| Summary: | The study was designed to provide quantifiable information on both within- and between-herd variation in pig growth rate from birth to slaughter and to examine how this was influenced by moving pigs at a common age to a common environment. Five litters were selected from each of eight pig herds in Northern Ireland with varying growth performance. All eight herds were offered the same nutritional regime. Five pigs (three boars and two gilts) were selected from each litter. In each herd, 22 pigs (12 boars and 10 gilts) were weighed individually, every 4 weeks, from 4 to 20 weeks of age. At 4 weeks of age (weaning) three non-sibling boars were taken from each herd and brought to a common environment where they received medication, were housed individually from 6 weeks of age and offered the same dietary regime. They were weighed and feed intakes were recorded twice weekly. A growth rate difference of 61 g/day (P < 0.001), 112 g/day (P < 0.01) and 170 g/day (P < 0.001) was observed on farm, between the top and bottom quartile of herds during 4 to 8, 8 to 12 and 12 to 20 weeks of age, respectively. This difference in growth rate equated to an average difference in cost of production of ¢13/kg carcass on a birth to bacon unit. When pigs from the different herds were housed in the common environment, large variation in growth performance (143 g/day (P < 0.01) and 243 g/day (P < 0.001) for 8 to 12 and 12 to 20 weeks, respectively) was also observed between the top and bottom quartile of herds. Although feed efficiency was similar, a significant feed intake difference of 329 g/day (P < 0.01) and 655 g/day (P < 0.001) between 8 to 12 and 12 to 20 weeks of age was observed. The variation in growth rate between pigs whether managed on farm or in the common environment was similar (variation in days to 100 kg on farm and in the common environment was 18 and 19 days, respectively). When housed in the common environment, although the top and bottom quartile of pigs converted feed equally efficiently, pigs in the top quartile had significantly higher feed intakes suggesting greater appetites. It is difficult to assess the extent to which these differences can be attributed to genetic effects or pre-weaning environment, and how much the effects of management, disease or genetics contributed to the variation between and within herds.
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