Alguns fatores que influem sobre a produção de leite de vacas mestiças simentais sob o sistema de retiros

Continuação

Resumo

1. The mean age of the heifers at freshening was 38.7 + .3 months. No relation was found between the month of the year in which the heifers were born and their age at first calving. The distribution of calvings throughout the year was about the same for heifers as for the rest of the cows.

2. The mean age at which the cows ended their last lactations before they left the herd was 112.5 +2 months. The relation between age at first calving and the productive life of the cows was negative, but insignificantly so.

3. The month of calving showed no significant influence on milk yield. The plus and minus deviations from the ge- neral mean were irregular, not indicating clearly any effect of season of freshening on milk production.

4. The influence of age of the cows on the actual milk yields was statistically significant but not important practi- cally. The variance was only 3 per cent less within groups of cows of the same age than it was among all cows.

5. The length of lactation period had the most pronoun- ced effect on total milk yield. The variance in yields was reduced 47.4 per cent when lactation period was held cons- tant. The correlation between yield and lactation length was +.69, showing a practically linear relation.

6. The effect of service period on milk yield was sta- tistically significant and the variance in yield was reduced 184 per cent when service period was held constant. The correlation between them was +. However, when the in- fluence of service period was separated from that of lacta- tion period, the partial correlation between service period and yield, length of lactation being constant, was —.23, show- ing little practical influence of service period on milk pro- duction.

7. The age of the cow had a significant efiect on the length of lactation period, the correlation being —.22 showing a de- crease in the length of L.P. as age increased. A significant difference of 36 days existed between the average lactation periods of the two groups calving at ages of 32.5 and 38.5 months.

8. When the average service period was calculated for the groups which calved in different months, all plus devia- tions from the general mean were in seven adjacent months (March to November), while all minus deviations were in the other five con ive months. The difference of these two groups was significant, but the effect was not important practically.

9. In this study 44 per cent of the variance in milk yields, standardized to a lactation length of 305 days, were due to permanent differences between individual cows, in- cluding inheritance and permanent environmental effects.

10. No regular decrease in production paralleled the increase in the percentage of imported blood but there was some indication of such a tendency, probably curvilinear, either because of heterosis or because the maximum yield comes from an intermediate percentage of imported blood. The influence of amount of imported blood on the cows’ li- fetime was significant, when analyzed by variance within and between classes, but an insignificant correlation (—.06) shows that the relation is curvilinear.