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Milk ejection during machine milking in dairy cows
Livestock Production Science 70 (2001) 121–124 www.elsevier.com / locate / livprodsci
Milk ejection during machine milking in dairy cows Rupert M. Bruckmaier Institute of Physiology, Technical University of Munich-Weihenstephan, Weihenstephaner Berg 3, D-85350 Freising, Germany
Abstract Only a small percentage of cisternal milk, usually less than 20 percent is immediately available for milk removal in dairy cows. Most of the milk, the alveolar milk, is stored in alveoli and small milk ducts and is fixed by capillary forces. Removal of the alveolar fraction requires active expulsion of milk into the cisternal cavities by the milk ejection reflex. Tactile teat stimulation causes release of oxytocin (OT) from the posterior pituitary into circulation. Myoepithelial cells, surrounding the alveoli, contract in response to elevated OT concentrations and the alveolar milk is shifted through the milk duct system into the cistern to be removed by the milking machine. The delay in milk ejection after the start of teat stimulation changes during the course of lactation and depends of the interval from previous milking. Milk ejection is not completed with the occurrence of alveolar milk in the cistern during early milking. The process of shifting milk continues until the end of milk removal, and continuously elevated concentrations of OT throughout the entire milking are essential to maintain milk ejection until the udder is emptied. In cows with very high milk flow rates during early milking, milk ejection may be a limiting factor for milk flow towards the end of milking. 2001 Elsevier Science B.V. All rights reserved. Keywords: Milk ejection; Oxytocin; Milk flow; Alveolar fraction; Cisternal fraction
1. Introduction Breeding of dairy cows has resulted in animals that are highly adapted to the requirements of dairy farming, including machine milking. The basic physiological regulation of lactation, however, has remained intact despite selection. Modern machine milking requires complete and fast removal of the stored milk under optimal hygienic conditions to maintain high milk yield, product quality and animal health at a low cost. The physiological regulation of milk ejection and milk removal needs to be considered for milking machine equipment and milking routine in order to meet the requirements of the animal and to optimise the interaction between cow and technology.
2. Distribution of milk fractions before milk ejection Between milkings, the milk secreted by the epithelial cells accumulates in alveoli and cisterns. While the milk in large mammary ducts and cisternal cavities (cisternal fraction) is immediately available for milking, milk stored in alveoli and small milk ducts (alveolar fraction) is fixed by capillary forces and requires an active expulsion into the cistern which is termed milk ejection or milk let-down. The cisternal fraction usually amounts to less than 20 percent of the total stored milk fraction after an interval of 12 h from the previous milking (Pfeilsticker et al., 1996). Shortly after milking there is almost no cisternal milk present (Knight et al., 1994;
0301-6226 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0301-6226( 01 )00204-4
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Bruckmaier and Hilger, 2001). The rate of milk transferred into the cisternal compartment is increasing with time from previous milking (Knight et al., 1994). Cisternal milk yield and fraction are highest at peak lactation and decrease towards the end of lactation (Pfeilsticker et al., 1996; Bruckmaier and Hilger, 2001). Thus, lowest amounts of milk available for milk removal before milk ejection are present after short intervals from previous milking and in late lactational stages. A close positive correlation between cisternal size and lactation number was shown (r 5 0.90) and the largest cisternal fractions were measured in the oldest cows (Bruckmaier et al., 1994a). The alveolar fraction represents most of the milk stored in the udder of dairy cows throughout lactation. To have this fraction rapidly and completely available during milking, synchrony between alveolar milk ejection and removal is essential.
