Animal Science

A large dose of oxytocin is often injected intramuscularly before each milking to increase milk production. If the injection is given after milking, a slightly different effect may be observed. This phenomenon has significant implications for industrial dairy management, which are discussed in the following text.

            Milk is synthesized in the secretory tissue of the mammary gland: the epithelial cells of the alveoli. It is then stored in the alveolus, ducts and cisterns. A full udder usually has less than 20% of the milk stored in the gland and teat cisterns, while the rest is stored in the ducts and alveoli (Svennersten-Sjaunja, 2004). When a cow is milked out, a fair volume of milk (10 to 20% of total milk) may still be left in the udder, known as residual milk. Large amounts of residual milk will result in lower daily milk yields and total production compared to cows with small amounts of residual milk. The amount of residual milk in the udder may be measured by injecting the cow with oxytocin and milking again after waiting a period of one minute (Najah, 2014). Oxytocin signals for the milk ejection reflex, which must be induced for milk to be let down but is not under conscious control by the animal. The milk ejection reflex is stimulated by the binding of oxytocin to myoepithelial cells lining the alveoli and the mammary duct system. The myoepithelial cells contract, increasing the intra-alveolar pressure causing expulsion of the milk from the alveoli through the ducts into the cisternal system. The release of oxytocin is needed throughout milking for fast and complete milk removal (Tancin & Bruckmaier, 2001).

            Imitating the natural stages of suckling stimulation may help to maximize milk production. Pre-stimulation, milk intake, and post-stimulation have physiological significance regarding milk let down and production. The following information obtained from the “Science Behind Milk Ejection” article by Kerstin Svennersten-Sjaunja outlines the effects of stimulation at various stages of milking on milk let down. Pre-stimulation involves cleaning the teats, drawing a small amount of milk, and massaging the teats. In a milking parlour these tasks are performed by laborors before the milking unit is attached. These stimuli evoke the milk ejection reflex by stimulating the natural release of oxytocin by the posterior pituitary. It has been found that manually stimulated cows have increased milk production by up to 30% and the lactation period has been prolonged (Phillips, 1984; Phillips, 1986). The stimulation of the teats is thought to stimulate oxytocin release to coincide milk let down with milking out by offspring or a milking unit. The physical stimulus carries an impulse from nerves in the udder or teats to the supraoptic nerve and paraventricular nuclei of the hypothalamus, which release oxytocin from their secretory terminals in the pituitary gland (Tancin & Bruckmaier, 2001). The oxytocin then stimulates the milk ejection reflex in the mammary gland, as previously described. This is important when udder-fill is low or milking interval is short in order to prevent milking on empty teats. The greater yield associated with pre-stimulation is probably due to the increased efficiency in udder emptying as a greater proportion of residual milk is removed.

            Post stimulation or stripping after milking have been shown to increase milk production over time. The likely explanation is that the residual milk is more efficiently removed from the udder, which also results in the removal of the inhibitor FIL. This gives opportunity for glandular secretions to accumulate in greater volumes post milking than in an udder where the residual milk is not removed. Inhibitor FIL signals to the epithelial cells of the alveoli that milk is present in the lumen, so inhibits milk secretion. Its removal results in increased production.

            Stimulation of the udder and teats results in the natural release of oxytocin from the pituitary gland, so it is not surprising that direct injection of oxytocin has the same effect as stimulation on milk production. Injecting with oxytocin just prior to milking results in a higher yield. Studies have found that let down of milk by a cow under the influence of injected oxytocin just prior to attachment of the milking cups will milk out more completely, thus maximizing production (DairyNZ, Milk Let Down, 2016). The advantage of oxytocin injection is that time is saved when manual stimulation is not required to induce oxytocin release, and actually increases production even more than physical stimulation alone. If the injection is given just after milking, the residual milk will be ejected from the alveoli into the cisterns and could be stripped or simply collected at the next milking. Stripping after milking is generally not practical for farmers. Whether it is applied before or after milking, the result of oxytocin injection increases milk production by roughly 3% (Ballou, Bleck, Bleck, & Bremel, 1993). By mobilizing more of the residual milk from the secretory tissue, more milk is obtained there is a greater volume of space available for milk secretion throughout the next grazing period, allowing increased gland output.

To maintain milk secretion a threshold level of oxytocin needs to be maintained throughout milking (Bruckmaier, Schams, & Blum, 1994), which means a high concentration of oxytocin must be injected before milking to maintain the threshold concentration. Unfortunately, repeated injections of high doses may have a negative feedback effect on the natural release of oxytocin. There is a refractory response, where normal hormonal functions do not resume for some time. The more frequently that oxytocin is used, the longer the refractory period. Eventually, the cow could develop a dependence on oxytocin injections, being unable to let down milk on her own because the hypothalamic-pituitary axis is insensitive to physical stimuli. This is known as central inhibition, where oxytocin fails to be released in response to pre-stimulation and milking (Bruckmaier, Schams, & Blum, 1994). Injections of oxytocin post-milking do not interrupt the natural response of oxytocin release by pre-stimulation and during milking. For this reason, my hypothesis is that routine injections of oxytocin post-milking could be used to increase milk production without the negative effects of dependence on synthetic oxytocin injections, although I have not found any studies coming to this conclusion.

Additionally, the cow may begin to associate milking with pain due to jabbing when oxytocin is routinely administered. The stress that this causes can inhibit milk let down on a peripheral level. The concentration of cortisol, a hormone known to be released in response to stress, is found to be significantly higher in cows with lower milk yields (Negrao & Marnet, 2006), and it causes oxytocin blockage. The tissue of the gland itself fails to respond to normal levels of oxytocin due to the action of catecholamines, which stimulate alpha-adrenergic receptors and cause contraction of the gland and teat cisterns so that milk ejection is inhibited (Bruckmaier, Schams, & Blum, 1994). This is a short term response that can be remedied by either ceasing oxytocin administration, or administering it with a method that puts the cow under less stress.

It is safe to say that injection of oxytocin either before or after milking out will increase the milk yield and total production of the cow. However, the increase in yield is due to different physiological factors that must be taken into account and the method of oxytocin application will depend on the priorities of the farmer. It is always advisable to use methods of administration that will reduce stress on the animal. If oxytocin is injected after milking, the subsequent collection of milk is likely to have a higher concentration of milk fats and proteins since it was in the alveoli where protein and fat are deposited. But, the impracticality of milking out twice for one cow at each milking means that injecting oxytocin after applying the cups is probably not a method a farmer would want to employ. It would increase the amount of time that cows are in the shed when they could be out grazing. The cost of spending extra time in the shed is probably greater than the potential profit of a bit of extra milk and possibly a higher concentration of milk solids. Injection pre-milking is the most convenient and profitable for the farmer because it results in faster milking out, meaning less time in the shed and more time in the field, more time for the farmer to do other tasks, and the cow milks out more completely.