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Comparative Lactation - Swine


Swine


This section discusses aspects of lactation in swine, including anatomy, mammogenesis, lactogenesis, suckling and milk ejection, milk synthesis, milk yield and mammary function, milk composition, mastitis, protection of the piglet, and piglet survival and growth.


Mammary Anatomy

External Structure : The number of mammary glands varies from 6 to 20 (3-10 pairs). The arrangement of glands is in two parallel rows, one on each side of the ventral median line and extending from the pectoral to the inguinal region. Each gland is separate and independent of secretory tissue from adjacent glands. The heritability of teat number is rather low (.10 to .20). The relationship between teat number and maternal performance renders it impractical to select for high teat number

Blood Supply : Blood supply to the glands arises from two branches of the arterial system, the common carotid artery supplies the anterior glands and a branch of the abdominal aorta supplies the posterior glands. There is an anastomosis of the anterior and posterior mammary arteries and veins between the second and fourth inguinal glands so that blood supplying the inguinal glands may pass forward through the anastomosis and blood supplying anterior glands may pass posteriorly through the anastomosis.


Mammogenesis

Lobuloalveolar development begins at about 45 days after conception (pregnancy is 114 days in pigs). Alveoli of the sow remain small during gestation and do not begin to distend until about 4 days before parturition. Fat globules cannot be detected within the alveoli until two days before parturition. There is little change in either DNA or RNA content of the mammary glands up to 50 days of gestation. By 100 days of gestation there is a large increase in both DNA and RNA. The increase in estrogen concentrations in the blood occurs about the same time that both lobular and alveolar growth begins and mammary DNA and RNA increase. Estrogen, progesterone, prolactin, growth hormone, and corticosteroids are required for lobuloalveolar growth.


Lactogenesis

Most studies have assessed lactogenesis subjectively as the time when milk can first be expressed from the teats. In the sow there is an abrupt increase in the concentration of lactose in the mammary secretion near parturition. Many studies indicate that some sows' secretions can be expressed for up to two days prior to the abrupt increase in concentration of lactose. There is a significant negative correlation between the concentration of progesterone in blood and concentration of lactose in colostrum. Transfer of maternal immunoglobulins to colostrum and lactogenesis must be synchronized to occur within a few hours of the birth of the first piglet. Due to the rapid initiation of lactation over the last two days of gestation in the sow, any slight abnormality in the timing of lactogenesis potentially threatens piglets survival.

[See Kensinger et al., 1986, J. Anat. 145:49]


Suckling and Milk Ejection

The sow has an exceptionally strong control of milk ejection and a number of studies have shown that the duration of milk flow is only 10 to 20 seconds. The average nursing interval is less than 1 hour, so that the suckling piglet normally receives more than 24 feedings daily. The nursing frequency tends to decrease with the advance of lactation. The normal time to first suckling by a piglet after birth is about 25 to 35 min. The first piglet born takes longer to find the teat than subsequent piglets. After birth, the piglet will sample each teat, then there is competition for teat order (fighting, aggression).

The teat order is established by ~4 to 6 hr. Teat order is highly developed in the pig. There is ~90% reliability of a piglet going to the same teats after 3 to 7 days. Occasionally they will suckle two teats (usually adjacent teats). The suckling drive lasts for ~ 4 to 6 hr, the piglet must be "rewarded" for the suckling drive to be maintained. Milk ejection can be induced by rubbing the front teats. Rubbing the rear teats will not cause milk ejection.


Milk Synthesis

Mammary glands utilize about 1/2 the total glucose entering the circulation. The major source of fatty acids in the milk are derived from plasma triglycerides. All essential and non-essential amino acid residues of milk protein are derived from their corresponding free amino acids in the plasma.


Milk Yield

Milk yield is more difficult to measure in the sow than in the cow since oxytocin must be injected to induce milk letdown when the milk is obtained by hand or machine during lactation (except during and immediately after farrowing, when milk or colostrum is easily stripped from the teats).

Milk yield is often estimated by weighing the pigs immediately before and after nursing, or by machine-milking, or by hand-milking of the sow at predetermined intervals after oxytocin injection.

