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What are Estimated Breeding Values (EBVs)?
An Estimated Breeding Value (EBV) is an estimate of the genetic worth, or merit, of an animal for a particular trait. It can be thought of as a picture of an animal's genes for that trait.
Examples of EBVs are:
CFW Clean Fleece Weight (kg)
BWT Body Weight (kg)
FD Fibre Diameter (micron)
FEC Faecal Egg Count
The Clean Fleece Weight and Body Weight EBVs are expressed in the units of measurement of the traits, eg, kgs of wool or kgs of liveweight. Fibre diameter EBVs are expressed in microns as a deviation from the average. FEC EBVs are an estimate of an animal's resistance to internal parasites. The more negative the FEC EBV, the more resistant to worms the progeny of that animal will be.
EBVs are a more accurate indicator of an animal's relative genetic merit for a trait than the raw data because they take into account:
· the heritability of the trait, ie, how much of the superiority is actually due to the animal's genes and can be passed on to its progeny;
· the number of relatives in the data set and their degree of relationship to the animal of interest;
· the measurements of other traits. Where two traits are affected by the same genes (ie, the traits are genetically correlated) the progeny records for both traits give us additional information to make the EBVs for both traits more accurate.
· Non-genetic, or environmental effects. These are factors that influence performance but are not passed on to the progeny. A simple example is that twins tend to be smaller (on average) and cut less wool than single-born lambs: This is not because they have poorer genes for body weight or fleece weight, but because they have had to share their dam's uterus (maternal nutrition) and milk supply (pre-weaning nutrition) with another lamb. Their environment has not (on average) been as good as that experienced by single lambs - this is a non-genetic influence that we need to account for in getting an accurate picture of the value of the genes.
Examples of using EBVs
EBV CFW (kg) EBV FD
Ram 1 +0.8 -1.2
Ram 2 +0.1 +0.8
Ram 1 has an EBV for clean fleece weight of +0.8 kg. That is, the progeny of Ram 1 are expected to be 0.35kg superior (0.8 - 0.1, divided by 2, as half of the genes come from the ram, half from the ewe) for clean fleece weight than the progeny of Ram 2 with an EBV of 0.1 kg.
Similarly, Ram 1 has an EBV for Fibre Diameter of -1.2m. Ram 2 has an EBV for Fibre Diameter of -0.8m. The progeny of Ram 1 are expected to be 1 micron finer (-1.2 - 0.8, divided by 2) than the progeny of Ram 2.
Breeding Objectives and Index Values
The breeding objective is what you want your breeding program to achieve. You might want to increase fleece weight and decrease micron both at the same time or you might want to decrease micron as much as possible without losing clean fleece weight. An index can be calculated to achieve your desired goals.
Indexes are just a way of determining which animals most closely match your breeding objective. Any index combines the performance of an animal for two or more traits into one number. The user of the index must be confident that the balance used to combine traits accurately reflects the breeding objective of the flock they are making selections for.
The index makes selection much simpler and more accurate. Without an index, the person making the selections in the yards must consistently balance the performance for a number of measured traits in their head, as well as assessing and balancing other visual assessed traits for each sheep.
Three different breeding objectives commonly used in the wool industry are:
Breeding Objective or Aim
Index Micron Premium Near maximum increase in fleece weight
3%MP 3% Reduce fibre diameter and increase fleece weight
6%MP 6% Greater reduction in diameter and maintain fleece weight
12%MP 12%
The 3% micron premium index ranks animals with high fleece weights more highly. It is valuable for those breeders who wish to maintain their fibre diameter and place maximum emphasis on increasing the fleece weight of their flock. The 12% micron premium index is useful for breeders who wish to place maximum emphasis on decreasing their flock fibre diameter, without losing fleece weight. A middle view is to use the 6% micron premium index which simultaneously increases fleece weight and decreases fibre diameter.
Explanation of Micron Premium
The micron premium tells you how much the price of wool increases if the fibre diameter decreases by one micron.
For example, what is an 8% micron premium ? If 20µ wool is worth $8.00/Kg clean, then 19µ wool is worth 8% more, or 1.08 x $8.00 = $8.64/kg.
For further explanation, please ask Sue Jarvis Phone 03 55 743298
