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Understanding EPDs (Expected Progeny Differences)
Expected Progeny Differences, or EPDs, are genetic evaluation tools that predict the average performance of an animal’s future offspring compared to a breed-wide baseline. EPDs are the most powerful objective tools available for making breeding decisions in both dairy and meat goats. By understanding what EPDs measure and how to use them, producers can make consistent genetic progress toward their production goals.What Are EPDs?
An EPD is a statistical prediction of the genetic transmitting ability of an animal. In practical terms, an EPD tells you how the offspring of a particular sire or dam are expected to perform, on average, compared to the progeny of an average animal in the breed.A Simple Example
If Buck A has a weaning weight EPD of +5.0 pounds and Buck B has a weaning weight EPD of +1.0 pound, the expected difference in the average weaning weight of their offspring is 4.0 pounds (assuming they are mated to comparable does). Buck A’s kids are predicted to be, on average, 4 pounds heavier at weaning than Buck B’s kids.Key Principles
- EPDs are relative, not absolute. They predict differences between animals, not the actual weight or production level of offspring.
- EPDs are breed-specific. You cannot directly compare EPDs across different breeds because each breed has its own genetic base.
- EPDs account for environmental effects. The statistical models used to calculate EPDs remove the influence of management, nutrition, and environment so that only genetic merit is reflected.
- EPDs become more accurate as more data is collected on the animal and its relatives.
How EPDs Are Calculated
EPDs are generated through a statistical process called Best Linear Unbiased Prediction (BLUP), which uses animal model genetic evaluations.Data Sources
- Individual performance records: The animal’s own measurements (birth weight, weaning weight, milk production, etc.)
- Pedigree information: Performance of parents, grandparents, siblings, and other relatives
- Progeny records: Actual performance data from the animal’s offspring (the most valuable data)
- Genomic information: Where available, DNA marker data can enhance EPD accuracy, especially for young animals with limited progeny data
What the Models Account For
The statistical models adjust for known environmental factors including:- Contemporary group (animals raised under the same conditions)
- Age of dam effects
- Sex of offspring
- Season and year of birth
- Management group
Common EPDs for Meat Goats
Meat goat breed associations, including the American Boer Goat Association (ABGA), publish EPDs for several economically relevant traits.Growth Traits
- Birth Weight (BW): Predicts the average birth weight of offspring in pounds. Lower (or more negative) birth weight EPDs are generally desirable to reduce kidding difficulty, although extremely low birth weights can indicate low vigor.
- Weaning Weight (WW): Predicts the average 90-day adjusted weaning weight of offspring. Higher values indicate faster pre-weaning growth. This trait reflects both the kid’s growth genetics and the dam’s mothering ability (milk production, care).
- Post-Weaning Weight (PW) or Yearling Weight (YW): Predicts weight at a later age, typically 150 to 365 days. This trait is important for animals destined for market or retained as breeding stock.
Maternal Traits
- Maternal Weaning Weight (MWW): Predicts the weaning weight of offspring attributable to the maternal (milking and mothering) ability of the animal’s daughters. This is a key trait for selecting replacement does.
- Number Born (NB): Predicts litter size. Higher values indicate greater prolificacy, which is economically important but must be balanced against kid survival and doe maintenance requirements.
Carcass Traits
Some associations are developing EPDs for carcass merit traits including loin eye area, fat thickness, and body wall thickness. These are becoming increasingly important as the meat goat industry matures and moves toward value-based marketing.Common EPDs for Dairy Goats
The American Dairy Goat Association (ADGA) publishes genetic evaluations for dairy goats through the USDA Dairy Goat Genetic Evaluation program.Production Traits
- Milk (lbs): Predicts the average difference in total lactation milk yield of offspring. This is the most widely used dairy goat EPD.
- Fat (lbs): Predicts total pounds of milk fat produced. Important for cheese yield and milk pricing.
- Protein (lbs): Predicts total pounds of milk protein produced. Protein is the most economically valuable component of milk for cheesemakers.
- Fat Percentage (%): Predicts the percentage of fat in milk. Useful for producers marketing high-component milk.
