Associations between β‐casein genotype and milk yield and composition in grazing dairy cows

Abstract

Milk samples from New Zealand dairy cows at pasture were analysed to investigate the relationship between β‐casein genotype and milk yield or composition traits. A total of 1661 milk samples from 21 North Island herds were analysed from individual cows in mid lactation, from a routine herd‐test taken between mid November 2000 and early February 2001. The cows, whose ages ranged from 2 to 11 years, were the daughters of 11 widely used Holstein‐Friesian bulls. The β‐casein genotypes of the sires (A1 or A2 alleles)were: A1A1 (n = 1), A1A2 (n = 7) and A2A2 (n = 3). The milk composition traits analysed were: protein%, fat%, somatic cell count (SCC), β‐casein%, κ‐casein%, β‐lactoglobulin%, lactoferrin%, and fatty acid composition (C4‐C24, including conjugated linoleic acid). Milk yield per day, fat colour (optical density measurement), milk‐fat melting point, and an index of milk value in dollars (combining the industry's economic values for milk yield, fat yield, and protein yield) were also analysed. Sire effects were significant for most of the traits. Linear contrasts were calculated between β‐casein genotypes on a pooled within‐sire basis for each trait. Expressing results relative to the mean of all samples analysed, there was a significant increase of 2.8% (±1.2%) in the daily fat yield of A2A2 over A1A2 genotype cows (P < 0.05), and the data suggested an increase (P < 0.10) in daily protein yield (a 2.2% margin of A2A2 over A1A1 genotypes, and a 1.9% margin of A2A2 over A1A2 genotypes). Milk solids yield was affected in a similar way to milk protein and milk fat yields. Milk value per day was significantly increased by 2.0% (±1.0%) in A2A2 cows over A1A2 cows (P < 0.05). β‐casein% and κ‐casein% were significantly lower in A2A2 than A1A1 genotypes, by 4.3% (±1.0%) (P < 0.001) and 4.5% (±2.0%) (P < 0.05) of their respective mean concentrations, with values in the A1A2 genotype being intermediate and significantly greater than in the A2A2 genotype (P < 0.05) for both β‐casein% and κ‐casein%. There was a lower percentage of medium‐chain fatty acids and a corresponding increase in long‐chain mono‐unsaturated fatty acids in the milk of A2A2 than of A1A1 cows (P < 0.05), whilst milk from A1A2 and A2A2 cows was similar in this respect. There was a significantly higher SCC (P < 0.05) for the A1A2 than the A1A1 genotypes, but the percentage of cows whose milk exceeded an SCC of 400 000 cells/ml was not significantly affected by β‐casein genotype. No other measured differences were significant. Overall, the milk value per day from A2A2 cows was worth 2.1 % more (±0.95%) than that from A1A1 and A1A2 cows together (P < 0.05). Using economic values in the Breeding Worth formula for the 2004 year, this advantage was equivalent to $15.8 per lactation or $4,740 in a herd of 300 cows.