Genetic analysis of California mastitis test records. I. Coded tests

Area of Science:

• Dairy Science
• Animal Genetics
• Veterinary Epidemiology

Background:

• Mastitis is a significant economic and welfare concern in dairy cattle.
• Early detection and genetic selection are crucial for mastitis control programs.
• The California Mastitis Test (CMT) is a widely used on-farm diagnostic tool.

Purpose of the Study:

• To investigate the effects of sire, parity, and calving season on California Mastitis Test (CMT) scores in Holstein cows.
• To estimate the heritability and genetic correlations of CMT scores for genetic improvement strategies.

Main Methods:

• Analysis of 15,965 Holstein lactation records with CMT scores.
• Categorization of CMT scores into normal (negative, trace) and elevated (1, 2, 3).
• Statistical analysis including sire, parity, calving year, and month effects; heritability estimation via paternal half-sister analysis.

Main Results:

• Elevated CMT scores significantly increased with parity and calving year.
• Sire genetics played an important role in the incidence of elevated CMT scores.
• Heritability estimates for CMT scores ranged from 0.11 to 0.48; genetic correlations between early and later lactation tests were moderate.

Conclusions:

• Parity and calving year are significant non-genetic factors influencing mastitis indicators.
• Genetic selection utilizing CMT scores can be effective in reducing mastitis incidence in Holstein populations.
• Heritability estimates support the potential for genetic progress in mastitis resistance.