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

摘要

Evaluation of feed efficiency (FE; calculated as energy-corrected milk yield/dry matter intake) and milknitrogen efficiency (MNE; calculated as milk N yield/Nintake) is needed to help farmers make decisions regardingthe economic and environmental sustainability ofdairy farms. Our primary objective was to compare FEand MNE data obtained from studies conducted withorganic versus conventional dairy cows. Specifically, 3data sets were constructed to meet this goal: (1) the organicJersey 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) constructedusing 19 experiments reported in the literature, and (3)the organic non-Jersey-breed (mostly Holstein, SwedishRed, and Norwegian Red) data set (ORG-NJE) createdwith 11 published studies. Comparisons were madebetween ORG-JE and CON-JE and between ORG-JEand ORG-NJE. A second objective was to compare theenteric methane (CH_4) emission data set from studiesusing organic Jerseys (n = 5) with those using conventionalJerseys (n = 4). Cows used in the ORG-JE dataset had lower FE (−16%) and MNE (−15.5%) thancows used in the CON-JE counterpart, possibly becausedry matter intake increased by an average of 10.4% inorganic cows. Feed efficiency and MNE computed fromcows belonging to the ORG-NJE data set were intermediatebetween ORG-JE and CON-JE. Measured CH_4intensity (g/kg of energy-corrected milk) from cowsin the ORG-JE CH_4 data set increased by 71% comparedwith that from cows in the CON-JE CH4 dataset. Estimated FE and enteric CH_4 emissions revealedthat Wisconsin organic dairies with the heaviest relianceon forage sources and longest grazing time duringthe summer were the least feed efficient and emittedthe greatest amount of CH_4 per kilogram of energycorrectedmilk at the animal and whole-farm levels.Overall, the comparisons of FE, MNE, and enteric CH_4emissions between organic and conventional dairies andwithin organic systems made in this symposium reviewshould be interpreted cautiously because they are basedon study means and small data sets. Research is neededto better characterize the performance, efficiency, profitability,and carbon emissions of forage-based organicdairies in the United States, including the fast-growing“grass-fed” segment, which relies exclusively on foragediets. The effect of large organic dairies on the economicand social sustainability of small and mid-sizeorganic dairy operations nationwide also deserves furtherinvestigation.