The objective of this study was to examine whether an oxytocin challenge test (OCT), evaluated by mea-suring the changes in uterine blood flow using transrectal colour Doppler ultrasonography, is a suitable non-invasive method to determine uterine contractility in puerperal dairy cows. The changes in uterine blood flow during uterine contractions induced by oxytocin were evaluated on days 2 and 5 postpartum (pp). Twelve clinically healthy Holstein cows were randomly assigned into two groups: (1) oxytocin group (n = 7), 50 IU oxytocin injected IM and (2) control group (n = 5), 5 mL saline injected IM. Blood flow volume (BFV) and pulsatility index (PI) in the uterine arteries were determined before and after injection for 120 min on days 2 and 5 pp.
BFV declined and PI increased rapidly after oxytocin injection on day 2 (P < 0.05), whereas oxytocin on day 5 pp did not cause changes in blood flow parameters. The result confirmed that uterine responsive-ness to oxytocin decreases with time postpartum in healthy cows. The same OCT was applied in cows with retained fetal membranes (n = 6) on day 2 pp, however uterine blood flow showed no change after oxytocin injection. The results showed that an OCT on day 2 pp may be a useful method for investigating the uterine contractile response to oxytocin (reflected as the decrease of uterine blood flow) and the potential pathophysiology of uterine involution in cows.