The number of pigs a sow can produce annually continues to be a popular measure of production efficiency. The common metric used to compile sow productivity level is the well-recognized pigs/sow/year (P/S/Y).

Throughout most of the 1970s and 1980s, 20 P/S/Y was considered the standard of excellence. By the late ’90s, the target rose to 25 P/S/Y. Today, producers are striving for 30 P/S/Y
or more.

From a management perspective, achieving a high level of P/S/Y involves management of females during two distinct periods, the developmental phase and the functional phase, which are connected by a transitional event — first mating. Figure 1 helps illustrate the functional period for P/S/Y.

Anything the gilt encounters prior to her first mating has the potential to influence her reproductive performance as an adult, because her reproductive organs are undergoing developmental changes. Once she is bred, her reproductive system is activated, and subsequent management conditions influence how effectively it functions as it cycles through pregnancy, lactation and rebreeding.

As producers strive to increase P/S/Y, management during the developmental and functional periods and the timing of when to transition females between the two are likely to play key roles in the relative success that is attained.

Developmental Period

The ovaries, uterus and reproductive centers in the brain begin to form between 30 and 50 days after fertilization and continue to grow throughout gestation. By birth, developing fetuses have the majority of the basic cell types they will use as adults.

This is most evident in the ovaries, which contain all the follicles and eggs a female will ever have at, or shortly after, birth. Consequently, the majority of the anatomical foundation for the sow’s subsequent reproductive potential is in place when she is born.

During the first 60 to 90 days after birth, cells within each of the reproductive organs begin to specialize. In the ovaries, groups of follicles begin to grow and differentiate. A similar process occurs in the uterus with some cells becoming the portion that will support development of fetuses, while others form the muscles that will be important during parturition.

Finally, during the later portion of pubertal development, probably somewhere between 140 and 160 days of age, the reproductive organs acquire the ability to respond to hormones and neural input from each other, which is how they will communicate among one another once females are bred and begin to produce live piglets.

A current challenge for the swine industry is to figure out how to evaluate how well gilts are maturing reproductively during the developmental period. This, in turn, is crucial for the development of management programs that will consistently produce females capable of 30+ P/S/Y. Because most of the framework for all the female reproductive organs is constructed during fetal development, the first opportunity that producers have to assess this is at farrowing.

There are well-established, positive relationships between birth weight and the size of most internal organs — heavier pigs have larger, more developed organs. This is certainly true for the size of the brain, intestines, liver and the number of muscle fibers. There is no reason to think that the relationship does not also hold true for the ovaries, uterus and reproductive centers of the brain.

Information about the relationships between birth weights and lifetime productivity are currently being collected and evaluated. Its usefulness as an early indicator of subsequent reproductive performance in some herds is encouraging.

Table 1 contains data collected within the multiplication phase of a small production system in which it was customary to “over breed” replacement gilts. After pregnancy diagnosis at Day 28, post-insemination, surplus pregnant gilts were marketed. This allowed for the evaluation of some important reproductive characteristics.

While these data are based on only about 50 gilts in each of the weight classes, they do show a positive trend between birth weight and reproductive performance early in the first pregnancy. The largest gilts at birth exhibited a tendency to perform better than their smaller counterparts. It is interesting to note that if no other reproductive losses occurred in each of these groups of females, then the ones with the heavy birth weights would be expected to produce around 12 pigs/litter total (15 ovulations x 83% survival), whereas those with light birth weights would farrow about nine (13 ovulations x 69%).

These data are preliminary, but they do seem to support the notion that birth weights may have some associa­tion with subsequent reproductive potential. It is reasonable to speculate that there are probably some minimum birth weights below which gilts simply do not have the reproductive machinery to produce 30 P/S/Y, regardless of how well they are managed later in life. For obvious reasons, this minimum threshold could vary among production systems.

It is up to producers to determine the relationships between birth weight and subsequent reproductive potential within their own systems. One way to accomplish this would be to record birth weights on all potential replacement gilts and then, after they enter production, retrospectively determine if and to what degree they are associated with the production of piglets as adults.

