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Sub-indexes are sometimes reported. These refer to the economic value for traits of a particular
type –e.g. Lamb Growth vs. Wool vs. Reproduction vs. Meat Yield (carcass quality).
All SIL indexes are expressed as cents per ewe lambing
INTERPRETATION
eBVs and selection indexes are a function of the population in which they are calculated. In
all cases they are comparable within a single flock.
For every analysis SIL sets 1995 as the benchmark year and the average genetic merit of
lambs born in 1995 is set to zero. Therefore, eBVs are measures of genetic merit for a ram
compared to the 1995 average for that evaluation. If two flocks had different average genetic
merit in 1995 the eBVs would have to be adjusted for the difference to make them comparable
When selecting a ram that is high genetic merit it is important to first select the flock the ram
was bred in. The ram should rank highly within that flock for the key traits you are interested in.
Index values provide the best guide to overall genetic merit across traits.
In summary, whichever flock you decide to buy a ram from, be sure to select one that is highly
ranked by SIL. This will guarantee improved genetic merit in your flock compared to using
rams selected without an accurate assessment of genetic merit.
EXPLANATIONS FOR COMMONLY USED SIL DATA
Abbreviations Definition
eBV eBVs (estimated breeding values) are expressed in the units the trait is
measured in, and are an estimate of the value of the ram’s genetic merit
for the trait
eBV Accuracy Indicates the amount of information used in the prediction of the eBV. Traits
that are more heritable and more related to other traits they are predicted
from have higher accuracy.
WWT eBV Weaning weight – “lamb growth”
LW8 eBV Body weight at 8 months of age – rates animals for post-weaning growth
FATY eBV (Whole carcass) Fat yield – kg per kg of carcass
EMA eBV Eye muscle area
EMAc eBV Eye muscle area corrected for carcass weight – a measure of muscularity
FAT eBV Weight of fat in carcass
FD eBV Fat depth ‘C’
NLB eBV Number of lambs born
FW12 eBV Fleece weight at 12 months of age
NZTW index New Zealand Terminal Worth is expressed in cents and is an estimate of
(previously TSO) genetic value for growth (TSG), meat yield (TSM) and survival (TSS).
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