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2 Growth of lactating tissue is dependent mainly upon two hormones, estrogen from the developing follicles and progesterone from the corpus luteum. The corpus luteum is the naturally regressed stage of the follicle after it has ovulated, releasing ova into the oviduct. Estrogen, which is cyclic, stimulates the duct development of the mammary gland. Progesterone is almost continuously secreted during pregnancy, causing secretory tissue development. As the gestation period nears its end, the mammary glands become capable of producing milk. After parturition, the rate of milk secretion increases for some time, reaches a peak, and then gradually declines. A loss of secretory epithelial cells occurs during involution, although some new cells are being formed. The general activity level of the individual cells declines also. At the cessation of milk production, secretory epithelial cells will totally disappear, leaving only myoepithelial cells. The frequent release of oxytocin may slow down the rate of the involution process and some goats may secrete milk for years continuously.
3 Milk secretion during lactation tends to inhibit the normal cycling of the estrus periods, with some temporary suppression of ovulation. With time, there will be a return to normal estrus cycles. Milk production will decline, and the glands will go into involution; although not complete, since the glands increase in size with successive pregnancies.
4 Hormones other than estrogen and progesterone that influence development of mammary glands or lactation originate from the hypothalamus and pituitary glands of the brain.
5 The anterior pituitary produces in response to stimulation of the hypothalamus six hormones, which exert either a direct or indirect influence on milk secretion. Somatotropic or growth hormone (STH); follicle stimulating hormone (FSH); luteinizing hormone (LH); prolactin (most intimately associated with initiating and maintaining lactation); thyrotropic hormone (TTH); and adrenocorticotropic hormone (ACTH).
6 The posterior pituitary stores two hormones from the hypothalamus both related to lactation. Vasopressin or antidiuretic hormone (ADH), decreases the amount of water lost in the urine, retaining it instead for use. Oxytocin is best known for its ability to cause milk ''let down'' in mammals, although it is also capable of stimulating contractions in other smooth muscles such as the uterus during estrus and parturition, as well as in the urinary bladder and intestine.
7 The pars intermedia of the pituitary secretes the hormone intermedin, which has some effect on water metabolism.
8 In mature goats secretions of FSH stimulate follicular development, which then becomes a source of estrogen. LH, working in the presence of FSH, produces ovulation; and from the corpus luteum progesterone is released.
9 Estrogen by itself generally stimulates duct development of the mammary glands, while estrogen and progesterone together cause lobulo-alveolar growth. In the goat, however, estrogen alone will initiate abnormal development of the mammary, producing dilated alveoli and even milk production in some cases. High levels of estrogen in circulation have a negative effect on feed consumption and milk production in goats. Estrogen and progesterone are required both for the complete development of the mammary system, along with the other pituitary hormones. Large amounts of prolactin are to be used just prior to parturition in response to the higher levels of estrogen and progesterone at this time. Prolactin matures the alveolar cells to a functional condition. STH and thyroxine, also exert an influence on the amount of milk produced. Small injections of estradioltestosterone produce a colostrum-like milk in goats, while large doses produce a watery, clotted milk secretion.
10 The hormone of most interest is oxytocin. It is responsible for milk let-down, causing the myoepithelial cells that surround the alveoli to contract, forcing the milk out into the ducts of the udder. Oxytocin release is initiated in several ways, the most natural being nursing. It will directly stimulate the afferent nerve fibers of the teat, causing the release of the hormone and transportation via the bloodstream to the mammary glands, with subsequent contraction of the myoepithelial cells. This milk let-down reflex can also be initiated by other repetitive occurrences, such as washing the udder, approaching the milking area, the sounds of the milking machine or milk buckets, or even the sight of the milker.
11 The process of milk let-down is subject to interference if the goat should become excited through some disturbance. A release of epinephrine (adrenalin) follows such excitation. Epinephrine causes constriction of the small arteries and capillaries of the udder and may prevent the myoepithelial cells from contracting. Therefore, anything that may cause a disturbance or alarm near milking time should be avoided.
12 After the initial stimulus for let-down, it takes a period of about 20-60 seconds for the response of oxytocin, which influence will last about 5-6 minutes. It is important to milk goats soon after their let-down and not a stimulate more does than can be milked immediately. Due to an inability to completely close off milk ducts, goats in contrast to cows cannot hold back milk flow entirely.
13 Artificial induction of lactation has been accomplished in goats with good results by injections of hexestrol at a daily rate of 0.25 mg. Pellet implantations of estrogen and progesterone have also successfully created udder growth, with final treatment of estrogens to initiate lactation. Art ificial induction of lactation has experimental value, but is not practical for several reasons. Injections must be given over a period of time on a daily basis, or tablet implants must be inserted. The methods are costly, labor intensive, give a low yield of milk, and do not produce income from the sale of the kids. There may also be questions of safety of the milk after using these injections.
