Residual milk in Friesland sheep and the galactopoietic effect associated with oxytocin treatment

Mature lactating Friesland ewes had a mean lactation yield of 293 +/- 26 kg during a lactation period of 35 +/- 2 weeks giving an average daily milk yield of 1.2 kg/d. Ewes were injected intravenously after normal milking with either saline (sham) or oxytocin and then remilked to determine the volume of residual (alveolar) milk. After a long milking interval of 16 h oxytocin treatment gave a significantly greater total daily milk yield than the sham treatment (oxytocin minus saline, morning milking, 0.199 +/- 0.038 kg, mean +/- s.e.m., P less than 0.01) Oxytocin had a small significant reverse effect after a short milking interval of 8 h (afternoon milking, -0.065 +/- 0.022 kg, P less than 0.05). The average increase in total daily yield over four stages of lactation was 0.133 +/- 0.029 kg (P less than 0.01), or 11% of the average daily milk yield. The galactopoietic effect of oxytocin was associated with the efficient removal of residual (alveolar) milk. Residual milk accounted for 7.4 and 27.2% of the total daily milk yield in ewes treated with saline or oxytocin respectively. Residual milk expressed as a proportion of daily total milk yield remained steady in ewes studied between April and July, but declined in September when yields were less than 1 kg/d.