S.I.L - The Answers

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Question 1.

Why is this small ram rated more highly for growth than that large ram I want to buy? Iíd never buy a small ram when growth of my lambs is so important.

Answer - Does actual size matter? It is reasonable to reject a ram with a physical fault. After all, he must work to deliver you income and profit. However, the actual size of an animal is not the best measure of genetic merit for growth we can use. You want to purchase the ram that will sire the fastest growing lambs.

Better measures are breeding values (BV) for bodyweights or the S.I.L sub-index (Goal Trait Group) for growth. These removes the influence of factors we know affect growth but are not genetic. For example, if we were looking at ram hoggets we would expect the sons of older ewes, born as singles, early in the season, to be larger than the sons of young ewes, born as multiples, late in the season. S.I.L corrects for these effects when estimating genetic merit. Another way of looking at it, is that a large animal will give you more meat to eat, but it may not give you more meaty lambs if you breed from it. We need to make breeding decisions, not eating decisions when we select rams!

S.I.L also takes account of the performance of relatives. If you had a choice of two animals with similar performance, you would be better to pick one from a high performance family than one from an average performance family. S.I.L adjusts the estimate of genetic merit on the basis of how ALL known relatives perform as well as an animalís performance in related traits. This measure of genetic merit is then of maximum accuracy for the data available.

Donít be put off by actual size. Small rams that are well bred, with genetic records to prove this, are a much safer bet than larger, well grown rams without records.

So to make fast gains in performance of your flock, select rams on the best measure of genetic merit available. For most buyers the simplest figure to use is the S.I.L sub-index for Growth, which summarises all known sources of relevant information.

- Sourced from Sheep Improvement Ltd website - FAQ 2


I want to improve lambing percentage and I only buy twin-born rams. Someone told me this was wrong. How can a single-born ram be any good?

Answer - Let us first look at an example. A very productive ewe may have a single lamb one year. What reason do we have to think that this lamb is any better or worse, genetically, than the multiple-born lambs this ewe had in previous years? The fact that this ewe has a single has little to do with the genes she passed to that lamb. All her lambs, single-born or otherwise, have an equal chance of inheriting her ďreproductionĒ or litter size genes.

Buying only twin-born rams has been used in the past to lift lambing percentage. It will work but gains are very slow compared to using the more accurate methods we now have AND, most significantly, it can severely restrict gains in other traits.

It is well known that reproductive rate (litter size or lambing percentage) is lowly inherited. In other words, a lot of the variation we see is not genetic. In this case, a small number of measurements, such as the ewes own lambing string (e.g. .2132) is not a good measure of genetic merit. S.I.L increases accuracy in the assessment of genetic merit, by using ALL available information on an animal and all itís relatives. It is a better bet to select an animal that is average but from a good family, than a good animal from an average family. This is particularly true for lowly heritable traits like reproduction.

An animalís own birth rank is just one bit of information. You will make faster improvement in lambing percentage (reproduction) if you select on an S.I.L measure of genetic merit for reproductive rate. S.I.L offers two measures, the breeding value for number of lambs born (NLB) and the sub-index (Goal Trait Group) for reproduction which is this breeding value multiplied by an economic weighting (cents per ewe lambing). S.I.L was set up to deliver these modern, accurate methods for you to use in ram selection.

Restricting yourself to the purchase of only multiple-born (or only twin-born) rams will mean you miss out on some high genetic merit animals that are born as singles and some of these may have high genetic merit for reproduction if you look at the S.I.L measure of genetic merit for this. In some cases, by selecting a twin-born animal you may be choosing one that is actually of low genetic merit for reproduction! You also reduce your selection options across all traits by rejecting single born animals, which may be very good in other traits. This is a big loss of potential gain.

Single trait selection is seldom a good idea. Farm income comes from a variety of sources so different combinations of performance traits can give the same income or profit. S.I.L takes account of this by using economic indexes, which rate animals for overall genetic merit, across key productive traits, in dollar (or cent) terms. Selecting on this index is usually the best way to improve genetic merit in your flock. You should ignore the actual birth rank of the animals.

To place extra emphasis on improving lambing percentage, S.I.L recommends that you select on overall index (the total package) first and then reject any animals with low values for the NLB breeding value or the Reproduction sub-index (Goal Trait Group).

- Sourced from Sheep Improvement Ltd website - FAQ 1

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Last revised: September 20, 2013