Selection responses for disease resistance in two selection experiments with Norwegian red cows

Area of Science:

• Animal Genetics
• Dairy Science
• Veterinary Epidemiology

Background:

• Genetic trends for clinical mastitis (CM), ketosis (KET), retained placenta (RP), and 305-day protein yield (PY305) were analyzed in Norwegian dairy cattle.
• Two selection experiments were conducted: 1978-1989 (high/low milk production) and from 1989 (high protein yield/low mastitis frequency).

Purpose of the Study:

• To calculate genetic trends for key production and health traits in Norwegian Red cattle under different selection objectives.
• To evaluate correlated responses in disease incidence due to selection for milk or protein yield and mastitis frequency.

Main Methods:

• Utilized a multivariate animal model analyzing 2.7 million first-lactation Norwegian Red cows for disease records.
• Extracted estimated breeding values from a large population analysis to determine genetic trends within experimental selection groups.
• Calculated genetic trends for PY305, CM, KET, and RP across different selection lines.

Main Results:

• Positive genetic trends for PY305 in high production groups (HMP, HPY); negative trends in low production groups (LMP, LCM).
• High milk production selection (HMP) showed increasing genetic trends for CM, KET, and RP.
• Low mastitis frequency selection (LCM) demonstrated decreasing genetic trends for CM, KET, and RP, indicating correlated responses against diseases.

Conclusions:

• Selection for high milk production can lead to undesirable correlated increases in disease incidence.
• Selection for low mastitis frequency effectively reduces genetic susceptibility to CM, KET, and RP.
• Significant genetic differences in disease prevalence were achieved between high protein yield and low mastitis frequency lines after five generations.