Abstract Thirty lactating cows were fed ad libitum six rations as complete feeds in a factorially designed experiment. Rations involved a control and an ammonia-molasses-mineral (ProSil) treated silage fed at 50%, and corn cobs treated with 3% sodium hydroxide and 1% calcium hydroxide at 0%, 12%, and 23% of the ration dry matter. The cobs replaced part of the corn in these rations. Crude fiber was 12%, 16%, and 20% of these respective rations. As-fed intakes were highest for 23% rations, but intakes of dry matter and net energy for milk production were highest for the 0% rations. Actual milk and 4% fat-corrected milk were lowest for the 23% rations. Milk fat content averaged 3.68% and was not affected by percent cobs. Milk protein was lowest for the 23% rations (3.11%) and highest for the 0% rations (3.34%). Treatment of the silage had no effect on yield of milk, 4% fat corrected milk, milk fat, or milk protein percentages. Mean daily milk production was 20.6kg for the untreated silage and 20.1kg for the treated silage rations. Intake of dry matter per unit of metabolic weight was lower for the treated silage. General performance favored the 0% and 12% rations whereas there was little difference between silage treatments.