Page 8 - Stud Ram and Ewe Fair 2020
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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.
       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.
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