The furthest thing from most producers’ minds when a replacement gilt is born is how long she will remain in the herd producing pigs. Instead, the immediate task at hand is to get her to survive until weaning. Nevertheless, whether they realize it or not, much of the reproductive potential that she will possess as an adult is already in place at birth. Moreover, the manner in which she is managed during the next three weeks while nursing her mother also has a significant effect on her future productivity as a sow. Although more research is needed, our current understanding of the relationships among birth weight, neonatal management and adult reproductive potential indicates that birth through weaning is a critical period that influences sow longevity and provides opportunities for most production systems to improve this important trait.

Birth weight and development of reproductive organs
The main female reproductive organs are the ovaries, uterus and portions of the brain. Collectively, they are often referred to as the reproductive engine or machinery of the sow, since they interact with each other to control all the reproductive events in a sow’s life: puberty, breeding, pregnancy and farrowing. All are formed about 40 days after fertilization and continue to develop during the rest of gestation. As is the case with most fetal tissues, the most rapid rate of growth and development occurs during the last three to four weeks of pregnancy. As a result, by the time the gilt is born, each of these organs, essentially, already contains many of the components that are present in adults. The best examples of this are the ovaries which contain, at birth, all the ova the sow will ovulate over her entire life. Consequently, the first opportunity that producers have to affect sow longevity potential is by measuring birth weights. There are well-established, positive relationships between birth weight and the size of most internal organs. This is certainly true for the brain, intestines, liver, muscle and bone. In addition, larger pigs grow faster and weigh more as adults than their smaller counterparts. Therefore, there is no reason to believe that similar relationships don’t also exist for reproductive organs and how they function in adult animals.

Information with regards to relationships between birth weights and lifetime productivity is still being collected and evaluated. However, its usefulness as an early indicator of subsequent reproductive performance is encouraging. One such example is shown in Figure 1. These data were collected as part of a project funded by the National Pork Board and illustrate the relationship between the birth weight of replacement gilts and how long they remained in production as adults. In this study, gilts were weighed at birth and allowed to nurse their mothers with minimal cross-fostering. The average number of pigs born alive was close to 12. After weaning, all the replacement gilts were sent to the same gilt development unit and then to two different commercial farms with similar standard operating procedures.The majority of the replacement gilts in this study had birth weights between 2.6 and 3.5 pounds. This was considered the normal range statistically. However, there were enough in the other two categories to make valid comparisons. It is clear that the replacement gilts that weighed less than 2.5 pounds at birth exhibited reduced lifetime productivity in that only about 11% farrowed six litters. An even more troubling observation was that most of these gilts failed to even produce one litter, since the largest loss was between their first breeding and farrowing. In contrast, approximately 32% of their counterparts with birth weights greater than 3.5 pounds produced at least six litters of pigs.

These data are preliminary, but they do seem to support the concept that birth weights have a positive relationship with sow longevity and that there probably is a minimum birth weight, below which gilts simply do not have the reproductive machinery to be efficient reproductively no matter how well they are managed later in life. For this particular farm this minimum threshold for birth weight appears to be 2.5 pounds. However, it could vary among production systems for obvious reasons and it is up to producers to determine relationships between birth weight of future replacement gilts and their 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 sow lifetime productivity.

It also may be necessary to consider the individual birth weight of a gilt with respect to the total birth weight of the entire litter in which she was born, or what has been described as her “litter of origin” by scientists at the University of Alberta. In other words, if two replacement gilts have the same birth weight, but one is that largest pig in the litter and the other is the smallest one, then an interesting question is whether their reproductive potential is similar. The answer to this is not known at the present time. However, all the possible combinations can be found on multiplication farms from which replacement gilts originate: large litters of small pigs; small litters of large pigs; small litters of small pigs; large litters of large pigs; and litters with both small and large pigs. Obviously, the ideal situation would be to select the largest gilts from the largest litters, but the frequency of these litters being born probably is low compared to large litters with small pigs, based on the potential of most genetic lines used to produce commercial sows.

As mentioned previously, scientists at the University of Alberta have identified a repeatable small birth weight phenotype. This was done by monitoring average piglet and litter birth weights over three consecutive parities. There are sows in multiplication units that consistently produce piglets with average birth weights between 2.2 and 2.4 pounds. The lifetime productivity of replacement gilts that originate from these litters is currently being studied. However, even if a gilt has a birth weight of, for example, 3 pounds or higher but her litter of origin was one with a low birth weight phenotype, then it is reasonable to speculate that her lifetime productivity may not be the same as a similar sized pig that was born in a litter with a different birth weight phenotype. One thing that is already apparent from this ongoing study, is that there are litters (predicted to be the low birth weight phenotype), from which no, or very few, gilts are preselected as potential replacements. If this trend is confirmed, it suggests that the efficiency of gilt production from nucleus and multiplication farms can be improved by early culling of sows with low birth weight phenotype.

