Quelques facteurs de variation du temps de coagulation des laits individuels de vaches

Abstract - Some factors of variation of the renneting-time of individual cows milks
The influence of different factors on the renneting time was studied from weekly tests on individual milks in the course of 107 lactations (63 Normandes (N) and 44 French Friesian (FFPN)).
The B.C.P. rennet test of DAVIS and MCCLEMONT (1939) was used. The main results obtained are:
Renneting times of heifers are slightly smaller than those of cows in their second and third lactation.
The stage of lactation has a marked influence on the renneting time tc ; after a phase of increase spread out over the first 200 days of lactation, a maximum is reached, which is only slightly marked, and then a gentle decrease during the last 100 days of lactation. Actually, tc varies, on the average· from 75 to 115, if one assumes its mean value throughout the whole of the lactation period is equal to 100.
The general trend of the renneting time with the stage of lactation seems to be largely related to the variation of the calcium / total nitrogen ratio in the milk.
Important sources of variation are the breed and, even more, the individual cow: the average renneting time for the whole lactation is 16,6 ± 0,5 mn for the Normandes and 20,0 ± 0,6 mn for the FFPN. Even in the same herd the coefficients of variation of the average renneting times are very high: 24,8 p. 100 for Normandes and 18,7 p. 100 for Friesian.
The analysis of the results obtained from 8 pairs of monozygotic twins and 8 pairs of dizygotic twins clearly shows that the character "rate of coagulation" of the milk is partly dependent on genetic factors. Refering to the great variability of this characteristic, it should be possible, to improve it by selection