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.