve the %DE can be an effective mitigation strategy in reducing GHG emissions. In this way, the forage genetic selection, substitution of the cultivar or species and supplementation aiming to increase feed digestibility implies in environmental gains in beef cattle production systems.