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UNDERSTANDING SIL INFORMATION

SIL (Sheep Improvement Ltd) is the New Zealand sheep industry’s performance recording
and genetic evaluation database. SIL estimated breeding value (eBV) and selection index
information is provided in some sale catalogue listings. These assist ram selection by giving the
best estimate of genetic merit available.

Breeding values
Breeding values (BVs) are the best estimate of the animal’s genetic merit for traits. They allow
rams to be compared, with all rams included in the same analysis. Mostly, this means all rams
from the same flock, but it can be used to compare animals from different flocks, if the analysis
has been performed across these flocks.
Breeding values are expressed in the units the traits are measured. For example, weight traits
are in kilograms.

Indexes
An index provides a summary of overall economic breeding merit of animals. The economic
value of each trait is summed to give a total economic value of the ram’s genetics. The
economic value of a trait depends on the production system; therefore a number of indexes are
used to describe different systems. For example, the terminal sire index (NZTW) describes a
system where all progeny are slaughtered.
All SIL indexes are expressed as cents per ewe lambing.

Interpretation
BVs and indexes are a function of the population in which they are calculated. In most cases,
they are only comparable within a single flock. Although across-flock analysis can allow values
to be compared between several flocks, provided there are good genetic connections between
the flocks.
It is not possible to compare the genetic merit of rams from different flocks, unless the flocks
have conducted an across- flock analysis and there are good connections between the flocks.
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, the BVs are a measure of the merit of a ram
compared to the 1995 average for that flock. If two flocks had different average genetic merit in
1995, the BVs would have to be adjusted for the difference to make them comparable.

The New Zealand Terminal Worth Index is expressed in cents per lamb
NZTW index born for terminal sheep, based on lamb growth (TSG), meat yield (TSM)
and lamb survival (TSS).
gBV: the breeding value expressed in units the trait is measured in
Actual: The actual value of the trait recorded.
Note: Blank cells indicate there is no value for the animal. Only performance-recorded rams
with data available in the NZGE, can generate a value.
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