Beef cattle breeding is rather small in the Netherlands. In countries like Australia or the USA, or within Europe European countries like France or the UK, beef cattle breeding is a much larger business. The cows graze on large areas of land and are not handled at regular basis. Therefore, AI is not a very useful tool in the reproduction. Most farmers buy bulls for natural mating and let them graze with the cows. The very large farms also breed their own bulls.
Consider a population of beef cattle that is selected for increased growth. The heritability is 0.35 and the phenotypic standard deviation (σp) is 0.2 kg / day. The females are selected on their own performance. As the population size is supposed to remain constant and females can produce about three calves in their lives, 2/3 of the females need to be selected to produce sufficient animals for replacement (remember that both male and female calves are born!). A selected proportion of 0.67 results in a selection intensity (if) of 0.54. The accuracy of selection for selection on own performance is equal to to √h2 so so rIH,f = 0.59.
The males are selected based on the performance of 100 progeny, resulting in a rIH of 0.95. Each male is mated to 10 females, resulting in a selected proportion of 0.10 * 0.67 = 0.067. The selection intensity thus is 1.95 (check in the table in 9.5.1).
Finally, the genetic standard deviation is equal to the square root of h2 * = 0.35 * 0.22 = 0.118. What is the genetic gain in this population?
Filling all that information into the formula results in a genetic gain per generation of:
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Scaling the genetic gain per generation to the generation interval results in a genetic gain per year of:
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Thus: selection intensity and accuracy of selection may differ between males and females. Selection response in each of these selection paths are calculated separately, and afterwards combined into a genetic gain for the entire population. |
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