Effects of animal activity and air temperature on methane and ammonia emissions from a naturally ventilated building for dairy cows

摘要

Knowledge of how different factors affect gas emissions from animal buildings can be useful for emission prediction purposes and for the improvement of emission abatement techniques. In this study, the effects of dairy cow activity and indoor air temperature on gas emissions were examined. The concentrations of CH_4, NH3, CO_2 and N_2O inside and outside a dairy cow building were measured continuously between February and May together with animal activity and air temperature. The building was naturally ventilated and had a solid concrete floor which sloped towards a central urine gutter. Manure was scraped from the floor once every hour in the daytime and once every second hour at night into a partly covered indoor pit which was emptied daily at 6 a.m. and at 5 p.m. Gas emissions were calculated from the measured gas concentrations and ventilation rates estimated by the CO_2 balance method. The animal activity and emission rates of CH_4 and NH_3 showed significant diurnal variations with two peaks which were probably related to the feeding routine. On an average day, CH4 emissions ranged from 7 to 15 g LU~(-1) h~(-1) and NH_3 emissions ranged from 0.4 to 1.5 g LU~(-1)h~(-1) (1 LU = 500 kg animal weight). Mean emissions of CH_4 and NH_3 were 10.8 g LU~(-1) h~(-1) and 0.81 g LU~(1-) h~(-1), respectively. The NH3 emissions were comparable to emissions from tied stall buildings and represented a 4% loss in manure nitrogen. At moderate levels, temperature seems to affect the behaviour of dairy cows and in this study where the daily indoor air temperature ranged from about 5 up to about 20℃, the daily activity of the cows decreased with increasing indoor air temperature (r = -0.78). Results suggest that enteric fermentation is the main source of CH_4 emissions from systems of the type in this study, while NH_3 is mainly emitted from the manure. Daily CH_4 emissions increased significantly with the activity of the cows (r = 0.61) while daily NH_3 emissions increased significantly with the indoor air temperatures (r = 0.66). Daily CH_4 emissions were negatively correlated to the indoor air temperature (r = -0.84). This suggests that increased daily indoor air temperatures due to seasonal changes may bring about decreased animal activity which may decrease the release of CH_4 from dairy cows. Finally, changes in daily NH_3 emissions were influenced more by the indoor air temperature than by the activity of the cows.