29 January 2024

13 minute read

By: Penn State University

North America

Pig reproduction is an adventure for most new swine enthusiasts. Often, the excitement of fruitful events, such as the birth of a litter of many new piglets, is often paired with a frustrating struggle of how to actually bring a gilt into season for her first breeding. In this article, you will find an overview of swine herd management, including typical markers of performance, different breeding and farrowing management systems, puberty (maturity for breeding), and techniques for breeding.

Measuring reproductive performance

The reproductive herd is for the multiplication of maternal and paternal lines that are prized for economically valuable reproductive traits, or, traits that generate desirable offspring reliably. Typically, prized reproductive traits include litter size, particularly total number of pigs born, and pigs weaned, weight of the litter of piglets at birth and weaning, wean-to-estrus interval (WEI), and pigs per sow per year (PSY). In more recent years, length of time a sow remains productive in the herd (longevity or survivability) is increasing in priority for major genetic lines. These traits are prioritized due to the impact on value of the sow to the herd.

Commonly, these records are collected to measure productivity in the reproductive herd and can be defined as follows:

• Litter size: The number of pigs per litter. As the number of pigs born per litter increases, generally there is an increase in the number of pigs born alive and the number of pigs weaned. This number is currently scrutinized as a primary economic focus, as in recent years there has been evidence of reduced birth weights due to increased litter sizes, which is associated with increased mortality rates and decreased growth rates in offspring. However, in the same period of time, offspring have been selected and now grow at the fastest rate in history, while mortality rates have generally remained the same. Litter size remains a top priority for most pig producers.
• Total born, born alive: Total born is the total number of pigs a sow has, including born alive, stillborn, and mummies. The number born alive is all piglets born alive, excluding number of pigs born still (stillborn, fully developed but not alive at birth) and mummies (partially decomposed due to death early in gestation and before full development).
• Total Weaned: The total number of pigs an individual sow weans. Marker of sow's ability to provide enough milk and mothering ability to rear pigs.
• Birth and Wean weight: Birth weight is the weight of piglets born alive within the first 24 hours, typically, this includes all piglets that were born alive, even if some of those pigs died before weighing. Wean weight is the weight of the piglets that were weaned from one litter. Weights can be individual, but are usually collected by producers as an entire litter.
• Wean-to-Estrus Interval (WEI): the amount of time that passes from the time a litter is weaned from a sow to the time she returns to estrus. A good sow should be reliable in her ability to return to a productive heat or estrus in a timely manner post weaning (usually 4-6 days). It is very common for producers to give sows up to 7 days post weaning to show signs of estrus. After 7 days, the opportunity to catch a reliable heat is usually dependent on the sow's ability to enter back into a normal estrous cycle. Depending on your system, you may wait until the next cycle (18-21 days) while watching for heat and trying to breed again.
• Heat-No-Service: As explained in the name, a heat or signal of readiness to breed is observed in the sow, however she is not inseminated (bred or 'serviced') by a boar or Artificial Insemination. Instead, she is allowed to continue in the estrous cycle (18-21 days) until her next heat or estrus period is reached. A purposely skipped heat is different than a missed heat. Purposely skipped heat stages are associated with management decisions that are for the benefit of the sow or the system. A missed heat is either a lack of management or a reproductively unpredictable sow; in either case here, a missed heat generally contributes to a poor understanding of herd performance.
• Days Non-Productive (DNP): The number of days that a sow is not in gestation (pregnant), lactation, or the 7-day window post weaning. This number can be used to evaluate overhead cost of caring for sows in the herd and act as a measure of reproductive performance when culling decisions are made or management changes occur.
• Pigs per Sow, per Year (PSY): The number of piglets born per sow in a given period, expressed on a yearly basis, divided by the average herd size at mating. This is often used to broadly evaluate sow productivity within the entire herd and across parities. PSY has traditionally been the benchmark of reproductive efficiency, however current pig production models place less emphasis on this single measurement.
• Longevity: how long a sow stays productive (continues to have litters) in the herd. Longevity of sows in pig operations can and should be measured across a variety of indicators of sow removal, such as replacement rate, culling rate, percent of gilts in the herd, and mean parity (number of litters) of females in the herd. Each marker of removal can indicate a different reason for sows exiting the herd, and should be carefully considered.

