Greenhouse gases inventory and carbon balance of two dairy systems obtained from two methane-estimation methods

The adoption of carbon inventories for dairy farms in tropical countries based on models developed from animals and diets of temperate climates is questionable. Thus, the objectives of this study were to estimate enteric methane (CH₄) emissions through the SF₆ tracer gas technique and through equations proposed by the Intergovernmental Panel on Climate Change (IPCC) Tier 2 and to calculate the inventory of greenhouse gas (GHG) emissions from two dairy systems. In addition, the carbon balance of these properties was estimated using enteric CH₄ emissions obtained using both methodologies. In trial 1, the CH₄ emissions were estimated from seven Holstein dairy cattle categories based on the SF₆ tracer gas technique and on IPCC equations. The categories used in the study were prepubertal heifers (n = 6); pubertal heifers (n = 4); pregnant heifers (n = 5); high-producing (n = 6); medium-producing (n = 5); low-producing (n = 4) and dry cows (n = 5). Enteric methane emission was higher for the category comprising prepubertal heifers when estimated by the equations proposed by the IPCC Tier 2. However, higher CH₄ emissions were estimated by the SF₆ technique in the categories including medium- and high-producing cows and dry cows. Pubertal heifers, pregnant heifers, and low-producing cows had equal CH₄ emissions as estimated by both methods. In trial 2, two dairy farms were monitored for one year to identify all activities that contributed in any way to GHG emissions. The total emission from Farm 1 was 3.21 t CO₂e/animal/yr, of which 1.63 t corresponded to enteric CH₄. Farm 2 emitted 3.18 t CO₂e/animal/yr, with 1.70 t of enteric CH₄. IPCC estimations can underestimate CH₄ emissions from some categories while overestimate others. However, considering the whole property, these discrepancies are offset and we would submit that the equations suggested by the IPCC properly estimate the total CH₄ emission and carbon balance of the properties. Thus, the IPCC equations should be utilized with caution, and the herd composition should be analysed at the property level. When the carbon stock in pasture and other crops was considered, the carbon balance suggested that both farms are sustainable for GHG, by both methods. On the other hand, carbon balance without carbon stock, by both methods, suggests that farms emit more carbon than the system is capable of stock. - Highlights: • IPCC underestimate methane emissions from medium-, high-producing and dry cows; • IPCC equations overestimate methane produced by 200-kg heifers; • Discrepancies between IPCC and SF₆ were nullified due to the herd composition; • IPCC can be used to estimate carbon balance depending on the herd composition; • Carbon stock in pasture and crops helps to offset the CO₂e emissions.