I disagree with some of the assertions made by Mark Eisler and colleagues about how to provide a growing human population with high-quality protein from limited arable land (see M. C. Eisler et al. Nature 512, 371; 2014). Their conjecture that cutting animal productivity by 20% would result in 80% better welfare and environmental impact is particularly unrealistic.
On the contrary, milk production is being sustainably intensified by effective management of high-yielding herds. Dairy cows have become more productive through genetic selection and better nutrition, housing and health, with a concomitant drop of 63% in greenhouse-gas emissions per unit of milk produced (J. L. Capper et al. J. Anim. Sci. 87, 2160–2167; 2009).
Typically, the diet of dairy cows in intensive farming systems is 60% forage fibre by weight (see go.nature.com/h3w7yy). Feeding these cows grain does not divert an equivalent mass of food away from humans, because the concentration of protein and other nutrients in milk surpasses that in grain. The male calves of Holstein dairy cows are already used for meat production, and manure is widely used as fertilizer on dairy farms (see go.nature.com/ebm5a9).