INCIDENCE OF SUBCLINICAL AND CLINICAL MASTITIS IN PURELINE AND CROSSLINE DAIRY CATTLE

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Abstract
Bi-monthly California Mastitis Tests (CMT) scores and the number of cases of clinical mastitis in 758 lactations of the Holstein-based H line, 376 lactations of the Ayrshire-based A line and 409 lactations of their reciprocal crossbreds housed under intensive management were studied to examine effects of sire line, dam line and sire line by dam line interaction using mixed model methodology. The incidence of subclinical mastitis as judged by CMT score, number of cases of clinical mastitis during the lactation, proportion of cows showing clinical mastitis at least once during the lactation and the cost of drugs for the treatment of clinical mastitis were lower in the crossline cows than pureline cows. The heterosis for these traits ranged from 2.5 to 7.8%. Sire line effect was significant for CMT score, cost of drugs for clinical mastitis and most of the traits measuring clinical mastitis. Dam line effect was small and nonsignificant for most of the traits studied. Significant sire line and dam line interaction was found for number of clinical cases in right front and proportion infected in right front and left hindquarters. Pathogenic organisms were isolated from 10.4, 34.2, 56.3, 77.7, and 84.8% of the composite milk samples, showing a CMT score of negative, trace, 1, 2, and 3, respectively. Staphylococcus was the most frequently isolated organism from the composite milk samples. Key words: Subclinical, clinical, mastitis, dairy cattle