14 Milk Composition Milk differs in structure and composition from the blood from which it originates. Milk is composed of protein, fats, sugar and mineral salts. The osmotic pressure of milk is essentially equal to that of blood. The pH at 6.5 is slightly acidic. Milk contains higher levels of sugar, lipids, calcium, phosphorus and potassium than blood, but lower amounts of protein, sodium and chlorine. Proteins also differ, since milk protein is composed primarily of casein, with smaller amounts of albumin, globulins and others. Blood protein, is primarily composed of albumin and globulins. Fat composition differs, as milk lipids are predominantly triglycerides and blood lipids consist of phospholipids and cholesterol. Non-protein nitrogen compounds are also found in milk, such as urea, uric acid, creatine, creatinine and ammonia. Some of these are from the blood system, while others are waste products of the mammary gland.
15 Lactose, the milk sugar, is a disaccharide carbohydrate that is unique to the mammary gland. Blood glucose is the primary precursor of lactose, besides propionic acid, while acetate (another VFA) is predominantly used in milk fat synthesis. Butyrate is generally distributed fairly evenly between lactose, casein and fat constituents of the mammary secretion.
16 Milk lipids (fat) consist primarily of triglycerides, although there are small amounts of phospholipids, cholesterol, fat-soluble vitamins, free fatty acids, and monoglycerides. Fat exists in small globules in the freshly secreted milk, averaging under 4 microns in diameter in goats and being smaller than for cow milk. The outer layer of the globule contains phospholipids, cholesterol, protein, and vitamin A and stems from the alveolus. Acetate is the major precursor of milk fat in goats. Goat milk fat does not rise or ''cream-line'' as easily as cow milk fat since it lacks the coalescing factor besides being of smaller average globule size. Rupturing the milk fat globule membrane can add to off-flavor problems of goat milk. Since milk fat synthesis depends on the supply of acetate from the rumen, any feeding regime, such as high grain feeding which lowers the production of rumen acetate, will also lower the fat content of goat milk. Supplementing the feed ration with more crude fiber sources, such as hay, sunflower seeds, peanuts in their shells, etc. will restore milk fat levels to normal.
17 Most major vitamins are found in goat milk, some in abundance. B-complex vitamins are manufactured by the rumen flora. Vitamin K is synthesized in the rumen and the intestine. Vitamins A, D and C tend to be diet related; expecially vitamin A and the precursor carotenoids. Milk levels of vitamin D are usually increased during commercial milk processing. Vitamin A in goat milk is related to blood levels; and no carotenoids are found in goat milk.
18 Minerals in goat milk consist mostly of calcium, phosphorous, sodium, potassium, chlorine, magnesium and sulfur. Trace amounts of aluminum, boron, bromine, cobalt, copper, fluorine, iodine, iron, manganese, molybdenum, silicon, silver, strontium and zinc are found in milk in less than 1 part per million.
19 Colostrum The first milk after a doe kids is of great significance to the young. This colostrum has a high nutritional value and contains antibodies essential for the survival of the newborn. Goats like other ruminants have a 5-layer placenta through which no antibodies can be transmitted from the dam to the kid in utero. Thus the kids depend on the colostrum as their source of antibodies, providing passive immunity until they are developing their own active immune system. The period in which the immunoglobulins can be absorbed through the kid's GI tract lasts 3 to 4 days in goats. Beyond this time, the digestive enzymes in the gastrointestinal tract of the kid will break down the protein structures of the antibodies, rendering them ineffective. The high percentage of these immunoglobulins in colostrum, along with albumin, gives it its thick, sticky consistency.
20 Globulin, having a high proline content, is also important for the formation of hemoglobin in the young kid. Normal milk is much lower in globulin levels, having instead a higher level of casein. The functional importance of casein is that it is the only milk protein forming a curd upon coagulation in the abomasum thereby creating a slower moving food reserve for the young kid.
21 The dry matter content of colostrum is much higher than in normal milk, primarily due to the large amount of proteins, especially albumin and globulins. The vitamin content of colostrum is also higher than normal; the lactose content is low. The colostral content of iron is generally about 15 times greater than in normal milk, while vitamin A and vitamin D levels are about 10 and 3 times those of normal milk.
22 Colostrum or milk bypasses the reticulorumen area because of the formation of the esophogeal groove, thus preventing the milk from laying in the rumen and turning rancid, which would then develop scours. The formation of this groove is in response to the sucking reflex of the young kid. Actual suckling need not occur however, as a kid can drink from a bucket and still bypass the rumen.
