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 (A 1 or A 2 alleles)were: A 1 A 1 (n = 1), A 1 A 2 (n = 7) and A 2 A 2 (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), milkfat 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 A 2 A 2 over A 1 A 2 genotype cows (P < 0.05), and the data suggested an increase (P < 0.10) in daily protein yield (a 2.2% margin of A 2 A 2 over A 1 A 1 genotypes, and a 1.9% margin of A 2 A 2 over A 1 A 2 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 A 2 A 2 cows over A 1 A 2 cows (P < 0.05). β-casein% and κ-casein% were significantly lower in A 2 A 2 than A 1 A 1 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 A 1 A 2 genotype being intermediate and significantly greater than in the A 2 A 2 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 A 2 A 2 than of A 1 A 1 cows (P < 0.05), whilst milk from A 1 A 2 and A 2 A 2 cows was similar in this respect. There was a significantly higher SCC (P < 0.05) for the A 1 A 2 than the A 1 A 1 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 A 2 A 2 cows was worth 2.1 % more (±0.95%) than that from A 1 A 1 and A 1 A 2 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.

Genetic polymorphism of milk protein variants and their association studies with milk yield in Sahiwal cattle

“…But according to Ng- Kwai-Hang et al (1984), Bovenhuis et al (1992), Ortner et al (1995) and Cardak (2005), the beta casein genotypes had an effect on milk. Morris et al (2005) showed superiority of A2 genotype with reference to milk yield in Holstein cattle. Tolenkhomba and Yadav (2012) also studied beta casein genotypes in Indian Sahiwal cattle and showed that cows with AB genotype produced more milk (11.81±2.10) than those with AA genotype whose values were 6.48±0.41.…”

Frecuencias genotípicas y alélicas de la b-caseína en el bovino Criollo Lechero Tropical de México

“…Ikonen et al (2001) reportaron un incremento en la producción de leche en vacas de primera lactancia con presencia del alelo A2, Morris et al (2005) refieren al genotipo A2A2 como el más favorable para producción lechera y Olenski et al (2012) concluyeron que el alelo A2 es un marcador para mayor producción de leche y proteína.…”

Conversión de las explotaciones de vacuno de leche a la producción de leche A2 ante una posible demanda del mercado: posibilidades e implicaciones

“…La frecuencia del alelo que codifica la proteína A2 en los animales genotipados es de 0,55 semejante a la estimada en otras poblaciones europeas actuales de Holstein (Massella et al, 2017) y a la estimada hace décadas en las poblaciones de Holstein estadounidenses (van Eenennaam y Medrano, 1991). No obstante, es superior a la estimada en las poblaciones de Holstein canadiense (Ng- Kwai-Hang et al, 1984) y neozelandés (Morris et al, 2005), y en la población española de Holstein-Frisón de la década de los ochenta (Tejedor, 1986), en las que el alelo que codifica la proteína A1 era el más frecuente. Las frecuencias genotípicas estimadas han sido 0,31 para A2A2, 0,48 para A1A2 y 0,21 para A1A1, indicando que la población está en aparente equilibrio de Hardy-Weinberg (p < 0,05) para la β-caseína.…”

“…Estos resultados son coincidentes con las asociaciones referidas en otras poblaciones Holstein; el alelo que codifica la proteína A2 se asocia a un aumento de la cantidad de leche y proteína y un menor contenido de grasa, lo que le llevó a ser considerado como un mar-cador para la selección de cantidad de proteína y leche (Lin et al, 1989;Morris et al, 2005;Olenski et al, 2010). Sin embargo en otros trabajos, el alelo de la proteína A2 por sí sólo no mostraba relación con la producción de leche y proteína, presentando únicamente relación en combinación con el gen de la κ-caseína (Ojala et al, 1996).…”

“…A lo sumo cabría esperar una muy ligera reducción (≤ 1%), favorable del recuento de células somáticas y desfavorable de la longevidad funcional. Morris et al (2005) encontraron un mayor recuento de células somáticas en el genotipo A1A2 que en el A1A1, aunque el porcentaje de vacas con más de 400.000 células/ml no variaba significativamente. Por su parte, Lin et al (1989) no encontraron relación entre el genotipo para β-caseína y la longevidad de las vacas al analizarla durante los cinco primeros años de vida.…”