Symposium review: Comparisons of feed and milk nitrogen efficiency and carbon emissions in organic versus conventional dairy production systems.

Evaluation of feed efficiency (FE; calculated as energy-corrected milk yield/dry matter intake) and milk nitrogen efficiency (MNE; calculated as milk N yield/N intake) is needed to help farmers make decisions regarding the economic and environmental sustainability of dairy farms. Our primary objective was to compare FE and MNE data obtained from studies conducted with organic versus conventional dairy cows. Specifically, 3 data sets were constructed to meet this goal: (1) the organic Jersey data set (ORG-JE) built with studies (n = 11) done at the University of New Hampshire Burley-Demeritt Organic Dairy Research Farm (Lee, NH), (2) the conventional Jersey data set (CON-JE) constructed using 19 experiments reported in the literature, and (3) the organic non-Jersey-breed (mostly Holstein, Swedish Red, and Norwegian Red) data set (ORG-NJE) created with 11 published studies. Comparisons were made between ORG-JE and CON-JE and between ORG-JE and ORG-NJE. A second objective was to compare the enteric methane (CH4) emission data set from studies using organic Jerseys (n = 5) with those using conventional Jerseys (n = 4). Cows used in the ORG-JE data set had lower FE (-16%) and MNE (-15.5%) than cows used in the CON-JE counterpart, possibly because dry matter intake increased by an average of 10.4% in organic cows. Feed efficiency and MNE computed from cows belonging to the ORG-NJE data set were intermediate between ORG-JE and CON-JE. Measured CH4 intensity (g/kg of energy-corrected milk) from cows in the ORG-JE CH4 data set increased by 71% compared with that from cows in the CON-JE CH4 data set. Estimated FE and enteric CH4 emissions revealed that Wisconsin organic dairies with the heaviest reliance on forage sources and longest grazing time during the summer were the least feed efficient and emitted the greatest amount of CH4 per kilogram of energy-corrected milk at the animal and whole-farm levels. Overall, the comparisons of FE, MNE, and enteric CH4 emissions between organic and conventional dairies and within organic systems made in this symposium review should be interpreted cautiously because they are based on study means and small data sets. Research is needed to better characterize the performance, efficiency, profitability, and carbon emissions of forage-based organic dairies in the United States, including the fast-growing "grass-fed" segment, which relies exclusively on forage diets. The effect of large organic dairies on the economic and social sustainability of small and mid-size organic dairy operations nationwide also deserves further investigation.