Substitution of maize silage with sorghum silages in cow ration: effect on global warming potential of milk production

About 45% of Green House Gas (GHG) emission from livestock sector is methane. In cow milk production, enteric emissions load for nearly 45% on the total GHG emissions and CH4 represents about 50% of GHG from enteric and manure fermentations. Sorghum is growing in popularity as alternative silage crop for dairy cattle feeding, because of lower requirements of water and agronomical inputs in comparison to maize. The aims of the study were to estimate the e ects of replacing maize silage with sorghum grain and sorghum forage silages on the global warming potential (GWP) of milk production and to compare the results obtained using both estimated and in vivo measured enteric methane emissions. The study compared three di erent scenarios where cropping systems and cow diets were based on maize silage (MS), whole plant grain sorghum silage (WPGS) and forage sorghum silage (FS). The GWP (kg CO2 eq. 100-year horizon) of milk production in the scenarios was evaluated through a ”cradle to farm-gate” Life Cycle Assessment. The emissions related to on-farm activities (forage production, fuel and electricity consumptions, manure and livestock management), o -farm activities (production of fertilizers, pesticides, bedding materials, purchased feed, replacing animals, electricity, fuel) and transportation were considered. Functional unit was 1 kg Fat and Protein Corrected Milk (FPCM). Enteric emission of methane of lactating cows was obtained from in vivo experiment with respiration chambers or from the equation by Ellis et al. (2007) based on ration DM, NDF, ADF and DMI. Methane estimated from enteric fermentation and manure storage was the major contributor (46%) to GHG for milk production and 73.1% of estimated methane produced derived from enteric emission. Daily enteric methane emission per cow estimated by equation was higher than the average emission measured from in vivo trials with the three diets (363 vs 312 g/d per cow). The average value of GWP using the estimated values of methane was 1.53 g CO2 eq./kg FPCM. The lowest CH4 emission per kg FPCM was obtained with MS diet, particularly when in vivo CH4 enteric measurements were used. As a consequence MS diet was the best option, in terms of GWP per kg FPCM. Purchased concentrate feed showed a great load on GWP (29% on average), especially in the FS scenario, because of the high o -farm purchased energy feed. Contribution of crop production on-farm to GWP was lower for sorghum scenarios, particularly FS, due to reduced water and fertilizer use.