EPDs (Expected Progeny Differences) are commonly used by cow/calf operators to select herd sires either to buy semen for artificial insemination or bulls to breed cows naturally. Even though EPDs are an accepted herd sire purchasing tool, their definitions, correct use and significance are not always clearly understood. To be reliable, EPDs should be compared between animals of the same breed. In other words, EPDs do not provide accurate comparisons between an Angus and Hereford bull. To use EPDs successfully, a producer needs a herd breeding plan and a clear understanding of the desired characteristics of his cattle.
Scott Greiner, Virginia Tech University explained, “EPDs estimate an animal’s genetic values and are calculated with performance data from the individual, its ancestors and relatives. Breed associations collect performance data, calculate EPDs using complex statistical equations and models and publish the results in annual sire summaries.”
“Most breed associations report birth weight, weaning weight, yearling weight and milk EPDs,” wrote Stephen Hammack and Joe Paschal of Texas AgriLife Extension. “Other reported traits might include direct calving ease, gestation length and scrotal circumference.”
Birth weight is an important EPD because most calving difficulty is caused by delivery of heavy calves. Considerable interest should be placed on birth weight EPDs when selecting bulls for use on heifers.
Greiner gives an example of birth weight EPD interpretation. Assume that Bull A has a birth weight EPD of +5 and that Bull B has +1. According to this data, Bull A is expected to sire calves that average four pounds heavier at birth than calves sired by Bull B.
“Weaning and yearling weight EPDs are indicators of growth genes that will be passed from an animal to its progeny,” wrote Greiner. “Weaning weight EPDs are a prediction of calf weights for a standard weaning age of 205 days. Rapid early growth is an important selection criterion for cow-calf producers since feeder cattle are sold by the pound. Yearling weight EPDs predict calf weights at a year of age or 365 days and is the most useful growth rate indicator for feedyards. This EPD indicates how long it will take an animal to reach a good harvest weight.”
“Milk EPDs reflect milking ability of an animal’s daughters,” Greiner further stated. This difference in milking ability is expressed as additional pounds of weaned calf. Milk EPDs are important in bull selection when replacements are retained in the herd. Bull selection should be based on milk EPDs that match feed resources and operation environment. More milk is not necessarily better since heavier milking cows may require more nutritional inputs to maintain body condition and reproductive efficiency.”
“Direct calving ease is the percentage or ratio of unassisted births in relation to total births,” stated Hammack and Paschal. “This EPD depends primarily on the size of the calf. If it is available, the direct calving ease EPD should be emphasized instead of birth weight. Birth weight only indirectly estimates calving ease.”
Gestation length EPD is reported by some breed associations and is defined by Greiner as a prediction of difference in gestation length in days for a bull’s progeny. “Bulls with lower gestation length EPDs are expected to sire calves that are born earlier. Shorter gestation lengths have been associated with slight decreases in birth weights and improved calving ease.”
“Scrotal circumference EPD is expressed in centimeters and predicts differences that will be passed on to progeny,” stated Greiner. “Bulls with larger scrotal circumference EPDs are expected to sire daughters that reach puberty at an earlier age and have earlier calving dates. Scrotal circumference is also an indicator of semen quantity produced by bulls.”
Only a few of the common EPDs are discussed here. Others will be highlighted at a later date.
Which EPDs do you find helpful in your bull selection? Let us know by typing your comment into the “Share your thoughts” area below.