New Zealand Journal of Agricultural Research abstracts
Associations between β‑casein genotype and milk yield and
composition in grazing dairy cows
C. A. Morris
S. M. Hickey
N. G. Cullen
C. G. Prosser
AgResearch Ltd
Ruakura Research Centre
Private Bag 3123
Hamilton, New Zealand
R. M. Anderson
M. L. Tate
AgResearch
Invermay Agricultural Centre
Private Bag 50 034
Mosgiel, New Zealand
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), 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 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.
Keywords milk composition; β‑casein; genotype;
cattle
A05018; Received 29 April 2005; accepted 5 August 2005; Online
publication date 11 October 2005
New Zealand Journal of Agricultural Research, 2005, Vol. 48:
441–450
0028–8233/05/4804–0441 © The Royal Society of New Zealand 2005
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