The digestibility of nutrients in milk replacers were determined with Ayrshire bull calves from 7 to 28 days of age. The mean apparent digestibilities of dry matter (DM), gross energy (g.e.) and nitrogen declined from 0.84 to 0.79; 0.85 to 0.78 and 0.79 to 0.76 when cooked potato flour replaced an equivalent weight of spray‐dried whey in liquid diets. Potato starch has a mean apparent digestibility of 0.65. Nitrogen retention was unaffected by diet, but the potato diet had a lower metabolisable energy value than the control diet (15.6 and 17.5 MJ per kg DM respectively). Glucose levels in the jugular blood were lower for calves fed the potato based milk replacer. The apparent digestibilities of dry matter, gross energy and nitrogen showed marked improvements with age, particularly on the potato based diet, with starch digestibility improving from 0.50 to 0.74 between 10 and 24 days. While blood glucose concentration increases with age, the relative differences between treatments remained.