Low average birth weight and a larger birth weight variation within a litter are results of striving to achieve larger litter size Getty Images

Low average birth weight and a larger birth weight variation within a litter are results of striving to achieve larger litter size.

Large litter size vs. low average birth weight — it’s a production trade-off

Blueprint October 2016 Issue

The selection for larger litter size has resulted in a reduction of average birth weight and in an increase of within-litter birth weight variation in pigs (Quiniou et al., 2002). Low birth weight offspring are a major concern for the swine industry and have significant negative impacts within pork production systems. In commercial pigs, low birth weight pigs have been shown to have increased mortality rates, slower growth and comprised carcass quality. In replacement females, in addition to the negative effects on growth, low birth weights have adverse impacts on reproductive performance and sow lifetime productivity. The detrimental effects of low birth weight are not only restricted to small pigs within a litter, but also may extend to entire litters that are prenatally programmed to have a lower than average birth weight (low litter birth weight phenotype) and compromised postnatal growth performance. Within a commercial multiplication program, this presents an additional risk, as the low litter birth weight phenotype is a repeatable trait in a sow’s productive life, and the sow passes these traits on to her progeny.

Low birth weight

Within the current literature, there is general consensus showing the risk of low birth weights on future growth and reproductive performance of replacement gilts. Gilts weighing less than 1.0 kilogram at birth have increased preweaning mortality rates and have little chance of surviving until weaning (Magnabosco et al., 2015). Those gilts that do survive past the nursery phase have poor growth until finishing and are significantly lighter than their higher birth weight litter mates. In fact, the authors stated that gilts as heavy as 1.28 kilograms had relatively high mortality at the end of the nursery period.

Preliminary work completed as part of a National Pork Board-funded project to investigate links between LBW phenotype and SLP, conducted in collaboration with Holden Farms Inc., shows similar results: Gilts with low birth weights have lower retention rates to various stages of production and are lighter at selection than heavier gilts at birth. Additionally, as future replacement females, their low birth weights negatively impact their reproductive potential. Flowers (2015) suggested that below a minimum birth weight of 1.1 kilograms, gilts simply do not have the reproductive machinery to be efficient reproductively no matter how well they are managed later in life. In gilts, the variation in birth weight is negatively correlated to ovarian and uterine development (Deligeorgis et al., 1984). Preliminary data from the University of Alberta shows a positive relationship between birth weight and uterine and ovarian weight, and perhaps an indication of future performance. Magnabosco et al. (2016) reported that gilts weighing less than 1.0 kilogram at birth but still selected as replacements at 170 days of age produced fewer pigs over three parities and remained in the herd for less time.

Low birth weight phenotype

A “low litter birth weight” phenotype at nucleus and multiplication level carries all the same risks described above for individual low birth weight gilts but as a “litter” trait. A negative relationship between total born (litter size) and litter average birth weight (average of all pigs in the litter) exists (Figure 1). Over the entire population of litters shown, total litter size explains part of the difference in litter average birth weight (R2 > 0.25), representing a 600-gram difference between the smallest and largest litters. In contrast, the variation in litter average birth weight among litters with the same total born was greater (mean = 1,200 grams, range 900 to 1,500 grams). As litter size increases, there is an increasing lack of high birth weight litters due to increased prolificacy.

As shown in Figure 1, the litter average birth weights of the most prolific sows with more than 20 total born are lower than the population average litter birth weight of around 1.4 kilograms. In contrast to these effects of increasing prolificacy on the upper range of litter average birth weights, across the entire range of litter sizes from eight to more than 20 pigs total born, there is a population of approximately 15% of sows that repeatedly have low birth weight litters that cannot be attributed to prolificacy in the sense of total pigs born. As described previously, the low average litter birth weight phenotype in these sows appears to be related to a more hidden prolificacy trait, arising from the interactions between ovulation rate and the dynamics of embryonic and early fetal survival (factors that determine litter size in utero in early gestation) and placental function and uterine capacity (factors that affect prenatal development) (Patterson et al., 2016). Selecting for increased overall prolificacy appears to have indirectly created an imbalance in a subpopulation of sows between the numbers of developing embryos in utero and functional uterine capacity to support the optimal development of surviving fetuses to term.

In any population, sows can be identified that consistently exhibit the “low” birth weight phenotype. In most cases, sows giving birth to a LBW litter, again give birth to a low birth weight litter, or at best a medium birth weight litter (MBW), at the next farrowing. Smit (2013) reported the correlation coefficient to determine the repeatability of average litter birth weight across successive parities is reasonably high (r=0.49 in later parities). Therefore, litter average birth weight is repeatable, and thus predictable, within sows. The ability to predict the future litter phenotype is an important management consideration with considerable ramifications on SLP.

In the NPB-funded study, based in a production nucleus/multiplication farm, individual birth weight and gender were recorded over at least three successive parities, and an average litter birth weight phenotype was calculated for these litters. A cohort of gilts produced from sows with a known average litter birth weight phenotype were then individually identified shortly after birth over an 14-month period, and the fate of these potential replacement gilts was then monitored through the weaning, nursery and grower stages of development. Preselection for entry to off-site and on-site GDUs and actual selection for entry to the breeding herd were then monitored, and SLP of selected gilts with known “litter of origin” will be tracked to at least third parity.

