Studies have repeatedly shown that belly nosing is the most common abnormal behaviours exhibited by early-weaned pigs, and as weaning age decreases, the incidence of belly nosing increases (Gonyou et al., 1998; Worobec et al., 1999). Furthermore, the effect of early weaning on behaviour persists well into the grow-finish phase of development (Gonyou et al., 1998; Worobec et al., 1999). Among members of the same litter and between litters, there is considerable variation in the incidence of belly nosing as well as in the amount of time it takes for the behaviour to commence following weaning (Fraser, 1978; Li and Gonyou, 2002). Li and Gonyou (2002) studied the temporal association of belly nosing with other behaviours in an attempt to identify the proximate cause of the behaviour. The authors found that the social environment can have a profound effect on the incidence of belly nosing, possibly explaining some of the variation in belly nosing observed between litters. Welfare concerns regarding the incidence of belly nosing in early-weaned pigs have reflected the environmental contribution to the development of the aberrant behaviour while the genetic component has not been researched beyond preliminary findings (Bench, 2005). The effect of enrichment and breed line on belly nosing in early-weaned pigs was investigated over two studies. In a preliminary study, 291 piglets were weaned at 7 days-of-age and observed over two consecutive days (pigs ranged from17 to 30 days-of-age at time of observation). Piglets were fed a liquid milk replacer diet for either 7 or 14 days following weaning. The pen environment was enriched by providing either an air-filled inner tube (Tube), rubber nipples in the feed trough (Nipple), or neither (Control). Pens were segregated by sex with 14–16 pigs per pen. Within pens, there were both Duroc and Yorkshire pigs. Instantaneous scan sampling by two observers was used to determine the incidence of belly nosing, belly sucking, and nosing and sucking other parts of the body. Yorkshire line pigs engaged in more belly sucking (3.97% versus 1.59%; P < 0.01), total sucking (4.30% versus 2.21%; P < 0.05), and belly-directed behaviour (9.2% versus 6.21%; P = 0.089) than did Duroc line pigs. Nipple enrichment effectively reduced the level of belly sucking, overall sucking and belly-directed behaviours in the Yorkshire line pigs, while Tube enrichment reduced other nosing and oral–nasal vices directed away from the belly in the Duroc line pigs. The second study investigated the effect of sire breed and sire within breed on the proportion of time early-weaned pigs spent belly nosing and belly sucking. Two hundred and forty-two crossbred pigs sired by Duroc (n = 120) and Large White (n = 122) boars were weaned at 14 days-of-age and observed at 18, 23, 28, 50, 63 and 91 days-of-age for belly nosing, belly sucking, other nosing and sucking, tail biting and other biting behaviours during the nursery and grow-finish phases of development. Continuous observations done live at 21 and 35 days-of-age were used to determine mean belly nosing and belly sucking bout durations. Pigs sired by Large White boars spent a greater proportion of time belly nosing (2.040%; P<0.01) and belly sucking (0.440%; P < 0.01) compared with pigs sired by Duroc boars (1.597% and 0.308%, respectively). In contrast, Duroc-sired pigs spent a greater proportion of time conducting other nosing (0.356%; P < 0.01) and other sucking (2.496%; P < 0.001) behaviours compared with Large White-sired pigs (0.173% and 2.063%, respectively). Individual sire also had a significant effect on belly nosing (P < 0.001), belly sucking (P < 0.001) and other sucking (P < 0.01) behaviours post-weaning. It was concluded that breed line differences might affect the incidence of nosing and sucking behaviours in early-weaned pigs, which may be reduced through the use of environmental enrichment tailored to the specific abnormal behaviours being performed.