In the existing commercial sow populations, variation in postnatal growth performance of their offspring may be already preprogrammed before birth. Because these limitations in postnatal growth may only become apparent in the late grower and finisher stages, sorting pigs by weight at the nursery and grower stages will not resolve variation in growth performance that appears at the finishing stage.

Prenatal programming affects muscle development early in gestation. A low number of muscle fibers in low-birth-weight pigs limit their muscle mass (lean yield) at market weight. However, gut development and health status of low-birth-weight pigs are also affected by prenatal programming and reduce survivability through lactation and the nursery stages.

We have increasing evidence that the changing dynamics of prenatal survival in mature sow populations are resulting in increasing variance in grow-finish performance. It is likely that this is partly due to prenatal programming problems.

The critical questions for producers are, how do we recognize the problem of prenatal programming at the production level, and what can be done about it?

Muscle Development Before Birth

The biphasic pattern of muscle fiber development (myogenesis) in the pig is illustrated in Figure 1.

In the first phase, Day 35 to Day 55 of gestation, a primary generation of so-called “primary myofibers” develops.

In the second phase, which lasts until Day 90 of gestation, the formation of secondary myofibers occurs. Over 20 secondary myofibers cluster around each primary myofiber. Considering the fact that an increase in muscle fiber number (muscle hyperplasia) ceases by around the 90th day of gestation, the number of primary and secondary muscle fibers formed by Day 90 ultimately determines the total number of muscle fibers at birth. Importantly, the total number of muscle fibers at birth is lower in smaller fetuses compared to larger fetuses.

In experimental studies, even modest levels of intrauterine crowding of embryos early in gestation have a negative impact on fetal muscle fiber development. Given the apparent imbalance in modern prolific sows between ovulation rate, early embryonic survival and uterine capacity — and consequences for fetal and postnatal development — the reproductive characteristics of prolific dam lines need careful consideration.

Although the primary goal of increasing the number of pigs born per litter may be achieved in prolific sow lines, associated adverse effects on prenatal programming are apparent. As a result, selection for increased litter size at birth has led to an increased between-litter variation in average piglet birth weight, as well as to an overall decrease in birth weight. In the extremes of high litter size born, the growth potential of the live born pigs that survive to weaning are seriously compromised by intrauterine competition with an increasing number of stillborn pigs that, obviously, never enter the nursery and grow-finish stages of production.

Birth Weight, Growth and Carcass Quality

Both the type and the total number of muscle fibers is fixed at birth and, between them, determine the lean growth potential of the pig. As the size and length of existing muscle fibers increases after birth, an increase in total muscle mass is apparent. Low-birth-weight pigs with low muscle fiber numbers, therefore, are expected to have impaired postnatal growth.

Experimental studies of within-litter variation in birth weight clearly demonstrate the associations among birth weight, carcass characteristics and meat quality traits.

For example, in the study shown in Table 1, pigs of low birth weight exhibited the lowest total number of muscle fibers, the largest muscle fiber size and the highest percentages of abnormal “giant” muscle fibers in the muscles investigated. These pigs also had the lowest percentages of muscle tissue, the lowest total protein and the lowest semitendinosus muscle weight, yet the percentages of internal organs, skin, bone and total water were highest, compared to their heavier littermates.

When slaughtered at a fixed age of 182 days, the pigs of low birth weight were lighter, had lower meat percentages and smaller loin eye area averages, although their omental (abdominal) fat percentages tended to be higher than pigs of high birth weight. With respect to meat quality, higher drip losses were determined in the longissimus muscle of low-birth-weight pigs.

Other studies confirm that low-birth- weight pigs required an extra 12 days to reach the same slaughter weight, and their feed conversion ratio was inferior. Of great importance to consumer satisfaction, the low-birth-weight pigs exhibited a lower score for loin meat tenderness compared with high-birth-weight pigs. Collectively, research indicates that pigs of low birth weight develop lower carcass and meat quality.

Birth Weight Variance

Comparisons between the largest pigs in a litter at birth and the smallest are most frequently used to study the impacts of birth weights on postnatal growth performance. However, if the limitations in functional uterine capacity in hyper-prolific sows results in prenatal programming of entire litters, we must also try to understand how the average birth weight variation between litters is a major cause of variance in postnatal performance.

Indeed, selection and production strategies that address the problem of between-litter variation in birth weight may be the most important aspect in addressing postnatal growth potential.

As Figure 2 shows, both the mean and the variance in birth weight decrease as litters get bigger. The birth weight of most pigs born in litters larger than 15 is relatively low. Likewise, between-litter variation in average birth weight is relatively low in these larger litters because the sow's limited uterine capacity is unable to support a higher birth weight. Furthermore, there appears to be a lower limit of average birth weight of around 2.2 lb., which is more or less independent of litter size.

