After selection the animals are mated for the first time and offspring will be born. In this example animals on average will have two batches of offspring (single or litters). The length of the generation interval is equal to the age in between the births of both batches. Assumption in this figure is that the number of offspring born in each batch is the same. If not, then the (length of the) generation interval needs to be weighted weighed according to the number of offspring in each batch.
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For animals that are selected on the performance of their first progeny the ‘counting’ only starts from the second batch of offspring onwards. This is presented in the lower part of figure 4. Otherwise the principle is exactly the same as with selection based on own performance or sibs. It is clear that the generation interval will become longer if selection is based on progeny testing.
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The genetic gain thus far was expressed per generation. Now that we have calculated how many years are in a generation, we can express the genetic gain per year:
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If we briefly go back to the example with the jumping rabbits: The breeder was happy when selection was based on performance of 12 offspring. However, he may want to look into the matter in more detail because it will depend on the litter size whether this number can be achieved with a single batch of offspring, or whether multiple batches are required. Multiple batches mean more time and the generation interval in rabbits is low. In such situations it may be a consideration to accept a slightly lower accuracy of selection, but manage more generations of selection in the same time frame. It may result in more genetic gain per time unit in the longer run.
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Thus: |
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optimizing genetic gain will require a balance between increase of the accuracy and increase of the generation interval |