Cows were assigned randomly to nine groups of four animals each. One group was slaughtered at the onset of the study to obtain baseline data. The remaining eight groups were assigned randomly to two treatments, a control group or a group implanted with 30 cm of testosterone propionate implants, and fed for two lengths of time, 42 and 84 d. This resulted in a 2 x 2 factorial arrangement of treatments with two pens of four animals per treatment. They had ad libitum access to a completely mixed 50% roughage diet for the first 7 d, a 35% roughage diet for the second 7 d, a 25% roughage diet for the third 7 d and a 15% roughage diet for the remainder of the trial (Table 2). All diets were balanced to meet or exceed NRC (1984) requirements for an ADG of 1.1 kg.