Genetic Evaluation

Each EPD reported is accompanied with an Accuracy (ACC) value. ACC is a measure of reliability regarding the EPD evaluation for a performance trait. Accuracy is reported as a decimal number between zero and one: large values indicate greater accuracy and more certainty the EPD will show little change as additional progeny information is obtained.

Birth Weight EPD (BW) The expected difference in average birth weight (pounds) of progeny. Birth weight reflects prenatal growth.

Calving Ease Direct (CE) is expressed as a difference in percentage of unassisted births in first calf heifers. A higher value indicates greater calving ease. It predicts the average difference in unassisted births with which a sire's calves will be born when bred to first-calf heifers.

Calving Ease Maternal (MCE) is expressed as a difference in percentage of unassisted births in first calf daughters. A higher value indicates greater calving ease. It predicts the average difference in unassisted births with which a sire's daughters will calve as first-calf heifers when compared to daughters of other sires.

Weaning Weight EPD (WW) The expected difference in average weaning weight of calves. The evaluation reflects the genetic influence on pre-weaning growth rate.

Yearling Weight EPD (YW) The expected difference in average yearling weight of progeny. The evaluation reflects genetic influence on both pre-weaning and post-weaning growth rate.

Maternal Milk EPD (Milk) The genetic ability of a sire’s or dam’s daughters to express in pounds of weaning weight in her calves due to her maternal ability through mothering instinct and milk.

Total Maternal EPD (MTL) A value to predict the weaning weight performance of calves from a animal’s daughters due to genetics for growth and maternal ability. Total Maternal is calculated by adding ½ the WW EPD to the Maternal Milk EPD.

Scrotal Circumference EPD (SC) The expected difference in scrotal circumference (expressed in centimeters) of a bull’s or dam’s male offspring at yearling compared to progeny of all other animals evaluated. Research has also indicated a relationship between increased SC EPD and decreased age at puberty for daughters.

Carcass records are adjusted to an age constant endpoint. Therefore selection based on any or all of the carcass merit EPD are comparable among cattle at the same age endpoint. For example selection based on increased EPD for carcass weight will result in heavier carcass weights than those animals with lower EPD for carcass weights when the cattle are harvested at the same age.

Carcass Weight EPD (CW) Expected progeny differences for Carcass Weight is a predictor of pounds of retail product at a constant age endpoint. Selection for increased values should result in heavier carcasses, while selection for decreased values should result in lighter carcass weights at the same age endpoint. Carcass Weight EPD are expressed in pounds and is a predictor of the differences in hot carcass weight between parents progeny at an age constant endpoint.

Ribeye Area EPD (REA) Ribeye area is measured from a cross-sectional area of the longissimus dorsi muscle at the 12th rib. Ribeye area is a major component of the USDA yield grade equation and selection for increased ribeye area should result in larger ribeyes and lower yield grades between animals with the same carcass weight. Ribeye area has a positive relationship with weight, the larger the animal the larger the ribeye area. Ribeye Area EPD are expressed in square inches and is a predictor of differences in ribeye area between parents progeny at a constant age endpoint.

Fat Thickness EPD (FAT) Fat thickness is measured at the 12th rib and is the primary component to the USDA Yield Grade equation. Fat thickness has a negative relationship to cutability; therefore, selection base on decreased fat thickness should result in lower yield grades and leaner cattle given the same age endpoint. Fat Thickness EPD are expressed in inches and are a predictor of differences in fat thickness between parent’s progeny at an age constant endpoint.

Marbling Score (MARB) Marbling is a subjective measure of the amount of intramuscular fat in the ribeye muscle. Marbling score is the primary component of USDA Quality grade and selection for increased Marbling Score EPD should result in cattle with higher quality grades at the same age endpoints. Marbling score has a small genetic correlation with fat, therefore producers may select for increased marbling score EPD while not changing external fat thickness when cattle are harvested at the same age-constant endpoint. Marbling EPD is a prediction of the differences in the USDA subjective marbling score between parent’s progeny at an age constant endpoint. Marbling is expressed in the same units as the USDA Marbling Score (see table below), see the accompanying table for USDA Marbling Score in the Carcass Section.

Terminal Sire Index (TSI) The AICA Terminal Sire Index (TSI) is a formal method of combing Expected Progeny Differences (EPD) – BW, WW, YW, REA, CW, MARB and FAT – into one single value on which to base selection decisions. The TSI uses estimates of the genetic relationships between traits with an economic default value based on three year rolling USDA data.

The TSI represents a dollar index per terminal progeny produced for a bull in the AICA database, ranking them for profit potential. This dollar index is to be interpreted much like single trait EPD. For example, if Sire A’s index is $191.66 and Sire B’s index is $200.00, then we would expect Sire B’s offspring to average $8.34 more net return ($200.00 minus $191.66) than Sire A’s offspring.

Heritibility may be defined as the proportion of the observed phenotypic variation that is due to genetic variation. For example, when analyzing a group of Charolais calves from the same sire, one would expect some variation in the weaning weights. Since weaning weight is 25 percent heritable, 25 percent of the observed variation is attributable to genetics while the remaining 75 percent of the observed variation is due to environmental influences.

Traits with low heritability estimates are influenced more by environment than by genetics, thus genetic progress from selection may be slow. Traits with low heritability respond greater to the effects of crossbreeding. Since heritibility is already an integral part of the EPD calculation, EPD reflect actual differences and require no adjustment.

Accuracy values give us an indication of how close our estimates are to an animal’s true genetic value. Accuracy values are extremely useful to breeders in determining the reliability of an EPD. An accuracy value can range from 0.0 to 1.0, depending on the amount of information that is known about an animal for any one of the reported traits. As the amount of information included in the analysis of a trait for an animal increases, the accuracy value for that trait increases accordingly.Table 1 shows the standard error of prediction (possible change value for an EPD) at various levels of accuracy for the traits reported. You will notice that as accuracy level increases, the possible change value decreases. Still, an EPD can change from year to year even though it may have a high level of accuracy. The point to remember is that the expected change of an EPD with a high ACC is correspondingly less than those of an EPD with a lower ACC value.

The possible change of identical EPD, given different levels of accuracy can be seen in the example. Two sires have YW EPD of +30, but different ACC values. This figure illustrates the possible range within where the true genetic value is. Sire A has an ACC of .40 with a possible change value of 23 pounds, and Sire B has an ACC of .80 with a possible change value of 8.5 pounds.