23 Milk Production Milk production consists of secretion and excretion. Secretion is the formation of milk from its blood precursors within the alveoli. Excretion is the discharge into the lumen, ducts, cistern, teat and final harvest by the milking person. As the collecting ducts begin to fill with the accumulated excretion of the alveolar cells, they experience difficulty in ridding themselves of milk. When the pressure in the udder rises, the cells and lumina of the alveoli become distended, compressing the small collecting ducts. This prevents a pressure overload on the teat sphincter and any leaking of milk.
24 There is little milk production going on while milking is being done. Udders can extend themselves greatly to hold high volumes of milk. During the first hour after milking, there is no discernible mammary pressure; but a steady, gradual increase in pressure occurs until the time of the next milking. Milk let-down results in a dramatic increase in mammary pressure which will gradually subside, even if no milk is removed.
25 While much has been said about the virtues of milking at equal intervals (2 x 12 hrs or 3 x 8 hrs) in order to keep up good production levels, studies have shown that the drop in goat milk production may be quite small. However, stress and mastitis incidence can be reduced significantly by equal milking intervals or by 3-times milking for very high milkers.
26 Rapid removal of milk after stimulation of letdown is essential for complete milking. There is always some residual milk left in the udder after milking, but it is normally less in goats than in cows. Regular and complete milking is one of the requirements for continuance of lactation. The stimulus of nursing prevents mammary gland regression. The response is due to release of prolactin from the anterior pituitary. Lack of this hormone hastens mammary involution and drying off of the doe.
27 Stage of Lactation Effects There is a great difference in the composition of milk during the various stages of lactation among does. Management practices such as the length of the dry period, feeding program and general health practices play a significant role in the quality and quantity of milk that is produced during lactation.
28 Colostrum appears to be a waste product from the new development of secretory tissues; and while essential to the kid, is not used for human consumption normally. In some does, during the first few weeks for milk production, there can be evidence of some blood in the milk. This is more common in the heavy producing, first-time freshner and likely the result of rupturing some tiny blood vessels in the udder. Milking the doe 3 or even 4 times a day may alleviate the problem. A lack of calcium may also be involved, and should be supplemented in the diet. Forceful milking is another possibility.
29 The production level of goat milk increases for about 20-30 days after kidding. During this period of lactation, there is an inverse relationship between levels of milk and fat content. The percentage of total fat as well as the composition of milk fat varies. Towards the end of lactation, fat and protein contents rise while milk yields decrease.
30 Calcium and phosphorous levels in milk are high in colostrum, then decrease constantly until near the end, when they rise again. During this period, the overall salt content of milk tends to increase, affecting the taste of milk. Somatic cell numbers are also very high normally in late lactation as well as in colostrum.
31 Persistency of milk secretion throughout lactation can be mathematically expressed by determining the average percentage of decrease in milk for each month, compared to the previous month. Goats, given proper feed and not being rebred, will continue to give milk with a high degree of persistency for a long time. High production on a yearly basis must combine high initial production and good persistency, which is a heritable characteristic.
32 Age Effects Milk volume increases with age up to the fourth or fifth year. After that, the volume decreases with advancing age. The rate at which production decreases is slower than the rate at which it increased to maximum yield. The average milking life of a doe maybe about 12 years.
33 Body Size The relationship of size to milk production provides a misleading picture. Large does are not necessarily more efficient producers. Based on gross energetic efficiency, there is little difference in milk production due to species. Although a goat produces more milk per unit of bodyweight than a cow, the actual net energy efficiency is close. Goats have, however, a relatively high basal metabolic rate and therefore tend to have among the single-purpose dairy breeds a relatively high dairy merit and net efficiency.
34 Estrus It appears that upon coming into estrus, the doe goes through a decrease in milk production. This is only temporary, and is usually compensated for by a brief period of higher than normal production after the estrus cycle. There may also be an increase in the level of fat produced during the estrus period, as is often the case when milk production is lowered.
35 Disease Effects Most diseases, including mastitis reduce milk yields. Fat content will rise due to lower milk production. The solids (and minerals), albumin, globulin and non-protein nitrogen levels will increase, while casein and lactose contents decrease.
36 Dry Period A dry period for the doe is necessary to rebuild her body reserves, especially minerals and to prepare for the period of heavy production in the next lactation. Fattening during the dry period can lead to ketosis or pregnancy toxemia problems. A 60-day dry period is considered normal. Goats without a dry period tend to produce less in the next lactation.
37 Season Effects Temperature, humidity, management practices and feeds tend to vary with seasons, thereby affecting milk and fat production. Does test lower in the summer than in the winter, not necessarily due to drop in milk production. Does which freshen later in the spring or early summer will usually have a higher test average for the year than does freshening at other times.
38 There are many variables that can affect quality and quantity of goat milk. Many may be hard to control. Careful and efficient management with a willingness to learn and try new ideas, is certain to increase productivity of the goats and the quality of their milk.