Measuring ram success in breeding

Good ram management has economic benefits and determines the success of your breeding program.

You must make sure rams are not just healthy but also content, as they are more likely to breed and be productive.

You should understand when a ram is not performing to the required standard, whether anything can be done to improve performance, or if it is time to replace or cull.

Measuring improvements in your rams

You need to objectively measure improvements in rams so you can be sure that your flock is improving at the same rate as the stud's.

Look for improvements in traits that:

  • will help you reach your breeding objective
  • you are paid for, such as fibre diameter, fleece weight and bodyweight.

Consider your breeding objective

You should always keep your breeding objective in mind and be prepared to make periodic assessments of the progress of your own flock.

You can enter sheep in bloodline comparisons, or use on-farm trials to ensure that you are heading in the desired direction.

Fibre diameter, fleece weight and body weight

Fibre diameter, fleece weight and bodyweight typically account for up to 95% of the variation in Merino sheep incomes. Fibre diameter normally accounts for 50–70% of the variation in clean Merino wool price. Your stud should provide this as basic information, as long as they also use this same information for selecting their stud sires.

You are likely to find deviations from the average of the drop from which the rams came. This is due to environmental influences such as nutrition and management, which are not heritable. For example, if you are given a deviance of -1 micron, +0.5kg fleece weight and +2kg bodyweight for fibre diameter, the minus deviation figure indicates that the ram will breed progeny with wool finer than the flock average. A plus sign indicates that the ram will breed progeny with wool stronger than the average.

Fleece weight and bodyweight information can also be presented as percentages of the flock average, indicating how a ram performs relative to the average. Rams with fleece weights in excess of 100% will breed progeny that are heavier cutters than the progeny of an average ram. Those below 100% will breed lighter-cutting progeny.

Measuring fibre diameter

Average fibre diameter is the major price determinant of wool. The 2 ways that fibre diameter is measured, the coefficient of variation of fibre diameter (CVFD) and standard deviation (SD), can provide you with valuable information on the variability of fibre diameter of fleeces.

CVFD measures the relative spread of fibre diameter around the mean, expressed as a percentage. The lower the CVFD the more uniform the diameters of the individual fibres will be within the fleece.

SD is another measure of the degree of variation in fibre diameter around the average fibre diameter. The higher the standard deviation, the more variable is the fleece sample.


Curvature measures the crimp of fibre. Low-curvature wool is generally less than 50 degrees per mm (low crimp frequency), medium curvature 60–90 degrees and high curvature greater than 100 degrees (high crimp frequency). Current machines used to measure curvature cannot provide consistent or accurate results, so you should treat all curvature measurements sceptically.

Test dates

A critical factor when assessing the quality of a ram is the age at which wool tests are conducted on sale rams, and the amount of wool grown before the test.

The test will generally be more accurate the older the sheep and the longer the period of wool growth. Ideally, tests at 15+ months with 12 months' wool are best, but this is often not possible.

Tests at 9 months of age are of less use, as they will include wool grown under maternal environmental influence. Wool grown over a short period will reflect the specific seasonal effects of the animal and not accurately reflect the characteristics of the annual fleece.

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