Improvements in sow lifetime productivity (SLP) will require the ongoing development of dam-lines best suited to specific production systems and marketing opportunities. Appropriate lifetime management of the gilt and sow is then needed to capture this genetic potential at production level. In terms of the present National Pork Board initiative, the goal is to increase SLP nationally by 30% in the immediate future, with a focus on increasing the number of “quality weaned pigs” per sow lifetime.

The focus on “quality weaned pigs” is important, because of increasing evidence that successful selection for sow prolificacy over several decades has increased the number of low birth weight pigs born. This became most apparent in highly prolific dam-lines developed in France, in which there was an increasing divergence between the increase in total pigs born and the total number of pigs actually weaned. Genetic selection for “the right female” continues to be an important issue and adjustments in selection strategies can make an important contribution to the goal of increasing the number of “quality” pigs weaned. Regardless of the dam-line being used, substantial differences in production performance between the “industry average” and the top percentage of farms raise several issues.

Firstly, the genetic potential of existing commercial dam-lines is consistent with excellent SLP in the best farms. Therefore, identifying key management changes needed in “average performing farms” to improve SLP is an important deliverable for this Blueprint issue, and for ongoing applied research projects funded under the NPB’s SLP initiative. However, it may be equally important to ask another question: Are the deficiencies in management practice in fact deliberate and reflecting a “least-cost” production model that is considered to represent the best “bottom line” outcome? Put another way, although excellent SLP in the best performing farms is achieved through commitments to extra skilled labor and facility improvements that support good gilt development programs and allow close attention to sows and litters around farrowing, the costs of incorporating these additional inputs may not improve net revenues in other production systems. Another possibility is that improvements in breeding management may not be the highest priority, or provide the best return on investment, if limited resources are available for improvements across the entire production system.

Nevertheless, the purpose of the forthcoming series of Blueprint articles is to look at different aspects of gilt and sow management from three perspectives. Firstly, how do producers identify gilts in early development that have the greatest potential for retention in the breeding herd and for achieving excellent SLP? Secondly, what key management practices promote retention and increase the number of quality weaned pigs? Finally, at different levels of the production cycle, what are the key risk factors for decreased SLP and what key performance benchmarks can be identified to offset these risks? Because the NPB SLP initiative will involve applied research projects with large populations of gilts in a commercial environment, and manipulated variables will be linked to differences in SLP measured up to at least third parity, some realistic measure of the true economic value of these studies should be possible.

Ongoing genetic selection of dam-line females
Although available dam-lines from major genetic suppliers can be highly productive, there is still an opportunity to match specific dam-lines to particular production systems to achieve the best return on investment. However, in all dam-lines studied there continues to be a consistent negative association between total pigs born and litter average birth weight (Figure 1).When all litter sizes are considered, the association with total born explains something like 20 to 25% of the variance in litter average birth weight, and represents something like a 35 to 40 gram decrease in litter average birth weight for each additional pig born. More than 16 total pigs born will limit the average birth weight (1.8 to 2.0+ kilogram) of the heaviest litters born and the most “prolific” litters (20+ total pigs) born to mature sows have an even more extreme low birth weight phenotype (0.8 to 1.4 kilogram). At management level, these trends require increased attention to post-farrowing management to avoid the increased risk of high pre-weaning mortality in low birth weight litters.

However, as shown specifically in Figure 1, even in litters of between 10 and 16 pigs born there is a big difference in the litter birth weight phenotype, regardless of the number of pigs born. Current thinking is that a repeatable low litter average birth weight phenotype in more mature sows is driven by high ovulation rates (25 to 30), linked to excessive crowding of embryos in utero in early gestation. This limits placental development at this early stage and the subsequent pre-natal development of those fetuses that survive to term. Because it is not possible to directly select for a more modest ovulation rate, increased selection pressure for traits such as increased litter average birth weight, a better ratio of live to dead pigs born, better pre-weaning survival, and an increase in average weaning weight, together with ongoing selection for increased numbers born, will make an important contribution to increasing the number of quality pigs weaned per sow lifetime. At production level, the impact of a repeatable low litter birth weight phenotype on efficient gilt selection needs considering. Is selection of gilts from a low birth weight litter already compromising the goal of increasing the number of quality pigs weaned? If so, can management strategies be developed that mitigate these risks? The next paper in this series by Billy Flowers, provides convincing preliminary evidence that of gilts early in development can interact with their birth “phenotype” to affect lifetime productivity.

Growth and sexual maturation of replacement gilts
Continued selection for improved lean growth performance and feed conversion efficiency in dam-line females is reflected in the impressive growth performance of healthy replacement gilts in well-managed systems. This requires a continual reassessment of the relationship between growth and sexual maturity. In general growth performance of even the slower growing gilts now exceeds the minimal growth rate (around 600 grams per day to around 160 days) below which the onset of pubertal estrus would be delayed. Indeed, the continuing improvement in growth performance means that gilts are increasingly heavier, but not younger, at first estrus, with important implications for gilt development and selection strategies. Convincing evidence for these trends in growth performance is presented in the article by Jeff Vallet and others.

