Size of Cow and Efficiency of Milk Production

Abstract

Summary McDowell's (4) published figures of 109,116 yearly records of cows 5 to 9 years of age in dairy herd improvement associations in the United States have been reworked to show, by live weight of cow and breeds: the energy yield in pounds of 4 per cent milk (340 calories); the theoretical coefficient of efficiency, that is, the percentage of feed energy recovered as milk energy; the recorded feed cost in dollars per hundred-weight of 4 per cent milk. As between grade and registered cows energy yield is substantially equal for the Guernsey and Ayrshire breeds; but materially greater for the registered cows in the case of the Jersey and Holstein breeds. In all 4 breeds the feed cost per hundred-weight of 4 per cent milk is lower for the grade cows, to the extent of about $0.10. The average feed cost is $0.90 per hundred-weight of 4 per cent milk for all records, the average yearly yield being 7,737 pounds of 4 per cent milk. For summary of remaining points all breed classes may be combined. Energy yield increases with size of cow in a linear manner and at the rate of about 250 pounds of 4 per cent milk per year for each 100 pounds increase in weight. Since this is about half the corresponding figure found in advanced registry records and the average yield also about half, it appears that the weight-yield relation is similar in the two classes of records if level of yield is used as a base. The theoretical coefficient of efficiency decreases with increasing weight. Interpreted in terms of feed cost this should mean an increase in feed cost per unit of milk with size of cow, amounting to about $0.03 per hundred-weight of 4 per cent milk for each 100 pounds increase in live weight. The recorded feed cost per hundred-weight of 4 per cent milk decreases with live weight of cow in the range 600 to 800 pounds; and is practically constant in the range 800 to 1,600 pounds. The cause of the discrepancy between the theoretical efficiency and recorded feed cost cannot be determined from the data at hand. It is suggested that it may be due to systematic errors in the bookkeeping which tend to overcharge the small cow and undercharge the large cow for feed energy actually consumed, particularly in the matter of pasture.