By W.L. Flowers, North Carolina State University Department of Animal Science

Reproductive management programs seem to be becoming more complicated as our understanding of the sow’s reproductive system increases. This is actually a good thing since it should help producers avoid management conditions that limit lifetime productivity.

However, when one considers all the recommendations that are available, it can be quite a daunting task to take a look at what is currently being done within a production system and then decide what new strategies might be beneficial and where to incorporate them. This is a continual process that is likely to be different from farm to farm. Based on work examining factors that affect sow longevity conducted over the past 20 years, a reasonable strategy for most production systems to begin this process is what has been referred to as “the two beginnings and the end.”

The first beginning is the neonatal management of the litter in which future replacement gilts are born. The second beginning is how the mature gilt is handled from the time she arrives until she is bred and enters production on commercial farms. The end is examination of reproductive tracts of sows and gilts that are culled prematurely.

The primary intent of this article is to review studies conducted on commercial farms in these three areas and discuss how each can be used to enhance sow lifetime productivity.

Preweaning growth

Sows are born with most of the follicles that will become their future piglets, and during their first four weeks of life, these follicles begin to acquire the ability to respond to hormones that will determine the effectiveness of their adult reproductive cycles. In young pigs, body weight gain is highly correlated to the functional development of most organ systems, and there is good evidence that the same relationship applies to the ovaries and follicles.

This has stimulated interest in examining relationships between neonatal management and sow longevity. The National Pork Board has funded at least four of these research projects over the past 15 years, and a consistent finding in all of them is that there is a positive and highly significant relationship between weaning weight of replacement gilts and their future longevity as sows.

One of the first studies to demonstrate this concept was conducted within a commercial production system in North Carolina, and relied on strategic cross-fostering to create large differences in neonatal litter sizes.

In this study, future replacement gilts nursed in litters that averaged either 6.8 or 11.8 piglets. The corresponding weaning weights and proportions of sows that farrowed six litters for these two groups were 20.6 pounds and 38% versus 16.1 pounds and 18%. It is not practical or possible to reduce the number of piglets nursing to seven or fewer on today’s multiplication farms given the prolificacy of modern maternal line genetics. However, the same relationship among number of piglets nursing, weaning weight and sow retention still exists within these systems.

In another study that is currently underway, farrowing barn staff followed farm standard operating procedures and created litter sizes that averaged 12.7 and 15.8 piglets nursing. Weaning weights for replacement gilts in these litters were 15.6 and 13.2 pounds, respectively. Currently, the majority of these gilts have farrowed their first litter, and 76% of the ones raised in litters of 12.7 are still in the herd, compared with only 58% of their contemporaries from litters of 15.8.

Comparable retention rates for P1 sows from the study mentioned earlier were 85% and 77% for gilts from litters of 6.8 and 11.8, respectively. Collectively, numbers of pigs nursing, weaning weights, and P1 retention rates vary from 6.8 to 15.8 piglets, 20.6 to 13.2 pounds, and 85% to 58%, respectively, across these two studies. These data indicate that for each piglet that was fostered off, there was at least a 1-pound increase in weaning weight, which translated into a 4% increase in gilts producing at their first litter.

Obviously, this is just a preliminary estimate and is likely to change somewhat as additional information is collected. However, it does illustrate the importance of weaning weight on sow longevity and attempts to quantify this relationship numerically. Consequently, enhancement of preweaning growth for future replacement gilts should be a high priority for multiplication farms.

In addition to cross-fostering, there are a variety of other strategies that have potential for increasing weaning weights, including early critical care programs that emphasize colostrum intake, supplemental feeding systems and techniques such as split-suckling and partial weaning directed toward maternal-line barrows. Implementation of these and other techniques likely will be farm-specific. However, if they increase weaning weights, then improvements in sow longevity should follow.

Gilt management specialists

Management of gilts from their arrival on commercial farms through their P1 rebreeding is crucial to their lifetime productivity since this is when most are culled for reproductive failure. During multiplication, the primary focus is on the growth and development of gilts and their reproductive systems. Once they are delivered to commercial farms, the main goals are to activate their reproductive systems via boar exposure and then to manage them in such a way that they function at a high level by becoming pregnant with large litters of piglets. Much of what should be done during this critical period in terms of gilt management has been the subject of previous Blueprint issues. What seems to vary in practice is how these tasks are implemented at the farm level.

In a recent study, litter mate pairs from the same multiplication flow were sent to two commercial farms with historically different sow retention rates. Their productivity was monitored until they were culled and management differences between the two farms were recorded (see Table 1). This approach is the exact opposite of what was used for the neonatal litter size studies discussed previously. In those, different populations of gilts were deliberately created during multiplication and then sent to the same commercial farm. In this study, gilts were managed similarly during multiplication and then similar populations (litter mates) were sent to different commercial farms with similar management SOPs, but different sow retention rates.

