Effects of exogenous oxytocin on production and milking variables of cows.

Five doses of oxytocin (.5, 1.0, 1.5, 2.0, 3.0 IU) were injected through the subcutaneous abdominal vein (milk vein) of 15 Holstein Friesian cows 1 min before machine attachment. After teat cup removal, cows were given 10 IU of oxytocin via the milk vein for measurement of residual milk 1. An additional 10 IU of oxytocin was given 15 min later for estimating residual milk 2. Total milk yield was slightly higher for cows receiving 2.0 and 3.0 IU oxytocin. Milk fat yield was highest for cows receiving 2.0 and 3.0 IU oxytocin. Percent milk protein was less for cows receiving higher doses. Administration of 2.0 or 3.0 IU oxytocin significantly reduced total residual milk yield as compared with other doses. Fat percent in total residual milk was not affected by administration of oxytocin. Machine-on time, peak flow rate, and average milk flow rate did not differ with oxytocin treatment. A dose of either 2.0 or 3.0 IU oxytocin given via the milk vein will lead to slightly higher milk yields but will not affect milk flow dynamics as compared with lower doses. Moreover, milk fat yield is increased and total residual milk yield is decreased with either 2.0 or 3.0 IU oxytocin, suggesting that these doses result in more efficient extraction of milk. Administering more than 3.0 IU of oxytocin intravenously to promote milk ejection should be of no advantage.