SEASON OF DELIVERY, AGE OF FIRST FERTILISATION AND MILKING YIELD DURING THE FIRST LACTATION PERIOD OF HEIFERS

In recent years, in some regions of the Republic of Bulgaria production of cow's milk has been focused on the construction and establishing industrial cow breeding farms (complexes). In order to meet the need to ensure relevant increase of cows population for such large farms, it is important to provide young reproduc-tive animals. The aim of this study is to evaluate correlation between the age of fertilisation versus the birth season (autumn, winter, spring, summer) and the quantity of cow's milk during the 305 lactation days. Many authors share the opinion about impact of the season of birth of female calves and the type nutrition as key factors influencing all these indicators. We have set us the task to find out, what is the role of these issues in a large, industrial, cow breeding complex of 6.270 large ruminants (bovines). Within our surveys, the age of the first fertilization of the heifers included in reproduction campaign is really dependent on the season of their birth. The earlier is the fertilization of those born in summer-autumn, respective time gap being 24 days compared to those born in the autumn-winter part of the year. The average milk yield of 616 primers (of first calf) for 305 lactation days is 8.164 kg. This milking rate is by 355 kg more than the one demonstrated by those born during the spring-summer season compared to those born in the autumn-winter season.