Gordon King, Animal Science Department, University of Guelph, Canada

The Importance of Reproductive Performance

Livestock farmers generally appreciate the time and input costs required to grow animals from birth to market weight. What is sometimes overlooked is the substantial expense involved with maintaining the mature breeding animals that are necessary to produce any stock for subsequent growing and finishing. Profitability in any commercial livestock-breeding unit is related closely to reproductive efficiency. Unfortunately, some producers and even many of their "expert" advisers pay insufficient attention to this aspect.

No presently available procedure allows prediction of current or future fertility. Thus, accurate monitoring of immediate past performance is essential to identify problems quickly and to initiate solutions before consequences become too severe. Some considerations for the assessment of performance and suggestions for problem solving are presented later in this section.

Boar Selection

Boar Management

Boar to Sow-Gilt Ratios

Natural service - 1 boar for every 20 females

AI + natural - 1 boar for every 20-30 females

All AI - 1 boar for every 40-50 females. Larger herds require fewer boar's

Gilt Management

Factors Influencing Puberty

- Yorkshire and Landrace usually mature slightly earlier than Hampshire or Duroc.
- crossbreeding advances puberty.

- restricted feed intake can delay puberty slightly.
recommend to allow only 0.5 kg gain per day after market weight.
- obesity interferes with normal ovarian function.

- moving or mixing gilts - fight
- tends to cause puberty 4-7 days later
- contact with boars may advance puberty
- related to pheromones
- direct exposure most effective
- exposure should not be too early (after 155 days of age)
- older boars more effective.

- some evidence for advantage of increase light
- may decrease age at puberty
- may increase litter size

When to mate

Selecting the proper time to breed replacement gilts is an important part of reproductive management. A few precocious females may begin estrous cycles and show estrous signs by five months but most do not reach sexual maturity until around six months of age. Since the ovulation rate increases at each subsequent estrus period, many farmers delay mating until the second or even the third cycle to increase litter size. Although the number of piglets born will increase if mating is postponed, there are other factors to consider in making the decision about when to mate gilts. The results from a number of research trials comparing litter size after mating at the first or third estrus are shown in the following table.
Mating estrus Piglet increase per day of delayed mating
First Third
Brooks & Cole, Livst. Prod. Sci. 7:67 8.8 9.9 0.026
Pay & Davis, Anim. Prod. 17:85 7.9 9.3 0.033
MacPherson et al, Anim. Prod. 24:333 8.4 10.4 0.062
Young & King, J. Anim. Sci. 53:19 9.6 10.6 0.023

Mating gilts on the third observed estrus resulted in larger litters but also required feeding the dams for an additional six weeks. Depending on costs for feed and housing, the extra piglets may or may not compensate for the additional expenses.

Pregnancy rates were 69.6% for animals bred at first estrus and 77.4% when mated at the third estrus in the study conducted by Young and King. Although this difference was not statistically significant in this trial, subsequent experience also indicated a somewhat lower fertility at the first cycle. An even lower pregnancy rate is usually obtained in the University of Guelph research herd when sexual maturity is induced with hormone injections (about 55% for six month old gilts inseminated on the induced estrus vs. 80% in sows).

Perhaps the best practical advice for producers is to wait for the second or later heat whenever this is convenient. However, if replacement animals are needed to fill groups and no older females are available, gilts can be mated on their first estrus. If gilts are mated at the first cycle, use herd boars rather than expensive semen purchased from an outside supplier.

Estrous signs

Good Practices for Detecting Estrus

Back Pressure Test

Mating Practices

Timing & number of services

Feeding Replacement Gilts

Housing for Breeding Females

Pregnancy Diagnosis

Sows and replacement gilts require considerable attention in the mating area but only casual observation once they are pregnant. Thus, producers usually place pregnant females in a gestating area but need some reliable method to check that conception occurred before animals are moved from the mating unit. All attendants should be aware that domesticated animals rarely exhibit any signs of sexual behavior one cycle interval after a fertile mating. Thus, for animals mated on known dates, demonstration of sexual receptivity when the next estrus period is due indicates no conception from the previous service. In contrast, the absence of sexual behavior is strong evidence for pregnancy. Sows and replacement gilts should remain in the mating area, with daily boar contact to check for estrus, for around 25 days after service. Once they pass three weeks without estrus signs they can be presumed pregnant and moved to the gestating area.

Other methods for pregnancy testing are also available.