3. Oxytocin release and milk ejection Milk ejection is the active transport of alveolar milk into the cisternal compartment. It consists of: (1) contraction of the myoepithelial cells that surround the alveoli like a basket; and (2) transfer of the milk through the milk duct system. Milk ejection is an innate reflex that is not under conscious control of the animal and occurs in response to tactile stimulation of the mammary gland through a neuroendocrine reflex arc (Crowley and Armstrong, 1992). Neural receptors which are sensitive to pressure are mainly located in the teat tissue. Via these receptors the neural signal of the tactile stimulus is carried to the brain, terminating at the supraoptic and paraventricular nuclei of the hypothalamus. These nuclei are clusters of nerve cell bodies that synthesize the nonapeptide oxytocin that travels, attached to the carrier protein neurophysin I through the pituitary stalk via the nerve cell axons. Oxytocin is stored in neurosecretory terminals of the posterior pituitary gland (Crowley and Armstrong, 1992). Within the hypothalamic nuclei, the same neurons that produce oxytocin receive excitatory signals by cholinergic neurons to transmit the signal to the pituitary gland. Release of oxytocin, i.e. exocytosis of the neurosecretory granules, is a Ca 21 -dependent event that is
normally induced by the depolarization of secretory terminals by invading action potentials from magnocellular neurosecretory neurons. In response to elevated oxytocin blood concentrations, stimulation of the oxytocin receptors causes myoepithelial cell contraction (Soloff et al., 1980). As a consequence, alveolar milk is forcefully shifted in the larger free-draining ducts and then into the cisternal space. This in turn, causes a rapid increase of pressure within the cistern (Bruckmaier and Blum, 1996) and an enlargement of the cisternal cavity size (Bruckmaier and Blum, 1992). However, not all alveolar milk can be ejected if no milk is removed from the udder. Further milk is ejection during the course of milk removal (Bruckmaier et al., 1994b). Milk ejection occurs up to a maximal intramammary pressure as soon as oxytocin surmounts a threshold concentration (Schams et al., 1984; Bruckmaier et al., 1994b). Only exogenous oxytocin, injected in supraphysiological amounts causes additional milk ejection. This effect is experimentally used to determine the milk remaining in the udder (residual milk) after normal milking.
4. Milk ejection at the start of milking The lag time from start of tactile teat stimulation until onset of milk ejection normally ranges from 1 to 2 min and depends on the degree of udder filling (Bruckmaier et al., 1994b; Bruckmaier and Hilger, 2001). The degree of udder filling mainly depends on the stage of lactation and on the actual interval from the last milk removal. Mainly towards the end of lactation, milk production decreases and the udder does not well fill up during normal milking intervals at twice daily milking. If milking intervals are in addition very short, as is the case in automatic milking systems with voluntary milking of the cows, milk ejection may occur only 3 minutes after the start of tactile stimulation (late lactation, 4-h interval between milkings). In an experiment, mean lag time from start of stimulation until milk ejection commenced was 50 s in early lactation after 12 h and 91 s in late lactation after 4 h from previous milking (Bruckmaier and Hilger, 2001). The variable occurrence of milk ejection is not due to different oxytocin release. During the course of lactation the release of
R.M. Bruckmaier / Livestock Production Science 70 (2001) 121 – 124
oxytocin at the start of milking is rather increasing than decreasing (Mayer et al., 1991). The delayed milk ejection at low degrees of udder filling is likely due to a delayed response to the oxytocin at the level of the mammary gland. Much more myoepithelial contraction is needed to press milk out of an incompletely filled alveolus than of a completely filled alveolus. More contraction of myoepithelial cells likely needs more time and the occurrence of alveolar milk ejection in the cistern is delayed. However, the course of milk ejection does not depend on the absolute amount of stored milk. Cows at different production levels, but similar lactational stages, i.e. comparable degrees of udder filling, had similar patterns of milk ejection (Wellnitz et al., 1999).
5. Importance of prestimulation The liner of the milking machine during normal milking mimics the stimulatory effect on oxytocin release and milk ejection of manual prestimulation (Bruckmaier and Blum, 1996). However, the milk flow pattern varies between the treatments. Ideal prestimulation would involve the induction of milk ejection without the simultaneous withdrawal of milk. Because of early induction of oxytocin release during prestimulation, the period to be sustained by the cisternal milk before milk ejection is reduced. Therefore, milking without prestimulation causes mostly a transient reduction or even total interruption of milk flow if the cisternal milk is almost removed and before the alveolar milk is ejected. Depending on the intensity of the interruption of milk flow, milking without prestimulation may be a more or less pronounced transient milking of empty teats. At low degrees of udder filling, i.e. at short milking intervals and in late lactation, small amounts of cisternal milk come along with a late occurrence of milk ejection (Bruckmaier and Hilger, 2001). Therefore, transient milking of empty teats is very likely under these conditions. This process causes penetration of the milking vacuum into the teat and gland cistern, collapsing cavities, and climbing of the cluster, which can reduce milkability during further milking, even after delayed milk ejection has occurred (Bruckmaier and Blum, 1996).
