What is the current significance of low birthweight pigs on commercial farms in Northern Ireland in terms of impaired growth and mortality?

There is little modern data addressing the differential lifetime growth of commercially reared low and average birthweight pigs born into large litters (>14 piglets). As such, the main aim of this study was to quantify the lifetime growth and mortality rate of low and average birthweight pigs on commercial farms in Northern Ireland. It was also aimed to analyze the level, stage and cause of mortality within each birthweight category. A total of 328 low birthweight (low BW; <1 kg) and 292 average birthweight (Av BW; 1.3 to 1.7 kg) pigs were individually identified across four commercial farms and one research farm. Animal growth and mortality were monitored on an individual basis from birth until slaughter age. Av BW pigs were heavier than low BW pigs throughout the trial (P < 0.001), with a weight advantage of 1.16 kg at weaning increasing to over 9 kg at slaughter age. Av BW pigs recorded a superior average daily gain (ADG) to low BW pigs throughout the trial (P < 0.05), with the greatest difference recorded immediately postweaning between weeks 4 and 8 and weeks 8 and 12 when a 77 and 85 g/d difference was recorded, respectively. AV BW pigs which were cross-fostered were significantly lighter than those remaining with their birth mother at weaning (0.9 kg), week 8 (1.7 kg), and week 12 (3.1 kg) (P < 0.05, respectively). The variance of weight was significantly greater for the AV BW pig population than the low BW pig population at week 4 (P < 0.001) and 8 (P < 0.05). Preweaning mortality of low BW pigs was over three times greater than that of Av BW pigs (21% vs. 6%; P < 0.001), with low BW deaths occurring earlier (9.2 d vs. 15.4 d; P < 0.001) and at a lighter weight (1.2 vs. 2.4 kg; P < 0.001) than Av BW pigs. There was a clear association between birthweight and cause of preweaning death (P < 0.05), with starvation (49%) and overlying (28%) accounting for the majority of low BW mortalities. Birthweight had no effect on rate, age, or weight of postweaning mortalities (P > 0.05). The alimentary tract (27%) and respiratory tract (27%) were the most commonly implicated body systems following postmortem examination of postweaning deaths. In conclusion, this study quantified the inferior weight, growth rate, and mortality of low BW pigs, identifying the lactation and immediate postweaning periods as having the greatest potential in reducing this birthweight associated growth differential.