What are EPD's? (Estimated Progeny Differences)
By Mike Safley
WHAT ARE EPDs?
An EPD 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.
(See Table 12.2)
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 HERDSIRES BE COMPARED UNDER DIFFERENT MANAGEMENT CONDITIONS?
The calculation of EPDs uses data from many different flocks, 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.