The lactational variation in milk protein composition and renneting
properties and their relationship to the cow's body condition at calving
were
investigated in 39 Danish Holstein first lactation cows fed on a well balanced
standard diet. All milk characteristics measured were
significantly affected by stage
of lactation (P<0·01). Casein as a proportion of
total milk nitrogen reached a
maximum in mid lactation. The proportion of αs- and
κ-casein in total casein
decreased and the proportion of β-casein increased
systematically during lactation
while the proportion of γ-casein was lowest in mid
lactation. The α-lactalbumin
content of milk and its proportion of total whey proteins decreased
during lactation.
Renneting time was highest and curd firmness lowest in mid lactation. These
results
appeared to reflect a low degree of proteolysis in late-lactation milks
compared with
several other investigations, probably because of the good nutritional
state of the
cows. The body condition at calving affected proteolysis and the renneting
properties
of milk. A good body condition increased the content of whey protein in
total milk
nitrogen and of γ-casein in total caseins
(P<0·05); in addition, curd firmness was
improved (P<0·01) and aggregation time was reduced
(P<0·05). We suggest that
these effects were related to the fat metabolism and energy status
of the cows during
lactation. The interrelationships between the milk characteristics were
evaluated by
factor analysis to support the interpretation.