THE EFFECT OF YEAR AND SEASON OF BIRTH ON THE GROWTH INTENSITY OF REPLACEMENT HEIFERS

The effect of the year and season of birth on the growth intensity of replacement heifers was studied in 1262 animals of Ukrainian Red and Black-and-White Dairy and Holstein breeds born between 2013 and 2021 in the breeding herd of LLC "Ahrofirma "Svitanok" in Mariinka district, Donetsk region. The dynamics of live weight and average daily gains from birth to 18 months of age were evaluated. It was found that the season of birth has a multidirectional effect on the growth rate of animals during the first year of life. The most significant effect is observed during the period of intensive sexual maturation (6–12 months). In the first three months of postnatal development (the milk period), heifers born in the autumn and winter seasons were distinguished by higher average daily live weight gains. Due to compensatory growth mechanisms, the intergroup difference in live weight of heifers is practically eliminated by one year of age and remains low up to 18 months. The year of birth has a more significant impact on the dynamics of heifers' live weight compared to the season. Heifers born in the later years of the study (2017–2021) showed higher live weight and gain indices, which is consistent with the increase in the conventional bloodline of the Holstein breed. Analysis of variance confirmed the significant effect of the year of birth (4.2–28.3% of phenotypic variability in live weight and 2.8–28.2% of its average daily gains). The influence of the season was noticeably lower (correspondingly 0.04–13.2% and 1.4–24.8%). Thus, the established significant influence of farm and climatic conditions in different years of birth on the age-related dynamics of live weight and growth intensity of heifers, along with the increase in the conventional bloodline of the Holstein breed, may have the most significant impact on the subsequent dairy productivity of cows.