Genetic and Phenotypic Relationships between Clinical Mastitis, Laboratory Criteria, and Udder Height

Genetic aspects of mastitis resistance were investigated where quantitative scores were determined for clinical mastitis, bacterial infection, and leucocyte count.Heritability and repeatability estimates were obtained for these scores, and phenotypic and genetic correlations between scores were estimated.Correlations also were obtained between udder height and the scores for clinical mastitis, bacterial infection, and leucocyte count.Herit:ability estimates for clinical mastitis obtained from the paternal sister correlation and daughter-dam regression were inconsistent.Repeatability for clinical mastitis was estimated at .31.Estimated genetic correlations from the two analyses were .29 and .23 for clinical mastitis with bacterial infection, and .80 and .98 for clinical mastitis with leucocyte count.Udder height was found to be negatively correlated with scores for clinical mastitis, bacterial infection, and leucocyte count.Environmental and managemental influences generally are conceded to be important causes of mastitis, and there seems little doubt that strict attention to managemental details would sharply reduce mastitis incidence in many herds.However, there is also considerable variation in the occurrence of mastitis between cows managed alike in the same herd, and between herds where the management appears to be much the same.These observations indicate that genetic differences also may be of considerable importance in determining the incidence of mastitis, and Lush (6) and Legates and Grinnells (5) have reported heritability estimates for mastitis resistance of .38 and .27,respectively.This paper reports further study aimed at a more detailed assessment of the genetic and phenotypic aspects of clinical mastitis.Additional data from ]ab~)ratory examinations of the foremilk for leucocyte count and presence of bacteria also were obtained to bolster the clinical histories.Whether this additional information would permit more rapid genetic improvement depends largely upon the genetic relationships between laboratory criteria and clinical mastitis.These relationships were studied, as was the relationship between udder height and mastitis.Although the pendulous udder has long been considered as predisposing to mastitis, there is no known experimental evidence to substantiate such a relationship. EXPERIMENTAL PROCEDURENature of the data.Data for this study were collected from November, 1954, to January, 1958, from four state-oxvned dairy herds.Two of the herds included animals of the Ayrshire, Guernsey, Holstein, and Jersey breeds, whereas the