Increasing evidence suggests that dietary factors influence the development of abnormal behaviours. In this study, the behaviour of 17 foals from the age of 2 to 40 weeks was examined. Each foal received either a conventional starch and sugar (SS) diet or a fat and fibre (FF) diet. The two diets differed in the fat and non-structural carbohydrates balance. The baseline behaviour of the foals was observed every 2 weeks and additional behavioural observations were conducted when the foals were weaned by one of two methods (i.e. paddock-weaning or barn-weaning). Approximately 2 months after weaning the temperament and tractability of the young horses was assessed using standard tests. Responses to a novel object, to a novel person, and during a handling test were observed and quantified. The horses grew well on both diets with no apparent effects of diet on growth rate or baseline behaviour. Immediately after weaning, horses received the FF diet cantered less frequently, for a shorter duration and appeared to be more settled. Foals that were barn-weaned appeared more stressed than foals that were paddock weaned. In the temperament tests, horses receiving the FF diet spent significantly more time investigating, and less time looking at, the novel object than horses receiving the SS diet. They also spent less time walking away from the novel person and their time taken to complete the handling test was significantly lower. Overall, the horses that received the FF diet appeared less distressed immediately after weaning and seemed calmer and more inquisitive during a range of temperament tests.