Effect of replacement rate on longevity and methane emissions for dairy herds in New Zealand

A high replacement rate in dairy herds is desirable to accelerate genetic gain. However, higher replacement costs are incurred, and average age and maturity of the herd is decreased, which can diminish overall herd productivity and profitability, but effects on methane emissions have not been reported. This study examines the relationship between herd replacement rate, longevity and methane emissions of the New Zealand dairy herd under grazing conditions. Three herd replacement scenarios were examined, low (13%), base (17%) and high (19.5%) using a farm model to quantify the annual metabolizable energy requirements for growth, maintenance, pregnancy and milk production for the national herd. Farm income was the sale of dairy products, culled cows and surplus calves, minus processing costs. Methane emissions were calculated using dry matter requirements and were expressed as CO 2-e . An increase in the replacement rate from 13% to 19.5% resulted in a reduction of 400 days in the life expectancy of a cow. The methane emission at the low replacement rate was 7.97 kg CO 2 -e/kg fat + protein, while methane emission at the high replacement rate was 8.81 kg CO 2 -e/kg fat + protein. By increasing the lifetime of the cow producing milk, the methane burden decreased from 3.0 kg total CH 4 /kg lactational CH 4 when the cow is 3 years old, to 2.1 kg CH 4 /kg CH 4 when the cow reaches 10-years. A better alignment between profitable production and consumer needs, animal welfare and an improved environment is needed.