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Massachusetts State College
ABSTRACT
Individual regression of daughters on dams in dairy cattle is of the order of about 0.3 for amount of milk and 0.45 for butterfat test.
Group regressions are of the order of 0.5 to 0.6 both in amount of milk and butterfat test.
The "Equal-Parent" index is based on a regression of 0.5 for milk and butterfat test, but this fact is perhaps not generally appreciated, nor are the respective breed averages generally used in computing EP indexes. The "Equal-Parent" index formula using normal expectation and breed average is w+2(d-e) where w=breed average, d = daughters' average, e = daughters' expectation =
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A NEW method of indexing, based on normal expectation and breed average, is suggested herein, its formula being w + d – e.
The NEW method compares daughters' actual production with their normal expectation rather than with their dams' actual production.
The variability of NEW indexes is of the same order as is that of groups of cows.
The variability of EP indexes is of the same order as is that of individual cows.
Either EP or NEW index method will place bulls in the same order of rank from highest to lowest.
The NEW index makes it possible to ascertain a bull's worth from a production standpoint by comparing the index directly with the respective breed averages, so much better or worse than breed average, through its incorporation of ,normal expectation– as a measuring stick of any bull's transmitting performance.
Either the EP or NEW index system used in the appropriate manner with the average record of a group of cows will predict the production of future daughters reasonably accurately and perhaps a little more accurately on the average than will the average of an earlier group of daughters. Daughters' records alone, however, are not a safe guide to the genetic level of a bull or at least not as good as indexing, which automatically includes a consideration of the mates' level of production; an important item so far as their daughters' ability is concerned.
The NEW method of indexing would appear to make pedigrees look more nearly like what they are genetically than does the EP method. It would provide a measure of conservatism so often and so badly needed in pedigree writing and interpretation.
1 The writer wishes to thank C. T. Conklin and the Ayrshire Breeders' Association; H. W. Norton, Jr., and Holstein-Friesian Association of America, Leonard Tufts, Dr. J. F. Kendrick and Dr. R. G. Schott for their cooperation and helpful suggestions. He is especially indebted to Dr. Jay L. Lush of Iowa State College for his constructive criticisms and guidance. Dr. Lush's paper on ascertaining the optimum regression factor for use in indexing dairy bulls is on the pages just following this article.
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