|Genetic Evaluations and the ABC’s of ETA’s, PTA’s and PTI’s|
|Genetic evaluations for yield are calculated twice annually and annually for type traits. Yield evaluations are released to the dairy goat industry in August and December, and type evaluations are also released in December per the schedule kept by the Council on Dairy Cattle Breeding (CDCB). The genetic evaluations include relative’s information regardless of breed, registered or not, and regardless of the herdbook. It is done on the basis of identifying the animals in the pedigree. Pedigree information is sent to CDCB by ADGA and is constantly updated monthly. Yield evaluations require a registered sire. Type evaluations are done at the end of the year so that the current year’s scores can be used. Raw scores are uploaded by ADGA to CDCB where they are age adjusted. Yield information comes to CDCB from the record centers. The production records are standardized for use in the evaluations. Records must pass certain edits as well.
Once evaluation information is uploaded into ADGA database, ADGA can use the PTA values from the evaluation to re-calculate the index values which are the basis for Superior Genetics awards and calculate the ETAs. Read on!
ETA’s, which are the basis for screening young bucks in the ADGA-Sire Development Program (SDP) are estimates (E) of a young buck’s transmitting ability (TA) before he has daughters that are of an age to contribute information regarding yield and type.
ETA’s are based on the production and type information of his ancestors. Genetic evaluations are reported as (P)redicted (T)ransmitting (A)bilities (PTA’s). A PTA is the genetic merit that an animal is expected to contribute to its offspring and is based on milk records from the Dairy Herd Improvement program and from linear appraisal data from ADGA.
For young bucks, an ETA is the PTA without any daughter information being available.
The sources of information that contribute to this process, starting with generating PTA’s, are complex. They are statistically determined and then calculated through algorithms that were developed at USDA’s Animal Improvement Programs Laboratory (AIPL). These variables include such items as parent average and doe yield deviation (or for bucks,the average daughter yield deviation). Offspring evaluations are taken into account. The effects of management on the herd, herd-sire interaction which takes into account any differential use of sires across herds, and permanent environment are also weighted by factors dependent on lactation length, and are all included in the formula.
Type traits of dairy goats are evaluated with a multi-trait animal model, which adjusts the type record for age, the effect of herd appraisal date, the doe’s genetic merit, and the effect of a doe’s permanent environment. Like yield evaluations, type data are analyzed across breeds. However, the resulting PTA evaluations are not adjusted for breed averages and, therefore, are comparable among breeds. Yield however, is adjusted for breed on the basis of breed variation between milk and component characteristics and is comparable only within the breed.
Genetic evaluations of the PTA’s for yield and type are combined into a production-type index (PTI) that represents the economic merit of the traits evaluated for the animal. This calculation is done at ADGA and updated after every evaluation. Fat Corrected Milk (FCM), based on an accepted conversion formula for milk represents yield, and PTA for adjusted final score as discussed above represents type. Two PTI’s with differing weights are provided by ADGA to dairy goat producers: one with yield weighted twice as much as type and the other with type weighted twice as much as yield allowing the producer to choose the emphasis. PTI’s are a relative index; the numbers are relative to breed averages. It is important to remember that comparisons should not be made between breeds.
Applying the information from the genetic evaluations to the SDP requires the PTI information. In order for a doe to have a PTI, she must have at least one completed DHI production record and have been type evaluated. A buck must have at least 5 daughter records in from the DHI program and at least 3 daughters with type information. These daughters, and their resulting records, contribute to what is known as a Sire Summary for the buck. The Sire Summary summarizes the gathered information for both production and type.
For determining PTI’s, as mentioned, production is expressed in terms of fat corrected milk and will be corrected to each breed’s average fat percent. A Standard Deviation of FCM is also necessary which is also breed specific. A Standard Deviation is a statistic used as a measure of the dispersion or variation in a distribution of values. The PTI formula also requires a weighting factor for production or type, which is either 1 or 2. This is how the 2:1,1:2 ratios are generated as discussed above, that provides the user an emphasis on either yield or type.
For type, the PTA for final score is needed as well as the Standard Deviation for that PTA. The Standard Deviation is also breed specific.
Once the PTI’s are calculated, the ETA’s can be determined. This calculation uses the PTI’s and subtracts a Qualifying Level (QL) that is set for each breed. Each year, the QL for each breed is analyzed in respect to the ratio of bucks that are registered and have ETA’s, to the total doe registrations, then adjusted if necessary,creating a new QL for that breed.
For breeds with small numbers of animals being registered, the QL is likely the mean of the distribution curve. That is, half of the bucks that have ETA’s in that breed will qualify for the ADGA-SDP. That is not what is ultimately desired. The QL is raised to limit the number of bucks that qualify in the breeds with larger numbers of animals making the QL above the mean for these breeds and meeting the overall goal which is that the higher the QL can be set, the better, as it indicates a group of higher quality young bucks. Ideally, the goal is that qualifiers should rank in the top 15% or higher of bucks of their breed having ETA’s.