Live Weight Versus Metabolic Body Size in Dairy Cows and Goats

Abstract

This article considers lactating maintenance and milk-energy yield, in cows and goats, as proportional to live weight versus proportional to metabolic body size kg. 34. It may be accepted as experimentally established that lactating maintenance per unit live weight as a species average is about twice as high in goats as in cows, or lactating maintenance is proportional to metabolic body size as between the two species. It is equally well established that the same relation does not hold within either one of the two species. Consistently, within species lactating maintenance is proportional to a power of live weight in excess (not significant) of unity. The weight-yield relation is similar to the weight-maintenance relation, provided live weight is measured as W1, age is not restricted, and milk-energy yield is measured as FCM8. Within cows the weight-yield relation is more variable than the weight-maintenance relation, both inherently and environmentally. Validity of the two postulates as indicated by observed values of FCM8 and W1 is tested by determining, by least squares the value of 5 and its standard error in the equation, FCM8/W1 = a + bW1 or FCM8/kg. 34=a+bW1. If one of the postulates conforms closely to the observations the other postulate will be invalid by the 5 per cent statistical standard with about 50 cows and 125 lactations in case of CV in FCM8/W1 = 16 and CV in W1 = 12. These numbers may be easily reduced to 10 cows and 40 lactations if CV in FCM8/W1 is reduced or if CV in W1 is increased. Six. sets of data on FCM8 and W1 are available on cows, each set with number of lactations large enough to give reliability in the statistical sense. 10 Brody (1, p. 857) continues his repeated citations of a “700-lb.” Jersey cow with 1000 FCM12/W = 101.6. Taken at face value this is a phenomenal performance. (From what we know about feed requirements of lactating cows at such enormous yields this 700-lb. Jersey must have consumed an average of about 35 lb. of grain per day for 365 consecutive days!) He uses the case to show the improbability, for example, that any 2000-lb. Holstein can ever attain 1000 FCM12/W =101.6. That is, dealing with the highest individual Jersey and the highest individual Holstein, the FCM8/W1 philosophy is shown to be untenable, as he sees it. Such extremes are of great interest but hardly rule out the need of comparing herd averages with respect to FCM8/W1 for the Jersey and Holstein breeds. Years ago Eckles developed a famous herd of Jerseys at the Missouri Station. How does the average FCM8/W1 for the Missouri Jerseys compare with the Nebraska Holsteins on the basis of all lactations of all cows? The average life spans of Holsteins and Jerseys are approximately equal and by Rubner's philosophy (footnote 9) it may be anticipated their average FCM8/W1s are also approximately equal. Adequate experimental observations must answer the question. Four sets support the FCM8/W1 philosophy almost perfectly; one set supports neither one of the two philosophies, but the metabolic body size philosophy less poorly; one set falls about half way between the two philosophies. The weight of experimental evidence as between cows distinctly supports the philosophy that FCM8 tends to be proportional to W1; and distinctly contradicts the metabolic body size theory of milk-energy yield (FCM proportional to kg. 34). © 1946, American Dairy Science Association. All rights reserved.