Data from purebred and crossbred cattle, consisting of the Afrikaner (A), Simmentaler (S) and Hereford (H) breeds managed in a relatively intensive but high stocking rate environment, were analysed to estimate breed direct effects, individual heterotic effects, breed maternal and maternal heterotic effects for birth weight (BW) and weaning weight (WW). Simmentaler breed direct effects, expressed as deviation from the general mean, were positive (P ≤ 0.01) for both traits. Hereford and Afrikaner breed direct effects were negative (P ≤ 0.01) for both traits while the Afrikaner breed direct effects exceeded those of the Hereford. Afrikaner direct maternal effects were positive (P ≤ 0.01) for both traits. The Hereford direct maternal effect was negative (P ≤ 0.05) (-2.9%) for WW. Simmentaler maternal effect was negative (P ≤ 0.01 ) for BW but non-significant (P ≥ 0.05) for WW. Individual heterotic effects for BW were significant (P ≤ 0.01) in H x S (3.5%) and S x A (11.0%) only. Individual heterotic effects were positive (P ≤ 0.01) for WW, with that of the H x A (9.8%) and S x A (6.7%) crosses exceeding the S x H (3.1%) cross. Maternal heterotic effects were non-significant (P ≥ 0.05) for both BW and WW .

Data from a multibreed composite beef cattle population, managed under intensive irrigated grazing conditions, were used to estimate direct additive heritabilities (h 2 a ), maternal heritabilities (h 2 m ) and maternal permanent environmental effects (c 2 ) for birth weight (BW) and weaning weight (WW) of the calf and cow efficiency of the dam (CE: WW/dam weight 0.75 x 100). Calves born between 1968 and 1993 (n = 52628) had varying levels of Hereford genes, ranging from 0% to 100%, with an average of 19.3%. Direct heritabilities, fitting unitrait models, were 0.72, 0.54 and 0.19 for BW, WW and CE, respectively, with corresponding estimates of maternal heritabilities being 0.14, 0.21 and 0.42. Genetic correlations between direct and maternal effects (r am ) were negative for all three traits, varying from -0.40 to -0.65. Maternal permanent environmental effects were 0.06, 0.12 and 0.11 for BW, WW and CE, respectively. Direct breeding values and maternal breeding values for BW and WW decreased with increasing proportion of Hereford. Direct breeding value for CE increased, while maternal breeding value for CE reached a minimum value at 0.62 proportion of Hereford.

Data from a multibreed composite beef cattle population managed under intensive irrigated grazing conditions were used to estimate direct additive heritabilities (h 2 a) and maternal heritabilities (h 2 m) for birth weight (BW) and weaning weight (WW) of the calf and cow efficiency of the dam (CE; WW/dam weight 0.75). Calves born between 1968 and 1993 (n = 52 628) had varying levels of Simmentaler genes, ranging from 0% to 97%, with an average of 33.4%. Direct heritabilities fitting unitrait models were 0.66, 0.53 and 0.21 for BW, WW and CE respectively, with corresponding estimates of h 2 m being 0.22, 0.36 and 0.59. Genetic correlations between direct and maternal effects (r am) were negative for all three traits, varying from-0.32 to-0.63. Direct breeding values for BW and WW decreased and maternal breeding values increased with increasing proportion of Simmentaler. Cow efficiency was unaffected by an increase in the proportion of Simmentaler.

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