The Past NSIP used EPDs
NSIP used to use EPDs. This is their definition. I have just received (as of June 20, 2011)new information from David Notter with NSIP EBV Descriptions.
Expected Progeny Difference (EPD) is an estimate of the genetic merit of an animal for a single trait. Specifically, the EPD of an animal is the expected difference between the performance of that animal’s progeny and the average progeny performance of all the animals in the breed, for that trait.
How are EPDs Calculated?
NSIP collects recorded performance data from purebred producers such as weights, number of lambs born, wool characteristics, etc. This information is entered into an Excel Spreadsheet provided by NSIP. The spreadsheets are sent to the Data Coordinator via e-mail.
Each breed group has a Data Coordinator who checks the data for errors and forwards the spreadsheets to the NSIP computer. For each breed, NSIP collects these performance records from purebred flocks across the country, breed by breed. This data comes from sheep reared under many different management systems, year after year after year.
The NSIP computer then identifies the genetic linkages between these flocks and across years—like when rams are sold or traded, or when progeny are distributed into many flocks—and puts this data into one massive calculation for each breed. The NSIP dataset for a breed also includes all the data from previous years, for all the relatives, across generations. The EPD calculations even 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, a good preweaning weight for a fast-growing lamb suggests it will also have a good postweaning weight). These calculations produce EPD values on every trait for every ram, ewe, and lamb in the system. And these EPDs are recalculated annually (or more often for accelerated flocks), after the performance records from each new production cycle are entered into the computer.
How are EPDs reported?
An EPD is reported in the normal units of a trait, such as +0.5 pounds (for weights) or -0.3 microns (for wool 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. Therefore, 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 trait. For example, an Weaning Weight EPD of +0.5 pounds is good (i.e. more weight of lamb at weaning), but an Fiber Diameter EPD of -0.3 microns can also be good (i.e. smaller diameter fiber, which is more valuable to fine wool producers). Not only can we compare sheep with positive and negative EPDs, but we can also use EPD values to compare animals who both have positive EPDs. For example, a ram with a Weaning Weight EPD of +1.0 is good, but a different ram with a Weaning Weight EPD of +2.0 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.
What Traits Does NSIP Evaluate?
Maternal Traits – NSIP evaluates all individual animals within a flock for three very important maternal traits: (1) number of lambs born per ewe lambing, and (2) maternal milk, and (3) Milk+Growth. To obtain an accurate evaluation of genetic merit for each of these traits, producers record information on all ewes exposed for breeding and all lambs born in each production cycle.
Growth Traits – NSIP evaluates growth for three possible weights: weaning weight, postweaning weight, and yearling weight. Farm flocks and range flocks are analyzed differently because their weighing schedules are so different. Farm flocks receive 60-day weaning weights and 120-day postweaning weights. For farm flocks, the cutoff point between weaning weight and postweaning weight is 90 days. Range flocks receive 120-day weaning weights and yearling weights. Some range flocks also chose to take 60-day preweaning weights, and those weights are used in their genetic analysis. NSIP accepts generous time windows around each age to weigh lambs, so that any flock can arrange convenient weigh dates to fit its management schedule.
Wool Traits – NSIP calculates EPDs on three wool traits: grease fleece weight, fiber diameter, and fiber length. These measurements only need to be taken once during an animal’s lifetime, usually at a year of age. These measurements must be taken on a full year’s growth of wool. A producer can also record codes for face cover and skin folds, although no EPDs are calculated on these traits.
Carcass Traits – Carcass traits are still under development but will be incorporated into NSIP very soon. These traits will be fat thickness, ribeye area, and an index trait called the “Carcass Value Trait”, which will be calculated from the age of a lamb, the weight of the lamb, ribeye area, and fat depth between the 12th and 13th ribs. Producers will record values for these traits either from direct measurement of the carcass or from ultrasound measurements on the live animal.
Accelerated Traits – For those breeds using accelerated lambing systems, NSIP is working on two important traits: date of first lambing, and lambing interval. Currently, implementation of these two traits are still under development.