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Live Weight Versus Metabolic Body Size in Dairy Cows and Goats

Illinois Agricultural Experiment Station, Urbana

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.
).

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.
= 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/W₁'s 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.
).