Frequent references are made to the “average age of first estrus” in different studies and different production systems, but in itself may have little relation to actual variation on true age at sexual maturation (sexual precocity) in any particular group of gilts. However, age at stimulation has a major impact on apparent age at first estrus: When puberty stimulation starts early, say 135 days, the true average age at which gilts can reach puberty is seen to be around 160 days. The later a stimulation program starts after 135 days, the later the “average” age at puberty appears to be.  If we accept that early sexual maturation is generally associated with greater lifetime fertility, by delaying puberty stimulation protocols to 180 or 190 days of age, we lose the ability to identify the true age at pubertal estrus in the most sexually precocious gilts. However, assuming that the more precocious gilts will almost immediately show pubertal estrus once boar stimulation is provided, the earlier responding gilts will still represent the gilts with the best potential fertility. Also, the number of “select” gilts identified in estrus each day of the stimulation program is greater, and the time to select the required percentage of select gilts is shorter, if stimulation is applied at a later age. Later stimulation protocols are, therefore, more efficient and can still discriminate the population of gilts reaching puberty before say 200 days of age from later-maturing gilts, without incurring excessive non-productive days to reach this decision.  Further information on these important aspects of gilt development is discussed by Robert Knox and co-authors.

As discussed by Vallet, a high percentage of gilts will eventually reach pubertal estrus if the stimulation program continues to 260 days. However, there needs to be a balance between efficient use of labor-intensive gilt development unit facilities and limited gilt non-productive days on the one hand, and the required selection rate expected from the population of gilts entering the GDU on the other. The overall efficiency of the final selection program, and the ability of staff to properly identify “heat-no-service” gilts, is dependent on the quality of the boar stimulation program used. Ongoing consideration of the biological constraints for effective puberty stimulation and management options for increasing selection efficiency in the GDU is important, particularly if different gilt selection strategies can be linked to differences in SLP. These issues, and management practices in the critical pre-breeding period that lead to optimal reproductive performance in the gilt, will be discussed in the paper by Knox and co-authors.

Growth rate, physical size and longevity in the breeding herd
Selection pressures that improve post-natal growth performance are thought to be linked to a gradual increase in mature body size. This is perhaps most evident in the increasing mismatch between the physical space limitations of older farrowing crates and the size of higher parity sows in today’s production systems. As discussed above, the pig is following the same trend seen in poultry, in that a particular physical size or point on the growth curve, no longer has a predictable relationship with onset of puberty and the ability to be bred. As discussed later, high growth rates in gilts during development carry the risk that they will be over accepted target weights for breeding (135 to 150 kilograms: 300 to 350 pounds) and for farrowing their first litter (>185 kilograms: 400 pounds). As already frequently discussed in the literature, and addressed by Vallet, limiting the body size of gilts and sows during the production cycle may become a critical issue for lifetime “fitness”. The highly complex and restrictive feeding programs used in the broiler-breeder industry provides a more extreme example of the difficult decisions that need to be increasingly addressed by the pork industry. In situations in which feed intake becomes a limitation for productivity in the sow (hot and humid summer conditions being a good example), a larger body size carries with it a greater maintenance requirement, which will tend to increase the risk of limited feed intake decreasing the productivity of the sow. Selection of dam-lines that perform best in such challenging environments is already being discussed as one of the next important advances in genetic selection programs. More complex interactions among growth performance and the space allowances provided even at the nursery stage of gilt development are discussed in the article by Mark Estienne and co-authors. It is clear that housing density can affect the sexual maturation process independent of direct effects on growth rate.

Body conformation and general physical fitness as the key component of improved lifetime performance
Although culling for poor reproductive performance is often reported to be a major issue for gilt retention to parity 2 and 3, the early culling of gilts and sows for lack of “structural soundness” is also common. In older sows, feet and leg problems are a serious concern for the industry and these problems are aggravated when sows carry excessive body weight because they were overweight as first-bred gilts. Furthermore, as discussed by Joe Stock and Ken Stalder, lameness may be an important contributor to poor reproductive performance, and is likely under-reported as the primary reason for culling. Evaluation of structural soundness is, therefore, an important part of any gilt selection program. As with other key traits such as an observed standing estrus and an appropriate weight at breeding, individual and objective measures of structural soundness need to become an essential part of replacement gilt management if the goal of improving SLP is to be realized.

Growth, sexual maturation and lactation performance – is there a link?
Comparative studies in other mammalian species indicate a link between the time of sexual maturation and the final differentiation of the secretory cells of the mammary gland. When growth and the onset of puberty have a more “natural” relationship, it has been suggested the increase in circulating sex steroid hormones at the time of puberty is the functional trigger for this final differentiation of mammary secretory tissue. Given that we have already altered the relationship between the overall growth of the gilt and the time of puberty (gilts are not younger but increasingly heavier at puberty), there is a serious lack of information about how these changing relationships are affecting the differentiation of mammary tissues and thus the milk producing potential of the modern breeding female. A number of existing selection traits will already be driving an increase in lactation performance. However, information on the interactive effects of growth rate and age at puberty in large gilt populations, on mammary gland function and lactation performance, might reveal some interesting possibilities for considering gilt selection programs from yet another important perspective. Regardless of the earlier processes of mammary gland development, the management of the first parity sows during lactation has ongoing consequences for SLP. The production inefficiencies represented by the “second parity drop” in numbers born, and the failure to re-breed a proportion of weaned first parity sows, are still key risk factors for poor SLP in many farms. Information on the interactions that occur during the first lactation to place sows at risk is, therefore, still relevant to the discussion of improved SLP and is discussed in the article by Nathalie Trottier.

Concluding comments
This brief introduction has attempted to put the development and performance of contemporary gilts and sows in a historical perspective. It has highlighted the evolving relationships between key phenotypic traits and the implications of these changes for different components of production efficiency and management responses that are needed to offset new risks for improved SLP. Hopefully, this introduction provides a useful background against which to read the complete Blueprint issue that will deal with specific aspects of gilt development and SLP.