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6. Milk ejection during the entire milking process During normal machine milking, oxytocin concentration in the blood remains elevated throughout milking (Bruckmaier and Blum, 1996). The importance of this continuously elevated oxytocin level could be experimentally demonstrated either by blocking oxytocin release itself (Bruckmaier et al., 1994b) or by oxytocin receptor blockade (Bruckmaier et al., 1997). During milking in unfamiliar surroundings, when oxytocin release was inhibited, a maximum of 50% of the total milk could be ejected before milking and without simultaneous milk removal. Further milk removal was only possible after the start of milk removal and occurred only as long as exogenous oxytocin was administered. Milk flow ceased as soon as oxytocin concentrations decreased towards basal concentrations (Bruckmaier et al., 1994b). During normal machine milking with normal release of oxytocin, milk ejection remained incomplete and milk flow ceased before the udder was emptied when an oxytocin receptor blocking agent was administered after prestimulation or during milking (Bruckmaier et al., 1997). Thus, adequate stimulation by the milking machine is necessary throughout the entire milking procedure to maintain elevated oxytocin concentrations and to continue milk ejection. In cows with very high peak milk flow rates milk ejection may become a limiting factor for milk flow towards the end of milking. Because the cisternal cavities are emptied in short time, and the rate of ejected alveolar milk is permanently decreasing towards the end of milking, high flow rates during early milking cause low and slowly decreasing milk flow at the end of milking. Therefore, extremely high peak flow rates are often not advantageous on the milkability of cows, but can be associated with an increased risk of mammary infection.
7. Conclusions Milk ejection is essential for complete milk removal in cows. Therefore, a stimulatory effect of the milking equipment on the release of oxytocin is required not only at the start of milking, but through-
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R.M. Bruckmaier / Livestock Production Science 70 (2001) 121 – 124
out the entire milking process. The importance and duration of the prestimulation before the start of milk removal depends on the degree of udder filling and thus the amount of cisternal milk and the lag time from start of stimulation until milk ejection occurs.
References Bruckmaier, R.M., Blum, J.W., 1992. B-Mode ultrasonography of mammary glands of cows, goats and sheep during alpha- and beta-adrenergic agonist and oxytocin administration. J. Dairy Res. 59, 151–159. Bruckmaier, R.M., Blum, J.W., 1996. Simultaneous recording of oxytocin release, milk ejection and milk flow during milking of dairy cows with and without prestimulation. J. Dairy Res. 63, 201–208. Bruckmaier, R.M., Hilger, M., 2001. Milk ejection in dairy cows at different degrees of udder filling. J. Dairy Res., in press. Bruckmaier, R.M., Wellnitz, O., Blum, J.W., 1997. Inhibition of milk ejection in cows by oxytocin receptor blockade, Alphaadrenergic receptor stimulation and in unfamiliar surroundings. J. Dairy Res. 64, 315–325. Bruckmaier, R.M., Rothenanger, E., Blum, J.W., 1994. Measurement of mammary gland cistern size and determination of the cisternal milk fraction in dairy cows. Milchwissenschaft 49, 543–546.
Bruckmaier, R.M., Schams, D., Blum, J.W., 1994b. Continuously elevated concentrations of oxytocin during milking are necessary for complete milk removal in dairy cows. J. Dairy Res. 61, 323–334. Crowley, W.R., Armstrong, W.E., 1992. Neurochemical regulation of oxytocin secretion in lactation. Endocr. Rev. 13, 33–65. Knight, C.H., Hirst, D., Dewhurst, R.J., 1994. Milk accumulation and distribution in the bovine udder during the interval between milkings. J. Dairy Res. 61, 167–177. Mayer, H., Bruckmaier, R.M., Schams, D., 1991. Lactational changes in oxytocin release, intramammary pressure and milking characteristics in dairy cows. J. Dairy Res. 58, 159– 169. Pfeilsticker, H.U., Bruckmaier, R.M., Blum, J.W., 1996. Cisternal milk in the dairy cow during lactation and after preceding teat stimulation. J. Dairy Res. 63, 509–515. Schams, D., Mayer, H., Prokopp, A., Worstorff, H., 1984. Oxytocin secretion during milking in dairy cows with regard to the variation and importance of a threshold level for milk removal. J. Endocrinol. 102, 337–343. Soloff, M.S., Chakraborty, J., Sadhukan, P., Senitzer, D., Wieder, M., Fernstrom, M.J., Sweet, P., 1980. Purification and characterization of mammary myoepithelial and secretory cells from the lactation rat. Endocrinology 106, 887–899. Wellnitz, O., Bruckmaier, R.M., Blum, J.W., 1999. Milk ejection and milk removal of single quarters in high yielding dairy cows. Milchwissenschaft 54, 303–306.
