MILK PRODUCTIVITY OF THE FIRST-CALF CALVING HEIFERS OF HOLSTEIN AND SIMMENTAL BREEDS WITH DIFFERENT LEVEL OF REPRODUCTIVE QUALITIES
The aim of the research is to study the effect of reproductive qualities on the milk production of the first-calf calving heifers of Holstein and Simmental breeds. The results indicate a significant effect of the age of the first insemination on milk yield for 305 days of lactation of the first heifers of the Holstein breed. In animals of the Simmental breed, there was a tendency for the growth of milk yield depending on a decrease in the time of the first insemination. At the same time, there has been a significant increase in the amount of milk fat, protein, fat and protein content in first-calf heifers of both breeds, which may justify fixing in the technology of reproduction of the age of first insemination less than 17 months. Similar patterns were obtained by analyzing the age of the first calving. In animals of the Holstein and Simmental breeds with an earlier calving age, the milk yield was significantly higher than in their peers calving at the age of 26-28 months. In first-calf heifers of both breeds, a pronounced tendency for an increase in milk yield per 305 days of lactation was revealed with a service period of 90-110 days. It was shown that in Holstein animals with a longer service period, the fat content in milk increased, while in their peers of Simmental breed, the percentage of fat showed a downward trend. Among the heifers of Holstein breed, a higher percentage of protein in milk was found in animals with a shortened and elongated service period. Among their peers of the Simmental breed, there were no significant differences in this indicator of milk productivity between groups. The integral characteristic of reproduction - period between calving - showed an advantage in terms of milk productivity of cows with a duration of about one year, which confirms the physiological validity of such a length of the reproduction cycle.