Production, Management and the Environment: Dairy 1

Excretion of excess dietary N from dairy cows contributes to environmental pollution, with ammonia, nitrate, and nitrous oxide being the major pollutants. Apparent efficiency of feed N use for milk protein production is typically about 25%, depending on diet. However, there are long periods when a dairy cow is not productive (during heifer growth and when dry) which reduce lifetime apparent N use efficiency (NUE) because the animal excretes N with little useful output at these times. This situation is exacerbated as cow longevity decreases, and the ratio of non-productive to productive periods of life increases. A simple modeling exercise was carried out to investigate the effect of dairy cow productivity and longevity on lifetime NUE of a dairy herd yielding an assumed fixed quantity of milk. Opportunistic losses in milk production caused by disease and infertility were included to take into account the number of extra animals required for target herd milk yield. N losses during growth (including heifer mortality), pregnancy, and culling were included. With an assumed baseline NUE largely determined by nutrition (e.g., 25%) herd longevity of milking cows with a mean lactation yield of 7,500 kg resulted in lifetime NUEs of 11.8, 18.6, 21.0, 22.1 and 22.7% for lactations 1 to 5 respectively. The response was well described (R2 = 0.99) by a general saturation curve model (Morgan-Mercer-Flodin). For a fixed herd milk yield, increased individual animal productivity resulted in better lifetime NUE because of fewer cows in the herd (e.g., 19.7, 21.0, 21.8% for 5,000, 7,500, and 10,000 kg cows completing 3 lactations). This equates to less N excretion per kg milk produced by higher yielding cows. However, this assumes rates of disease incidence and longevity are unaffected by productivity, which may not be achieved in practice. In conclusion, for herds with very high replacement rates, overall lifetime NUE is significantly affected by animal longevity. For animals surviving for about 3 lactations or more the major factor determining lifetime NUE efficiency is nutrition, and the unproductive periods of the cow’s life have a relatively minor influence.