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Shaver Composites and Heterosis
by shaverbeef on Saturday 05 November 2005 Hybrid vigour is expressed in economically important traits such as calving ease, growth rate, carcass quality, yield and possibly short gestation calves . It also overcomes antagonistic traits like lean carcass vs. marbling. Some of these traits have a low heritability and are difficult to breed for in purebreds. More heterosis comes from more breeds in a composite; Shaver Composites are made up of 9 breeds. Heterosis (Hybrid vigour) is the measureable production increase in the progeny of two breeds above what would be expected from the progeny average of the two parents. Retained heterosis is that hybrid vigour passed on to the next generation, and the more breeds that go into a Composite animal, the more retained heterosis will be expressed in the progeny. Heterosis in an animal is measured as a percentage (n-1)/n x100 where n is the number of breeds making up that composite. For example, a purebred animal will have (1-1)/1 = 0% hybrid vigour, a crossbred animal made up of 2 breeds will have (2-1)/1= 50% heterosis (the progeny (g2) of crossbreds (g1) will revert to 0% heterosis, as the parents (g1) are effectively the same breed) while a 3 way crossbred animal will have (3-1)/1= 66.6% heterosis. Shaver Composites are composed (like a Symphony) of 9 breeds giving (9-1)/9x100= the Optimum 88.9% retained heterosis. (Using 10 breeds would give you 90% retained heterosis, but this is only 1.1% more than a Shaver) In other words, there is a large enough gene pool in Shaver Composites that the progeny from "purebred" Shaver parents will still have hybrid vigour. Also, heterosis is free! References |
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