Milk ejection during machine milking in dairy cows Rupert M. Bruckmaier Institute of Physiology, Technical University of Munich-Weihenstephan, Weihenstephaner Berg 3, D-85350 Freising, Germany
Abstract Only a small percentage of cisternal milk, usually less than 20 percent is immediately available for milk removal in dairy cows. Most of the milk, the alveolar milk, is stored in alveoli and small milk ducts and is fixed by capillary forces. Removal of the alveolar fraction requires active expulsion of milk into the cisternal cavities by the milk ejection reflex. Tactile teat stimulation causes release of oxytocin (OT) from the posterior pituitary into circulation. Myoepithelial cells, surrounding the alveoli, contract in response to elevated OT concentrations and the alveolar milk is shifted through the milk duct system into the cistern to be removed by the milking machine. The delay in milk ejection after the start of teat stimulation changes during the course of lactation and depends of the interval from previous milking. Milk ejection is not completed with the occurrence of alveolar milk in the cistern during early milking. The process of shifting milk continues until the end of milk removal, and continuously elevated concentrations of OT throughout the entire milking are essential to maintain milk ejection until the udder is emptied. In cows with very high milk flow rates during early milking, milk ejection may be a limiting factor for milk flow towards the end of milking. 2001 Elsevier Science B.V. All rights reserved. Keywords: Milk ejection; Oxytocin; Milk flow; Alveolar fraction; Cisternal fraction
1. Introduction Breeding of dairy cows has resulted in animals that are highly adapted to the requirements of dairy farming, including machine milking. The basic physiological regulation of lactation, however, has remained intact despite selection. Modern machine milking requires complete and fast removal of the stored milk under optimal hygienic conditions to maintain high milk yield, product quality and animal health at a low cost. The physiological regulation of milk ejection and milk removal needs to be considered for milking machine equipment and milking routine in order to meet the requirements of the animal and to optimise the interaction between cow and technology.
2. Distribution of milk fractions before milk ejection Between milkings, the milk secreted by the epithelial cells accumulates in alveoli and cisterns. While the milk in large mammary ducts and cisternal cavities (cisternal fraction) is immediately available for milking, milk stored in alveoli and small milk ducts (alveolar fraction) is fixed by capillary forces and requires an active expulsion into the cistern which is termed milk ejection or milk let-down. The cisternal fraction usually amounts to less than 20 percent of the total stored milk fraction after an interval of 12 h from the previous milking (Pfeilsticker et al., 1996). Shortly after milking there is almost no cisternal milk present (Knight et al., 1994;
0301-6226 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0301-6226( 01 )00204-4
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Bruckmaier and Hilger, 2001). The rate of milk transferred into the cisternal compartment is increasing with time from previous milking (Knight et al., 1994). Cisternal milk yield and fraction are highest at peak lactation and decrease towards the end of lactation (Pfeilsticker et al., 1996; Bruckmaier and Hilger, 2001). Thus, lowest amounts of milk available for milk removal before milk ejection are present after short intervals from previous milking and in late lactational stages. A close positive correlation between cisternal size and lactation number was shown (r 5 0.90) and the largest cisternal fractions were measured in the oldest cows (Bruckmaier et al., 1994a). The alveolar fraction represents most of the milk stored in the udder of dairy cows throughout lactation. To have this fraction rapidly and completely available during milking, synchrony between alveolar milk ejection and removal is essential.