Livestock producers must recognize that there is one inherent problem that will continue to be associated with even the most reliable pregnancy test. No matter how accurate the procedure, the results are only valid for the time when the sample was collected or the physical examination conducted. There can be no guarantee that the embryonic or fetal piglets present at the time of testing will continue to develop normally throughout the remainder of gestation.

Early Embryonic Mortality

Unless fertility depressing abnormalities or mistimed matings occur, fertilization rates in pigs approach 100% so almost all ovulated oocytes have a chance of developing into viable piglets. Unfortunately, a substantial proportion of these early zygotes do not successfully complete gestation and this is a major cause of reduced litter size. Most of the losses occur during the first 3 to 4 weeks and are designated as early embryonic mortality. When death occurs at this very early stage of pregnancy, the conceptus is resorbed completely in the uterus so losses remain undetected. Some embryonic mortality undoubtedly results from chromosomal abnormalities present in one or possibly both fertilizing gametes. Abnormal karyotypes have been detected in a high proportion of aborted human fetuses but not in those from domesticated mammals. This might infer that chromosomal defects are relatively unimportant in livestock. However, the livestock conceptus may be lost at very early stages before karyotyping is possible, or the damage could be at a submicroscopic level. Some degree of embryonic mortality must be considered unavoidable and represents a natural mechanism for elimination of unfit genotypes but the extent of this in livestock is uncertain.

Preparation for Farrowing

Go to section on farrowing considerations.

Seasonal Influences on Pig Reproduction

Wild pigs are seasonal breeders, usually mating in the late fall and giving birth to piglets in early spring. Although selection over many generations has almost totally eliminated any seasonality of reproduction in domesticated pigs, some traces still remain. Close observation of large groups in several countries indicated slightly longer intervals from weaning to estrus, more animals not returning at all within a reasonable interval, and lower prolificacy in animals weaned during late summer and early fall. This infertility has been attributed to higher temperatures in some regions and undoubtedly this may be a factor in tropical climates. However, since these problems are observed routinely in cooler temperate regions such as in the UK (latitude 50 - 60o N) and in southern Australia (latitude 35 - 40o S), photoperiod may be an even more important contributor. Producers wishing to insure they have sufficient pregnant animals to keep farrowing units operating at full efficiency should always cull a few less sows and keep a few more replacement gilts during the summer and early fall.

Assessing Reproductive Performance

No presently available procedure allows prediction of current or future fertility. Thus, accurate monitoring of immediate past performance is essential to identify problems quickly and to initiate solutions before consequences become too severe. For sows, true productivity must combine litter size, piglet survival and farrowing interval, so a useful measure is Piglets Weaned per Sow per Year.

Many producers delude themselves, or try to impress their neighbors, by omitting culled females and gilts from the calculation. However, all members of the breeding herd are part of the overhead and should be included in any efficiency calculation. Gilts may be added to the monthly inventory as soon as they pass market weight and are retained as potential breeding stock or when they are first mated. When the performance is calculated on an annual basis, proportional figures can be used for females kept for periods less than a full year. Litter sizes are sometimes calculated and even published in official production statistics with gilt litters excluded. Since gilts produce 30 to 50% of total litters in commercial piggeries, such figures may look impressive but are quite misleading.

Another simple way to monitor productivity of a large continuously farrowing sow unit is by calculation of the Piglets Weaned per Sow per Month in operations where weaner pigs are sold or Finished Animals Marketed per Sow per Month in Farrow-to Finish herds. The actual number of piglets weaned or marketed is divided by the total number of mature females on the herd inventory. A monthly average of 1.7 corresponds to over 20 piglets per sow per year and this figure should be maintained or exceeded in most problem-free operations.

Suggestions for Performance Targets and Potential Problem Values for Pig Reproduction



Potential problems


> 22

< 19


> 9.5

< 9


> 2.4

< 2.1

Piglets born live/litter


< 10

Stillborn piglets (%)

< 5

> 8

Mummified piglets (%)

< 0.5

> 1

Preweaning mortality (%)

< 8

> 12

Nonproductive sow days

< 50

> 75

Weaning to estrus days

< 7

> 9

Farrowing rate (%)

> 85

< 80

Sow mortality (%)

< 2

> 3

Postweanig loss (%)

< 2

> 3

Finishing loss (%)

< 1.5

> 2

Age at 1st mating (mo)

< 7.5

> 8

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