The Babcock Institute University of Wisconsin Dairy Updates

212 Milk letdown, in most of the species studied, occurs after the activation of a neuroendocrine mechanism that releases oxytocin in the blood stream. The released oxytocin contracts the myoepithelial cells that surround the alveoli, forcing the expulsion of the milk. However, the pattern of milk ejection varies remarkably from species to species and could be related to the need or degree of oxytocin release at milking. Other factors that may interfere in the physiology of milk ejection reflex among species are number of offspring, frequency and duration of nursing, and the anatomical structure of the mammary gland (e.g., presence of mammary cisternae). This paper compares stimulation requirements for milk letdown of some species, and in particular, results of a study carried out in Brazil with crossbred cows (Holstein and Zebu type cattle), where the presence of calves during milking was not associated with enhanced milk letdown. 1 Debora Costa is a Graduate Research Assistant in the Department of Dairy Science; and Dr. Douglas J. Reinemann is a Professor in the Departments of Biological Systems Engineering, Dairy Science and the UW Milking Research and Instruction Lab at the University of Wisconsin-Madison. 2 Paper presented at the 100th Centenary International Dairy Federation World Dairy Summit Special Conference: 100 Years with Liners and Pulsation. September 2003, Bruges, Belgium. Comparative Physiology of Milk Removal A common mechanism of milk ejection is the neuroendocrine mechanism in the majority of species studied. However, there are differences among species in the need or degree of oxytocin release at milking [1]. Two animal models are used to explain the different patterns of the milk ejection reflex. In the rabbit model, initial suckling by the litter induces the release of a single pulse of 20 to 50 mμ oxytocin and milk removal is completed in two to five minutes. In the rat model, multiple pulses of 0.5 to 1.0 mμ of oxytocin are released at intervals of 5 to 15 minutes throughout suckling periods of 30 to 60 minutes. The sow’s milk ejection is similar to the rabbit model whereas human and ruminants’ are more similar to the rat model [14]. There are large differences in the proportion of total milk stored within the cistern among ruminant dairy species. Specialized dairy cows store less than 30 percent of the total milk yield volume in the cistern after a normal milking interval [4]. In contrast, the cisternal milk accounts for up to 75 percent in dairy goats [27], and in sheep it ranges from more than 50 percent for dairy breeds [29] to less than 30 percent for meat breeds [12]. It is argued that milk ejection may not be essential for adequate milk removal in The Need for Stimulation