Section 3.4: EPDs

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What are EPDs?

An EPD (expected progeny difference) is an estimate of the genetic merit of an animal for a single trait. The EPD is the expected difference between the performance of an animal's progeny for a specific trait and the average progeny performance, for that trait, of all the animals in the breed at the time the genetic evaluation program is established. Changes in mean EPDs over time quantify the long-term genetic improvement that has been achieved. 

How Are EPDs Calculated?

Alpaca breeders need to record the performance values for their animals. This information would be posted to a central database via a web-based tool. Most of the necessary information would be compiled from a fleece report or histogram, (Information graph) and then downloaded into a central database. The data would come from alpacas reared under many different management systems.

A computer software program then identifies the genetic linkages between the alpacas across years. The dataset for the alpacas will include all the data from previous years, for all the relatives, across generations. The EPD calculations include data from related traits, because an animal's performance in any trait gives information on how it will perform in a similar trait (for example, fleece weight and staple length). These calculations produce EPD values on every trait for every alpaca herdsire, dam and cria in the system. And these EPDs are recalculated, annually, after the performance records from each new production cycle are entered into the database.

EPDs are not constants; they are estimates of genetic merit that change over time as new information accumulates on an animal and its relatives. Thus each year results in a new set of EPDs for all animals, with progressive increases in the accuracy of the estimates and in the breeders’ ability to discriminate among prospective breeding animals.

How Are EPDs Reported?

An EPD is reported in the normal units for the trait, such as +0.75 pounds (for fleece weights) or -0.4 microns (for fiber diameter). It's important to note that an EPD value is not a ratio or an index. EPDs are expressed as deviations (+ or -) from the average population value, which is considered to be zero. This average of the herd’s performance, which is used to establish the baseline (zero) for comparison purposes, might be recalculated periodically, probably every five years. EPDs always have a positive (+) or negative (-) sign in front of them.

The positive and negative symbols don't always mean better or worse--it depends on the particular trait. For example, a fleece weight of +0.75 pounds is good but a Fiber Diameter EPD of -0.3 microns can also be good, i.e. finer fiber. These EPDs are used to compare herdsires, for instance, a stud with a fleece weight EPD of +3.0 is good, but a different stud with a fleece weight EPD of +4.0 is better. EPDs may take a little getting used to, but once you get the hang of them, they give the most objective and reliable estimation of genetic value possible.

All of the individual records that are collected by the entity processing the data, usually a university, are kept confidential. The EPDs for individual animals are reported to the owners. The collective results are made available to the participating breeders in the form of genetic evaluation reports.

Can Alpaca EPDs Be Compared Across Different Conditions

The calculation of EPDs uses data from many different herds, and this procedure is mathematically valid across herds, so long as none of the herds is genetically isolated from the others. But the feed regimen at all farms must be good enough to permit good performance.

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