Impact of Oxytocin-Milking Method on Lactation Performance and Lactation Length of Sheep

Determination of milk yield potential using an accurate method is essential for assessing nutritional re-quirements, evaluation of genetic potential, lamb growth and survival, management decisions and improv-ing performance traits of sheep flocks. Exogenous oxytocin injection is applied to estimate milk secretion rate in sheep. Oxytocin is a neurohormone produced in the hypothalamo-neurohypophysial system, which is released into the blood in response to teat stimulation. It has a galactopoietic effect and plays an important role in lactation acting on milk ejection via the secretory activity and contraction of myoepithelial cells. One hundred singleton-bearing and rearing Makui breed ewes were assigned into two treatments: group I (hand milking and stripping following the intramuscular injection of 2.5 IU of oxytocin) and group II (hand milking and stripping). The study aimed to examine the effect of exogenous oxytocin injection on milk yield, milk composition and lactation length of ewes during the entire lactation period. Data were recorded at fortnightly intervals from post-partum day 14 onward. Oxytocin treatment group produced significantly more average daily milk yield (562.3 vs. 301.4 g) and total milk yield (93.6 vs. 38.5 kg) compared with non-treated animals (P < 0.05). Lactation length was 145.7 days and 115.2 days for the oxytocin and the control group, respectively (P=0.038). Oxytocin injection increased milk peak yield and lactation persis-tency (P < 0.05) while shorter peak time (34.1 vs. 39.2 days) was observed for oxytocin group (P=0.043). Oxytocin administration increased milk fat percentage (6.73 vs. 5.86) in treated compared to control ewes (P=0.029). However, no difference was detected in the percentage of protein and lactose associated to the oxytocin injection (P > 0.05). The development of milk ejection inhibition throughout lactation during the exclusively suckling period, followed by hand-stripping, entails the need for oxytocin to sustain mammary cells and lactation persistency in ewes. It further confirms that oxytocin allows overcoming the negative feedback mechanisms exerted on milk ejection, enhancing the intracellular transfer of secretory vesicles and contraction of myoepithelial cells and thereby increasing milk production and lactation length, which is a useful technique to ensure complete alveolar milk let-down and optimum milk withdrawal from the cistern.