3. Oxytocin release and milk ejection Milk ejection is the active transport of alveolar milk into the cisternal compartment. It consists of: (1) contraction of the myoepithelial cells that surround the alveoli like a basket; and (2) transfer of the milk through the milk duct system. Milk ejection is an innate reflex that is not under conscious control of the animal and occurs in response to tactile stimulation of the mammary gland through a neuroendocrine reflex arc (Crowley and Armstrong, 1992). Neural receptors which are sensitive to pressure are mainly located in the teat tissue. Via these receptors the neural signal of the tactile stimulus is carried to the brain, terminating at the supraoptic and paraventricular nuclei of the hypothalamus. These nuclei are clusters of nerve cell bodies that synthesize the nonapeptide oxytocin that travels, attached to the carrier protein neurophysin I through the pituitary stalk via the nerve cell axons. Oxytocin is stored in neurosecretory terminals of the posterior pituitary gland (Crowley and Armstrong, 1992). Within the hypothalamic nuclei, the same neurons that produce oxytocin receive excitatory signals by cholinergic neurons to transmit the signal to the pituitary gland. Release of oxytocin, i.e. exocytosis of the neurosecretory granules, is a Ca 21 -dependent event that is
normally induced by the depolarization of secretory terminals by invading action potentials from magnocellular neurosecretory neurons. In response to elevated oxytocin blood concentrations, stimulation of the oxytocin receptors causes myoepithelial cell contraction (Soloff et al., 1980). As a consequence, alveolar milk is forcefully shifted in the larger free-draining ducts and then into the cisternal space. This in turn, causes a rapid increase of pressure within the cistern (Bruckmaier and Blum, 1996) and an enlargement of the cisternal cavity size (Bruckmaier and Blum, 1992). However, not all alveolar milk can be ejected if no milk is removed from the udder. Further milk is ejection during the course of milk removal (Bruckmaier et al., 1994b). Milk ejection occurs up to a maximal intramammary pressure as soon as oxytocin surmounts a threshold concentration (Schams et al., 1984; Bruckmaier et al., 1994b). Only exogenous oxytocin, injected in supraphysiological amounts causes additional milk ejection. This effect is experimentally used to determine the milk remaining in the udder (residual milk) after normal milking.
4. Milk ejection at the start of milking The lag time from start of tactile teat stimulation until onset of milk ejection normally ranges from 1 to 2 min and depends on the degree of udder filling (Bruckmaier et al., 1994b; Bruckmaier and Hilger, 2001). The degree of udder filling mainly depends on the stage of lactation and on the actual interval from the last milk removal. Mainly towards the end of lactation, milk production decreases and the udder does not well fill up during normal milking intervals at twice daily milking. If milking intervals are in addition very short, as is the case in automatic milking systems with voluntary milking of the cows, milk ejection may occur only 3 minutes after the start of tactile stimulation (late lactation, 4-h interval between milkings). In an experiment, mean lag time from start of stimulation until milk ejection commenced was 50 s in early lactation after 12 h and 91 s in late lactation after 4 h from previous milking (Bruckmaier and Hilger, 2001). The variable occurrence of milk ejection is not due to different oxytocin release. During the course of lactation the release of
R.M. Bruckmaier / Livestock Production Science 70 (2001) 121 – 124
oxytocin at the start of milking is rather increasing than decreasing (Mayer et al., 1991). The delayed milk ejection at low degrees of udder filling is likely due to a delayed response to the oxytocin at the level of the mammary gland. Much more myoepithelial contraction is needed to press milk out of an incompletely filled alveolus than of a completely filled alveolus. More contraction of myoepithelial cells likely needs more time and the occurrence of alveolar milk ejection in the cistern is delayed. However, the course of milk ejection does not depend on the absolute amount of stored milk. Cows at different production levels, but similar lactational stages, i.e. comparable degrees of udder filling, had similar patterns of milk ejection (Wellnitz et al., 1999).
5. Importance of prestimulation The liner of the milking machine during normal milking mimics the stimulatory effect on oxytocin release and milk ejection of manual prestimulation (Bruckmaier and Blum, 1996). However, the milk flow pattern varies between the treatments. Ideal prestimulation would involve the induction of milk ejection without the simultaneous withdrawal of milk. Because of early induction of oxytocin release during prestimulation, the period to be sustained by the cisternal milk before milk ejection is reduced. Therefore, milking without prestimulation causes mostly a transient reduction or even total interruption of milk flow if the cisternal milk is almost removed and before the alveolar milk is ejected. Depending on the intensity of the interruption of milk flow, milking without prestimulation may be a more or less pronounced transient milking of empty teats. At low degrees of udder filling, i.e. at short milking intervals and in late lactation, small amounts of cisternal milk come along with a late occurrence of milk ejection (Bruckmaier and Hilger, 2001). Therefore, transient milking of empty teats is very likely under these conditions. This process causes penetration of the milking vacuum into the teat and gland cistern, collapsing cavities, and climbing of the cluster, which can reduce milkability during further milking, even after delayed milk ejection has occurred (Bruckmaier and Blum, 1996).