Unfortunately, producers have very little control over changing birth weight, since it is influenced mostly by the total number of pigs born. As total number increases, the average birth weight decreases and becomes more variable. This is basically the result of supply and demand.

The uterus has a finite capacity to support fetal growth. As the total number of fetuses increases, the uterine space available for each one decreases, and the end result is reduced growth and development. This phenomenon is referred to as intrauterine growth retardation and is particularly relevant for highly prolific, maternal sow lines. These females produce replacement gilts in modern production systems and it is not unusual for them to have 14 or more total piglets born. As a result, there usually are small pigs in each litter and variation in individual birth weights can be significant.

Strategic Crossfostering

There is some evidence that strategic crossfostering after birth may provide a relatively simple way to counter some of the effects associated with intrauterine growth retardation. In a recent study conducted in a large commercial production system, future replacement gilts were raised in litters of either 11 or seven piglets. The average litter size in which replacement gilts were born was 12 piglets and the average birth weight was 2.8 lb., which is consistent with litters experiencing mild to moderate growth restriction during gestation.

After weaning, gilts were commingled, sent to commercial farms and managed similarly until they were either culled or gave birth to their sixth litter. Reduction of the litter size in which replacement gilts were raised significantly increased sow longevity (35.5% vs. 17.3%), improved farrowing rate (88.7% vs. 83.3%) and tended to increase number of pigs born alive (11.0 vs.10.5) over six parities.

Replacements gilts raised in the small litters were heavier at weaning and maintained a significant weight advantage throughout the rest of their productive lifetime, compared with their counterparts from large litters. Although the exact physiological mechanisms associated with this response are not fully understood, a reasonable explanation is that reducing nursing competition provided an environment that increased not only the overall growth of the piglets, but also enhanced the early development of their reproductive organs. In other words, reduced postnatal competition helped counter some of the prenatal restriction that resulted from being born in a large litter.

What is not known is how much of a reduction in litter size during lactation is needed in order to realize an improvement in subsequent reproduction as an adult. The previous study was designed to reduce the nursing competition by about 50%. This probably is not practical in most multiplication systems, especially those with high indexing, maternal-line sows.

It is quite possible that the best guide might also be birth weight. Just as there probably is a minimum birth weight below which subsequent reproductive development is permanently compromised, it is reasonable to speculate that there is also a range over which strategic crossfostering can improve lifetime productivity in adults. It is conceivable that the birth characteristics of a litter of future replacement gilts might dictate whether all, some, or none of them may benefit from strategic crossfostering. If subsequent research proves these speculations to be correct, then production systems that have the ability to examine birth weights of replacement gilts retrospectively, in conjunction with their lifetime productivity, will have a distinct advantage in terms of implementing strategies such as this and reaching the goal of 30 P/S/Y.

Gilt Management

Considerable research has been conducted on managing replacement gilts from nursery through finishing phases and the relationships between growth rate and subsequent reproductive performance. The general consensus from these studies is that gilts that attain a lifetime growth rate (weaning-to-puberty) of 1.5 lb./day or greater tend to reach puberty earlier and have less breeding problems as adults, compared with gilts that do not.

Results from other studies show that reduced competition, either in the form of fewer animals per pen or increased square footage per pig, has a positive influence on reproductive performance in the first parity. In those studies, replacement gilts with more space or fewer penmates grew faster than their counterparts, so monitoring the growth of gilts between weaning and breeding most likely is a fairly good indication of how well replacement gilts are adjusting to their production environment. Those that grow 1.5 lb./day or greater are adapting well, while those that don’t, are not. Since most producers collect growth data on their replacement gilts, they have the information for their genetic lines and are in the best position to determine the threshold level of postweaning growth within their own system that supports acceptable reproductive performance as an adult.

The most important reproductive milestone during the developmental period is the ability of the replacement gilts to exhibit estrus and ovulate in response to boar exposure. This is the best physiological indicator that her reproductive organs have developed to a point where they can function as an adult.

There is considerable evidence that females that respond early to boar exposure have greater longevity and productivity as sows compared with those that do not. This seems to be true regardless of when they are bred relative to the expression of their first estrus. In essence, the ability of gilts to respond early to boar exposure might be the best way to assess how well they have been managed during the developmental period.

