Heat stress impact on productive efficiency and GHG emission intensity in dairy cow

Abstract

In this study, we assessed the effect of heat stress on greenhouse gas emission (GHG) intensity (emissions produced per kg fat and protein corrected milk: FPCM in kilogram of carbon dioxide equivalents: kg CO2e) in dairy cows. A commercial farm milking about 1,100 cows/day was considered. Data on milk yield, fat and protein as percentage, number of cows milked for day and digestible energy and protein of diet were used to estimate the enteric methane emissions under heat stress and thermo-neutral scenarios using the IPCC-based equations. Temperature Humidity Index (THI) was calculated from data recorded in the nearest weather station and used to define heat stress conditions. Months of June, July, August, September and October showed an average maximum THI greater than 70 unit and were considered under heat stress, while the other months were considered as thermo-neutral. Productive parameters considered were 27.4 liter/cow/day; 3.8% and 3.2% under heat stress compared to 28.9 liter/cow/day, 3.7% and 3.3 % under neutral climate for milk yield and for fat and protein as percentage, respectively. Diet did not change during the periods studied then 73% of digestible energy and 16.7% of protein were considered for both scenarios. Methane emission intensity, was found as 0.400 and 0.388 kg CO2eq/kg FPCM for heat stress and thermo-neutral scenario, respectively. Under heat stress, emissions were12 grams CO2eq/kg FPCM or about 60 tonsCO2eq (considering the total milk yield in the study period) higher than that of thermos-neutral conditions. The preliminary results suggest that the effect of heat stress on the production efficiency may affect the emission intensity of GHG. However, further investigations are needed to well understand how heat stress modules enteric methane and others sources of GHG emissions in dairy cows. Therefore, a further study will focus on using HolosNor, a farm scale model, to account for all significant GHG emissions and to compare different levels of heat stress on farm GHG emissions.