Technical Abstract: In order for a beef cow to continue in an annual production cycle, she must rebreed within three months after calving. Malnutrition during this period frequently results in failure of the cow to become pregnant. The energetic needs of the cow are increased by lactation and additional energy is required for growth of the primiparous cow. Determining energy expenditures during the first 40-60 d postpartum is critical to developing feed programs that will allow cows to successfully become pregnant with a second calf. Sixty-seven balance trials were conducted on 25 MARC III cows (four breed composite: 1/4 Hereford, 1/4 Angus, 1/4 Red Poll, and 1/4 Pinzgauer) that were between 3 and 53 d in milk. Cows’ body weights were 481 +/- 4 kg. Metabolizable energy intake ranged from 14.8 through 28.9 Mcal/d. Milk yields ranged from 4.7 through 13.3 kg/d. Retained energy increased linearly 0.693 +/- 0.037 with increased metabolizable energy intake. Forty-four observations were conducted with cows in negative tissue energy balance and 23 observations were conducted with cows in positive tissue energy balance. Estimated zero energy balance from regression analyses of retained energy on metabolizable energy intake was 146 kcal metabolizable energy/kg BW**0.75. Efficiency of conversion of metabolizable energy to milk energy for cows in positive tissue energy balance was 73% and for cows in negative tissue energy balance it was 76%. The efficiency for conversion of metabolizable energy to tissue energy for cows in positive tissue energy balance was 71% and the conversion of tissue energy to milk energy was 91%. Our findings suggest that even though their milk production is lower, the overall efficiency of energy retention in young beef cows is similar to that of dairy cows.