Effect of dairy production system, breed and co-product handling methods on environmental impacts at farm level
Résumé
Six dairy farms with the same on-farm area and milk production were compared. One farm (G-No) used grass as the sole forage for a herd of Normande cows, a dual-purpose breed. Three farms, with Holstein cows, varied forage for the herd from grass only (G-Ho) to low (G/LM-Ho) or high (G/HM-Ho) proportion of maize silage in the total forage area. Finally, two farms based on G/LM-Ho and G/HM-Ho systems aimed to increase omega-3 fatty acids in the winter diets of cows (G/LM/O3-Ho, G/HM/O3-Ho). Allocation methods (biophysical, protein, economic allocation) and system expansion applied for co-product (milk and meat) handling were examined. The impact categories considered were climate change, climate change including the effects of land use and land use change (CC/LULUC), cumulative energy demand, eutrophication, acidification and land occupation. The impacts per kg of fat-and-proteincorrected milk (FPCM) of G-No were highest, followed by those of G-Ho, G/LM-Ho and G/HM-Ho, regardless co-product handling methods and impact categories (except for eutrophication). CC/LULUC per kg FPCM of G/LM/O3-Ho and G/HM/O3-Ho were both 1% and 3% lower than those of G/LM-Ho and G/HM-Ho, respectively, but other impacts were higher. With system expansion, impacts per kg FPCM were lower than when allocation methods were used. Enteric fermentation was the greatest contributor (45e50%) to CC/LULUC, while grass production was the most important contributor to other impacts. The highest CC/LULUC (for G-No) can be explained by (1) G-No having the lowest milk yield/cow (though it produced the most meat) and (2) the fact that grass required more N fertiliser, but had lower yields than silage maize, even though grassland sequestered C. Among Holstein systems, increasing cow productivity by increasing feed intake (including maize silage and supplementing with concentrate) decreased impacts of milk. Reducing replacement rate and age of first calving also decreased impacts of milk. Increasing cow productivity reduced the amount of on-farm area required to produce a given amount of milk. Thus, the “liberated” on-farm area of Holstein systems was used to produce cash crops, and total impacts of these systems were lower than those of G-No (except for eutrophication and land occupation).