Breeding systems

Management of the reproductive herd is commonly conducted with a combination of closed herd genetic approach, where genetics are maintained from stock on-site or through purchased sows, boars, or semen through breeders, live auctions, or boar studs. Producers with large numbers of pigs often work in a contract with a swine genetics company. Ultimately, all pigs feed into pork production in some fashion. However, when prioritizing genetic performance, it is good practice to keep either a portion of your total herd dedicated to prioritizing reproductive traits. Working with a consistent third party source (breeder or boar stud) can help you achieve your reproductive herd goals.

All breeding systems require identification of nucleus females (those used for maintaining highly reproductive genetics), requiring close evaluation and selection program, and management of the genetic makeup of the herd. This focus on quality in the nucleus herd likely includes a dedicated gilt development program, that focuses on optimizing the rearing of the replacement sow through puberty before service. This herd should be managed differently than the terminal herd. The terminal herd focuses genetics and performance for the purpose of pork production. Crossbreeding systems offer several advantages to producers including maximizing heterosis (improved or increased function of any biological quality in a hybrid offspring), providing greater product consistency, easier to implement and manage, and typically prioritizes best use of genetically selected sire and dam lines.

Breeding and farrowing management systems

Batch operation

In a batch operation, sows are organized into groups that allow mating and farrowing to occur at distinct intervals. Sows will farrow and be weaned in groups. Typically, farrowing happens in a narrow range of days, as all pigs were bred on the same day or within a very narrow (less than 1 week) time frame. Group sizes typically are dependent on the amount of space available for sows, including the number of farrowing rooms and farrowing stalls, as well as target weaning age for the offspring. This system often works best with small herds and becomes difficult to manage a true batch with a large herd. A batch operation system is one of the best management systems to adopt an all-in/all-out pig management system to prevent the spreading of diseases. In addition to this systems ability to prevent the spreading of diseases, personnel can pay more attention to sows and offspring on farrowing day, leading to improved piglet survival. Batch operations also improve efficiency for use of resources, which in turn improves profitability through improved growth rates, feed conversions, and health through reduced medical costs. However, this operation makes it difficult to manage females that recycle and is difficult to get returns (sows or gilts that fall out and need to be re-bred and returned to a new group) back into a group. Additionally, it is difficult to ensure that nurse sows will be available for extra pigs or fallback (runts or pigs that fail to thrive) pigs. Batch operations also have a high investment cost to implement.

Continuous operations

In contrast to batch operations, in a continuous operation, breeding and farrowing are happening daily, or continuously, with no breaks. Mating, farrowing, and weaning are done on a daily basis and labor is spread out across all tasks. Typically, events are organized into weekly groups for moving sows through breeding, gestation and farrowing facilities. Sows are moved through these facilities on individual timelines, so the facilities are never completely emptied because pigs and sows are always moving through it. Due to this continuous flow, it is difficult to break a disease cycle unless large, segmented facilities are available to attempt to break the cycle. However, it is easier to manage the females that recycle and return them back into a group, while also creating an even flow of pigs and distribution of labor throughout the year, which is beneficial for companies that may be underutilizing their available staff.

Puberty in gilts and boars

The onset of puberty is the time point when both metabolic changes, such as hormone levels, and physical changes, such as body growth and fat accretion, lead to the achievement of sexual maturation. Once the first estrous cycle takes place, the gilt is now capable of conceiving and producing a litter. Gilts with an earlier onset of estrus have greater productivity once in the breeding herd. Notably, genetic background (breed type) has a substantial influence on the age of onset of puberty, however this influence can be quite variable across different herds and within the same herd. In most cases, a group of replacement gilts will reach puberty between 170-220 days of age.

Similar to the gilt, early onset of puberty in the boar improves breeding capacity. A boar is commonly put into service around 225-270 days of age, however, there are many factors that influence the onset of puberty in boars. Genetics play a significant role in the age of puberty in a boar, particularly, selection for larger testes, which is associated with improved sperm production.

Variation in puberty attainment in both gilts and boars is largely due to variation in physiologic age, which is a measure of how a body is functioning relative to actual age. Physiological age is dependent on genetics as well as environmental factors such as housing, nutrition, or exposure to the opposite sex or older animals. For example, gilts can ovulate in response to exogenous gonadotropin and estradiol after 100 days of age, however, this mechanism does not completely mature until after 160 days of age. In addition to this variation, and variation induced by care, the development of gilts and boars should be carefully recorded for any operation looking to improve herd reproductive performance.

Breeding sows and semen delivery

Techniques for breeding sows are classified by semen delivery, these techniques can be referred to as: pen mating, hand mating, or artificial insemination.

