THE CONTROL OF MILK PRODUCTION AND LET-DOWN
Nutrition and genetics are clearly the greatest factors affecting the level of milk production, and as is explained later in the chapter, mastitis can also influence yields. This short section deals primarily with the hormonal and managemental factors involved.
The induction, or start, of lactation is controlled by a hormone called prolactin which is produced by the pituitary gland, situated at the base of the brain. Chapter 8 will discuss the factors which maintain high levels of progesterone during pregnancy. Immediately prior to parturition, blood progesterone levels fall. This allows prolactin levels to rise, and prolactin then produces the changes in the udder tissue needed to start milk production. In many species milk yield is maintained by high levels of circulating prolactin, and the higher the level of prolactin, the greater will be the level of milk production. This is not so in the cow, however, where the continuation of milk yield appears to be controlled by a combination of growth hormone (BST) from the pituitary gland, thyroxine, produced by the thyroid gland in the neck (see Plate 12.3), and steroids from the adrenal glands situated beside the kidneys.
Bovine somatotrophin (BST)
BST is a natural growth hormone secreted by the pituitary gland, a small organ situated at the base of the brain. High yielding cows have higher levels of BST circulating in their blood than lower yielders and cows at peak more than late lactation animals. Being a simple protein in structure it can be synthesised by means of biotechnology, viz by injecting the gene into bacteria, inexpensively producing large quantities which can then be injected into cows to boost their yields. BST alters the cow’s metabolism so that a greater proportion of her food is used for milk production, thus making her more efficient.
At the dosages suggested, yields will be increased by around 10-20% or 4-6 litres per day for an average early lactation cow.
This is approximately the same effect as that gained by milking three times a day (see page 178). Some four to six weeks later there will be an associated increase in food intake and dry matter appetite capacity and this will further increase her efficiency. In trials carried out to date, there have been no adverse effects on either the health or longevity of treated cows. BST is detectable in minute quantities in the milk of normal cows, and cows under treatment do not have detectably higher levels. (This is despite the fact that tests are very sensitive - sufficient to detect the equivalent of one second in 32 years!)BST is totally harmless to man. Being a protein hormone it is destroyed in the intestine by the normal processes of digestion and even if it was accidentally self-injected there would be no adverse effect. Massive doses were even once used for the treatment of dwarfing in man, but with no beneficial results, since the growth hormone required for man has a different structure.
However, even with all these assurances of safety, there could still be considerable consumer resistance to the thought of drinking milk from ‘hormone treated’ cows. In addition, the product has to be given by regular intramuscular injections, which some people might consider unacceptable. There could also be problems with pedigree breeding programmes, since careful surveillance would be needed to compare BST-treated with normal cows. As of May 1999 the product has not been licensed for use in the EU, although it is used in many states in the United States.
Milk let-down
When milk has been synthesised, oxytocin, yet another hormone produced by the pituitary gland, is needed to eject the milk from the udder. Oxytocin causes the contraction of small muscle-like myoepithelial cells surrounding the alveoli, shown in Figure 7.1. This forces the milk down into the ducts and hence into the udder cistern and then to the teat cistern, where it is ready for withdrawal by the calf or the milking machine.
The overall process is known as milk let-down, and oxytocin is released from the pituitary gland by what is known as a reflex action, that is in response to a consistent stimulus which the cow associates with milking. This stimulus may be udder-washing or foremilking, but neither is necessary. The cow can be trained to produce milk let-down (in response to oxytocin) simply by entering the parlour.
One point is important, however. The stimulus for oxytocin release must be the same at every milking. If the proper stimulus is not provided, or if it is inhibited, then milk stays in the alveoli and as little as 50% of normal production may be obtained. Oxytocin has a very short duration of action - approximately 20 minutes. If the milking machine is not applied soon after the teat fills, the myoepithelial cells will relax, the alveoli enlarge once again and milk will be drawn back into the udder. It is absolutely vital that a constant routine is established in the milking parlour therefore, so that the cow knows precisely when the unit will be applied and she can train herself to let-down accordingly.
In addition this whole process can be inhibited by the action of the hormone adrenalin, which is produced by the adrenal gland. Adrenalin is sometimes known as the ‘flight or fight’ hormone: in man it causes a thumping heart, cold hands and sweating, all of which are associated with fear. Anything which disturbs the cows - unusual noise, strangers, rough handling etc. - will lead to adrenalin release and may interfere with milk let-down and therefore overall production. It is adrenalin which produces the defaecation associated with excitement - a phenomenon I expect every stockman will have witnessed! Treatment of milk let-down failure is discussed at the end of this chapter.
Milking frequency
Frequency of milking can have a marked effect on milk production. If cows are milked once a day, yields will fall by some 40%. The majority of farms milking twice daily do so at intervals of fourteen hours and ten hours.
Trials suggest that only in the highest yielding herds does this produce significantly lower production than precise twelve-hourly milking.However, increased frequency of milking does increase yields and hence the interest in automatic milking. Cows could then go through the milking system as frequently as they wish and yields would rise even further. Changing from two to three times daily milking leads to increased yields of:
• 10-15% in cows
• 15-20% in heifers
Because of the flatter lactation curve produced, three times daily milking has to be continued to the end of lactation to obtain its full beneficial effect.
It is not the pressure of milk within the udder which limits further milk production, but the presence of an inhibitor protein in the milk which reduces further milk synthesis. Increased frequency of milking leads to more frequent removal of the inhibitor protein and more milk is produced. There are two stages to this:
• Initially the existing milk-producing cells simply work harder and more efficiently.
• After one to two months of frequent milking, more milk-producing cells form, that is there is an overall increase in productive tissue within the udder.
By more frequent flushing of the teat canal, three times daily milking also decreases the incidence of mastitis, and is something to be considered as welfare-friendly in high yielding herds.
Residual milk
Machine stripping, that is additional manual pressure applied to the cluster at or towards the end of milk flow, may lead to a secondary release of oxytocin but it may also train cows to ‘hold back’ some milk for this period. For this reason, and to reduce the risk of teat end impacts (page 192), machine stripping should not be done. For the average yielding cow, leaving small quantities of milk (e.g. 1-2 litres) in the udder is not too important in terms of overall yield, and if on one occasion a cow leaves the parlour only half-milked you will certainly not lose any more than a small part of the next milking’s production, and possibly nothing if she is not a particularly high yielder.
If a cow is consistently undermilked however, for example if only 60% of the milk is withdrawn for seven to ten days, then this will result in a lowering of production. This effect can even be seen in an individual quarter, for example a quarter badly affected by teat end damage.The dry period
At the end of lactation the old milk-producing alveolar cells die off and are replaced by new tissue during, and especially towards the end of, the dry period. A dry period of six to eight weeks is ideal, and if the cow is not dried off at all, the next lactation may be as much as 30% lower. Situations such as this can occur when a bull is run with the herd continually and no pregnancy testing is carried out. In addition to having a very short or non-existent dry period, some cows may conceive so soon after calving that both their 305 day lactation and their annual production will be depressed.