The objective of this work was to compare breed types with increasing percentage of Bos taurus on cow/calf energy requirements and preweaning efficiency. Forty mature, lactating, nonpregnant cows [10 Nellore (NL), 10 Canchim x Nellore (CN), 10 Angus x Nellore (AN), and 10 Simmental x Nellore (SN)] were randomized in blocks by calving date. Calves from cross-bred cows were sired by Canchim bulls (5/8 Charolais + 3/8 Zebu), whereas calves from NL cows were sired by Nellore bulls. Cows were individually fed a pelleted diet with 50% hay (alfalfa and coastcross) and 50% concentrate from calving to weaning (20 to 180 d post-partum). Estimated diet ME content was 2.24 Mcal/kg of DM. Individual cow DMI was adjusted every 14 d to keep shrunk BW and BCS constant. Shrunk BW and BCS were 430 +/- 12 kg and 4.7 +/- 0.09 for NL, 449 +/- 10 kg and 4.8 +/- 0.09 for CN, 496 +/- 10 kg and 5.0 +/- 0.09 for AN, and 507 +/- 12 kg and 5.1 +/- 0.09 for SN. At 40 d calves were allowed ad libitum access to the same diet. Milk yield was recorded using a weigh-suckle-weigh technique. Increasing B. taurus percentage had a linear effect (P < 0.01) on ME intake (MEI) of cow/calf pairs: 21.9 +/- 0.38 for Nellore, 23.6 +/- 0.35 for 31.5% B. taurus (CN), and 25.6 +/- 0.27 Mcal/d for 50% B. taurus (AN and SN). Bos taurus percentage was also positively associated with milk production. Nellore calves had lower (P < 0.05) weaning weight (kg) than crossbreds: 167 +/- 12 vs. 206 +/- 10 for 3/4 Canchim 1/4 Nellore (3/4C), 220 +/- 11 for 1/2 Canchim 1/4 Angus 1/4 Nellore (1/4A) and 228 +/- 11 for 1/2 Canchim 1/4 Simmental 1/4 Nellore (1/4S). Calf body composition was estimated at weaning using the 9-10-11th-rib section. Retained energy (Mcal) was greater (P < 0.05) in 1/4A (384 +/- 19.9) than in Nellore (298 +/- 21.6) and 3/4C calves (312 +/- 19.8), and was intermediate in 1/4S calves (333 +/- 21.6). Cow/calf energetic efficiency (kcal deposited/Mcal of MEI by the pair) was greater (P < 0.05) for AN (103 +/- 5.3) than NL (84.9 +/- 5.9), CN (83 +/- 5.3), and SN pairs (83.5 +/- 6.1). Difference (P < 0.05) between Continental and British crosses was shown in linear contrasts; SN pairs had greater MEI and lower retained energy than AN. Increasing the B. taurus percentage increased inputs as well as outputs. However, a relatively greater increase in outputs resulted in significantly greater preweaning efficiency. Purebred Nellore had lower ME requirements; however, in an environment without nutritional constraints, crossbreeding improved preweaning biological efficiency.