In the study involving strategic crossfostering noted earlier, all gilts, regardless of whether they were raised in a small or large litter, were given boar exposure around 150 days of age. Gilts that exhibited a standing reflex within 28 days of their first exposure to a boar were classified as early responders. The proportion of early responders was 24% higher in females raised in small litters vs. large litters (77% vs. 53%).

From a physiological perspective, early responders have a more robust reproductive system in that their organs respond to the boar exposure and to a greater degree, compared with late responders. Alternatively, boar exposure might produce identical changes between the two groups, but early responders have an increased sensitivity to these changes and therefore respond sooner. Either one of these situations is consistent with females with increased longevity and the likelihood of producing 30+ P/S/Y.

Boar Exposure

At what age boar exposure should begin in order to have the best chance of identifying early responders is an important question. For most genetic lines, any age between 160 and 220 days is probably fine. However, adjustments in the criterion used to classify an early responder may be necessary.

At the younger ages within this range, the overall population of gilts is less mature, so the average female will likely take longer to react to boar exposure. At the older ages, the opposite is true. All gilts, including the early responders, should exhibit estrus sooner. In the former situation, a 28-day window might be necessary to identify early responders, while in the latter scenario, only 14-21 days may be needed.

Boar exposure must be handled correctly in order to accurately identify early responders. For maximum stimulation, these rules apply:

  • Use mature, active boars that produce lots of saliva;
  • House gilts away from boars in the barn and move gilts to the boar- housing area for estrous detection;
  • Place gilts in the same pen with boars during heat detection; and
  • Gilts (or pens of gilts) should be given at least 10 minutes of quality boar exposure per day.

It may not be possible to follow all of these guidelines in every situation. However, the consequences of skipping one or more steps will reduce the effectiveness of boar exposure and make it more difficult to identify early responders, especially if boar exposure is initiated at an early age.

First Mating

A replacement gilt’s first mating represents the transition from her developmental phase to her functional phase. The first mating should occur as soon as she is able to reproduce in an effective and efficient manner.

Historically, age, weight, number of estrous periods and body condition have all been used as reference points for when the first mating should occur. It is very difficult to examine each of these factors individually because they all are interrelated.

For example, older gilts tend to be heavier and have more body condition than lighter gilts. In addition, there is typically an increase in ovulation rate between the first and third estrous periods after puberty, but these differences are also affected by the age at which the first estrus occurs.

The fact that all of these traits are intertwined is the reason that many production systems have focused on making sure that gilts are within a specified weight range and have exhibited at least one estrus before they are bred. It appears that for most modern genetic lines, a weight between 260 and 320 lb. seems to be associated with acceptable reproductive performance and longevity. It is certainly true that this range is dependent on genetics and unique aspects of various production systems.

For example, additional weight at first mating might provide some additional benefits for sows housed in pens during gestation vs. their counterparts kept in stalls. There is probably a minimum age and amount of body condition that is necessary for optimal reproduction, even if gilts are bred within the specified weight range. There is no reason to believe that if replacement gilts have a postweaning growth rate of at least 1.5 lb./day, and are close to the target weight range at first mating, that they are not ready to transition to the functional phase of production.

Functional Phase

Once replacement gilts are bred, they enter the functional phase. Their P/S/Y is determined by the number of piglets they produce in each litter and by the number of litters they produce each year.

Number of piglets per litter is a function of ovulation rate, fertilization rate and embryonic/fetal survival. The number of litters per year is influenced by conception rate, gestation length, lactation length and the wean-to-estrus interval. All of these events are influenced by management during breeding, gestation and lactation. However, management in lactation has the potential to influence each of these to the greatest extent and, therefore, is the most critical for producers striving to achieve 30+ P/S/Y.

Lactation is a time of recovery and resumption. The female has just carried a litter of piglets for about 114 days. Parturition is very stressful in itself, especially for the uterus. Therefore, a sow’s reproductive organs need time to recover before they are capable of resuming normal activity.

