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Genetic Progress Home SIREMAX At Siremax® we know what’s going on inside the sheep ® The Leader in Lean-Growth Genetics

In 2006, Cindy Wolf (Sheep Improvement Company) entered four Siremax ram lambs into the Wisconsin Ram Test for March born lambs.

Click below to see the results (opens in a PDF)


An Expected Progeny Difference (EPD) is an estimate of an animal’s true breeding (or genetic) value.

EPDs provide an objective estimate of the genes an animal carries for a range of traits of economic importance.

PWWT estimates the genetic difference in liveweight (pounds) between animals adjusted to 225 days of age. Weight taken at scanning.

PFAT estimates the genetic difference in fat depth (inches) between animals, adjusted to 100 pounds liveweight. Fat measurements taken at the 12th rib.

PEMD estimates the genetic difference in eye muscle depth (inches) between animals, adjusted to 100 pounds liveweight. Ribeye measurements taken at the 12th rib.

Selection differential is the difference between the average genetic merit of two selected parents and the average genetic merit of the flock from which they come.

Selection differential is our primary tool to drive genetic improvement.

EPDs do not necessarily reflect the animal’s observed performance, which is a combination of both genetic and environmental influences.

EPDs are estimates of the genetic component of that performance.


The goal of SIREMAX is to produce composite terminal sires that are superior to established terminal sire breeds in lean growth, commercial fitness and longevity, and whose selection is based on objective performance recording and BLUP based analysis. BLUP (Best Linear Unbiased Prediction) is the statistical model used by Lambplan to calculate the genetic value of an animal for the trait being evaluated.

Selecting breeding sires is our most important activity, whether the rams are intended for our use or for sale.

The activities that we engage in can be called genetic technology, that is the time spent identifying sheep, setting up breeding groups, assembling data, conducting Lambplan analysis etc.. This genetic technology is based on a branch of science called population genetics - the study of genetics in groups of animals; small groups and large groups.

The technology that Siremax uses is primarily concerned with improving the average performance of the whole Siremax flock, and each new lamb crop. We do identify top ewes, rams, and lambs whenever we sort by index or trait, and we identify the bottom end too. However, we are not in the business of making elite sheep, or seeking blue ribbons for 'best-in-show'.

What we do is illustrated best by this edited extract from the Lambplan Breeder's Guide. The complete Breeder's Guide can be found online here.

To calculate EPDs, all the data we gather on a lamb’s individual performance are submitted to the Lambplan database, and then combined with data from related lambs in all Siremax locations, and in current and all previous generations.

Lambplan provides an analysis of all these data in the form of Estimates of Breeding Value, which Siremax translates into Expected Progeny Differences (EPDs), the version that is more familiar to US livestock producers.

An EPD is an estimate of an animal’s genetic value as a parent, expressed as the expected change in performance of its sons and daughters.

EPDs are sent back to Siremax for each trait we measure: Post Weaning Weight, Back Fat, and Ribeye Depth, for each lamb.

The EPDs for these traits are then combined into Lambplan’s Carcase Plus index that is calculated to meet our breeding objectives.


These genetic technologies will gain credibility when other breeds adopt them. We would be glad to help composite and purebred terminal sire breeders get started.

BWT estimates the genetic difference in liveweight (pounds) between animals at birth. Since the Carcase Plus selection index does not include BWT, the average EPD  for birth weight has only increased by ¼lb from 2001 to 2010.

Until 2010 the emphasis placed on Post Weaning Weight (PWWT) was 60% of the index, the emphasis on reduction in back fat (PFAT) was 20% of the index, and the emphasis on ribeye muscle depth (PEMD) was also 20% of the index. In 2011 the make-up of the Carcase Plus index has changed to 65% PWT, 5% PFAT, 30% PEMD.


Lambplan conducts data runs for terminal breeds on the 1st and 15th of every month. We try to submit our data in time for the October 1st deadline, and we usually receive our analysis inside a week. This gives us a month to decide which rams we will use inside Siremax, which ewe lambs we will retain, which ram lambs we will grow out for sale as breeding rams, and which we will sell as market lambs. More urgently, we have to decide which rams we will move around the country to link our flocks, and allow time to move them, and let them settle.

Lambplan returns our analysis as PDF files and as a password protected spreadsheet on the Lambplan website. Our Webmaster, Oliver Essame, exports these data into the Siremax database, which he maintains as a Siremax resource. We extract all kinds of information from this database. Examples can be found HERE.

We can only give you a glimpse of the database here, but closer scrutiny would show that not all lambs are superior. However, the performance of the worst lambs is as important as the performance of the best. Every lamb that provides a complete lambing and scanning record helps to build the four pillars that support the integrity of the Siremax project. These are:

1 Performance

A huge benefit of the Lambplan’s statistical model is that it works. It is not just a record keeping system; it is a procedure that ensures genetic improvement. Simply put, if you obey the rules, genetic improvement will happen. When the selection of breeding stock is based on EPDs, genetic improvement is certain, cumulative and permanent.

2 Accuracy

You can calculate EPDs without reference to related sheep, for example for just one ewe and her lambs, but they would not be useful. Over a lifetime the same ewe could have 12 lambs, and her sisters could have 100 lambs, and her father could sire another 100 lambs. Lambplan uses the performance of all these lambs, and their progeny, to calculate genetic value (EPDs) for this ewe and her lambs. This is useful. The more records that contribute to one EPD, the more accurate it will be.

3 Honesty

From time to time, we are asked for assurance that we don’t cook the books, presumably by submitting data about an animal that are less than completely accurate. This can’t be done. At the most, only about a third of the information that goes into an EPD comes from the lamb itself. The other two thirds come from its relatives. Imagine the ripple effect of submitting inaccurate data. It wouldn’t be cooking the books, it would be causing the kettle to boil dry.

4 Confidence

There is nothing more discouraging than being shown a pen of good looking ram lambs, all the same size and condition, and there is nothing to tell us which one to buy. We have all been there. We need the same window sticker that you find when you look for a new pickup. EPDs provide this information and future performance proves their value.