ROTATION CROSSES AND INTER-SE MATINGS WITH ANGUS AND HEREFORD CATTLE FOR 5 GENERATIONS

A total of 3768 calves and 4950 cow mating-years were involved in a st udy of heterosis retention. Crosses included first and subsequent inte r-crosses between Angus and Hereford breeds as far as the fifth calf g eneration (F1, F2,...F5), back crosses (B1) using both Fl sims and Fl dams, and rotation crosses as far as the fifth calf generation (R2, R3 , R4 and R5). Records collected were calf traits to the yearling stage , cow reproduction and maternal traits, and steer carcass data. Effect s were estimated for 27 breed types for calves and 23 for cows. Breed type effects were then replaced by multiple regression coefficients fo r genetic effects including: an average individual (Angus) breed effec t (g(I)), average maternal (g(M)) and average grandmaternal (g(MG)) br eed effects, individual (d(I)), maternal (d(M)) and paternal (d(P)) do minance effects, and epistatic effects. From these, individual (h(I)) and maternal (h(M)) heterosis and other parameters were derived. For t he Angus breed, g(I) was generally negative and g(M) positive for dire ct calf weights and cow weight; g(I) was positive for calf weaning wei ght (WW) as a maternal trait. The Angus breed was superior to the Here ford for net reproduction, productivity (weight of calf weaned per cow mated) and an efficiency ratio (productivity/average cow weight). Rel ative to the average of the purebreds, the superiorities of F1, F3 to F5, and R4 animals were respectively 2.1, 3.2 and 5.0% for birth weigh t, 6.5, 3.7 and 7.9% for yearling weight, 6.7, - 2.2 and 3.4% for cow weight, 10.3.0.7 and 11.7% for calf WW as a maternal trait, 7.4, - 1.3 and 5.7% for number of calves weaned per cow mated, 19.4, 1.0 and 18. 4% for productivity, and 11.2, 2.1 and 14.0% for the efficiency ratio. For calf traits and cow weight, d(M) and h(I) were generally signific ant and positive (except negative for calving date), whilst only h(I) was generally significant for calf weights as maternal traits. For the reproductive traits, the trends were for h(I) to be positive, but sta ndard errors were large. For carcass traits, the interbreds and rotati on crosses were generally like the heavier, fatter (i.e. Hereford) pur ebred. Overall, for productivity and the efficiency ratio, die rotatio n crosses (especially R4) were superior to the purebreds but generally not to the F1s, whilst the interbreds were similar to the purebred av erage.