The most striking management difference that was observed involved farm personnel given the responsibility for managing the gilts. On the high-longevity farm, all activities associated with gilts from the time they arrived until they were moved into farrowing were the responsibility of one person. In essence, this employee was their gilt development specialist. In contrast, a team approach was used on the low-longevity farm, and all breeding barn employees performed the same tasks. It was clear during monthly visits to both farms that the gilt development specialist on the high-longevity farm possessed a superior knowledge of gilt-to-gilt variation than his contemporaries on the low-longevity farm. This was particularly evident when it came to heat detection and breeding activities. The gilt development specialist used the timing and intensity of heat-no-serve events to identify eligible gilts and as a reference for how to handle their subsequent inseminations. This resulted in fewer gilts being removed from production on the high-longevity farm, and postmortem examinations performed on culled gilts revealed a significantly reduced incidence of missed heats compared with the low-longevity farm.

The concept of having production specialists oversee functions critical to swine production systems is not new and has been used successfully for a number of years. Consequently, the distinct advantage observed on the high-longevity farm in terms of getting delivered gilts bred and into production should not be surprising. Establishment of their first pregnancy is a critical event for gilts from both a physiological and management perspective, so it seems reasonable to assign this responsibility to a specialist.

Postmortem examination

It is often said that more can be learned from failures than successes. This is certainly true with regards to sow longevity. The most common reproductive failure is when females fail to show estrus. For sows, this is most prevalent after weaning, while for gilts it happens most often after their arrival on commercial units. Routine postmortem examinations of reproductive tracts of gilts and sows culled for “no heats” are very useful for identification of management conditions that negatively affect sow longevity.

The physiology behind estrus and ovulation in sows and gilts is basically the same. Follicles begin to grow, and as they increase in size, they produce estrogen. Estrogen is the hormone that stimulates all the classic behavioral signs of heat and eventually causes the release of another hormone, LH, which stimulates ovulation. Once follicles ovulate they are transformed into corpora lutea, which produce progesterone. At the end of the estrous cycle in gilts or pregnancy in sows, corpora lutea regress and become corpora albicantia, which are avascular tissues that remain on the ovary.

Gilts delivered to commercial farms that are exhibiting normal reproductive activity should either have corpora lutea or follicles and corpora albicantia on their ovaries depending how long it has been since they were detected in heat. When a sow farrows, the corpora lutea from her current pregnancy have regressed and formed corpora albicantia. During lactation, the suckling action of the piglet inhibits follicular growth, so in addition to the corpora albicantia, there would be small-to-medium follicles on the ovaries. At weaning, these medium-sized follicles begin to grow. They should ovulate in four to six days if the sow has been managed correctly, and form corpora lutea.

When these sequences of events don’t happen as they should in either gilts or sows, then this is reflected by the structures present on the ovaries. A brief summary of the most common ovarian morphology found in gilts and sows that failed to show heat is contained in Tables 2 and 3, respectively.

Table 2 contains pictures of ovaries from gilts that were delivered to commercial farms but never found in heat or were found in heat once but never exhibited a second one. The first column describes the ovarian structures that are present. The second column provides a physiological explanation, followed by the most common management conditions that can lead to each situation.

Table 3 is organized the same way for weaned sows that were not found in heat 14 days postweaning. The list of possible management conditions that could contribute to each of the ovarian morphologies in each table should be considered as a partial one that contains the most common reasons. 

The best way to explain how to use this information is with some examples. If most of the ovaries from gilts have corpora lutea, then the main problem is with heat detection. Follicles grew normally and ovulated since corpora lutea are present. In contrast, if the majority of the ovaries have no structures, then ovulation never occurred and this usually is associated with situations where the ovary doesn’t develop at the same rate as the other organ systems. It is interesting to note that this was the predominant ovarian morphology for the culled gilts with low weaning weights in the studies examining preweaning growth and sow longevity discussed previously. Similar situations would be true for postweaned sows: corpora lutea on the ovaries indicate the problem is with heat detection, whereas the presence of the other structures shown probably is related to some aspect of management during lactation.

Routine postmortem examinations of reproductive tracts of culled gilts and sows should be a component of every reproductive management program. It plays the same role as serological profiling in disease management. When production is good, they validate that management conditions are enhancing the reproductive systems of sows and gilts, and when production is bad, they provide invaluable information that can be used to find where problems exist.

Summary

There is still much to be learned about the reproductive physiology of sows and factors that affect their lifetime productivity. However, it does appear that how they are managed after birth in multiplication units and after their arrival on commercial farms are two areas that are crucial to their longevity and productivity. There is convincing evidence that increasing weaning weights of future replacement gilts increases the likelihood that they will become productive sows. Establishment of their first pregnancy is a critical period of time for replacement gilts. If this fails, they have no longevity.

As a result, how they are managed and who does the managing from the time they arrive on commercial farms until they are bred is important. It makes sense that placing this responsibility in the hands of a “specialist” enhances sow lifetime productivity.

Finally, routine postmortem examinations of culled sows and gilts are relatively inexpensive, simple and underutilized tools for which, currently, there is no alternative. They can provide invaluable information as to what management conditions contributed to reproductive failures in both gilts and sows.