At the other extreme, litters of less than 10 pigs should not have suffered from extreme intrauterine crowding in early gestation. The average litter birth weight tends to be higher in these litters.

Given these extreme effects on litter birth weight, the greatest likelihood of finding variation in average litter birth weight appears to be in litters of between 10 and 15 total born. When only these litters are taken into account, the overall impact of number born on average litter birth weight is relatively small (<40 g or 0.09 lb. for each additional pig born between 10 and 15).

In contrast, the difference in average birth weight between the heaviest and lightest litters in the range of 10 to 15 total born is over 2.2 lb. Clearly, some factor other than total born/litter is driving these major differences in average litter birth weight.

The fact that low-average birth weight litters have more pigs born dead and less piglets weaned is consistent with the notion that these litters are subjected to prenatal programming in the uterus. Also, the lower within-litter standard deviation of birth weight in the low-average-birth weight litters may be a consequence of the prenatal loss of the smaller and weaker pigs, thus already reducing the variation in litter birth weight at term.

In contrast, in litters not subjected to extremes of intrauterine crowding, pigs across a wider range of birth weights have the opportunity to survive to term, and this would explain the higher variance in birth weights we observed in the higher-average birth weight litters.

We conclude from these analyses that between-litter variance in birth weight is a major contributor to variation in postnatal growth performance.

A study of phenotypic data from 600 litters born to multiparous commercial sows suggests that low-average-birth weight is the result of intrauterine crowding earlier in gestation and prenatal programming.

Necropsy was performed on a subset of stillborn pigs that fell within the mid-weight range for their respective litters. In addition, data on organ weights were used to estimate “brain sparing effects” as a measure of prenatal programming (Figure 3).

Between-litter variation in average birth weight was again the biggest source of variation in birth weight in litters of 10 to 15 pigs born. More importantly, the stillborn pigs from lower average birth weight litters carried all the negative phenotypic characteristics associated with prenatal programming of poor postnatal performance.

These data further support the suggestion that one of the major causes of variation in postnatal growth performance will be between-litter variation in average birth weight. Linking back to the extensive data on the impact of birth weight on postnatal growth performance reviewed earlier, the postnatal growth potential of low-birth-weight litters should be a major concern for all pork producers.

Bottom Line for Better Production Systems

If prenatal development affects postnatal variation in growth performance, what are the practical resolutions to this problem?

The continued selection for increased litter size born, without understanding implications for variation in average litter birth weight in litters born to higher-parity sows, seems questionable. Feed costs to finish market hogs continue to rise, and the livestock industry will increasingly compete with other industries seeking to divert these feedstocks to other industrial processes — most notably ethanol production.

The efficiency of feed utilization, and minimal environmental impacts of food-animal production, will become increasingly important issues in our ability to sustain pork production. The net efficiency with which we can produce a pound of high-quality pork is critical to the competitiveness of our industry.

As intensive pork production systems continue to evolve, greater attention is being paid to the concept of segregated management systems. The reasons for adopting segregated production flows vary, but the underlying principle remains the same ? the net advantages that come from managing particular subpopulations of the pork production chain to achieve greater efficiency and consistency of production.

Segregation may be a spatial concept, in a geographic sense, to improve the control of disease transmission at different levels of the production pyramid. Increasingly, segregation involves separation of subpopulations on the basis of their susceptibility to disease challenges compared to, say, more mature animals, or because segregation allows specialized management to be applied in a cost-effective way to these segregated populations.

Segregation in this instance can be on-site within a farm, or even within-barn, depending on the situation and goals.

In light of the above discussion, we suggest that segregation of entire litters on the basis of average litter birth weight may do more to address variation in postnatal growth performance than existing programs of extensive crossfostering, irrespective of average litter birth weight, and the successive sorting of pigs by weight in the later stages of production. Neither of these strategies accepts the major inherent differences in postnatal growth potential that clearly exists between litters with high and low average birth weight.

Table 1. Association Between High and Low Birth Weights Within a Litter and Postnatal Growth, Carcass Characteristics and Pork Quality Traits*

Within-litter birth weight grouping

Low (2.0 lb.) >High (4.0 lb.)
Average daily gain, lb. 1.3 1.4
Live weight, lb. 233.4 255.2
Hot carcass weight, lb. 185.2 203.5
Drip loss, % 6.6 4.5
Myofiber* area, µm2 3,900 3,200
Myofiber number × 1,000 900 1,200
“Giant” myofibers, % 0.44 0.07
Myofibers are the muscle fibers making up the muscle mass in newborn pigs.
*from Rhefeldt et al., 2004