- Protein Percentage (%): Predicts the percentage of protein in milk.
Combined Indexes
- Production Type Index (PTI): A multi-trait index combining production and type traits into a single selection tool. Higher PTI values indicate animals that are expected to produce more while maintaining functional type.
- Cheese Merit Index: Some evaluations include indexes specifically targeting cheese production profitability by weighting fat and protein yield.
Type Traits
ADGA publishes evaluations for linear type traits assessed during official appraisals, including:- Final Score: Overall type composite reflecting the animal’s structural correctness and functional capacity
- Mammary System: Udder attachment, texture, and capacity
- Dairy Character: Angularity and openness indicating dairy productivity
- Body Capacity: Barrel depth and width, reflecting feed intake ability
- Feet and Legs: Structural soundness for longevity
Reliability and Accuracy Values
Not all EPDs are created equal. The amount of data behind each EPD determines how much confidence you can place in it.What Is Reliability?
Reliability (sometimes expressed as accuracy) is a measure of the stability of an EPD. It indicates how likely the EPD is to change as more data becomes available. Reliability ranges from 0% to 99%.- Low reliability (below 40%): The EPD is based on limited data, typically pedigree and genomic information only. The EPD could change substantially as progeny data is collected.
- Moderate reliability (40% to 70%): Some progeny data is available. The EPD is becoming more stable but could still shift.
- High reliability (above 70%): Extensive progeny data supports the EPD. It is unlikely to change significantly. Proven sires with many recorded offspring typically have high reliability.
Practical Implications
- When comparing two animals, an EPD difference is more meaningful when both EPDs have moderate to high reliability
- A young buck with a high EPD but low reliability might ultimately prove to be average once progeny data is collected
- Conversely, a young buck with genomic-enhanced EPDs may offer accurate predictions even before progeny are measured
- For high-stakes breeding decisions (selecting a herd sire), prioritize animals with higher reliability values
Using EPDs in Sire Selection
Sire selection is the single most impactful genetic decision a producer makes, because a single buck can be mated to many does, amplifying his genetic influence across the herd.Define Your Breeding Objectives
Before examining EPDs, clearly define what you are trying to improve:- Meat producers: Prioritize weaning weight, post-weaning growth, and maternal traits
- Dairy producers: Prioritize milk, fat, and protein production along with udder conformation
- Seedstock producers: Balance growth or production traits with structural soundness and breed character
Compare Within Contemporary Groups
Always compare EPDs among animals evaluated on the same genetic base. Most breed associations publish percentile rank tables that allow you to see where an animal ranks relative to all evaluated animals in the breed.Avoid Single-Trait Selection
Selecting exclusively for one trait can cause unintended negative consequences in correlated traits. For example, selecting solely for maximum birth weight could increase kidding difficulty. Use multi-trait indexes when available, or balance multiple EPDs against each other.Practical Selection Strategy
- Set minimum thresholds: Establish minimum acceptable EPD values for your most important traits
- Rank candidates: Among animals meeting your minimums, rank by your primary trait of interest
- Consider reliability: When two animals have similar EPDs, prefer the one with higher reliability
- Evaluate the whole animal: EPDs predict genetic merit, but soundness, temperament, and adaptability to your environment also matter
- Avoid inbreeding: Check pedigrees to maintain genetic diversity in your herd
Breed Association Databases
EPD data is available through breed association databases and should be consulted before every breeding decision.American Dairy Goat Association (ADGA)
ADGA publishes genetic evaluations through its online database. Producers can look up individual animal EPDs, view breed percentile rankings, and access linear appraisal scores. The USDA cooperates in computing dairy goat genetic evaluations using DHIA production records.American Boer Goat Association (ABGA)
ABGA maintains a performance database with EPDs for growth and maternal traits. Animals with performance records and pedigree registration can have EPDs generated through the national genetic evaluation.Accessing the Data
- Register with the appropriate breed association
- Use online pedigree and performance databases to look up individual animal EPDs
- Request EPD summaries when purchasing breeding stock
- Review breed-wide EPD percentile tables to contextualize individual animal values