The mechanism for recovery is the nursing activity of the newborn piglets. Each time piglets nurse, nerves send signals to the brain that prevent the release of hormones responsible for stimulating follicles to grow and eventually ovulate. Thus, lactation not only provides nourishment for young piglets, it also provides a period of healing for the sow’s reproductive organs.

The ovaries need very little recovery time and, if stimulated properly, can resume normal function within hours after farrowing. However, the reproductive centers of the brain need between 12 and 14 days to resume normal production of luteinizing hormone (LH) and follicle stimulating hormone (FSH); both cause follicles to grow and eventually ovulate.

The uterus needs 14 to 16 days before it is capable of supporting another pregnancy. Keep in mind that these estimates are based on situations in which sows are managed perfectly during lactation, they consume enough nutrients to meet the demands of lactation as well as those needed for the reproductive system to recover, and there are no other environmental stresses, such as health issues, for them to deal with.

If these conditions are not met, recovery of the reproductive organs will take longer. Consequently, nutritional and health management of the sow herd cannot be compromised in herds expecting to produce 30+ P/S/Y. Details about sow herd health and nutrition are presented in articles elsewhere in this Blueprint edition.

Weaning and crossfostering strategies also have important implications for the recovery of the reproductive system and P/S/Y. Weaning is the event that stimulates the sow’s reproductive system to resume normal activity. When piglets are weaned, the suckling-induced inhibition of hormone secretion is removed. Follicles begin to grow in response to increasing levels of LH and FSH.

If weaning occurs before the sow’s reproductive organs are fully recovered, then a myriad of problems can occur. Cystic follicles, low ovulation rates and poor estrus intensity result from inadequate secretion of LH and FSH, while incomplete uterine recovery leads to high embryonic mortality and low conception rates.

As noted earlier, in theory, a lactation length of 16 days should be sufficient for complete recovery and is the earliest that weaning should occur. However, a more realistic goal would be to wean between 18 and 20 days, with the additional days serving as “insurance” in that they provide extra recovery time for any additional stresses encountered during lactation.

When analyzing lactation length, many producers concentrate on the average of the sow herd. The problem with averages is there are always individuals on either side. For example, a herd with an average lactation length of 18 days is likely to have a significant number of sows that lactate for 16 days or less. These sows have not had sufficient time for complete recovery, and their subsequent reproductive performance is likely to be poor.

Adhering to a minimum weaning age that allows sufficient recovery time for all sows in conjunction with tracking the herd average is a prudent approach for producers trying to increase P/S/Y.

Large piglets in a litter nurse more intensely and more frequently than small piglets, so they provide most of the inhibition of the reproductive hormones during lactation.

Whenever these piglets are removed from a litter, such as partial weaning or crossfostering, sows may respond as if the entire litter was weaned.

For example, in a litter of 11, if the four biggest pigs are weaned early or fostered off and replaced by smaller pigs, the reduction in the suckling intensity might be enough to stimulate an increase in LH and FSH and the resumption of reproductive activity. If this occurs before the sow is fully recovered, the same problems associated with weaning the entire litter too early are likely to occur. Production systems that rely heavily on practices such as bump weaning, partial weaning, split-suckling or aggressive crossfostering throughout all of lactation need to make sure that these strategies aren’t negatively affecting the subsequent reproductive performance of the herd.

Breeding Management

Breeding management contributes to P/S/Y through fertilization rate, which in turn affects pigs per litter and conception rate. Good breeding programs that achieve high P/S/Y consistently inseminate sows with the freshest semen available, closest to when they are likely to ovulate. Good heat detection and insemination techniques are obviously necessary for accomplishing this goal, but if problems exist in these areas, it is important to identify and correct them quickly. Subtle things, such as proper semen handling, are often overlooked.

