A diallelic experiment between Hereford (HH) and Aberdeen Angus (AA) beef breeds, developed between 1994 and 2002 at the Bernardo Rosengurtt Experimental Station, College of Agriculture-University of the Republic, Uruguay (32°35′ S, 54°15′ W, and 94 of altitude). The data set totalized a set of 57 sires and 1365 dams. The individual (gⁱ_A − gⁱ_H) and maternal (g^m_A − g^m_H) breed differences, as well as individual (hⁱ_AH) and maternal (h^m_AH) heterosis were estimated using additive linear models for cow body condition score at calving (BCS), calf birth weight (BW) and weaning weight (WW), and generalized linear models for weaning percentage (%W). Effects of genetic group, year and month(year) at birth, dam age, and calf sex were included as fixed effects, and individual or sire as random effects. The effect of the difference between individual breed genetic (gⁱ_A − gⁱ_H), as well the heterosis were not significant for BW (P > 0.05). For WW, the difference between maternal effects (g^m_A − g^m_H) was positive, with 4.87 ± 1.01 kg (P < 0.0001), and individual heterosis was 5.12 ± 1.24 kg (P < 0.0001). The hⁱ_AH for %W was 0.22 ± 0.03. The h^m_AH was significant for WW (11.58 ± 1.48 kg, P < 0.0001). Crossbred dams were higher than pure breed by 0.04 points in BCS (3.79 vs. 3.75), being different HH and HA vs. AA and HA (P < 0.05). At birth, calves HH, AH and HA were heavier than AA. For %W, crossbred dams averaged 24.7 points higher than pure breeds (85.0% vs. 60.3%). Crossbred breed dams weaned calves significantly heavier (means 159.1 ± 4.1 kg) to purebred cows. Hereford breed dams weaned calves with significantly less weight (means 143.3 ± 2.7 kg). The estimations of breed genetic effects show reductions in birth weight related to a Angus breed (gⁱ), as well an increment in weaning weight due to a maternal Angus effect (g^m). Superiority in crossbred dams were evidenced in individual and maternal heterosis.