Cereal silages are used extensively in the diets of feedlot cattle all over western Canada. Plants are generally harvested at dough stage as this stage is believed to represent the best compromise between feeding value and dry matter yield. However, under the growing conditions of eastern Canada, where cereals are grown as cover crops, it has been argued that harvesting at the boot stage would be beneficial to the establishment of the under sown crop and could improve yields of digestible dry matter. Previous experiments have suggested that cereal silages harvested at boot stage vs soft dough stage (Acosta et al., 1991), although the former were more digestible, resulted in no benefit in terms of animal performance. Our work with grass silages (Berthiaume et al, 1996) showed that this could be due to the highly degradable nature of protein in immature silages, and that protein supplements would be beneficial. This study evaluated the effect of harvesting oats for silage at boot vs milk stage with or without the addition of a protein supplement on nitrogen utilisation and growth of calves.