GENETIC EVALUATION OF DAIRY CATTLE IN THE USA
The genetic evaluation of dairy sires began in the United States in 1935.
In 1974, the Modified Contemporary Comparison (MCC) index was implemented as an improved method of evaluating animals.Every six months, in January and July, the United States Department of Agriculture calculates the genetic values of cows and bulls whose records are collected by the Dairy Herd Improvement (DHI) programs.
Since July 1989, the Predicted Transmitting Abilities (PTA) of cows and bulls have been computed using a statistical procedure called the “Animal Model.” In the animal model, males and females are evaluated at the same time using an extremely powerful computer.
TRAITS EVALUATED
The five production traits for which the evaluation is performed are:
1) Milk yield;
2) Fat yield;
3) Protein yield;
4) Fat percentage;
5) Protein percentage.
In addition, conformation traits (primarily udder, feet and legs, body form, and dairy capacity) are also evaluated. Genetic evaluation of bulls for productive life (life expectancy of daughters in the herd) and somatic cell score (a measure of resistance to mastitis infections) have been available in the United States since January 1994.
PREDICTED TRANSMITTING ABILITY
Transmitting ability is the average genetic value for a certain trait that an animal transmits to its offspring. The transmitting ability of a trait may be calculated with a certain degree of confidence (called reliability) using three sources of information:
1) The genetic merit of the parent;
2) Performance of the animal itself (when applicable);
3) The distribution of the records for a particular trait in the offspring of an individual (progeny test).
The predicted transmitting ability value of a bull is an average number; it is our best estimate of the genetic merit of a bull.
The genetic value and performance of a particular daughter still contains an unpredictable component because each daughter receives a different set of genes from the bull. Chance determines the actual genetic merit of the offspring at the time of fertilization of the ova by the spermatozoon. In other words, the genetic merit of an animal cannot be predicted at the time of mating.
For example, when two animals of high genetic merit are mated, the genetic value of the offspring will not necessarily be high.
Although the offspring is more likely to be above average, it is possible that its genetic merit will be below average. The performance of the daughters of a bull is distributed according to the bell shaped curve, whether a bull has a PTA of 1000 kg or 0 kg for milk. It is important to realize that although Sire A has a PTA of 1000 kg, a few of Sire A’s daughters may have a genetic merit below some daughters of Sire B, who has a PTA of 0 kg (Figure 1). However, the important point is that more daughters of the high-average sire (Sire A) have a PTA higher than daughters of the low average sire (Sire B).
Genetic Base
Genetic base is the reference point used to express the predicted transmitting ability of an animal for a trait. All PTA values are expressed as a deviation from the genetic base.
The base is defined by setting the average predicted transmitting ability to zero for a group of animals.
For example, in the United States until 1994, the PTA for milk yield was set at zero for all cows born in 1985 and still milking in 1990. There is a separate base for each trait and each breed of cow. Strictly speaking, there is no need to modify a genetic base. However, as genetic progress is made, the PTAs continue to increase. It is only a matter of convenience that the genetic base is periodically upgraded.
Some countries change genetic bases every year (Canada), whereas others change it only once every 10 years. In the United States, the genetic bases are modified every five years. The impact of changing the genetic basis illustrated in Figure 2 for milk production. Changing the genetic base does not change the genetic value of an animal or the ranking of an animal relative to others in the population; only the reference point has been modified.
Reliability
Reliability is a measure of the confidence that should be given to the PTA estimate. Reliability reflects the amount of information in the PTA evaluation and depends primarily on:
- The reliability of the PTA of the parents and other relatives;
- The number of records (cows);
- The number of herds in which the daughters are located (sires).
It takes about 30 daughters in 30 different herds to obtain a bull’s PTA evaluation for milk yield with a reliability of about 70%. One hundred daughters in 100 different herds increases the reliability of the PTA to about 88%. The greater the amount of information available, the higher the reliability and the less likely it will be that future PTAs will be different from current PTAs. A PTA with a reliability of 70% is likely to change in the future. The direction of the change is unknown; as more information is available, the PTA value may go up, go down, or remain almost unchanged.
Thus the use of a sire with a low reliability entails a greater risk, but at thes ame time a greater opportunity than a sire with a highly reliable PTA (more than 90%), which is no longer expected to change greatly.
Confidence Range
Confidence range reflects the interval within which the animal’s true transmitting ability is expected to be 68% of the time (two out of three times). A confidence range is useful because it gives a realistic expectation of the magnitude of change that may affect the PTA of a bull. Confidence range is easy to calculate because it depends only on reliability and the standard deviation for the trait of interest. The lower and upper limits of the confidence range can be calculated as follows:
Lower limit = PTA - deviation;
Upper limit = PTA + deviation;
1- reliability x genetic standard deviation.
Let us calculate the confidence interval for two bulls, both with a PTA for milk of 1,000 lbs, but with reliabilities of 70% and 99%, respectively.
As indicated in the footnote of Table 1, the genetic standard deviation for milk yield is 560 lbs.
Thus for a bull having a PTA with a reliability of 70%, the deviation from the PTA is 1- 0.7 x 560 = 307 lbs.
The lower limit of the confidence range is 1000 - 307 = 693 lbs, and the upper limit is 1000 + 307 = 1307 lbs.
Thus we would predict that two out of three times the bull’s true PTA value lies between 693 lbs and 1307 lbs. This also means that one out of three times the bull’s true value lies outside this range; one out of six times the true PTA will be less than 693 lbs of milk, and one out of six times the true PTA will be more than 1307 lbs of milk
Once the reliability of the PTA is 99%, the confidence interval is quite small. In our example, a bull with a PTA of 1000 lbs and a reliability of 99% has a true transmitting ability between 944 lbs and 1056 lbs two out of three times (Figure 3).
