Effects of xylazine on early bovine pregnancy

Abstract

The effects of xylazine on early pregnancy in cattle were determined in 39 beef heifers in the first trimester of gestation. Fetal heart rate ( n = 13), maternal heart rate ( n = 8) and uterine contractility ( n = 13) were observed at 3 min before and 3, 10, 60 and 120 min after the administration of xylazine (20 mg i.v.). All heifers were examined for fetal heart beats 1 week after the administration of xylazine. There was a decrease ( P < 0.01) in fetal and maternal heart rate at 3 min after injection. The fetal heart rate returned to pretreatment values within 60 min, whereas maternal heart rate was still depressed at 120 min after treatment. Uterine contractility was markedly increased ( P < 0.001) at 3 min after xylazine administration, but returned to normal within 120 min. Fetal heart beats were observed in 38 of the 39 heifers 1 week after xylazine treatment. A second experiment was designed to assess the effects of increased uterine activity on fetal heart rate after the injection of oxytocin (20 IU i.v., n = 11), the PGF 2α analogue cloprostenol (500 μg i.v., n = 7), and the uterine relaxant clenbuterol (300 μg i.v., n = 11) administered together with xylazine (20 mg i.v.). Oxytocin and cloprostenol caused an increase ( P < 0.05) in uterine contractility, but no changes in fetal heart rate. The injection of xylazine with clenbuterol resulted in a shorter, less pronounced increase in uterine contractility and less depression of fetal heart rate than xylazine alone. It was concluded that the decrease in fetal heart rate observed after the administration of xylazine to the dam is most likely the result of a specific pattern of increased uterine contractility and/or α 2 − receptor-mediated vasoconstriction, resulting in reduced uterine blood flow and subsequent fetal hypoxia and bradycardia. The changes in uterine contractility and fetal heart rate caused by a single injection of 20 mg xylazine in heifers in the first trimester of gestation have no short-term adverse effects on pregnancy.