Nitrogen Transactions in Ruminants

The primary goal of ruminant nutritionists is to achieve maximum output of proteinaceous materials in products such as milk, meat and wool with a minimum of dietary crude protein (CP) inputs. In practice, this nitrogen (N) output to input ratio is relatively low. For example, it can vary from 13% for milk protein production in pasture-fed dairy cows (Wanjaiya et al., 1993) to 31% in dairy cows grazing on Lolium perenne based pasture (Delagarde et al., 1997). However, since 40-45% efficiency coefficients are theoretically possible in dairy cows (Van Vuuren and Meijs, 1987; Hvelplund and Madsen, 1995), there is scope for considerable improvement in nutritional management of our grazing livestock. Moreover, increasing the efficiency of use of protein N by livestock, leading to lower N excretion, is becoming an environmental imperative in many countries (Castillo et al., 2001).