Breed effects and heterosis in advanced generations of composite populations for birth weight, birth date, dystocia, and survival as traits of dam in beef cattle. 
Heterosis effects were evaluated as traits of the dam in F2 progeny of F1 dams and F3 and 4 progeny of F2 and 3 dams in three composite populations of beef cattle.  Traits included birth weight, birth date, calving difficulty percentage, and survival percentage at birth, 72 h, and weaning for calves with dams of different age classes.  Breed effects were evaluated for the nine parental breeds (Red Poll [R], Hereford [H], Angus [A], Limousin [L], Braunvieh [B], Pinzgauer [P], Gelbvieh [G], Simmental [S], and Charolais [C]) that contributed to the three composite populations (MARC I = 1/4 C, 1/4 B, 1/4 L, 1/4 H, 1/8 A; MARC II = 1/4 G, 1/4 S, 1/4 H, 1/4 A; and MARC III = 1/4 R, 1/4 P, 1/4 H, 1/4 A).  Among calves with 2-yr-old dams, breed effects were significant for birth weight, birth date, calving difficulty percentage, and survival percentage at birth but not at 72 h and weaning.  Calf survival at weaning was lowest for smallest (less than mu - 1.5 sigma) and largest (greater than mu + 1.5 sigma) birth weight classes and did not differ among intermediate birth weight classes.  Calves with difficult births with 2-yr-old dams were significantly heavier at birth (39.6 vs 35.4 kg) and had significantly lower survival at 72 h (87.1 vs 92.2%) and at weaning (77.4 vs 85.1%) than calves with 2-yr-old dams that did not experience difficult births.  Among calves with dams greater than or equal to 3 yr old and from dams of all ages, breed group effects generally were significant for the traits analyzed.  Important breed group effects on dystocia and survival traits were observed independent of breed group effects on birth weight.  Effects of heterosis were significant for birth weight for each generation of each composite population and for the mean of the three composite populations.  Generally, heterosis effects for calving difficulty percentage were not significant.  Effects of heterosis generally were significant for date of birth (earlier) for each composite population and for the mean of the three composite populations.  Heterosis effects on survival to weaning percentage generally were positive but generally were not significant.  Heterosis retained for birth weight, birth date, and survival percentage in combined F3 and 4 generation progeny of combined F2 and 3 generation dams did not differ (P greater than .05) from expectation based on retained heterozygosity.  These results support the hypothesis that heterosis in cattle for these traits is the result of dominance effects of genes.