Modelling the effect of the stage of pregnancy on milk composition

The effect of pregnancy on the fat and protein contents of milk was studied on a 414-lactation sample. One hundred and forty-nine lactations of non-pr egnant cows managed under identical conditions served as controls. The diff erence between individual fat or protein contents of each pregnant animal a nd the mean corresponding values in controls was computed weekly from the w eek of conception. The effect of pregnancy on fat and protein concentration s began to be significant from the 20th week of pregnancy, regardless of th e week of conception. It was higher in the Friesian breed than in Holstein or Montbeliarde cows. In Holstein and Montbeliarde cows, the effect of preg nancy was higher in high producing multiparous cows (+2.8 and +2.9 g/kg for fat and protein concentrations, respectively, at the 29th pregnancy week) than in primiparous or low producing multiparous cows (+2.0 and +1.4 g/kg, for fat and protein concentration, respectively). In each of these three la ctation groups (Friesian, high producing multiparous Holstein and Montbelia rde cows, other Holstein and Montbeliarde cows) the following linear model Y = a(Pw - 18) + b(Pw - 18)<SUP>2 </SUP>teas fitted, where Y is the difference between fat or protein concent ration of pregnant and non pregnant cotes at a given stage (week), Pw is th e pregnancy week, and a and b are parameters. This increase in milk fat and protein concentrations in late pregnancy cannot compensate the concomitant decrease in milk yield, so that fat and protein yield decreased in the sam e manner as milk yield during pregnancy (-77 g/day and -68 g/day at the 29t h week of pregnancy, for fat and protein yield, respectively).