Animal Breeding - Selection

Question 1

Goal of selection

Answer 1

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

Question 2

Does selection produce new genes

Answer 2

– 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

Question 3

What are the two types of selection

Answer 3

Natural selection

Artificial selection

Question 4

What is Natural selection

Answer 4

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

Question 5

What is Artificial selection

Answer 5

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

Question 6

What does selection involve

Answer 6

• 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

Question 7

gene frequency

Answer 7

is the proportion of loci in a given

allelic series occupied by a particular gene.

Question 8

Genotypic frequency

Answer 8

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?

Question 9

What is the two additional conditions for Hardy-weinberg Equilibrium

Answer 9

– Equal fertility of parental genotypes

– Equal viability of offspring genotypes

Question 10

Hardy-Weinberg Law

Answer 10

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

Question 11

For selecting more than one
important trait, there are three
main methods of selection:

Answer 11

– Tandem selection
– Independent culling levels
– Index selection

Question 12

Tandem selection

Answer 12

 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.

Question 13

Efficiency of tandem selection

Answer 13

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

Question 14

Independent Culling Levels

Answer 14

– 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

Question 15

Disadvantage of independent culling

Answer 15

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

Question 16

Index selection

Answer 16

– 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

Question 17

When is Index selection the most efficient method

Answer 17

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

Question 18

Examples of Selection Indexes:

Indexes used in the Beef Industry

Answer 18

 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

Question 19

What Can One Expect from the BREEDPLAN

Membership

Answer 19

• 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

Question 20

There are four standard selection indexes calculated for Australian Angus animals:

Answer 20

• Angus Breeding Index (ABI)
• Domestic Index (DOM)
• Heavy Grain Index (GRN)
• Heavy Grass Index (GRS)

Question 21

Angus Breeding Index

Answer 21

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

Question 22

Angus Domestic Index

Answer 22

• 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

Question 23

Angus Heavy Grain Index

Answer 23

• 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.

Question 24

Angus Heavy Grass Index

Answer 24

• 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.

Question 25

Examples of Selection Indices in Dairy

Cattle (USA)

Answer 25

• NM$ (Net Merit)
• CM$ (Cheese Merit)
• FM$ (Fluid Merit)
• TPI (Total Performance Index)
• JPI™ (Jersey Performance Index

Question 26

Examples of Selection Indices in Swine

Answer 26

Sow Productivity Index (SPI)
Maternal Index (MI)
Terminal Indexes (TI)

Question 27

Sow Productivity Index (SPI)

Answer 27

– Provides a measure of sow productivity and is especially useful
when culling sows
• Prolificacy is measured by the adjusted number of pigs born alive in a
litter
• Milking ability is measured by the adjusted weight of the litter at 21
days of age

Question 28

Maternal Index (MI)

Answer 28

– Is intended to put emphasis on maternal
characteristics and is useful for selecting boars to
produce replacement gilts and in selecting replacement gilts
– Because barrows, and gilts that are unacceptable
for replacements, are residuals of this type of mating, there is some emphasis on growth rate, backfat and feed efficiency

Question 29

Terminal Indexes (TI)

Answer 29

– Puts emphasis on growth, efficiency, and backfat

– Should be used for selecting animals to be used in terminal crosses

Question 30

Australian Dairy Cattle Selection Indices

Answer 30

Balanced performance index (BPI)
Health Weighted Index (HWI)
Type Weighted Index (TWI)

Question 31

What is Balanced performance Index (BPI)

Answer 31

Economic Index
Blends Production, type and health traits for maximum profit
In line with farmer preference

Question 32

What is Health weighted Index (HWI)

Answer 32

Fast track fertility and Mastitis resistance

Question 33

What is type weighted Index (TWI)

Answer 33

Fast track type

Question 34

For all the australian Dairy cattle Selection indices, which is the most important factor

Answer 34

ASI - Production