The peak daily milk yield is reached at around 21 days of lactation. However, piglets often are weaned at the end of the second or third week. The average milk yield for a sow will vary depending onmany factors including parity, litter size, breed, body size, nutrition, and other management factors. A firwst lactation sow nursing 10 piglets may produce about 10-12 kg milk/day.

Effects of daily injections of porcine somatotropin (pST) on milk yield have been variable depending on the study. [For example, see Harkins et al., 1989, J. Anim. Sci. 67:1997; Cromwell et al., 1992, J. Anim. Sci. 70:1404.]


Milk Composition

Fat, protein, and lactose constitute approximately 60%, 22%, and 18% respectively, of the total energy content of sow's milk. Milk constituents are derived partly from synthesis within the mammary gland (lactose, fatty acids, some milk protein fractions) and partly by the filtration or active transport from the blood (serum albumin, immunoglobulins). There appears to be little variation among glands on the same sow in composition of the milk produced, with the exception of fat, which does seems to be more variable between glands. Fatty acid composition of the milk is sensitive to changes in the composition of dietary fat.


Mastitis

Sows are susceptible to getting mastitis at farrowing time. This is often associated with uterine infection (metritis) and agalactia (inhibited lactogenesis). For review see Wagner, 1982, Vet. Clin. N. Amer. (Large Anim. Practice) 4;333. Also see Smith and Wagner, 1984, Suppression of prolactin in pigs by Escherichia coli endotoxin. Science 224:605.


Protection of the Piglet

Accumulation of colostral immunoglobulins must occur rapidly over the last two days of pregnancy. Despite the transfer of immunoglobulin into colostrum the serum concentration of IgG does not show a significant decrease in the sow. A liter of sow's colostrum contains approximately 1/3 of the blood pool IgG. Colostral immunoglobulins are rapidly absorbed across the small intestine of the piglet and maximum serum concentrations are reached 12-24 hours after birth. There is a relationship between low immunoglobulin values at 24 hours postpartum and piglet mortality. The quantity of immunoglobulin in sow colostrum increases with parity. Many farms practice heavy culling of sow herd; the quantity of immunoglobulin is therefore minimized rather than maximized.


Piglet Survival and Growth

Piglets glycogen stores 271 kJ at birth. The piglet requires an external energy source during the first few hours of life to prevent hypoglycemia. Colostrum contains 586-628 kJ/100ml. The piglet needs to consume 250-300ml of colostrum to remain in energy balance. Postnatal mortality in piglets can be nearly 12% between d 0 and d 7 postpartum (MLC, 1994). More than 60 % of preweaning mortality is caused by maternal factors such as insufficient nutrient supply and over-lying (English et al 1977; Dyck and Swierstra 1987; Prime et al 1987). Piglets are born with low organic reserves, hence the availability of energy from milk immediately after birth is critical to survival. Even beyond the immediate postpartum period, low milk production by the sow has long range effects resulting in reduced growth rates prior to weaning and suboptimal growth postweaning through the grower and finisher stages (Hartmann et al 1984).

Even under typical management systems, newborn piglets achieve only a fraction of their potential growth rate while nursing their dam (Harrell et al 1993; Zijlstra et al 1996). While milk yields of sows generally have increased in the past 30 years (King 2000), sow milk yield and yield of milk nutrients are limiting to optimal piglet growth rates. Milk production by sows becomes limiting to piglet growth at about 8 or 9 days of lactation and this difference between piglet need for milk nutrients and level of milk production progressively increases through lactation (Harrell et al 1993). This is a significant limitation to optimal piglet growth whether piglets are weaned at 14, 21 or 28 days. Current early weaning management strategies, with weaning at around 14 days, only serve to further intensify the challenge of meeting the pig’s requirements for preweaning growth and development.


The images below will give you some examples of how much mammary development may occur normally in late-pregnant and lactating sows.


Pregnant sows:
Top - ~day 100, underdeveloped
Bottom - ~ day 106, developed

Lactating sow in nursing position



Lactating sows


Sows after weaning
Note involuted glands


 
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