Reproduction and lactation in pigs

In order to be able to control reproduction the pig farmer must understand the factors that affect it.

Puberty in gilts

Puberty is the onset of estrus and ovulation with continued regular estrus cycle and is affected by the following:

Age, liveweight and growth rate

These factors are interrelated. However they are not indicators of maturity because a gilt of say 40kg can show signs of estrus while another of the same weight may not. Growth rate is the best indicator among the three because it shows the activities of the pituitary.

Nutrition

Lysine and methionine supplements seem to induce puberty

Breed

Certain breeds of pigs attain puberty earlier than others e.g. landrace attains puberty earlier than most breeds.

Heterosis

Hybrid vigor reduces the age at puberty. Crossbreeds attain puberty earlier than purebreds.

Sire dam puberty age difference

Polland china x          Duroc             less 11 days

Duroc              x          Yorkshire        less 36 days

Photoperiod

The length of day influences the onset of puberty such that long days tend to induce puberty.

Crowding

Crowding induces puberty probably because of interaction and production of hormones.

Boar contact

Gilts kept close to boars will attain puberty early. Optimum exposure period is 150 to 170 days of age. If exposed at an early age of 120 days they tend to get used to the boar and will not reach puberty early. If over 200 days in age there will be no effect.

The Estrus cycle

The pig will cycle on average after every 21 days but this varies between 18 and 24 days.

Signs of estrus

• Mounting on other females but will not tolerate being mounted. This is very early signs
• Swollen vulva
• Discharge from the vulva - this may not be very pronounced
• Stands to riding test

Signs may last 3 to 4 days but gilts will allow mating only for 2 to 3 days.

Ovulation

This occurs in late estrus usually the second day after the onset of estrus. The ova released will remain fertile for 24 hours therefore conception can optimally occur if mating or artificial insemination is done in late estrus. The highest conception rate will be achieved if insemination is done 6 – 10 hours before ovulation.

Depending on the ovulation rate, litter size varies from 7 to 16 piglets.

Factors affecting ovulation rate

Estrus number

Whether the pig is cycling for the first, second or third time etc. The number of ova released increase with each cycle. The number of ova released at a single estrus gradually increases through the first three cycles. Litter size represents only about 70% of the number of ova released.

Breeds of pigs

Chinese breeds are very prolific with litter size going as high as 30. Yorkshire and Landrace are also prolific. Hampshire and Duroc are not.

Energy intake

High energy intake tends to increase ovulation rate (2 weeks prior to breeding)

Temperature

High temperature from 33⁰C and above will reduce ovulation rate. Low temperatures do not have any effect.

Artificial control of estrus

This can be done by parenteral gonadotrophin administration after weaning. This involves injecting the sow with Pregnant Mare Serum Gonadotrophin (PMSG) at weaning or anytime after and following it 60 to 70 hours with Human Chorionic Gonadotrophin (HCG). The sow will cycle in less than 3 days.

Conception rate

This is the percentage of mating that result in the development of viable fetus. A rate of 70% is acceptable for gilts and sows in the first service. It is difficult to attain 100% conception rate due to factors like cystic follicles, improper estrus timing, boar or female infertility and other anomalies.

Litter size in relation to prenatal death

Litter size refers to the number of piglets born to a sow. The size of a litter will depend on:

• The number of ova produced
• Percent fertilization
• The number of prenatal death and stillbirths

Usually prenatal death accounts for 40% and increases with gestation period, the possible reasons for prenatal being:

• Excess secretion of estrogen
• Uterine crowding
• High plane of nutrition after conception especially the first 3 weeks

Natural mating versus Artificial Insemination (AI)

Natural service is the most common system of mating for pig farmers in Kenya. It is an efficient way of mating but the cost of a boar can be quite high. Where AI is practiced fresh semen is used because of the poor keeping quality of boar semen. Currently with the use of extenders the semen can be kept fertile for 3 to 5 days. Whereas bull semen can be frozen at -96⁰C for life, boar semen cannot be frozen. Bull semen can be extended using citrate and phosphate extenders but these extenders are toxic to boar semen. However it is possible to use fresh semen for 4 to 5 days. Semen can be collected twice a week per boar.

Lactation

Milk production in the sow is a necessary process for the survival of the young piglets. Milk yield and composition of the sow can be estimated by weighing of piglets before and after suckling. Sometimes it becomes necessary to inject oxytocin in order to induce milk letdown.

Peak milk production is attained in the 5 – 6 weeks. By 9 or 10 weeks daily production is negligible. Average milk production is about 5 to 8kg per day over a period of 8 weeks.

Milk production depends on nutrition, body size and breed.

Milk from sows is characteristically deficient in iron and copper and however much iron is provided it does not pass through the milk.

Post natal development

The new born piglet is rather immature and therefore certain changes must occur to make it well acquainted to the environment. Piglets are born when they weigh 1.1 to 1.4kg on the average. Usually the birth weight of the piglet will determine its survival chances. The lower the birth weight the higher the mortality rate. Litter size also affect mortality rate indirectly. A large litter size tends to have high mortality rate due to low birth weights.

New born piglets have very low liver glucose levels and at the same time the fat content of the whole piglet is below 1% therefore there is no energy stored in the piglet. The piglet must therefore suckle immediately in order to get energy. If it does not suckle the blood glucose drops dramatically in the first 48 hours. Because of this such piglets will undergo hypoglycemic comma leading to death.

Digestive development

During the first 24 hours the digestive tract is permeable to intact protein. Following the intake of colostrum the ability to absorb such proteins is lost. When the new born piglet is allowed to suckle immediately after birth it will acquire passive immunity due to the immunoglobulin content of colostrum. Colostrum has certain antibodies and this will be present in the cells of the young piglet at least 6 hours after suckling.

Active immunity in young piglets start after 3 weeks but production of antibodies is minimal up to 5 weeks of age.

The newborn piglet will readily absorb glucose but the utilization of sucrose and polysaccharides are not as sufficient. Another carbohydrate that is readily utilized is lactose. The older pigs will utilize lactose very poorly because they have lower lactase secretion.

Complex carbohydrates like cellulose cannot be digested well by pigs. This is because the pigs do not have the ability to secret cellulose and has very little micro-flora in the true stomach. However certain microbes which have the ability to degrade cellulose are present in the pig in the large intestine and caecum. This microbes release volatile fatty acids which are absorbed in the large intestine for energy metabolism.

Pigs of all ages have the ability to secrete large amounts of pancreatic lipase therefore fat absorption is very efficient in the pig.

At birth intact protein will be absorbed, but following the absorption of alpha globulins the gut closes and intact proteins will not be absorbed anymore. Following gut closure dietary proteins must be hydrolyzed before absorption. The efficiency of protein absorption depends on the source of protein. E.g. milk protein is absorbed at a higher rate (92 – 95%) than soya bean protein (89%).