Is observation of horses when they are outdoors adequate for detecting individuals with abnormal behaviour?

It is reported that outdoor housing of horses reduces stereotypic (SB) and aberrant (AB) behaviour, however, observation of the horse only when outdoors may not preclude it from being stereotypic when in the stable. The study objective was to determine if the observation of horses when they are outdoors only is sufficient for detecting stereotypic individuals. The second objective was to compare SB and AB in outdoor versus outdoor maintained horses. The SB, AB and aggression of horses maintained either 1), indoor system (n=19; IS) with four hours of turnout or 2), continuously in an outdoors system (n=20; OS), were observed for 10 h/horse. Overall 0-58.5% of horses presented such behaviours at least once. In both systems, a similar number of horses performed crib -biting, weaving, tongue playing, licking, wood chewing, coprophagy, geophagy, pawing, and aggression toward humans and horses. Grid biting, incisors sliding, box -walking and wall kicking were only observed in IS horses and only when in the box, and wood chewing only when outside. The IS horses displayed weaving behaviour only in the box. However, only when outdoors, IS horses displayed wood chewing, nibbling of faeces and soil. In IS horses, the frequency of tongue playing, licking, pawing and acts of aggression toward other horses was similar to that when in the stable. Thus, the observation of the horse only when outside the stable may be misleading when assessing stereotypies in horses. While paddocking reduces the SB and AB, it is possible that this could also be due to the lack of substrates for displaying them. It is concluded that in welfare assessment protocols for horses, maintained most of their time in the stable, the periods when they stay in their home boxes should be considered.