Body Size and Lactation Rate

The influence of age on lactation rate can be determined when the effect of body size is derived independently. This derivation can be made on the basis of the theory that lactation capacity is proportional to the metabolic body size (34 power of body weight) of the cows. There are two reasons why the effect of body size on lactation rate cannot be calculated from records within one herd: first, the variability of lactation rate, aside from the influence of size, ranges from±10 to±20 per cent of the mean rate, even in well-bred herds kept under uniform conditions; second, size differences within each of those herds are usually rather small. The unreliability of results from a recent attempt to calculate size effects on lactation rates within a herd is demonstrated. A table is calculated showing the number of cows necessary to distinguish significantly between production rate per unit weight and production rate per unit of metabolic body size. A table is supplied in which metabolic body size of cows can be read directly when body weight in pounds is given. Lactation rate per unit of metabolic body size (34 power of body weight) is a sound basis for calculating the effects of age on lactation rate. The average daily milk production during a 10-month period, expressed as milk energy or fat-corrected milk, and divided by the mean metabolic body size, is suggested as an important result in summaries of production records. When the cows have been kept under quasi-optimal conditions, such a result may be known as relative lactation capacity. This term, which expresses quantitatively the inherent ability of cows for milk production, would be useful as a major criterion for breeding dairy cattle.