Effects of β-casein A1/A2 milk and β- and κ-casein genotypes on the performance, body composition, and fecal score of dairy calves

The perceived health benefits of β-casein (β-CN) A2 milk for humans have driven increased selection for homozygous β-CN A2 dairy cattle. However, little is known about the effects of feeding milk with different β-CN variants to calves, or about the influence of β-CN genotypes on calf health and performance. We aimed to evaluated the performance, daily fecal score, and body composition of calves that carried β-CN A1A1, A1A2, or A2A2 genotypes and different κ-CN genotypes, fed homozygous β-CN A1 or A2 milk. Every day, during the first 14 days of life, calves were offered 7.5 L/day of either homozygous β-CN A1 or A2 milk (27 females and 25 males per group), and total energy-corrected milk intake was recorded for each calf. On Day 15, the body composition of all 104 calves was assessed using magnetic resonance imaging and dual-energy X-ray absorptiometry. No significant differences were observed in performance, daily fecal scores, or body composition between calves fed β-CN A1 milk and those fed β-CN A2 milk. Compared with β-CN A2A2 calves, calves with the β-CN A1A1 and A1A2 genotypes presented greater final body weights, average daily gains, and feed efficiencies and fewer days with diarrhea. These findings suggest that the exclusive selection of β-CN A2A2 animals should be reconsidered because of the potential impact of this genotype on calf health and growth performance.