Colostrum intake and development of reproductive organs

The importance of adequate colostrum intake for promoting baby pig health via passive immunity has been well documented. In addition, there is recent evidence from scientists at Auburn University and Rutgers University that colostrum is necessary for the normal development of the ovaries and uterus. In addition to antibodies for passive immunity, colostrum contains a rich mixture of compounds which collectively are referred to as mitogens. Mitogens stimulate cell growth by increasing cell divisions. Relaxin is a hormone that is produced by sows during farrowing and facilitates dilation of the birth canal. It also is present in colostrum and has been shown to promote normal ovarian and uterine development in young pigs.

Recently, an immunocrit assay was developed by scientists at USDA-Agricultural Research Service. This assay measures the amount of immunoglobulins in the blood of young pigs 24 hours after they are born and provides an excellent way to estimate colostrum intake because colostrum is the primary source of immunoglobulins for these pigs. These scientists observed that there was a positive relationship between immunocrit concentrations at 1 day of age and the number of pigs born alive, presumably due to increased colostrum intake after birth which, in turn, resulted in superior development of the reproductive tract. At the present time, additional studies are under way to establish definitive cause and effect relationships among colostrum intake, Relaxin, and adult reproductive function. However, it appears that adequate colostrum intake as early as day 1 is an important aspect of developing neonatal management strategies that enhance sow lifetime productivity, especially in multiplication units that have sows with the small birth weight phenotype discussed previously.

Strategic cross-fostering, pre-weaning growth and sow longevity

Unfortunately, there is really not much that can be done to change birth weight or the birth weight distribution within a litter. These are functions of uterine capacity and placental development. 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. In some situations, this fetal growth restriction affects the entire litter, while in others it affects only a portion of the piglets. However, the end result is that the variation in individual birth weights within a litter and between litters can be large.

There is good evidence that strategic cross-fostering after birth may provide a relatively simple way to counter some of the effects attributed to gilts that are born small. 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 pounds, which is consistent with litters experiencing mild to moderate growth restriction during gestation. After weaning, gilts were comingled, 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% versus 17.3%; Figure 2); improved farrowing rate (88.7% versus 83.3%); and tended to increase number of pigs born alive (11.0 versus 10.5) over six parities. Replacement 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.The magnitude of this response appears to be influenced by birth weight (Figure 3). For gilts that weighed more than 3.6 pounds at birth, there was no effect on the proportion that produced six litters as adults. In contrast, for medium-sized pigs (2.5-3.5 pounds), a robust response was observed: 40% of the gilts allowed to nurse in small litters were still in production after their sixth parity compared with only 17% of their counterparts raised in large litters. For the small birth weight gilts (< 2.5 pounds), reducing the size of the litter in which they nursed did improve longevity, but only by less than 10%.Although the exact physiological mechanisms associated with these responses are not fully understood yet, 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. This could be simply a general effect of growth or a selective one that  only affected the reproductive organs. In addition to the Relaxin in colostrum, normal sow milk also has an abundance of compounds that have been shown to stimulate development of reproductive tissues in adult animals, so it is reasonable to assume that they would have the same, if not a more pronounced effect, in young, immature pigs.

The minimum number of pigs that need to be fostered off in order to realize an improvement in subsequent reproduction as an adult is not known. The previous study reduced the nursing competition by about 50% which isn’t practical in most multiplication systems. It is quite possible that the best guide might be birth weight or litter of origin. Obviously, additional studies are needed, but for large piglets or for litters of large piglets, there probably is no need for any intervention. The same might also be true for small piglets or for large litters of small piglets. Even though small birth weight piglets responded to a reduction in litter size during lactation, the magnitude was small. It seems that the best opportunity for strategic cross-fostering to enhance lifetime productivity would be for the medium-sized piglets since they gained the most benefit from it in terms of longevity. Consequently, 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 cross-fostering. 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 to improve sow lifetime productivity

Summary

There is still much to be learned about the optimal environment to which replacement gilts should be exposed from birth to weaning. However, it does appear that understanding relationships between birth weight, colostrum intake and pre-weaning growth will prove to be useful in creating management systems that consistently produce sows with enhanced lifetime productivity. A consideration of birth weight in conjunction with litter of origin might prove to be the starting point upon which future decisions are made.

Based on what is currently known, it is conceivable there is a minimum birth weight below which adult reproductive performance would be marginal at best so these piglets would not be selected for gilt development programs. In contrast, the opposite probably is also true. There is an optimal birth weight and gilts at or above this level really do not require much intervention during the neonatal period to realize their full potential as sows, with the possible exception of adequate colostrum intake. Finally, lifetime productivity can be significantly enhanced for gilts with birth weights between the minimum and the optimum by the use of strategic cross-fostering and other management strategies.