Sheep genetics
Role of objective measurement
Ken Wilson and Warren Hunt, formerly of the Agency for Food
and Fibre Sciences
Revised by Nicole McLennan and Dr Roger Lewer, Agency for Food and Fibre Sciences
Key points
- Studs control the rate of genetic gain of their ram buying clients.
- Source rams from a stud that has a clearly defined breeding objective and uses objective measurement and selection indexes to select rams for use in their own flock. The superiority of these rams will be passed onto ram buying clients.
- Measurements on individual rams are not useful on their own; it is the ram's performance in relation to the average of the rest of the drop that counts.
- Date of birth, birth type and maternal effects can reduce the accuracy of selection on both objectively measured and visually assessed traits.
Introduction
Commercial producers can capitalise on objective measurement by sourcing rams from a stud with a comprehensive objective selection program. The benefits will be derived through improved future incomes.
A single ram produces more progeny in its lifetime than any given ewe and so will have greater impact on genetic improvement. It is wise to source rams from a stud that has a clearly defined breeding objective and uses selection based on objective measurement because this will more accurately identify sires that produce the most profitable offspring. The gains made from accurate selection in the stud control the rate of improvement in clients' flocks.
Objective versus visual selection in the stud
Visual classing is neither a consistent nor accurate method of selection for important production traits in sheep. This is clearly demonstrated in Table 1, which shows the results of a classer's visual selection versus index selection (a combination of fibre diameter (FD) and greasy fleece weight (GFW)) at "Brumich", Augathella.
Table 1. Classer results versus that of selection using objective measurements in an index.
|
Sheep selected by classer |
Sheep selected by objective index |
|
|
Top: 53 head |
||
|
Average GFW |
4.20 kg |
4.63 kg |
|
Average FD |
21.93 microns |
19.04 microns |
|
Value/head |
$32.57 |
$62.50 |
|
Middle: 471 head |
||
|
Average GFW |
4.10 kg |
4.33 kg |
|
Average FD |
21.47 microns |
21.01 microns |
|
Value/head |
$33.94 |
$37.67 |
|
Culls: 53 head |
||
|
Average GFW |
4.07 kg |
3.97 kg |
|
Average FD |
21.76 microns |
23.08 microns |
|
Value/head |
$32.43 |
$27.59 |
Use of the visual method alone would have resulted in many worthwhile sheep being sacrificed as culls. Indeed, the value differential achieved by the index would cover the testing of the entire group several times over. Add to this the fact that part of this dollar differential is passed to the progeny and it is clear that objectively based selection is money in the bank.
It is even more difficult to accurately determine how well an individual ram's progeny will perform compared with other rams in his drop using visual assessment alone. The use of objective measurement will provide the most precise information on how an animal is really performing in terms of his predicted progeny performance.
Many breeders tend to become pre-occupied with the actual measurements of individual rams (particularly fibre diameter), wrongly assuming that this is what is inherited. Not so. The measured value of an individual animal is not, on its own, very meaningful without information on the test group average. A ram's performance in relation to the average of rest of the animals in that drop is what counts. This is because nutrition, environment and management as much influence production performance as genetics. Clean fleece weight, fibre diameter, body weight and growth rate, will all change in line with seasonal variations. But at the same time, individual rams will generally tend to retain their ranking in relation to the rest of the animals from the same drop. What is important, therefore, is the deviation of a ram's performance from his group average, that is, whether he is above or below, and by how much.
What does this mean for commercial breeders?
So what does all this mean to the commercial breeder and how can this knowledge best be put to use when selecting replacement rams? To put it simply, you need to know three things:
- Does the stud have a clearly defined breeding objective that states where both the stud and its clients aim to be in, say, ten years?
- How the stud selects its rams ie is objective measurement and selection indexes used, or visual information, or a combination of both?
- The performance of a ram in relation to the average of the rest of the drop.
Commercial sheep breeders who consistently want rams that are significantly either side of the stud average will in many instances be selecting off-types and would be well advised to change to a stud that has a breeding objective more in line with their production objective.
Influences on objective measurements
Objective measurements need to be kept in perspective by remembering the following.
Account should be taken of environmental effects on production characteristics. However, this also applies to classing.
Fleece weight and fibre diameter measurements should be considered in conjunction with each other. Selecting animals on fleece weight alone will result in a slight increase in fibre diameter, while selecting for fibre diameter alone will cause fleece weight to be slightly reduced. By combining the two sets of figures in an index, sheep can be identified that will breed for both lower fibre diameter and higher fleece weight.
Most rams are presented for sale from about 18 months of age. In order to obtain adequate objective measurements and to allow time for sufficient wool growth at the time of selection, rams need to be shorn and measurements made no earlier than about 12 months of age. This in itself causes complications as measurements on young rams are normally different to their measurements when they are mature and there is usually an age difference between rams of the same drop.
The fibre diameter of rams 12 to 18 months of age is lower than it will be when they reach maturity. It is quite normal for studs with a long term average fibre diameter of, say, 23 microns to have a test average of around 20 microns at this age. This is in agreement with the fibre diameter profile of individual rams, which generally show an increase in fibre diameter with age. But as long as individual animals are considered in relation to the average of the drop while keeping in mind the long term average of the stud, then fibre diameter tests on young rams are a valuable indicator of progeny performance. Re-testing of older rams has little value.
There will normally be a two to three month difference in age in the young ram drop and this can have a significant bearing on fleece weight as well as other production traits. Also, environmental factors carried over from lambing may still be having an effect at 12 to 18 months of age. It is therefore likely that the measured information displayed may still be showing some of these effects. For example twins usually have a lower weight than singles and if the dam is older, she will most likely be a good milk producer and her lambs will benefit by having higher growth rates and resulting bodyweights. Ideally, production figures should be adjusted for known environmental differences prior to selection on objective measurement.
Acknowledgment
The Lamb Boost Project funded by Australian Wool Innovation produced these notes.
Further information
For further information contact the DPI Business Information Centre on 13 25 23 (Queensland residents) or (07) 3404 6999 (non-Queensland residents) between 8 am and 6 pm weekdays, or e-mail callweb@dpi.qld.gov.au.
This DPI Note is also published on the DPI's PrimeNotes CD-ROM.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
File No: SW0024 . Date created: May 1999 . Reviewed: January 2003
