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A total of 632 intact rams comprising three purebreds (Dorset Horn, Merino and Corriedale) and their six contemporary F1 crossbreds produced during phase I of the CSIRO Sheep Crossbreeding Experiment were used in the study. The rams were slaughtered at an average age of 21 months, at which time their average (�_ s.e.) carcass weight was 28.4 �_0.2 kg. This paper, the second in the series, is concerned with estimation of the linear effects due to heterosis (hI), breed-transmitted effect (gI) and breed-maternal effect (gM) on four adiposity traits: perirenal fat weight, subcutaneous fat depth, percentage ether extract of m. semimembranosus and adipocyte diameter from the perirenal fat sample. The results showed that, in general, breed-transmitted effects are relatively more important than breed-maternal effects in accounting for breed differences in adiposity traits. Further, data based on all the adiposity traits under investigation are consistent in placing the Corriedale as the 'fattest' of the three purebreds studied. The hI effect is variable, depending on both the trait and the breed combination. Thus, perirenal fat weight (%hI = 22.6) and subcutaneous fat depth (%hI = 13.9) are both more heterotic than percentage ether extract (%hI = 1.6) or adipocyte diameter (%hI = 2.8). In every trait, the estimates of hI are lower in value (actual unit or as a percentage) for the Merino-Corriedale breed combination than for either the Dorset Horn-Merino or the Dorset Horn-Corriedale breed combination, demonstrating the importance of breed as a source of variation in heterosis effect. |
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