however, it was only made possible by specialization and intensification of agricultural production after World War II. Average milk yield per cow in 1944 was 2,074 kg/y, compared with 9,193 kg in 2007. This improvement in productive efficiency facilitates the dilution of maintenance effect, by which the total resource cost per unit of milk is reduced (Bauman et al., 1985). The daily nutrient requirement of lactating cows comprises a specific quantity needed to maintain the vital functions and minimum activities in a thermo-neutral environment (maintenance requirement) of the animal plus extra nutrients to support the cost of lactation. As shown in Figure 2, the maintenance energy requirement does not change as a function of production, but the daily energy requirement increases as milk yield increases, thereby reducing the proportion of total energy used for maintenance. The total energy requirement per kg of milk produced is therefore reduced: a cow producing 7 kg/d requires 2.2 Mcal/kg of milk, whereas a cow yielding 29 kg/d needs only 1.1 Mcal/kg of milk (Figure 2).