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Factors Affecting Litter Size

16 February 2001

By Frank Aherne, & Roy Kirkwood, Alberta Pork Research . - Current farm data suggests that the herd average for pigs born alive is eleven. Although averages are useful for setting benchmarks, realistic targets for individual farms, must reflect their genotype, parity distribution, disease level, housing and management because these factors make each farm's production possibilities unique.

The most common measure of productivity for the sow herd is pigs weaned/sow/year (PSY). The number of litters farrowed/sow/year and number of pigs weaned/litter contribute equally to PSY (Figure 1). Many factors affect the number of piglets born alive (Figure 2) and the resulting litter size. We will discuss the factors that influence litter size in this article. 

Fig.1 Factors influencing the no. of pigs weaned/sow/year

Figure 1.



Fig.2 Factors influencing the number of pigs born alive

Figure 2.

1. Parity

Litter size is usually smallest in the first litter, rises to a maximum between the third and fifth litter and then remains constant or declines slightly with older parities. Guidelines for the number of pigs born alive for each parity are shown in Table 1.

Table 1. Pigs Born Alive by Parity
Parity 1 2 3 4 5 6
Pigs born alive 9.5 10.0 10.5 11.0 11.5 11.0

An average herd parity of 2.5 to 3.0, with over 45 percent of the sows in parity three to six is optimal for overall productivity. The parity distribution that optimizes litter size varies with genotype, management and facility type.

2. Genetics

Litter size and the percentage of large litters will continue to increase with the identification of genes affecting prolificacy, selection for prolificacy and the use of Chinese lines. There are large breed and genotype differences in litter size, however, the heritability of litter size is low (10 to 15%) so within-herd selection for litter size is unlikely to be beneficial. Selection for increased leanness and fast growth rate has not reduced litter size.

3. Ovulation rate and embryonic mortality

Ovulation rate sets the upper limit on litter size. Gilt selection on the basis of ovulation rate has not increased litter size because any increase in ovulation rate has been offset by an increase in embryo and fetal mortality. This mortality may be the result of a reduction in uterine space or blood supply per fetus.

4. Nutrition

The developing litter is given a very high priority for available energy and nutrients during pregnancy but, the requirement is very small. Therefore, under normal commercial conditions, it is unlikely that the feeding level during gestation will influence litter size. It has also been found that very high levels of vitamins do not increase litter size in herds using normal diets. Several experiments conducted at the University of Alberta have shown that severely restricted feed intake during any week of lactation will reduce ovulation rate and subsequent embryo survival.

Feeding gilts a good quality diet ad libitum for at least ten days prior to breeding will maximize ovulation rate and litter size but overfeeding gilts (greater than 2.5 kg feed/day) during the first 48 to 72 hours after breeding may reduce embryo survival.

5. Service Management

Gilts and sows should be served 0 to 24 hrs (optimum 12 hrs) before ovulation to maximize litter size. In order to time breeding according to ovulation it is necessary to determine the average duration of estrus for gilts and sows and assume ovulation occurs at 70 percent of estrus.

Sexual maturity (number of cycles), at the time of service, has a greater influence on litter size than the age or weight of the gilt. Serving gilts at their second estrus rather than at puberty can increase litter size by about 0.7 pigs. Service with a vasectomized boar at the pubertal estrus followed by a fertile boar service at the second estrus may increase litter size by 0.7 pigs/litter. It has not been determined whether these effects are additive. A combination of natural service and AI, for gilts, may increase litter size compared to AI alone.

There is no difference in litter size between sows served by a boar or AI, assuming both are done properly. An increase in the percentage of sows returning to estrus on days six and seven after weaning will decrease farrowing rate and litter size. At least 85 percent of sows should return to estrus on days three, four, and five after weaning.

6. Boar Effects

Select prolific boars and do not overuse boars. Boar usage depends on need and the age of the boar. Young boars, 8-12 months of age, can be used for two to four consecutive services per week. Mature boars may be used for up to six services per week. High ambient temperature (30%C) will reduce boar fertility and subsequent litter size.

7. Management

All services, natural or AI, should be quality services. Group housing, where group size is large (greater than 20 animals) tends to cause a reduction in litter size, especially in gilts. Do not relocate sows prior to completion of implantation (in the first 28 days after service).

8. Lactation Length

There is a slight (0.06 pigs/day) decrease in litter size as lactation lengths decrease from 18 to 12 days. It appears that the negative effect of lactation length can be overcome by high daily feed intakes (greater than 5.6 kg/day). Lactation lengths of 18 to 28 days have very little effect on litter size.

9) Reproductive Diseases

Various diseases affect litter size and whether a sow farrows. The usual culprits are parvovirus or PRRS. These viruses can cross the placenta, infect the litter and cause embryo or fetal death. This may appear as a high mummy rate or a reduction in litter size, depending on whether or not mummies are detected. Questions regarding these diseases and possible vaccination schedules should be directed to the herd veterinarian.

Further Information

For further detailed information on all aspects of Reproduction
BROWSE or SEARCH our Pig Health Database.

Alternatively, why not buy the book and learn at your leisure.
The following feature articles are also available on litter size:
Dealing with Litter Size Problems

Simultaneously optimizing farrowing rate and litter size

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