A 300 cow Brahman herd kept on improved pasture was subjected to a selection and management programme based on a limited breeding season. Artificial insemination using mainly progeny tested bulls was used in part of the herd and the rest were bred in single sire herds. Of the 200 sires used during the 30 year period, 82% were homebred and selected principally for high estimated breeding value of 18‐month weight. Variance components of birth (BW), weaning (205 W) and 18‐month (548 W) weights of 6130 calves born 1968 through to 1997 were estimated by the Restricted Maximum Likelihood method (REML) using uni‐ and bivariate animal models. For each weight the animal's direct and maternal genetic and the dam's permanent environmental effects were considered random and those of sex, year and month of birth and age of cow were considered fixed, but the models differed as far as the number of significant interactions included. Adjusted least squares means for BW, 205 W and 548 W were 28, 158 and 292 kg. Phenotypic and direct and maternal genetic trends from univariate analysis were for BW: 0.156, 0.061 and −0.001 kg; for 205 W: 0.471, 0.126 and 0.044 kg; for 548 W: 1.973, 0.486 and 0.251 kg per year. Direct and maternal heritabilities from univariate analyses were for BW, 205 W and 548 W, 0.33 and 0.08; 0.07 and 0.14; 0.13 and 0.08, respectively. Genetic direct‐maternal correlations for the three weights were −0.37, −0.13 and 0.49 and permanent environmental variance of the dam as proportion of phenotypic variance (c2) had values of 0.03, 0.16 and 0.01, respectively. Direct and maternal genetic correlations were for BW: 205 W, 0.64 and 0.74; for BW: 548 W, 0.35 and 0.74; and for 205 W: 548 W, 0.64 and 0.96. Future genetic work in the herd should put more emphasis on the improvement of cow efficiency for sustainable beef production on native and improved pasture.

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Summary In two experiments (E1 and E2), 2909 and 1514 calves were produced by artificial insemination of zebu cows on two ranches in the Venezuelan floodplains. Semen of progeny‐tested national Brahman (B) bulls were used to produce the two control groups and of Charolais (C), Limousin (L), Marchigiana (M), Romosinuano (Criollo, R) and Simmental (S) in E1 and Angus (A), Chianina (K), Gelbvieh (G), L and S in E2 to produce F1 progeny. Calves were raised with their dams on savanna until weaning and on savanna, and improved pasture after this until 18 months. Birth weight (BW), weaning weight (WW) and 18‐month weight (18MW) were analysed by least‐squares procedures; breed group, sex, year and month of birth, age of dam, sire within breed, and breed × sex and year × month were included in the model. Breed group had highly significant effects on all weights. Adjusted means for BW, WW and 18MW (548 days) were 29.8 ± 0.2, 165.3 ± 1.1 (age 198 days) and 268.7 ± 1.9 kg in E1 and 31.7 ± 0.2, 205.9 ±1.2 (age 282 days) and 282.3 ± 1.3 kg in E2. The overall mean and range of group means for crossbred advantage for the three weights in E1 were: 2.5 (− 6.8‐7.7) %, 8.4 (3.9‐11.7) % and 10.9 (7.6‐16.4) % in E1 and; 6.7 (3.5‐10.2) %, 11.3 (9.9‐14.3) % and 11.9 (10.7‐14.2) % in E2. F1 C in E1 S and F1 S in E2 were superior to all other F1 groups in WW and 18MW. Breed × sex interaction was significant for BW and 18MW and crossbred advantage was higher in females than in males. Male superiority over females was highly variable beetween the breed groups. The highest significant difference between progeny of bulls with extreme genetic values within breeds was 14.7% for BW, 15.1% for WW and 16.8% for 18MW. Zusammenfassung Wachstum von F1 Bos taurus × Bos indicus versus Bos indicus Fleischrindern in Venezuela. I. Geburtsgewicht, Absatzgewicht und 18 Monate Gewicht In zwei Versuchen (E1 und E2) wurden in zwei Herden im venezolanischen Tiefland (Überschwemmungsgebiet) durch künstliche Besamung von Zebukühen 2909 und 1514 Kälber geboren. Mit Sperma von nachkommengeprüften venezolanischen Brahman (B) Bullen wurden zwei Kontrollgruppen gebildet und mit Charolais (C), Limousin (L), Marchigiana (M), Romosinuano (Criollo, R) und Simmental (S) Sperma (E1) und mit Angus (A), Chianina (K), Gelbvieh (G), L und S Sperma (E2) F1 Nachkommen produziert. Die Kälber wurden bis zum Absetzen zusammen mit ihren Müttern auf Savanne gehalten und danach bis zum Alter von 18 Monaten auf Savanne und verbesserter Weide. Die varianzanalytische Berechnung der Geburts‐ (GG), Absatz‐ (AG) and 18 Monate (18MG) Gewichte wurde mit der Methode der kleinsten Quadrate durchgeführt, unter Berücksichtigung der folgenden Effekte: Rassegruppe, Geschlecht, Geburtsjahr und — monat, Alter der Mutter, Vater innerhalb der Zuchtgruppe, Rassegruppe × Geschlecht und Jahr × Monat. Rassegruppe hatte einen hoch significanten Einfluss auf alle Gewichte. Die korrigierten Mittelwerte für GG, AG und 18MG (548 Tage) waren in E1 29,8 ± 0,2,165,3 ± 1,1 (Alter 198 Tage) und 268,7 ± 1,9 kg und in ...

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