Understanding the tools of objective breeding - Note 5
Understanding the tools of objective breeding
Since domestication, selection has occurred within the sheep flock. This has been a mixture of both natural selection (survival of the fittest) and deliberate animal breeding. Breeding from genetically superior stock will result in permanent benefits such as a reduction in the cost of production, and these will build up year on year. It is each sheep breeder's goal to enhance the performance of their animals by breeding from the best. But how is best defined? The following explains the objective measurements, which are presently available to aid this selection process.
Best Linear Unbiased Prediction (BLUP)
In 1991, Best Linear Unbiased prediction 'BLUP', a statistical procedure, became available for processing performance data. BLUP separates out the genetic factors influencing an animal's performance from the non-genetic 'or environmental' factors, such as the way it is managed or fed. This is achieved by creating genetic links between groups of animals. For example, at least one individual in a group must be the offspring of a ram who also has offspring in another group. BLUP makes use of this linkage to take out differences in management and environment, which occur between the groups. This BLUP technology has replaced the need to bring animals together to a central test station. It produces a series of Estimated Breeding Values (EBV's).
A ewe and lamb from the RAMS flock at Greenmount College. this flock is a member of both Progressive Texel Sires (PTS) (N Ireland Sire Reference Scheme) and Elite Texel Sires (Great Britain Sire Reference Scheme)
What are EBV's?
EBV's are in the form of positive and negative values (for example, the eight week weight may be + 0.8 or - 1.1), indicating the superiority or inferiority of the genes an animal possesses for each trait. The values are expressed relative to a common baseline value of zero for the breed and are expressed in the same units as the recorded traits (for example, kg for liveweight).
EBV's have allowed stock selection to move away from the 'feed them and weigh them' approach, which led to the actual daily liveweight gains being published. This information was extremely misleading as the actual growth rate and weight was probably more a reflection of external factors such as when the lamb was born, whether it was a twin or single and how it had been managed, rather than its actual genetic ability
Downkillybegs Excelsior.
Excelsior was the reference ram selected in 1998 by PTS. He was chosen from the top performing lambs in the PTS flocks.
EBV's are a major step forward in breeding, as they supply information on an animals genetic ability, irrespective of how well it was fed, managed or the environment it comes from.
Table 1. Estimated Breeding Values available to sheep breeders
| EBV | Unit of measurement | A positive figure indicates... | A negative figure indicates... |
|---|---|---|---|
| Lamb growth to eight weeks | kg | Faster growing lambs | Slower growing lambs |
| Prolificacy | Number of lambs | More prolific ewe lambs | Less prolific ewe lambs |
| Maternal Ability | kg | Heavier lambs at eight weeks | Lighter lambs at eight weeks |
| Mature Size | kg | Lambs which are heavier at maturity | Lambs which are lighter at maturity |
| Lamb growth to 20/21 weeks | kg | Faster growing lambs | Slower growing lambs |
| Muscle Depth | mm | Greater muscling | Poorer muscling |
| Fat Depth | mm | Fatter carcase | Leaner carcase |
Accuracy
With each EBV there is also an accuracy value which is expressed as percentage points ranging from 0 to 100. This provides a guide to the likelihood of an EBV changing 'up or down', as more information on the animal and its relatives becomes available. The greater the value the more accurate the EBV. The main factors, which affect the accuracy of EBV's are:
- Whether the animal has been measured itself for the particular trait;
- The amount of information on the trait from relatives of the animal;
- The heritability of the trait, how easy or difficult it is to pass superior genes from one generation to the next;
- The amount of information available on other traits, which are related to the trait of interest, for example, the birthweight and 200 day weight will influence the 400 day weight of an animal;
- The number of animals within a recorded group, the more the better.
Sire referencing
Unlike in the beef world where artificial insemination (AI) has been prevalent for many years, creating genetic links between individual cattle herds throughout the United Kingdom, flock owners have only recently adopted this technology. This lack of genetic linkage between flocks means that rams and ewes from one flock cannot be directly compared with those from another flock. To overcome this, Sire Referencing Schemes have been developed. Within the scheme each member has a number of ewes mated to a sire reference ram using AI. This creates a genetic link between each and every member of the group and allows all the stock within the group to be directly compared. Each member of the group benefits from being able to use the best (genetically superior) animals produced within the group. This will increase the rate of improvement among individual members and ultimately across all participating flocks.
Multi trait indexes
EBV's can be combined into a multi-trait selection index for a specific breeding objective in order to maximise improvements. Economic weightings relevant to current market conditions are used to ensure each trait is given the appropriate amount of emphasis in the index.
Indexes for terminal sire breeds
The aim of these indexes is to increase carcase lean weight whilst minimising any associated rise in fat. The indexes are based on the relevant EBV traits, scan weight, muscle depth and fat depth. The Lean Index is produced for scanned flocks, which are not in a Sire Referencing Scheme. The average Lean Index for the current year's lambs within each flock is 100. Therefore lambs from within the flock which have a Lean Index above 100 are carrying the better genes for lean meat production. But remember this is within flock and two animals each from a different flock, both with Lean Indexes, cannot be directly compared. Only animals within a Sire Referencing Scheme which have a Scheme Index rather than a Lean Index can be directly compared. The average Scheme Index for all lambs, in the year the Sire Referencing Scheme started is, 100, and consequently as progress is made the average Scheme index will increase.
Maternal Index identifies best ewes within a breed
The aim is to produce more productive ewes. The Maternal Index is calculated using the EBV's for litter size, lamb growth, maternal ability and mature size. As the objectives for each ewe breed are different, for example, a lowland versus hill ewe, the influence of each EBV towards the Maternal Index reflects the breeding goals of each specific breed.
Pedigree breeders
- Only select animals that are physically correct and typical of the breed.
- Select stock rams from those in the top 10% for index. Preferably all females retained should have an index above the flock average.
- Where females are being retained for breeding, the rams selected should have a good maternal as well as lean or scheme index.
- Match male with female qualities to improve overall flock uniformity. For example, if a ewe lacks growth select a ram with a high EBV for 20/21 week weight, or if she lacks muscle select a ram with a high EBV for muscling.
- Always use ram lambs sparingly until the accuracy of their breeding values has improved. Remember estimates can move up and down considerably until sufficient information is available.
