Genetic gain in beef cattle has three components:
1. Heritability (h2).
This is the measure of how strongly a trait is inherited, which determines how much parental superiority is passed on to the next generation. For breeding calculations heritability is described mathematically on a scale from 0 to 1.0. Some important traits such as fertility have low heritability, but you can still make useful genetic progress from selection because of the large amount of variability in a population. Other traits such as growth and fibre production have high heritability.
2. Selection differential (SD).
This is the amount by which the parents are better than the mean of the group from which they were chosen. Because you need fewer males you can select sires further above the mean than you normally can with females.
3. Generation Interval (GI).
This is the rate of generation turn over. It is defined as the average age of the parents when the offspring are born. Keeping generation interval short speeds up genetic progress.
The three components go together in a simple formula:
Heritability is fixed and is somewhat outside your influence, except that the effective heritability can be improved by adjustment for environmental effects, the use of repeated records, and the use of marker traits of high heritability with close genetic relationships to important breeding objectives. The other two components of the formula are well within your control. Aim to keep the GI short and the SD high – it’s the core of practical animal breeding. A good livestock recording system will provide you with the information to achieve this aim.
Removing environmental effects
Removing environmental influences in selection decisions is one of the key elements of a soundly based recording system. Geneticists apply carefully devised formulas which adjust the figures recorded so they take account of environmental factors. It is very important to do this, as environmental effects can easily obscure the true genetic worth of an animal.
The main environmental factors are: age of dam, birth and rearing rank (single or twin), age of the animal, sex and mob or management group.
For lamb liveweights for example, the ranking figures are adjusted for all these factors. All lambs’ performances are adjusted to the equivalent of a single-born and reared, male lamb, out of a four-year-old or older (mature) ewe, and compared to the average of all lambs born of the same sex in that flock, with management differences removed.
Animalplan offers two options for adjusting records:
1. By using standard correction factors which are the same for all flocks and herds of the same breed in the recording system. These standard adjustment factors are most appropriate for small flocks and herds.
2. By calculating within flock or herd correction factors. This can only be done when there are sufficient numbers of animals with records for a particular trait within the system. The numbers required differ both with species and trait. If numbers are sufficient, within flock and herd correction factors increase the accuracy of adjustments because they are based on the specific environment of a particular flock or herd.