The environmental impact of fertility in dairy cows: a modelling approach to predict methane and ammonia emissions

Dairy cows are estimated to contribute about 20% of total UK atmospheric methane emissions and 25% of total UK ammonia emissions. Previous models of atmospheric emissions have linked dietary inputs to gas emissions, but fertility models have not included emissions by herd replacements. The objective of this study was to construct a model that linked changes in fertility to herd structure, number of replacements, milk yield, nutrient requirements and gas emissions. The model was then used to investigate the impact of changing fertility parameters on predicted total gas emissions at the herd level. Fertility has a major effect on the number of heifer replacements required to maintain herd size for a given milk quota or number of cows. The proportion of total gas emissions that is produced by herd replacements is up to 27% of methane and 15% of ammonia with commercially common fertility levels. Restoring fertility to 1995 levels is predicted to reduce methane emissions by 10–11% and ammonia emissions by about 9%. Further improvements in fertility could reduce methane emissions by up to 24% and ammonia emissions by about 17%. Nutritional strategies for improving fertility are likely to be complementary to dietary strategies already proposed for reducing emissions.