The purpose is to examine haylage 'intolerance' in horses, which is expressed as diarrhoeic faeces in some horses when they are fed haylage (sometimes also when fed hay). A typical appearance is two-phase faeces - one solid and one liquid. It is not known why or how this condition appears, and it has not been described previously for other feeds. Also, it is not known if this condition results in higher risk for the horse to suffer serious gut disturbances. We therefore want to perform a case-control study with 30 pairs containing one affected and one unaffected horse, where each pair is housed and fed similarly. We want to study feeding, feed quality and faecal composition. Faecal microbial composition is examined with molecular methods, and faeces also analyzed for biochemical/chemical composition. Feed composition will be analyzed and feeding routines mapped. By doing so, deviations (if present) in the affected horses can be identified and open for solutions to the problem.