Modelling the effect of the stage of pregnancy on milk composition
Résumé
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-pregnant cows managed under identical conditions served as controls. The difference between individual fat or protein contents of each pregnant animal and the mean corresponding values in controls was computed weekly from the week of conception. The effect of pregnancy on fat and protein concentrations began to be significant from the 20th week of pregnancy, regardless of the week of conception. It was higher in the Friesian breed than in Holstein or Montbeliarde cows. In Holstein and Montbeliarde cows, the effect of pregnancy 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 lactation groups (Friesian, high producing multiparous Holstein and Montbeliarde cows, other Holstein and Montbeliarde cows) the following linear model Y = a(Pw - 18) + b(Pw - 18)2 teas fitted, where Y is the difference between fat or protein concentration of pregnant and non pregnant cotes at a given stage (week), Pw is the 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 same manner as milk yield during pregnancy (-77 g/day and -68 g/day at the 29th week of pregnancy, for fat and protein yield, respectively).