Genetic and economic responses to breeding programs that consider mastitis

Area of Science:

• Animal Breeding and Genetics
• Dairy Science
• Animal Health

Background:

• Mastitis is a significant economic and health concern in dairy cattle.
• Breeding programs traditionally prioritize yield traits like milk and fat production.
• The genetic correlation between yield and mastitis resistance is complex and often antagonistic.

Purpose of the Study:

• To evaluate the impact of different breeding program strategies on milk yield, fat yield, mastitis incidence, and economic response in US dairy cows.
• To assess the effectiveness of including mastitis (clinical or somatic cell count) in selection indexes.
• To determine the economic desirability of simultaneously improving yield and mastitis resistance.

Main Methods:

• A simulation study using discounted gene flow was conducted over 25 periods (13 months each).
• Eight breeding programs were evaluated, modeling US dairy (Dairy Herd Improvement) cows served by artificial insemination sires.
• Economic responses and annual genetic changes were calculated for various selection strategies.

Main Results:

• A program without mastitis consideration yielded 53.5 kg milk, 2.24 kg fat, 0.020 mastitis cases, and $98.24 economic response annually.
• Including somatic cell count (SCC) or mastitis in selection indexes reduced yield trait improvement by 1-2% but slowed mastitis increase by 20-25%, with <1% economic gain.
• Restricted selection against mastitis reduced yield improvement by up to 27% and economic response by 17%.

Conclusions:

• Simultaneous genetic improvement of yield and mastitis is not economically optimal due to antagonistic genetic correlations and differing heritabilities/economic values.
• Direct selection against clinical mastitis is currently unrealistic due to data limitations.
• Indirect selection via SCC may be less efficient than predicted, partly due to incomplete testing program enrollment.