| AYRSHIRE BREED | HOLSTEIN BREED |
Lactation is the distinguishing characteristic of mammals. Milk is produced in the mammary gland and the young animal relies upon it entirely for nutrition and for passive immunity.
Milk is chemically very complex. Its major constituent is always water which constitutes 46 - 90 % of milk, depending upon species. Milk composition varies considerable between species. The other major components of milk are protein, fat and lactose. It is a good source of many minerals (particularly Ca, Mg, P) and vitamins. Immediately following birth of the young, high concentrations of maternal immunoglobulins (antibodies) are present to confer immunity to diseases. This milk is called colostrum.
The protein and fat components are present in complex structures. The major protein in milk is called casein. Casein exists in different forms with different prevalences in different species. The casein molecules are aggregated into bundles called micelles, stabilized by other components (Ca, phosphate, citrate etc.). The fat is present in small globules enclosed by a membrane derived from the cell that secreted it - the milk fat globule membrane. The membrane is easily damaged during machine milking. This makes the fat globule more susceptible to break down by other enzymes in milk. This is called lipolysis and leads to a rancid taste.
Caseins consist of amino acids. Casein contains all amino acids required by humans. Thus, it is a very high quality protein in nutritional terms.
Fat - fatty acids of varying chain lengths (2-26 carbons). Fat may be saturated (C-C-C) or unsaturated (C=C=C). Mainly triglycerides 16-18 carbons, monounsaturated.
Milk is manufactured in the mammary glands from groups of specialized secretory epithelial cells. These are collected together in spherical structures called alveoli. The alveolus is surrounded by contractile cells called myoepithelial cells. These cells contract in response to a hormone released from the pituitary gland - oxytocin. Mammary glands are: exocrine glands i.e. the external secretion of the alveoli is transported through a duct system to the teat where the young can remove it. Developmentally the mammary gland is a modified sweat gland.
Almost all the economically important species that are used for milk production are ruminants (sheep, cow, goat etc.). Exceptions are the camel which is milked commercially in the middle east and horses which are used to make yoghurt-like fermented milk drinks in Asia.
Why ruminants for milk production?
| BROWN SWISS BREED |
In the 19th century, mechanical devices called lactators were produced. These squeezed the outside of the teat with rollers in a simulation of hand milking. Some were very elaborate but were not very successful. Finally around 1900 the idea of using vacuum developed and led to the type milking machine we use today.
The physiological and developmental cycle on the dairy farm
What are the fundamental physiological requirements of the dairy industry? Lactation is designed to feed the young and follows birth. Consequently, in the dairy herd the reproductive cycle is manipulated with the objective of each animal bearing one calf a year and lactating for around 10 months. The sexually immature female does not have a developed mammary gland but a structure known as the fat pad which will begin to develop ducts and alveoli as sexual maturity develops. However, most mammary development occurs during pregnancy. There are many hormonal influences but estrogen and progesterone are the most important. They are produced by the ovary under the influence of FSH (follicle stimulating hormone) and LH (luteinizing hormone). Unlike the sheep, the cow is not a seasonal breeder and has a normal cycle length of 21 days. The cow is in heat (sexually receptive) for an average of 12 hours duration and ovulation follows the heat. The normal gestation length of the cow is 285 days. The goat cycle is 150 days, and the sheep 147-150 days.
Most dairy farmers breed their heifers (females which have not given birth) around 15 -18 months to calve at 24-27 months. They can be bred younger but are more likely to have problems, particularly if they have a large calf. The key is size of the heifer at breeding rather than calendar age. Careful rearing of heifers is necessary to optimize growth rate with mammary development. (800-875 lbs for a Holstein heifer at breeding). Too rapid a growth rate can inhibit mammary development and subsequent milk yield. In some countries seasonal calving is practiced. A good example is New Zealand where most calves are bred to calve in a 6 week period in the spring, The objective is to make the best use of grass. In these systems synchronization of estrus by injection of hormones is commonly used to get this narrow calving pattern. Most commonly this involves the use of prostaglandins.
During pregnancy, the final developmental of the mammary gland occurs to prepare for milk production (lactogenesis), which commences as the calf is born. The first milk produced is called colostrum and is particularly rich in antibodies. On the dairy farm it is used to feed the calves but is not collected for human consumption (in fact there are some old traditional European dishes made from colostrum).
After calving, lactation continues for an extended period, in the dairy cow typically 300 days. Hormonal influences are critical to the maintenance of lactation - prolactin, insulin, thyroid hormones, growth hormone (BST). A few weeks after parturition the cow will begin her estrus cycle again and show heat. She will be usually be artificially inseminated at an appropriate heat around 70-90 days after calving. The objective is to get the cow calving about once per year. Milk yield declines as pregnancy progresses. In fact the hormonal changes during pregnancy and the increasing flow of nutrients diverted towards the fetus serve to down regulate milk secretion. At around 300 days milking is usually stopped and the cow "dried off". Once milking stops lactation ceases and a process called involution begins, in which the secretory tissue regresses. In the late dry period a further round of mammary development occurs prior to calving and then lactation recommences.
The average number of calves and lactations completed per dairy cow is around three. If the calf is female (heifer) it will probably be retained and grown as a herd replacement. Around 25-30% of the herd is replaced annually. Bull calves will probably be sold for veal or beef.
Feeding the lactating dairy cow
The modern dairy cow can produce 30,000 lbs of milk/lactation (100lbs/day). This requires extremely effective nutrition and feeding management. Most commonly in the US this is achieved through the use of a mixture of forages, grain and minerals called a total mixed ration (TMR) which is balanced for a particular milk yield plus body maintenance. If the feed input is insufficient the cow will mobilize her own body reserves to produce milk and will lose weight and condition. To produce 80lbs of milk/day the cow will need 2.5 times the energy for milk production that she requires for maintenance. Rations have to be carefully balanced to ensure the correct balance of protein, energy, roughage, and minerals . This is often calculated using a computer program based upon analysis of feeds that are to be used in the diet. There are several diseases that affect dairy cows that are related to nutritional problems caused by the great difficulty in balancing rations for high producing dairy cows.
As milk production has increased beyond consumer demand the price paid to farms has dropped. Great emphasis has now to be placed on reducing costs of production. The predominant non-capital cost is feed. To reduce feed costs some farmers have attempted to utilize higher proportion of forages in the diet and reduce use of grain. Many are trying to do this via better grazing and pasture management practices. An alternative approach is to utilize by product feeds such as cotton seed and citrus pulp.
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