Effects Of Thermal Environment, Pregnancy And Level Of Production On Thermoregulation, Blood Constituents And Endocrine Responses In Crossbred Dairy Cows

Abstract

The main objective of this study was to evaluate the effects of seasonal changes in thermal environment and pregnancy on the physiological responses of crossbred dairy cows (Butana x Friesian). Also, it was intended to examine the nycthermal changes in physiological parameters in highand low producing cows under local tropical conditions. In experiment 1, crossbred multiparous dairy cows were divided into four groups according to reproductive state (empty, early, mid and late pregnancy). The cows were exposed to a normal grazing programme under natural summer and winter conditions. The rectal temperature (Tr) and respiratory rate (RR) were significantlyhigher during summer than during at different stages of pregnancy; there was an increase in Tr and RR with the advance of pregnancy. The packedcell volume (PCV) and haemoglobin concentration (Hb) were significantly higher in summer; however, both parameters decreased in late pregnancy. The cows had higher total leukocyte count (TLC) in winter only during early pregnancy. The TLC was significantly higher in winter comparedto the respective summer value in early pregnancy. For all experimentalgroups serum levels of glucose, cholesterol, triglycerides and urea were significantly higher during summer than winter. Plasma glucose levels decreased with the advance of pregnancy. There was a tendency for decrease in triglyceride level with the advance of pregnancy. The urea level tended to behigher during early pregnancy in both seasons. The activity of serum aspartate aminotransferase (AST) was higher, while alanine aminotransferase (ALT) was lower in empty group and late pregnancy during summer than during winter. The ALT activity increased significantly with the advance of pregnancy and the AST level was iv significantly higher during late pregnancy in both seasons. Serum calcium (Ca) level was higher in the empty group and mid pregnancy and serum phosphorus (P) level was lower in all experimental groups during summer. There was a decrease in Ca level and an increase in P level with advance of pregnancy. Serum magnesium (Mg) level was significantly higher during winter in empty and early pregnancy.Serum thyroid stimulating hormone (TSH) was elevated in summer than in winter. Serum triiodothyronine (T3) was higher in summer, and there was slight decrease with the advance of pregnancy in both seasons. Serum thyroxine (T4) was lower in summer in all groups. Serum cortisol increased in response to summer heat load, and was higher during late pregnancy in both seasons. In experiment 2, twelve crossbred (Butana x Friesian) empty multiparous dairy cows were selected and divided into two groups (high and low producing dairy cows). The nycthermal changes in physiological parameters were monitored for 48 hours. Rectal temperature (Tr ) increased in the afternoon (12:00 p.m.) in both groups. The high producing cows maintained higher Trvalues during the course ofthe study. Respiratory rate (RR) was increased the afternoon in both groups of cows; the high producing cows maintained higher RR values than low producing cows. Heamatocrit and Hb concentration were slightly higher in high producing cows than in low producing cows. Serum levels of glucose, cholesterol and triglycerides showed no significant diurnal variation in both groups of cows. Serum urea concentration showed relative instability in both experimental groups. Serum levels of glucose, cholesterol, triglycerides and urea were lower in high producing cows than inthe low producing cows.AST and ALT showed irregular diurnal variation in the both high and low producing cows. The high v producing cows maintained slightly higher aminotransferase enzymes activity. Serum cortisol level was higher at 12:00 p.m and 6:00 p.m. compared to values monitored in the morning (6:00 a.m.) and night (12:00 p.m). The cortisol level was higher in high producing cows than in low producing cows. Serum insulin level did not reveal significant diurnal variation in both groups of cows. Highproducing cows maintained higher serum insulin level than in the low producing cows. The results indicated that under local tropical conditions when the temperature humidity index (THI) exceeds 75 during summer, dairy cows were exposed to heat stress conditions which affect thermoregulatory response, serum metabolite level and endocrine responses that could impact negatively productivity and reporoductivity performance. The stage of pregnancy had significant influence onthermoregulation, erythrocytic parameters, serum levels of Ca, P and cortisol. The nycthermal effect did not influence most of the parameters, because there was no critical change in climatic conditions during the course of the study. High milk yield influenced thermoregulation, blood constituents and endocrine responses.