SOMATIC CELLS, MILK YIELD AND PRODUCTIVE LIFE

B.T. McDaniel and J. C. Wilk

The objective was to determine the value of genetic evaluations (PTAs) of Holstein and Jersey bulls for Somatic Cell Score (SCS), milk yield and components to predict Productive Life. The measure of productive life used was the PTA. All PTAs used were those computed by the Animal Improvement Programs Laboratory of USDA.

Sires included were required to have been widely used in AI long enough in the past that the PTAs for productive life and other traits were highly reliable. Minimums for Holsteins were 300 or more daughters with PTAs for productive life and milk and 100 for SCS. Respective numbers for Jerseys were 200 and 80. In addition at least 60% of the progeny had to have an opportunity for multiple lactations. A total of 940 Holstein and 94 Jersey bulls qualified.

Correlations of PTAs were generally in desired directions. In both breeds sets of half-sisters that had higher SCS did not live as long as those having lower SCS (table 1) and These advantages are expressed as the extra months of life expected for the different PTAs for SCS. To put these in perspective, the ranges in length of average productive lives of progeny of widely used Holstein bulls is only about five months. The effect of SCS was larger in Holsteins than in Jerseys.

As expected, sets of progeny with higher PTAs for milk yield also lived longer. Of the milk components, protein percent was useful in Holsteins and fat percent in Jerseys. An antagonism between SCS and milk was present in Holsteins but it was much smaller in these progeny of widely used bulls than previously reported for all animals within a breed (table 3).

Analyses where PTAs for both SCS and milk were used simultaneously to predict productive life showed a positive synergism between them. When considered jointly, the ability of SCS and milk as predictors was enhanced. Most likely this occurred because of the antagonism between milk and SCS. Some progeny groups lived longer as a result of high yields while others flourished because of resistance to subclinical and clinical mastitis. Those sets of half- sisters that excelled in both had high longevity.

These results show that the newest PTA available in dairy cattle, that for SCS, is probably more valuable than many others currently used to chose bulls. Certainly, AI bulls with desirable (low) PTAs for SCS have more value than generally recognized by many breeders.

Posted Nov 12th

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