Ever walked into a pen of purebred bulls or replacement heifers that look pretty much the same in terms of quality and wonder how in the world you are going to accurately pick the best genetics?
You might have a copy of their weights, adjusted weights, ratios and maybe some other records that tell you exactly what the animals weighed on a certain day, what they would have weighed if they were adjusted to a common age, sex of calf, age of dam or which were the highest in their trait.
Now suppose you go to a second breeder and a third and they all have the same records and pretty much the same high quality cattle. How do you decide what head to buy?
In the early 1970s a group of progressive cattle breeders and their breed associations pushed for the development of a new method of evaluating the performance of their purebred cattle. They wanted this method to take into account differences due to ranches and environments to improve and increase the ease of genetic selection.
The new tool was called Expected Progeny Difference (EPD), and it was an estimate of the performance of the sire's calves compared to the performance of the calves of other sires in the breed.
It does not predict the actual level of performance, just the level and direction - positive or negative. EPDs are now calculated on all animals within a breed, including animals that have few or no progeny. EPDs cannot be compared across breeds without adjustment.
Because they are widely used within breeds, the genetic trend or average EPD for the breed generally increases each year. For that reason, the breed average for most traits is not zero. So, in a breed with an average EPD for weaning weight of +8 lbs., a bull with a weaning weight EPD of +20 lbs, is only 12 lbs. above the breed average, not 20 lbs.
Associated with every EPD is an accuracy (ACC) of the EPD. The ACC determines how reliably the estimated EPD reflects the animal's actual genetic value for the trait.
The higher the ACC (closer to 1.0) the better the estimate. ACC is affected primarily by the number and type of records. Progeny records are best for improving the ACC of the EPD. Cattle that have more records (of all types of relatives and associated traits) used in calculating their EPD will have higher ACC as well.
Most of the purebred breeds of beef cattle in the United States calculate EPDs on the animals registered with their associations. Most of these are calculated from weights and measurements reported by breeders when they register their cattle. Most associations have EPD numbers for birth weight, weaning weight and yearling weight.
These are estimates of the animal's genetics for growth at these points in their life. These are easy to measure. Most EPDs also calculate a milk or a maternal weaning weight. The milk estimate is the additional pounds of weaning weight produced by the daughters of the selected animals due to increased milk production.
This is reflected in the additional weaning weight of the daughter's calves. The maternal weaning weight, often listed as maternal or total maternal, is simply 1/2 of the weaning weight EPD added to the milk EPD. This reflects the fact that a cow influences the weaning weight of her calves through both milk production and genes passed to her calf for growth.
Today, many breed associations have increased the number of traits from which they calculate EPDs.
Scrotal circumference in bulls, first calf calving ease in heifers, frame size or hip height (as a measure of mature size), stayability (probability of a cow remaining in the herd until six years of age), carcass merit (carcass weight, muscling, marbling score, cutability, etc), and even tenderness are now calculated traits.
To select for improvement, it should be emphasized that you need to decide which traits are important to you in your marketing effort. How many calves you sell, what they weigh when you sell them, what they bring at the market, and how much they cost to produce will determine how much profit you make, so your major selection emphasis should be primarily on the traits that affect your bottom line.