Utilisation and conservation of farm animal genetic resources 177
Chapter 8. Operation of conservation schemes
2.4. Selection and mating in small populations
he conserved live population may be well above the minimum efective size of 50 animals see paragraph 2.1 and some improvement of the genetic adaptations of the
breed may be desired as described in chapter 2. In this case two selection methods will be suggested:
Phenotypic selection, i.e., select for own performance records of the animals. Optimal contribution selection Meuwissen, 1997; Grundy
et al., 1998. Some form of Marker Assisted Selection MAS.
he irst selection method is very easy to implement, whereas optimal contribution selection is a rather high-tech method.
Note that if using BLUP Best Linear Unbiased Prediction for estimating breeding values, the optimal contribution method should always
be used. Otherwise the efective population size can be severely reduced below the actual size and thus increase genetic drit without the user being aware of it.
2.4.1. Phenotypic selection
Truncation selection for phenotypic values is the simplest method of selection, but, at equal rates of inbreeding, it can outperform BLUP-selection Quinton
et al., 1991. It is very easily implemented when the traits can be measured for both sexes. For
example, simply select the animals with the highest growth rate. If the animals are kept in diferent herds, which hampers a direct comparison of animals across herds,
• •
•
Box 8.2. Optimum Contribution selection to minimise inbreeding. It is useful to rewrite the minimum kinship selection problem, as described in the text, in matrix
notation: K
a
= c’Kc, Where K is qq matrix of coeicients of kinships q = number of selection candidates; and c is
the vector of contributions. It can be shown that K
a
is minimised when the contributions are: c = ½ K
-1
Q Q’ K
-1
Q
-1
1, where 1 is a column vector of ones; Q is a q2 incidence matrix of the sex of the candidates where
the irst column contains a one for male and a zero for female candidates, and the second column contains a one for female and a zero for male candidates. he contributions of the male and those
of the female candidates will sum to ½.
178 Utilisation and conservation of farm animal genetic resources
Theo Meuwissen
selection can be for the standardised deviation of the animals from the herd mean or herd-year-season mean.
When the trait is only recorded on one of the sexes, e.g., litter size, selection could be at random in the unrecorded sex. Alternatively, a number of female ofspring could be
obtained from the male selection candidates and selection could be for the phenotypic mean of the female ofspring or the dam of the males. he latter requires however, that
the population is of a quite large size.
Oten selection will be for more than one trait, i.e., several traits need to be improved. As before phenotypic selection will only be for the own performances of the animals,
but the own performances have to be combined into a selection index such that the population mean will change into the right direction. his involves three steps:
1. Determine the optimal direction of the selection, i.e. picture the animal that
is optimally adapted to its environment and market niche. he picture of this optimal animal should not be too optimistic such that it can be reached within a
reasonable time horizon. 2. Obtain a desired gains selection index to select the animals in the optimal direction
using only own performance records see Cameron, 1997. See Box 8.3 for a brief description of the desired gains selection index.
3. Calculate the selection response that will be achieved within the time horizon using Cameron 1997. If the size of the selection response deviates substantially from the
original goal of step 1, the goal of step 1 should be made more realistic and steps 2 and 3 should be repeated.
From step 2 a selection index can be calculated for every animal by weighing the own performances of the traits by the index weights. Selection proceeds as with single trait
selection, but with the individual trait replaced by the selection index.
he above desired gains index avoids determining economic values for every trait, i.e. they are implicitly determined by the desired gains Cameron, 1997. his seems useful,
because the calculation of economic weights can be very complicated for traits in which local breeds oten excel: fertility, disease resistance, longevity and quality of special
products. Selection for over-simpliied breeding goals can make the characteristics of the local breed equal to those of the introduced breed, which has usually been very
intensely selected for a simple breeding goal.
In situations where the calculation of economic weights is rather straightforward, the traditional optimal selection indices
should be used Cameron, 1997. Both desired gains and optimal selection indices require knowledge about genetic covariances amongst the traits. hese are not always
available for small breeds, but perhaps estimates from other related breeds can be used.
Utilisation and conservation of farm animal genetic resources 179
Chapter 8. Operation of conservation schemes
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