Effects of pregnancy and lactation on thyroid hormones, insulin, and metabolic blood parameters of modicana dairy cows
Pregnancy and lactation induce dynamic endocrine and metabolic adaptations in dairy cows oriented to the redirection of nutrients, in order to promote the fetal growing and the milk production. Thus, the aim of the present study was to evaluate the effects of different physiological stages (before insemination: non-pregnancy), of gestation (<25, 26-100,>100 days), and lactation (<60, 61-120, 121-180, 181-240, and >240 days) on thyroid hormones, insulin, and metabolic blood parameters in Modicana cows under a dairy semi-intensive management. Blood samples were in a one-year period from 10 healthy multiparous Modicana cows to measure circulating thyroid stimulating hormone (TSH), total and free triiodothyronines (T3, fT3) and thyroxines (T4, fT4), insulin, glucose, triglyceride, and total cholesterol concentrations. The gestational phase showed higher T4 at <25 and >100 d than non-pregnancy (P <0.01), higher insulin at 26-100 d than the rest of pregnancy and non-pregnancy (P <0.0001), and lower glucose concentrations at >100 d than non-pregnancy (P <0.007). During lactation, the lowest insulin concentrations were found at <60 d and the highest at >240 d (P <0.02), whereas the opposite trend was obtained for glucose (P <0.01). No other significant differences were found. Taking together the significant increase of both T4 (<25 and >100 d of pregnancy) and insulin (26-100 d of pregnancy) concentrations, it could be suggested that these hormones are metabolically involved along the whole physiological period. In addition, the highest concentrations of glucose at the start of lactation (>60) until 120 d, and the lowest values at the end, partially reflect the changes observed for insulin, confirming that glucose is the most important substrate for milk production and that insulin production and sensitivity is in part reduced for glucose uptake by peripheral cells, especially in the first part of lactation to mount the glucose uptake by the mammary gland. The assessment of breed-specific hormonal and metabolic changes allows to understand the adaptive response of local breeds that are not yet strictly selected for milk production but that could become.