Changes in Alveolar and Cisternal Compartments Induced by Milking Interval in the Udder of Dairy Ewes

Abstract

The aim of this work was to evaluate the effects of milking interval (4, 8, 12, 16, 20, and 24h) on cisternal size and milk partitioning (cisternal and alveolar) in the udders of dairy ewes. Twenty-four dairy ewes (Manchega, n=12; Lacaune, n=12) were used in a 2-wk experiment during mid-lactation. Cisternal and alveolar milk yields were measured and milk samples from each udder fraction were collected for analysis. Cisternal milk was obtained after i.v. injection of an oxytocin receptor antagonist, and alveolar milk was obtained after i.v. injection of oxytocin. Enlargement of the cisternal compartment due to milking intervals was measured by ultrasonography for each half udder. Volumes of cisternal and alveolar milk differed according to breed, being greater in Lacaune (888±43 and 338±25mL, respectively) than in Manchega ewes (316±43 and 218±25mL, respectively). Alveolar milk increased linearly to 16h in Manchega and 20h in Lacaune and remained constant thereafter. Cisternal milk accumulated linearly to 24-h milking intervals in both breeds. Cisternal area (values per udder half) increased as milking interval increased, reaching a plateau at 20h in Manchega (21±1cm2) and 16h in Lacaune (37±1cm2). Correlation between cisternal area and cisternal milk was the greatest at 8h (Manchega: r = 0.70 and Lacaune: r = 0.56). Cisternal area correlated with total milk (r = 0.80). Milk fat content varied markedly with milking intervals, increasing in alveolar milk (until 12h in Manchega, 8.90±0.18%; and 20h in Lacaune, 8.67±0.19%) and decreasing until 24h in cisternal milk (5.74±0.29% and 4.85±0.29%, respectively). Milk protein content increased in alveolar milk until 24h (Manchega, 6.46±0.11%; Lacaune, 5.95±0.11%), but did not vary in cisternal milk. Milk lactose content only decreased at the 24-h milking interval in the cisternal milk of Manchega ewes (4.60±0.04%). In conclusion, our results suggest that cisterns play an important role in accommodating secreted milk during extended milking intervals. Thus, long milking intervals could be a recommended strategy for large-cisterned dairy sheep. Evidence indicates that ultrasonography provides accurate estimations of udder cistern size and could be used as an indicator for selecting large-cisterned dairy ewes.