Body Size and Milk Production

BODY SIZE AND PRODUCTIVE CAPACITYThe senior author has suggested (1) that in general the total efficiency of energy utilization by animals energy in the product (milk, eggs, etc.) energy in the food is independent of body size.Involved in this theory is the assumption that an animal's capacity to take in food per day is, as a rule, a certain multiple of its maintenance requirement or also its rate of fasting katabolism.This assumption was supported by some data on the food capacity of baby chicks, rabbits, sheep, swine, and steers, which indicated that the rate of intake of total food energy in these animals was four to six times as great as their rate of fasting katabolism.According to this theory the capacity for rate of production (energy in meat, milk, or eggs per unit of time) of large and small animals should be proportional to their respective rate of fasting katabolism; or, since this metabolic rate is proportional to the 3~ power of body weight (2), the capacity for rate of production should also be proportional to the a/~ power of body weight.Thus the rate of lactation per unit of metabolic body size (kg.~)*should be the same for large and small animals.This deduction was supported by Eckles's data (3) on average milk production and body weight of Jersey and Holstein cows.The energy in the yearly milk production (calculated according to MSllgaard (4)) of the Jerseys was 25.1, that of the Holsteins 23.9 thousand kilocalories per kg.~, a difference of only 5 per cent, whereas it was 5.65 and 4.87 thousand kilocalories respectively per kg. of body weight, a difference of 16 per cent.If the milk energy is calculated according to Overman and Gaines (5), then the difference in milk energy per unit of the a/~ power of body weight is only 2 per cent; that per kg.body weight, 11 per cent.Our theory that capacity for rate of milk production is closely related to the 3/~ power of body weight has been rather well confirmed by Brody and his coworkers in comparing Jersey with Holstein cows (6, 7), goats with cows (8), and rats with cows (9).Rats as well as high-producing cows gave off about twice as much energy in their daily milk as they would have lost daily as heat during fasting. GAINES ~ THEORY OF BODY SIZE AND MILK PRODUCTIONIn contradiction to our 3~ power rule, W. L. Gaines (10) deduced in a