For an analysis of litter-of-origin effects on subsequent performance, sows producing potential replacement gilts were retrospectively classified into four groups based on average litter birth weight phenotype [low (<1.15 kilograms), low-medium (1.15-1.37 kilograms), medium-high (1.37-1.6 kilograms) and high (>1.6 kilograms)] such that the extreme phenotypes represented 15% of the total population.

Furthermore, within each phenotype, individual birth weight was classified by the same weight categories described above.

In the low birth weight phenotype, nearly 60% of piglets had an individual birth weight <1.15 kilograms and nearly 90% < 1.37 kilograms (Figure 2), essentially putting nearly all gilts born to these sows at risk for growth and compromised reproductive potential. At nucleus level, LBW litters will lead to a low selection of gilts from these litters as pure-line replacements, either because the gilt fails to survive or meet growth requirements, or because of negative impacts on reproductive performance, independent of growth-related effect. At multiplier level, for sows that repeatedly exhibit the LBW phenotype, it is not inconceivable that entire litters of replacement gilts may be culled prior to selection, and if they do meet selection requirements, subsequent reproductive performance may be compromised, and the LBW phenotype would be perpetuated in the herd. Of all low birth weight gilts born, nearly 40% were born to sows that expressed the LBW phenotype.

In another words, nearly 40% of low birth weight gilts, those that will have compromised growth and poor SLP, can be easily identified by measuring litter birth weight phenotype. Sows that repeatedly display the LBW are a very large contributor to all the low birth weight gilts born.

In the NPB study, gilts were classified by litter birth weight phenotype they were born to (Figure 3), and the percent that fell into each category and the corresponding retention rate at various stages of production is shown. Preliminary data shows that gilts born to sows with the LBW phenotype have compromised retention at several stages of production (approximately weaning, nursery exit, selection and service) compared to gilts born to high birth weight phenotype sows. Future work in the NPB project will examine SLP until third parity.

Sex ratio

The effect of the sex ratio of the litter in which the replacement female was born has been shown to affect performance of gilts. In rodents, a high male sex ratio has been shown to result in delayed puberty, compromised reproduction and increased aggression. In pigs, a high male-to-female sex ratio has been shown to result in delayed puberty, lower successful inseminations, lower mating success, and lower litter size. The sex ratio of the litter into which a potential replacement gilt is born should, therefore, also be considered when selecting future replacement females, using a bias against litters with a high male-to-female sex ratio.

Management interventions

There are several factors that should be considered when selecting future replacements in a commercial multiplication program. Selecting gilts with improved retention and reproductive performance will have beneficial effects on herd productivity and efficiency.

Unfortunately, these are not easy decisions to make and ones that cannot be made without collecting the necessary data to do so. As the saying goes, “If you don’t measure it, you can’t manage it.”

Particularly in a commercial multiplication program, it may be beneficial to record litter size, sex ratio and birth weight of litters born to the grandparent sows in order to make informed selection decisions. Identifying sows that repeatedly display the LBW phenotype, or gilts born in predominately male litters, may allow producers to selectively cull lower-performing gilts at birth. Data show that no nucleus/multiplication sows first giving birth to an LBW litter produced a high birth weight litter at subsequent farrowing, so producers can effectively select against extreme LBW sows (bottom 15%) without risking missing out on high-quality litters born in later parities.

The practical implications of a predictable LBW litter phenotype for the less extreme phenotypes are of immediate interest, and unfortunately, there is limited literature reporting appropriate interventions to more efficiently manage them (Smit, 2013). Producers could more effectively manage low birth weight litters by segregating sows into farrowing rooms based on expected birth weight phenotype for specialized management. Farrowing room management (postnatal management, split-suckling, strategic cross-fostering, ensuring colostrum intake, etc.) would address the increased risk of higher preweaning mortality in low birth weight litters.

Ensuring adequate colostrum intake provides essential nutritive and protective support to newborns as they transition from prenatal to postnatal life and promotes normal uterine development within the first two days of life (Bartol et al., 2014). Lack of colostrum intake could potentially negatively impact reproductive performance as adults.

Strategic cross-fostering, a reduction of the litter size in which replacement females are raised, is a management technique that significantly increases sow longevity, improves farrowing rate and tends to increase number of pigs born over six parities (Flowers, 2015). A reduction in nursery competition may increase overall growth and enhance early development of reproductive organs. Furthermore, 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. It may be beneficial to implement this strategy on all future replacement gilts. One of the outcomes of the NPB-funded study is to make future recommendations on the optimal suckling litter size for improved SLP.


It is clear that sows expressing the LBW phenotype and the offspring produced have negative consequences for the efficiency of genetic multiplication and for reproductive performance in the herd. In order to manage these impacts of birth weight phenotype, we suggest it is necessary to routinely monitor these phenotypic traits at production nucleus/multiplication level.

In general, the swine industry should strive to decrease the number of LBW litters. Indeed, breeding companies recognize the detrimental impact of LBW litters, and the selection strategies they are implementing are increasingly incorporating selection traits like preweaning survivability, robustness and litter birth weight. Despite these changes in genetic selection in the longer term, appropriate management interventions should be applied at the production nucleus/multiplication level to more efficiently account for the negative impacts of low birth weight litters on the efficiency of the genetic transfer program and on SLP of low birth weight gilts presently entering the replacement population.  

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