Substitution of corn silage with sorghum silages in lactating cow diets : In vivo methane emission and global warming potential of milk production

A study was conducted to determine whether sorghum silage can be a valid alternative to corn silage in lactating cow rations in terms of enteric CH<inf>4</inf> production and global warming potential (GWP) per milk unit. Diets containing corn (CS), whole plant grain sorghum (WPGS) or forage sorghum (FS) silages were fed to 6 primiparous Italian Friesian cows in a replicated 3 × 3 Latin square design. Diets were balanced to have 36.0 and 26.0% DM of NDF and starch, respectively. In each period cows were placed in individual respiration chambers to register CH<inf>4</inf> production. The GWP of milk production in the three different forage systems was evaluated through a "cradle to farm-gate" Life Cycle Assessment. All the emissions related to on-farm activities (forage production, fuel and electricity consumptions, manure and livestock management), off-farm activities (production of fertilizers, pesticides, bedding materials, purchased forages, concentrate feed, replacement animals, electricity, fuel) and transportation were considered. The functional unit was 1 kg FPCM. Enteric emission of CH<inf>4</inf> from cows was obtained with an in vivo experiment or predicted using an equation based on gross energy intake and on dietary NDF and ether extract.Dry matter intake (kg/day) tended to be higher for CS and WPGS (20.0 for both diets) than FS (18.2) (P = 0.07). Milk yield was 23.6, 24.6 and 25.4 kg/day for FS, WPGS and CS, respectively (P = 0.05 between CS and FS).On average, CH<inf>4</inf> from enteric fermentation and manure storage was the major contributor (45.4%) to GHG emission of milk production and 71.1% of the CH<inf>4</inf> was from enteric losses. Predicted CH<inf>4</inf> emission was slightly lower than the emission measured from in vivo trials (323 vs 340 g/day per cow on average). In vivo CH<inf>4</inf> production was not different among diets but intake energy lost as CH<inf>4</inf> was higher (P = 0.04) for FS (5.8%) in comparison with CS and WPGS (5.1 and 5.2% for CS and WPGS, respectively). Contribution of on-farm crop production to GWP was lower for sorghum scenarios, particularly FS, due to the reduced use of water and fertilizers. On the contrary purchased concentrate feed showed a great load on GWP (30%), especially in the FS scenario, because of the greater amount of corn meal needed to compensate for the low starch content of the sorghum silages. In conclusion the CS forage system gave higher milk production, lower CH<inf>4</inf> energy loss and lower GWP per kg FPCM than the sorghum forage systems.