It is absolutely critical for semen delivery schedules to be coordinated with a farm’s weekly breeding demands. Twice-per-week semen delivery is common in the industry.
To illustrate the importance of good scheduling, consider if weaned sows have a return-to-estrus pattern similar to that shown in Figure 2A (page 16), when the heaviest breeding days are Tuesday, Wednesday and Thursday. If semen is delivered to the farm on Mondays and Thursdays, it should be obvious that the oldest semen from the Monday delivery (solid blue line) is being used on the heaviest breeding day of the week, Wednesday. In essence, the majority of the semen delivered on Monday morning is sitting in the semen storage unit for at least 48 hours before it is inseminated. It would be better to have semen delivered on Tuesdays, so that the majority of the sows bred each week would be inseminated with semen that was 24 hours fresher.

Another important consideration is prioritizing whether sows should be bred with new or old semen doses. This is best illustrated by using the previous example and dividing sows into those receiving their first mating and those being bred for the second time. In Figure 2B (page 16), the daily breeding demands have been partitioned into first and subsequent matings. For example, of the 100 matings normally performed on Mondays (Figure 2A), about one-half are sows that are detected in estrus for the first time and about one-half are sows that were in estrus previously, most likely on Sunday. Although the timing of ovulation relative to estrus varies within and among herds, the general trend is for it to occur at about 70% of the duration of estrus. In other words, if a sow is detected in estrus in the morning and stands for two days, she most likely would ovulate during the early afternoon of the second day.

In this example, there would be at least two days (and possibly more) in which two different batches of semen with different delivery dates are available to inseminate sows. On Monday (and probably Tuesday), semen from the previous Thursday delivery and from the current Monday delivery are available. The same situation would exist for Thursday (and probably Friday).

Assuming that ovulation occurs later in estrus, semen delivered on Monday should be used first to breed sows receiving their second and third matings. The second and third matings should be considered priority matings and receive the freshest semen. The oldest semen (the doses delivered the previous Thursday) should be used to inseminate sows standing for the first time. This strategy ensures that the freshest semen is used to breed sows that, on average, are the closest to ovulation which, in turn, increases fertilization rate and 30+ P/S/Y.

Gestation Management

Gestation management affects embryonic and fetal development and obviously influences the number of pigs born alive. In addition, the complete loss of a pregnancy reduces farrowing rate and increases non-productive sow days (NPD). These, in turn, decrease litters/sow/year.

Partial or complete embryonic and fetal losses are not compatible with achieving high levels of P/S/Y. Fortunately, these tend to be rare occurrences in sows unless they have been exposed to adverse health or environmental situations.

The most common environmental challenge affecting gestating sows is summer or seasonal infertility. Unquestionably, exposure to combinations of high ambient temperature and humidity is a major reason why reproduction is worse in the summer and late fall. Proper maintenance and operation of the ventilation system during the summer months is absolutely critical, especially the supplemental cooling components.

Even with optimal ventilation, sows in some geographic locations are at risk of heat stress during the summer. Eliminating even a mild form of stress, such as moving sows to another location, vaccinations, blood sampling, etc., should probably be avoided during periods of potential heat stress.

The Bottom Line

High levels of P/S/Y are affected by management decisions that occur early in a replacement gilt’s life (developmental period), and by those made after she is bred and becomes a productive sow (functional period).

Unfortunately, there is still much that we do not know about the optimal environment for replacement gilts from birth to breeding. However, it appears that a greater understanding of the relationships between birth weight, pre- and postweaning growth, and subsequent reproductive performance might prove useful in creating management systems that consistently produce replacement gilts with the reproductive physiology to consistently produce 30+ P/S/Y.

Conversely, there is considerable evidence that females exhibiting estrus early in response to boar exposure have distinct advantages in terms of sow longevity and productivity. Production systems that can accurately evaluate and use this trait in gilt development programs will have distinct reproductive advantages.

Once females are bred and enter their functional phase, lactation management is where reproductive performance is likely to suffer the most if sows are mismanaged. Weaning ages and crossfostering strategies compatible with 30+ P/S/Y need to ensure that the sow’s reproductive system has had sufficient time to recover before it is stimulated to resume its normal activities.

Breeding and gestation management contribute significantly to P/S/Y, but are less prone to management problems than lactation. Adjusting delivery schedules and semen usage so that the freshest semen is used to breed the sows closest to ovulation, and minimizing the occurrence of heat stress during the summer months, are strategies consistent with top reproductive performance.