Animal Breeding - Selection Flashcards
Goal of selection
increase the frequency of desirable genes
Occurs whenever some individuals are permitted to produce more offspring than others
The genetically superior animals are allowed to produce offspring and pass on their genes
The inferior animals are culled and are not allowed to produce offspring
Does selection produce new genes
– Selection does not produce any new genes
– Animals with desirable genes and gene combinations to have more offspring than animals who lack these genes or gene combinations
What are the two types of selection
Natural selection
Artificial selection
What is Natural selection
selection that occurs in nature without human help
• “survival of the fittest”
• Weak and sickly animals die before they reach reproductive age.
• The stronger, more vigorous animals are more likely to
reach breeding age and produce offspring
What is Artificial selection
selection done by humans
• Selection - most powerful force available to animal breeders to change gene frequency
– Replacement Selection – selecting new animals to become parents first time or using artificial insemination rather than bulls on your farm
– Culling – the process that determines which parents will no longer remain parents
What does selection involve
• Selection involves saving or culling whole animals with all of their genes
– May have to cull some animals who have good genes for some traits, but bad genes for other traits
• Example, culling sires that are carriers of the dwarf gene, even if they have high yearling weights
gene frequency
is the proportion of loci in a given
allelic series occupied by a particular gene.
Genotypic frequency
the proportion of the N individuals in the population with a particular genotype.
– What proportion are AA? What proportion are Aa?
What proportion are aa?
What is the two additional conditions for Hardy-weinberg Equilibrium
– Equal fertility of parental genotypes
– Equal viability of offspring genotypes
Hardy-Weinberg Law
This law states that in a large, random mating population, the gene and genotypic frequencies remain constant from generation to generation in the absence of mutation, migration, and selection.
– If these conditions are true, the population is said to be in HardyWeinberg Equilibrium
For selecting more than one
important trait, there are three
main methods of selection:
– Tandem selection
– Independent culling levels
– Index selection
Tandem selection
Selection is practiced for only one trait until a
satisfactory level is reached, then a second trait is selected for, etc.
For example, a beef cattle breeder might select for
weaning weight until the herd mean reaches 500 lb
Then he forgets about weaning weight and selects for
yearling weight until a herd mean of 1,000 lb is achieved
Next he might select for ADG, or he might go back to
selecting for weaning weight
We select for only one trait at a time while ignoring all
other traits.
Efficiency of tandem selection
depends on the
genetic relationships between traits
– Positive genetic correlation
• Selection for one trait will result in improvement in the
other trait
• If we select for weaning weight, yearling weight will
also improve, because these traits have a positive
genetic correlation
– Genetic correlation = 0
• Selection for one trait will have no effect on the other
trait
• Negative genetic correlation
– Selection for the second trait will undo the
progress we made when selecting for the first trait
Example: there is a negative genetic correlation
between yearling weight and cutability:
• We select for yearling weight until our herd average
reaches 1,000 lb
• Then we select for cutability, while we are selecting for
cutability, yearling weight will decrease – we will undo
the progress we have already made in yearling weight
Tandem selection is the least efficient of these three
methods of selecting for two or more traits
Independent Culling Levels
– Selection is practiced simultaneously for two
or more traits
– A minimum culling level is set for each trait
• Any animal that falls below the minimum culling
level for any trait is culled
Disadvantage of independent culling
Superiority in one trait cannot offset inferiority in
another trait
For example, say that the independent culling level for weaning weight is 500 lb and for yearling weight is 1,050 lb
We have a bull that has a 450 lb weaning weight and a 1,200 lb yearling weight
With independent culling levels, this bull will be culled even though he has a good yearling weight, because he falls below the cut-off for weaning weight
Setting culling level is a subjective decision
Index selection
– Involves calculating a total score for each animal
• Add up the animal’s merit in each trait and arrive at a total score
– Most efficient of these three methods of selection
• Results in maximum genetic improvement
– Is a very objective measurement
– One problem with index selection
• Wait quite a while to obtain the data needed for the index (e.g., birth wt, weaning wt, and postweaning gain)
• Index selection to select for these 3 traits, we have to wait until all of the animals have completed the post weaning test before we can calculate the index values and make the selections
When is Index selection the most efficient method
the most efficient method of selection over the long run
– It will result in maximum genetic improvement for the traits considered
– Index selection is still subject to year-to-year environmental variation
• Index selection in one year may not make much improvement in a trait due to a bad environment
• Using index selection will maximize genetic progress over the years
Examples of Selection Indexes:
Indexes used in the Beef Industry
Multiple-trait selection indexes that combine EPDs for
several traits into a single economic value, which can then be used to make selection decisions
The difference in index value between two bulls is the
expected difference in average dollar value or performance of their progeny, when the bulls are bred to similar cows
BreedObject, a tool that combines the BREEDPLAN EBVs for an animal with an economic weighting (based on costs of production and returns on outputs), to produce a single Selection Index
What Can One Expect from the BREEDPLAN
Membership
- Receive a BREEDPLAN report for your herd which includes Estimated Breeding Values (EBVs) for your sires, dams, heifer progeny, bull progeny and steer progeny.
- Effective identification of animals with the best genetic package for you and your bull buying clients.
- Monitor the genetic progress of your herd across a range of economically important traits.
- Access to other genetic tools that assist selection and genetic progress such as Mating Predictor, TakeStock and Internet Solutions EBV related functions
There are four standard selection indexes calculated for Australian Angus animals:
- Angus Breeding Index (ABI)
- Domestic Index (DOM)
- Heavy Grain Index (GRN)
- Heavy Grass Index (GRS)
Angus Breeding Index
estimates the genetic differences between animals in net profitability per cow joined in a typical commercial self replacing herd using Angus bulls
• Suitable for commercial producers who sell progeny into different markets, or to seed-stock producers supplying bulls to commercial clients
• Not specific to a particular production system or market end-point, but identifies animals that will improve overall profitability in the majority of commercial grass and grain finishing beef production systems
Angus Domestic Index
- Estimates the genetic differences between animals in net profitability per cow joined in a commercial self replacing herd targeting the domestic supermarket trade.
- Steers are assumed to be finished using either grass, grass supplemented by grain or grain (eg. 50 – 70 days) with steers slaughtered at 490 kg live weight (270 kg carcase weight with 12 mm P8 fat depth) at 16 months of age.
- Daughters are retained for breeding and therefore maternal traits are of importance.
- Emphasis has been placed on eating quality and tenderness to favour animals that are suited to MSA requirements
Angus Heavy Grain Index
- Estimates the genetic differences between animals in net profitability per cow joined in a commercial self replacing herd targeting pasture grown steers with a 200 day feedlot finishing period for the grain fed high quality, highly marbled markets.
- Steers are assumed to be slaughtered at 760 kg live weight (420 kg carcase weight with 30 mm P8 fat depth) at 24 months of age.
- Daughters are retained for breeding and therefore maternal traits are of importance
- There is a significant premium for steers that exhibit superior marbling.
Angus Heavy Grass Index
• Estimates the genetic differences between animals in net profitability per cow joined in a commercial self replacing herd targeting pasture finished steers.
• Steers are assumed to be slaughtered at 620 kg live
weight (340 kg carcase weight with 12 mm P8 fat depth) at 22 months of age.
• Daughters are retained for breeding and therefore maternal traits are of importance.
• Emphasis has been placed on eating quality and
tenderness to favour animals that are suited to MSA
requirements.
