Indicators of lipid metabolism and their relationship with the thyroid status during the dry and post-calving periods in cows with different fertility

In black-and-white cows with reduced fertility, during the dry and early post-calving periods, changes in the functioning of the thyroid system are observed, which differ from those in individuals with high fertility. Such changes may lead to a lack of energy sources during the critical transition period.Purpose: To study the dynamics of changes in the lipid content before and after calving and its association with the content of thyroid hormones in the blood of black-and-white cows with various fertility.Materials and methods. Black-and-white cows of lactation 2-4 were used in the study. The animals were bled 6, 4 and 2 weeks before calving and 1, 3, 7 and 13 weeks after calving. In blood serum samples, the content of lipids was determined using a biochemical analyzer, as well as thyroid hormones and progesterone using ELISA. Cows were inseminated after treatment according to the Ovsynch protocol, and pregnancy was assessed on Day 33 based on ultrasound examination and progesterone levels in the blood.Results. The cows were divided into two groups: I – pregnant individuals (n=17) and II – individuals that remained infertile (n=12). The milk productivity of animals and their BCS before and after calving were similar in the compared groups. Thirteen weeks after calving, the cholesterol content in the blood of cows of group II was 1.2 times lower (p<0.05) compared to this content in animals of group I. At the same time, 6 weeks before calving, the concentration of phospholipids in the blood of individuals of group II was 1.3 times higher than that of individuals of group I (p<0.05). In addition, 3 weeks after calving, the level of triglycerides in cows of group I was 1.2 times higher (p<0.05) than that in animals of group II. During the postpartum period, the blood levels of cholesterol and triglycerides in animals of group I positively correlated with the content of total thyroxine, whereas such a correlation was not detected in group II.Conclusions. Thus, the metabolic state of cows with higher fertility was characterized by an increased supply of lipid energy sources during the post-calving period compared to animals with low fertility. At the same time, the reduced content of triglycerides at the early stage of lactation in the blood of cows that remained infertile points to their increased accumulation by the liver. A positive relationship between the concentration of total thyroxine in the blood and the concentration of triglycerides and cholesterol, revealed during the post-calving period in cows with high fertility, suggests the participation of the thyroid hormone in maintaining the reproductive function of these animals by modulating lipid metabolism.