From 1998 to 2007, the average number of fully formed pigs (born alive plus stillborns) in the United States has increased from 10.2 to 11.1 pigs per litter (PigCHAMP, 1998, 2007).

Studies have reported negative genetic correlations (Kaufmann et al., 2000) between birth weight and litter size and lower individual birth weight as the result of greater number born alive (Fix and See, 2008). This combination of factors suggests that an increased incidence of low birth weight pigs occurs within commercial swine production.

The objective of this study, conducted at North Carolina State University, was to quantify the effect of birth weight on the future growth, composition and mortality of commercial market hogs.

Materials and Methods

During a four-week period at a commercial sow farm, all pigs (n=5,747) were individually weighed and identified within 24 hours of birth. Individual mortality records were kept through the harvest phase. Pigs were weaned (20.74±0.03 days) in four groups and placed in a commercial nursery for seven weeks.

Individual weights were recorded prior to weaning (n=4,117) and at finisher placement (around 70 days of age) (n=3,447). All pigs from the second and third weaned groups were weighed at seven weeks (around 120 days of age) into the finish phase (n=1,628) and 16 weeks (around 182 days of age) into finish phase (n=1,592). During the 16-week weighing (off-test) pigs were measured for real-time ultrasound backfat (BF) and longissimus muscle area (LMA).

Statistical Analysis

All growth and composition data were analyzed using the Mixed procedure of SAS (SAS Institute Inc, Cary, NC). Mortality data were analyzed using the GLIMMIX procedure of SAS (SAS Institute Inc, Cary, NC).

Results

Growth

Birth weight had quadratic relationships with body weight measured at weaning (4.92±0.02 kg), finisher placement (23.24±0.08 kg), 7 weeks into the finish phase (58.53±0.23 kg), and off-test (103.14±0.34 kg). As birth weight increased, all weights increased at a decreasing rate. Individual relationships between birth weight and body weights collected at future weights are described below.

Weaning weight

Rate of increase for weaning weight differed between cross-foster status: effect of increased birth weight on weaning weight was greater in non-cross-fostered pigs compared to cross-fostered pigs (Figure 1). Cross-fostered pigs tended to be the smaller or poorer quality pigs within a litter, which may explain this observation. Regardless of cross-foster status, increasing birth weight from 0.7 to 1.7 kg, compared with increasing birth weight from 1.7 to 2.7 kg, resulted in a greater increase in weaning weight of around 2.5 versus around 1.5 kg.

Finisher placement weight

For weight at finisher placement, the increase from 0.7 to 1.7 kg birth weight resulted in a greater (9 versus 3 kg) increase in weaning weight than increasing birth weight from 1.7 to 2.7 kg (Figure 2).

Weight at 7 weeks into finish phase

As piglet birth weight increased from 0.7 to 1.7 kg versus 1.7 to 2.7 kg, weights at 7 weeks into the finish phase increased (around 15 versus around 1.5 kg; Figure 3).

Off-test weight

Weight at off-test increased around 25 kg as birth weight increased from 0.7 to 1.7 kg; however, weight at off-test did not change as birth weight increased from 1.7 to 2.7 kg (Figure 4).

Composition

Real-time ultrasound BF and LMA measured at off-test were adjusted to a mean weight of 103 kg. Birth weight did not affect LMA at off-test. Pigs that were heavier at birth had less BF at off-test. A 100-gramme increase in birth weight resulted in 0.02±0.003 cm less BF.

Survival

The relationship between birth weight and the likelihood of not surviving to weaning was also analysed. Piglets below around 0.7 kg were humanely euthanased and removed from the analysis due to the poor health aspects associated with their size. Few pigs were born heavier than 2.1 kg. Due to their small number, a response would be difficult to predict; therefore, those pigs were also removed from the analysis. Using the remaining pigs (n=5,262), the relationship was analyzed between birth weight and survival to weaning. As birth weight increased, the likelihood of survival to weaning increased (Figure 5). Once birth weight reaches about 1.5 kg, the probability levels off and increasing weight is not associated with an improved likelihood of survival. However, each incremental decrease in birth weight below 1.5 kg is associated with a greatly increased probability of not surviving to weaning.

Summary

Pigs with low birth weights grew slower, were fatter and were more likely to not survive until weaning. Based on these results, the authors recommend that further research be completed so a management strategy can be devised to reduce the magnitude of the effect of birth weight on economically important traits or reduce the incidence of low birth weight pigs. Also, potential selection criteria which may reduce the incidence of low birth weight pigs need to be explored.

References

Fix, J.S. and M.T. See. 2008. Sow parity and number born alive influence piglet birth weight along with subsequent growth, composition, mortality and endpoint value. J. Anim. Sci. Vol. 86, E-Suppl. 2, 293.
Kaufmann, D., A. Hofer, J.P. Bidanel and N. Kunzi. 2000. Genetic parameters for individual birth and weaning weight and for litter size of Large White pigs. J. Anim. Breed. Genet. 117: 121-128.
PigCHAMP. 1998. PigCHAMP 1998 Datashare. PigCHAMP Inc., Ames, Iowa. [click here] Accessed 1 Feb 2009.
PigCHAMP. 2007. PigCHAMP 2007 Benchmarking. PigCHAMP Inc., Ames, Iowa. [click here] Accessed 1 Feb 2009.
SAS Institute Inc. 1999. SAS/STAT® User’s Guide, Version 9.1.3. SAS Institute Inc., Cary, NC.