Reproduction

By Al Leman.

Boars reach puberty at about 6 months of age. They should not be used for regular mating until they are at least 8 months old. The quality and quantity of the sperm generally increase until the boar reaches about 12 to 15 months of age, after which it plateaus.

The useful life of a boar in a herd is from about 8 to 30 months of age. If boars are kept in a herd longer there is the possibility of father-daughter coatings. This inbreeding can increase the rate of genetic defects and cause a loss in hybrid vigor.

Ten billion sperm cells or more are needed for optimal fertilization. Mature boars produce at least 10 billion sperm a day, while young boars may require several days to produce this amount. Boars from 8 to 12 months should be used 2 to 3 times a week, while older boars can be used 4 to 6 times a week. Most herds need 1 boar for every 20 to 25 females.

Boars affect the conception rate, litter size, pig survival, growth rate, and carcass quality of their offspring. Since boars comprise half the genetic basis of a herd, producers should select them with appropriate care. The following is a list of generally accepted standards for boars.

The underline should contain 12 or more evenly spaced, well developed teats.

They should be free from lameness and have generally free and easy movement.

Boars should attain 230 pounds at an age of 155 days or less.

During their test period of about 60 to 230 pounds, boars should require not more than 2.75 pounds of feed per pound of gain.

Daily gain during the same test period should be at least 2 pounds per day.

Backfat at 230 days should be 1 inch or less.

Gilt Development

Gilts usually are selected on their body conformation, weight for days of age, and an estimation of backfat. Gilts usually reach puberty between the fifth and seventh month. The exact time of puberty is influenced by the breed, season, growth rate, exposure to boars, amount of daylight, pen space, and the number of animals per pen. When all these factors are optimal, 90 percent of the gilts should have had their first estrous cycle when 8 months old.

Estrous cycle of the pig averages 21 days, and most fall within 19 to 23 days. Estrus or sexual receptivity ranges from 1 to 3 days. Ovulation occurs just prior to the termination of estrous behavior. The optimum time for mating is 5 to 10 hours before ovulation or sometimes into the second half of the sexual receptivity period.

There are two generally accepted forms of mating. One is called pen mating, where the boars are mixed and housed with the females and mating occurs unmonitored and whenever there are receptive females.

The other type of mating is called hand mating where producers attempt to detect estrus in females and pen the estrous females together with the boars. The mating is observed and generally recorded for a future reference to the time of farrowing.

Pregnancy, Birth

Usual ovulation rate for the gilt is 12 to 14 eggs, while older sows may ovulate 15 to 18 eggs. Most of these eggs are fertilized. Embryonic losses soon occur, however. By day 25 after mating only about 75 percent of the fertilized eggs still remain viable.

Gestation in the pig is usually 114 to 116 days. A generally accepted standard for the industry is that 80 percent of the mated females should farrow a litter. The very best farms in highly controlled mating programs sometimes exceed 85 percent.

Animals that do not farrow may have never become pregnant, may have returned to heat normally or abnormally (that is, after day 25), or they may have resorbed their litter and come back into heat perhaps at day 40 to 50. Abortions also are possible. Up to a 2 percent annual abortion rate is generally accepted as being normal.

Farrowing begins with restless behavior by the sow. The sow's instincts cause her to attempt to build a nest. In certain confinement situations without bedding, however, this nesting is not possible.

Once the active process of farrowing starts, the sow usually finishes in about 2 to 4 hours. Average interval between the birth of 2 live-born pigs is about 20 minutes. The interval between a live-born and a subsequent deadborn pig is about 45 minutes.

The only source of nutrition for the baby pig during the first two weeks of life is the sow's milk. Pigs have a very strong nursing order and once they establish themselves on one of the teats they are likely to stay there until weaned.

About 40 percent of the pigs are born tailfirst, and about 60 percent headfirst. Last pigs in the litter to be born are the most likely to be stillborn, since most stillbirths result from insufficient oxygen during the birth process.

Within minutes after birth the baby pig is on its feet and searching for the mammary gland of the sow. The first milk is called colostrum and is very high in antibodies, which help protect the pig against the diseases likely to be encountered.

The biggest pigs at birth seem to have a strong survival advantage. They migrate toward the front mammary glands which usually yield the most milk.

Litter Size

Average litter size of U.S. sows is about 10 total pigs including about 1/2 to 3/4 pigs per litter dead at birth or stillborn. First litter females generally have the smallest litters. Litter size increases as these sows have their second, third and fourth litters.

The third to sixth litters generally produce the most pigs, after which litter size begins to drop somewhat. As sows get older, litter size drops slightly but there is a major increase in the stillbirth rate.

Average live-born litter size is about 9.3 pigs and the average litter weaned throughout the United States is about 7.4. Some well managed farms are doing much better, but this national average has remained unchanged for many years.

Lactation

The only source of nutrition for the baby pig during the first two weeks of life is the sow's milk. Sows can produce up to 20 pounds of milk a day. To do this, they must consume high amounts of energy and protein. There is considerable variation between the quality and quantity of milk from the different mammary glands on the same sow.

Pigs have a very strong nursing order. Once they establish themselves on one of the teats they are likely to stay there until weaned, even for a pig that attaches to a mammary gland producing very little milk. This pig will not grow well and will be a runt.

In the United States, about 20 percent of all liveborn pigs fail to reach weaning age. Death can result from: Crushing or overlaying by the sows, starvation, chilling, or a variety of baby pig diseases.

The control for these diseases is to provide the baby pig with a warm, clean environment and a good supply of milk. When these things are in place, natural defense mechanisms of the pig usually suffice to fight off diseases.

Weaning.

Peak milk production in sows is usually in the third to fourth week after farrowing. The average weaning age for pigs is probably between 4 and 5 weeks, although some pork producers now are trying to wean at 3 weeks.

Weaning pigs at 3 weeks, when they weigh between 10 and 13 pounds, requires an excellent nursery facility and high quality nursery feed for high survival and performance. Weaning at 3 weeks of age as compared to weaning at 4 weeks will result in a slightly smaller litter the next time the sow farrows.

Common Diseases

Leptospirosis is a bacterial disease that can cause late term abortion, stillborns, and weak pigs. It is best controlled by vaccination. Vaccination is common throughout the United States and the disease appears to be less important than it was 10 or 20 years ago.

Brucellosis can cause swine reproductive failure, although the disease is very uncommon. There are attempts to eradicate brucellosis from U.S. swine because it is a potential human health hazard, especially to workers at slaughterhouses.

Porcine parvovirus is the major viral reproductive disease of swine. It once was classified under the SMEDI syndrome which stands for stillbirths, mummified pigs, embryonic death and infertility.

This virus is widespread, and present on almost every farm. The animals at risk are those that do not become naturally infected and immunized prior to mating. On most farms this amounts to only 10 to 20 percent of the animals. New vaccines appear helpful in controlling parvovirus and in creating immunity in the animals that were previously at risk.

Another reproductive disease is pseudorabies. This disease is discussed in the fourth chapter of this section. It is capable of causing an abortion in sows.

Al Leman is Swine Extension Veterinarian, College of Veterinary Medicine, University of Minnesota, St. Paul.