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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.

INTRODUCTION TO SIL
Performance and physical appearance of animals are the result of a combination of factors; the
genes they get from their parents, management such as feeding, and other effects such as age
and rearing rank. To make genetic improvement we need to be able to assess how much of an
animal’s performance is controlled by the genes alone, as it is only the genetic component of
performance that is passed on to the next generation. Visual assessment of animals - or even
assessment on an individual animal’s performance - can be a poor guide to the genetic merit
of the animal.

SIL measures of genetic merit do not tell us everything about an animal. For example, in order
to function as a sire, a ram has to be physically sound. Visual inspection of soundness should
be used in conjunction with SIL figures to assess whether an animal will perform well as a sire
and will produce productive progeny.

SIL terminology can look confusing if you are not used to it. The following explanations of common
terms and variables can be used to interpret SIL data with reference to your breeding goals.

KEY DEFINITIONS
Estimated Breeding value (eBV): Breeding values are the best estimate of the animal’s genetic
value as a parent for traits. They allow rams to be compared with all rams included in the same
analysis. This may mean all rams from the same flock or all animals in an across-flock evaluation
where there are strong genetic links between the different flocks (see “Interpretation” below).

Estimated breeding values are expressed in the units the traits are measured, weight traits are
in kilograms, NLB in lambs per ewe lambing.

Indexes: An index provides a summary of overall economic breeding value 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 New Zealand Terminal Worth (NZTW)
index describes a system where all progeny are slaughtered. The New Zealand Maternal Worth
(NZMW) index describes a system where ewe lambs are retained as replacements for breeding
with surplus lambs going to carcass production.

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