The relationship of leptin and thyroid hormones with milk yield and some fertility parameters in Simmental dairy cows during transition period

: In this study, transition period cows were used to test the hypothesis that serum levels of leptin and thyroid hormones would correlate the production efficiency such as milk yield, body condition score, and fertility in dairy cows raised under the same controlled conditions. The study comprised 37 Simmental cows, which were 3 years old and between 257–265 days of gestation. Serum levels of leptin, T3, T4, FT3, and FT4 hormones were measured in blood samples obtained from the animals, which were in the period of 3 weeks before and 3 weeks after delivery. Prepartum serum leptin levels were significantly (p < 0.05) lower in cows having increased milk production compared to the first lactation peak value. Also, postpartum serum FT3 levels were significantly (p < 0.05) lower in cows having milk yield of ≥ 30 kg. Leptin levels were found to be inversely related with milk production and fertility. FT3 levels exhibited a negative correlation with milk production and similarly T3 levels showed an inverse correlation with body condition score. The present study proposed that routine monitoring of leptin and thyroid hormones would be useful for the reproductive management of cows with breeding problems.