Sow herd productivity has a major impact on the overall profitability of pork production and Canadian producers have been successful in their efforts over the past several years to increase average litter size. It is well known that piglets born into larger litters are on average smaller. However, it is not known if the variability in birth weight, that is the range between the heaviest and lightest pigs within each litter, increases as litter size increases. Additionally, recent work in Europe has shown that the fibre composition of the muscles differs between the very small birth weight piglets and their larger litter-mates. This could have an effect on the ultimate eating quality of the pork. We therefore, conducted a study with the overall objective of defining the relationship among birth weight, birth order, and post-weaning growth, on the eating quality of the pork. We saw no effect of birth order on birth weight or the other parameters, so it will not be discussed. The effects of birth weight on carcass and pork quality will be discussed in a later edition of Centred on Swine. This article addresses the relationship of birth weight on growth and the variability in growth. Moreover within the experiment we were able to examine the role of litter size on these parameters.
The protocol for this experiment required that data be collected from a minimum of 100 litters. This required the attendance at all farrowings for 5 consecutive weeks at PSC Elstow. During these weeks a technician was constantly present from 8:00 am Tuesday, until Saturday, 4:00 pm. Farrowing and piglet management, including cross-fostering, iron injections, castration and tail clipping followed normal barn procedures. At birth, the piglets were divided into one of 4 birth weight quartiles
(Table 1). Piglets weighing less than 800 grams at birth were excluded from the experiment. Table 1 describes the effect of birth weight on performance. The "large" birth weight piglets were almost 900 grams heavier than the "small" birth weight piglets at birth. This translated into a difference of 2.2 kg at weaning and 9.6 kg at 1st pull. Or, to put it another way, at birth, the "small" birth weight piglets weighed about 54% of the "large" birth weight piglets, but 90% at 1st pull. Thus, there is some "catch-up" by the lighter pigs. As intended, shipping weights were similar however days to market increased as birth weight decreased (P < 0.001; Table 1). "Large" piglets had a higher rate of gain throughout, (P < 0.002), but, the relative difference in the rate of gain became less as the pigs grew.
Table 2 describes the effect of litter size on numbers weaned, growth and variability. Variability is represented as the standard deviation of the mean. The grouping into litter sizes was arbitrary; we wanted approximately an equal number of pigs per grouping. We had an average of 9, 13 and 15 pigs born alive in the litters defined as small, medium and large, respectively. As expected, average birth weight decreased as litter size increased (P < 0.001) and the number of pigs weighing less than 800 grams tended to increase as litter size increased (P = 0.12). This difference in average body weight was no longer evident at weaning or later as the pigs grew (P > 0.20). The variability in body weight (SD) within a litter was not affected by litter size at any of the time points examined (P > 0.10). Average days to market was unaffected by litter size (P = 0.55) and the total kg shipped per litter increased by 200 to 300 kg as litter size increased from small to medium to large (P < 0.001).
The bottom line is that the light birth weight piglets never completely catch up. While increasing litter size resulted in a decreased mean birth weight; by 5 wks post-weaning, the average body weight was similar among litter sizes. Our data does not support the hypothesis that larger litters result in more within litter variation. Larger litters were no more variable than small litters and larger litters resulted in more pork produced per sow.