123
6. Milk ejection during the entire milking process During normal machine milking, oxytocin concentration in the blood remains elevated throughout milking (Bruckmaier and Blum, 1996). The importance of this continuously elevated oxytocin level could be experimentally demonstrated either by blocking oxytocin release itself (Bruckmaier et al., 1994b) or by oxytocin receptor blockade (Bruckmaier et al., 1997). During milking in unfamiliar surroundings, when oxytocin release was inhibited, a maximum of 50% of the total milk could be ejected before milking and without simultaneous milk removal. Further milk removal was only possible after the start of milk removal and occurred only as long as exogenous oxytocin was administered. Milk flow ceased as soon as oxytocin concentrations decreased towards basal concentrations (Bruckmaier et al., 1994b). During normal machine milking with normal release of oxytocin, milk ejection remained incomplete and milk flow ceased before the udder was emptied when an oxytocin receptor blocking agent was administered after prestimulation or during milking (Bruckmaier et al., 1997). Thus, adequate stimulation by the milking machine is necessary throughout the entire milking procedure to maintain elevated oxytocin concentrations and to continue milk ejection. In cows with very high peak milk flow rates milk ejection may become a limiting factor for milk flow towards the end of milking. Because the cisternal cavities are emptied in short time, and the rate of ejected alveolar milk is permanently decreasing towards the end of milking, high flow rates during early milking cause low and slowly decreasing milk flow at the end of milking. Therefore, extremely high peak flow rates are often not advantageous on the milkability of cows, but can be associated with an increased risk of mammary infection.
7. Conclusions Milk ejection is essential for complete milk removal in cows. Therefore, a stimulatory effect of the milking equipment on the release of oxytocin is required not only at the start of milking, but through-
124
R.M. Bruckmaier / Livestock Production Science 70 (2001) 121 – 124
out the entire milking process. The importance and duration of the prestimulation before the start of milk removal depends on the degree of udder filling and thus the amount of cisternal milk and the lag time from start of stimulation until milk ejection occurs.
References Bruckmaier, R.M., Blum, J.W., 1992. B-Mode ultrasonography of mammary glands of cows, goats and sheep during alpha- and beta-adrenergic agonist and oxytocin administration. J. Dairy Res. 59, 151–159. Bruckmaier, R.M., Blum, J.W., 1996. Simultaneous recording of oxytocin release, milk ejection and milk flow during milking of dairy cows with and without prestimulation. J. Dairy Res. 63, 201–208. Bruckmaier, R.M., Hilger, M., 2001. Milk ejection in dairy cows at different degrees of udder filling. J. Dairy Res., in press. Bruckmaier, R.M., Wellnitz, O., Blum, J.W., 1997. Inhibition of milk ejection in cows by oxytocin receptor blockade, Alphaadrenergic receptor stimulation and in unfamiliar surroundings. J. Dairy Res. 64, 315–325. Bruckmaier, R.M., Rothenanger, E., Blum, J.W., 1994. Measurement of mammary gland cistern size and determination of the cisternal milk fraction in dairy cows. Milchwissenschaft 49, 543–546.
Bruckmaier, R.M., Schams, D., Blum, J.W., 1994b. Continuously elevated concentrations of oxytocin during milking are necessary for complete milk removal in dairy cows. J. Dairy Res. 61, 323–334. Crowley, W.R., Armstrong, W.E., 1992. Neurochemical regulation of oxytocin secretion in lactation. Endocr. Rev. 13, 33–65. Knight, C.H., Hirst, D., Dewhurst, R.J., 1994. Milk accumulation and distribution in the bovine udder during the interval between milkings. J. Dairy Res. 61, 167–177. Mayer, H., Bruckmaier, R.M., Schams, D., 1991. Lactational changes in oxytocin release, intramammary pressure and milking characteristics in dairy cows. J. Dairy Res. 58, 159– 169. Pfeilsticker, H.U., Bruckmaier, R.M., Blum, J.W., 1996. Cisternal milk in the dairy cow during lactation and after preceding teat stimulation. J. Dairy Res. 63, 509–515. Schams, D., Mayer, H., Prokopp, A., Worstorff, H., 1984. Oxytocin secretion during milking in dairy cows with regard to the variation and importance of a threshold level for milk removal. J. Endocrinol. 102, 337–343. Soloff, M.S., Chakraborty, J., Sadhukan, P., Senitzer, D., Wieder, M., Fernstrom, M.J., Sweet, P., 1980. Purification and characterization of mammary myoepithelial and secretory cells from the lactation rat. Endocrinology 106, 887–899. Wellnitz, O., Bruckmaier, R.M., Blum, J.W., 1999. Milk ejection and milk removal of single quarters in high yielding dairy cows. Milchwissenschaft 54, 303–306.