• Pen mating refers to a method that allows a boar to run with females. Pen mating works best in a pen of pigs that are in various stages of the estrous cycle, as the boar is likely to find the sows in heat, but will not be over-worked by the entire pen showing heat at once. Pen mating, however, is difficult to keep good records on when a sow was serviced.
• Hand mating refers to a process that can be described as supervised natural mating. Similar to pen mating, a boar is utilized to deliver semen to the sow through natural cover. However, when using hand mating, one boar is penned with one sow, and the animal caretaker monitors the event, being sure to co-mingle the boar and sow repeatedly over 2-3 days and carefully recording each mating event.
• Artificial insemination (AI) is the most commonly used technique by pig producers. Utilization of AI requires that semen be collected from the boar by a human handler. The semen is then processed and packaged, then shipped for use on sow farms. Boars that reside on sow farms are then used predominantly for heat detection, and secondarily (if at all) used for breeding. The primary benefit of AI is the ability to obtain choice genetics for low costs and to readily add genetic variation to your herd. Unlike a cattle herd, however, use of AI does not remove the need to have a boar on farm. Uniquely, reproductive performance of gilts and sows still benefits tremendously from having a boar on farm. Allowing interaction between a boar and sow for estrus detection is still considered a best management practice among pig producers.

Daily tasks and housing

Temperature, ventilation, aggression, body condition, and nutrition management are all very important aspects to proper care of the reproductive herd. Aggression management is of particular importance when housing sows in groups. While being social animals, pigs are traditionally aggressive towards each other when first placed in group housing or with new animals. Fighting is very common within the first 2 weeks of mixing (placing pigs in a pen together), and injuries can be severe. 

Keen focus is placed on minimizing severity of fighting, some producers choose to group sows by of similar age, size, or consistent breeding/farrowing group to mitigate the severity of fighting and reduce losses due to injuries. Gilts and young sows tend to be less aggressive than mature sows, many producers house them separately to minimize fighting in this susceptible group and improve longevity in the herd. Temperature, ventilation, body condition, and nutrition management of reproductive stock should also be closely considered, individually and as they interact with each other. As temperature changes across seasons, so will the need to adjust ventilation and nutrition. Body condition scoring is used to make these estimations, however it is only a tool and needs to be considered in respect to current diet, temperature, and ventilation status of the barn at any time.

Factors affecting longevity

There are many contributing factors into longevity, including genetics, the impact of development, body condition score, nutrition, and health. Generally speaking, longevity is prioritized as a target for herd improvement through genetic selection and animal development to breeding. Genetic selection for longevity focuses on reducing culling rates due to structural-induced lameness and reproductive failure. Heritability for longevity, however, is fairly low, with studies reporting Landrace longevity heritability to be 0.13 and Yorkshire longevity heritability to be 0.16.

Gilt and boar development is also highly impactful on longevity of those animals through their productive time in the herd. However, little research is available on specific recommendations for gilt development, outside of prioritizing calcium and phosphorus in the diet for bone development. Some studies have reported gilts fed a higher energy diet during rearing may promote excessive weight gain and increased culling due to lameness, however, gilts with higher backfat tend to reach puberty earlier as well, further improving reproductive performance. Ultimately, while more research is conducted, good record-keeping on reasons for culling sows will move your sow herd performance closer to your reproductive goals.

Some of the records for culling are largely based on acknowledging why the pig was culled. Culling decisions can be classified in this manner:

• Culling refers to the active removal of live animals from the herd by the herd manager. There are a variety of reasons why animals might be culled, however, all culling is conducted to improve the herd in some manner. When discussing culling for the reproductive herd, we can categorize the reasons as voluntary or involuntary:
  • Voluntary culling decisions are carried out to manage the parity profile and remove any sows with sub-optimal performance. These sows may have shown any of the following: farrowing difficulties, poor litter sizes and born alive, poor lactation and rearing ability, poor maternal behavior, decreased productivity compared to herd average, poor conformation.
  • Involuntary culling decisions are forced decisions due to reproductive failure, such as anestrous, failure to conceive (typically after two or more failed breeding attempts), or abortion, as well as physical injury, lameness, or disease.

This overview is just a taste of herd reproductive management for pig producers. Based on the goals of your program, this information should help guide you to begin planning your reproductive herd management program. For more information or assistance in building your program, see these Extension resources below, or reach out to Penn State Extension educators and specialists for assistance.