Impact of corn or soybean in crops and lactating cow diets on estimated greenhouse gas emission from Wisconsin certified organic dairy farms

Abstract Text: This study used a partial life cycle assessment approach to estimate the impact of feeding strategies and associated cropping systems on greenhouse gases emission intensity (GHG-EI, kg CO₂-eq/t energy corrected milk, ECM) from Wisconsin certified organic dairy farms. Gases and sources of emissions included in the study were: Nitrous oxide (N₂O) from fields (row crop and pasture), enteric methane (CH₄), and manure management (N₂O and CH₄). An earlier study had identified four clusters from a survey dataset of 69 organic dairy farms. In cluster 1, 2, 3, and 4 daily DMI of lactating dairy cows was 22.1, 15.2, 20.9, and 18.1 kg/d, amount of concentrate fed was 8.0, 2.0, 6.0, and 6.0 kg/d, time grazing on pasture was 39.2, 53.5, 38.6, and 47.7% of the year, and ECM was 6,657, 3,857, 7,666, and 5,495 kg/yr per cow, respectively. Three combinations of corn grain (CG) and soybean (SB) as concentrate (100%CG, 75%CG+25%SB, and 50%CG+50%SB) were assigned to each cluster to study the substitution effect of protein vs. energy supplementation. Overall, GHG-EI was 1,273 ± 235 kg CO2-eq/t ECM with contributions of 57.4, 34.1, and 8.5%, for enteric fermentation, manure management, and field emissions, respectively. There was a strong inverse relationship between level of production and GHG-EI, which averaged 1,209, 1,622, 996, and 1,264 kg CO2-eq/t ECM for cluster 1, 2, 3, and 4, respectively. Grazing time was positively related with GHG-EI in part because longer grazing time was associated with lower ECM production and increased N₂O emission from manure deposited on pasture during grazing. The GHG-EI was the greatest in Cluster 2 with 50%CG+50%SB and the lowest in Cluster 3 with 100%CG (1,635 vs. 983 kg CO₂-eq/t ECM). The GHG-EI was predicted to increase with increasing SB in all four clusters and on average GHG-EI was 1,260, 1,273, and 1,285 kg CO₂-eq/t ECM for 100%CG, 75%CG+25%SB, and 50%CG+50%SB, respectively. Planting soybean decreased N₂O emission from cropland due to lower intensity of N fertilization and greater reliance on biological N-fixation compared with planting corn. Lowering CG for more SB in the diet reduced enteric CH₄ emission (because of greater fat content in the latter) but increased N₂O emission from manure (because of greater CP content of the latter). This study suggested that growing and feeding CG or SB might explain only a small fraction of the large differences in emission observed among clusters.

Keywords:

carbon footprint, organic dairy farms, LCA