Abstracts: Feed withdrawal ( FW ) is a frequent issue in open outdoor feedlot systems, where unexpected circumstances can limit the animals? access to food. The relationship among fasting period, animal behaviour during feed reintroduction ( FR ) and acidosis occurrence has not been completely elucidated. Twenty steers fitted with rumen catheters were fed a high-concentrate diet (concentrate : forage ratio 85 : 15) and were challenged by a protocol of FW followed by FR. The animals were randomly assigned to one of the four treatments: FW for 12 h ( T12 ), 24 h ( T24 ), 36 h ( T36 ) or no FW (control group) followed by FR. The steers? behaviour, ruminal chemistry, structure of the ruminal microbial community, blood enzymes and metabolites and ruminal acidosis status were assessed. Animal behaviour was affected by the FW?FR challenge (P