Using the New Somatic Cell Score Sire Evaluations

Genetic studies of dairy cattle have found that single-trait selection for higher milk production brings with it slightly higher rates of mastitis. The good news, however, is that not all high-production bulls sire high rates of mastitis.

Now with the advent of USDA sire rankings for Predicted Transmitting Ability for Somatic Cell Score (PTASCS), producers can select bulls on their ability to sire daughters with lower rates of mastitis. ("How Bulls Differ.")

A few words of caution are in order. First, high production and high rates of mastitis are genetically correlated. Therefore, if you select only those bulls with low SCS (PTASCS), you will also likely select for lower rates of improvement in milk yield.

In fact, studies show that selection programs that optimize total economic merit will not stop the increase in mastitis incidence. Most studies suggest such programs will simply slow the rate of mastitis increase by 20% to 25%.

Second, the level of genetic control--known as heritability--for SCS is estimated to be only 10%. In contrast, the heritability for milk production is about 25%.

As a consequence, genetic progress will be slower and evaluations for mastitis will be less accurate than for milk production for bulls with a given number of daughters. For example, a reliability of 73% is produced by 30 daughters in 30 herds for milk production. To achieve this same 73% reliability for mastitis, 100 daughters in 100 herds are needed.

Fortunately, like milk production, SCS data are collected monthly by DHIA and summarized every six months by USDA. As additional records and daughters enter a sire's progeny proof, the accuracy (reliability) of the proof will improve. In the end, all of this suggests that genetic progress for reducing mastitis will be slow. Traditional mastitis-control programs still must be of the utmost importance in controlling the disease in dairy herds. Milking machines still must be attached to clean, dry teats. And equipment still must be maintained for optimal milking performance.

Measuring Mastitis

Before progress can be made in genetic selection, the trait being selected for must be measurable. The problem is that in the U.S., there are no cheap or uniform methods of directly measuring mastitis.

As it turns out, however, somatic cell score (SCS) is an excellent substitute trait for mastitis. When a cow is challenged with a mastitis-causing organism, her immune system responds by sending white blood cells to the mammary system. These somatic cells attack the organism to eliminate the infection.

In fact, studies have shown that mastitis is the most important cause of an elevated SCS. Researchers have found that the genetic correlation between SCS and mastitis is 60% to 80%. This implies that daughters of sires with high PTASCS experience a high incidence of mastitis. Another advantage to using SCS as a substitute trait is that it is routinely collected at low cost by DHIA. SCS data are now available from millions of cows in thousands of herds. These data, averaged over a cow's entire lactation, are ideally suited for use in sire summaries.

Using the Sire Evaluations

The sire evaluations will be reported in terms of a bull's predicted transmitting ability for somatic cell score. For example, in the table (see the back flap), Bull A has a PTASCS of 3.12. Bull B has a PTASCS of 3.62. The difference is 0.5. If daughters of these bulls are housed in the same herd at the same time, the SCS of the bull's daughters is expected to differ by 0.5.

Other studies show that the rankings of bulls by PTASCS holds up across herds, regardless of mastitis-control levels in those herds. So both high- and low-SCS herds can benefit from selection of the same sires with low PTASCS.

An important fact to remember is that a lower PTASCS is better. That's because a lower SCS means that the cow has less subclinical mastitis--and therefore is less prone to milk losses--than a cow with a high SCS.

Also remember to not overemphasize PTASCS in your sire-selection program. To avoid overemphasis, SCS evaluations should be included in a selection index. Here, weights are assigned to various traits, depending on their economic importance. Most dairy geneticists suggest that production should receive 75% to 80% of the weight in these indexes. SCS should receive about one eighth of the weighting of production.

For example, USDA offers a net merit index that includes production (67%), herd life (28%) and SCS (7%). The SCS portion includes the cost of clinical mastitis and premium deduction for high SCS.

How Bulls Differ

The accompanying table shows differences in PTASCS for the most-frequent sires of Holstein bulls in the top-400 type-production list.

The table illustrates the real differences among bulls for PTASCS. For these top-10 sires of sons, the range in PTASCS went from a high of 3.73 to a low of 3.12. Unfortunately, Bull B--who has sired the second-highest number of top-400 TPI bulls--has a PTASCS of 3.62. This is the second-worst PTASCS of bulls on the list!

The important thing to remember is that these sires of A.I. sons have had tremendous impact on the breed. The 10 bulls listed in the table have sired 80% of the most commonly used A.I. bulls.

There is good news here. The weighted average of PTASCS of these top-10 sires of sons is 3.33, or slightly below the Holstein breed average of 3.40. Remember, this is favorable since a lower PTASCS means less mastitis.

For years, A.I. sire analysts have selected bulls that sire shallow, well-attached udders. Since there appears to be a strong positive correlation between SCS and udder depth, sire analysts have probably been indirectly selecting for low SCS for some time.

With the publication of the new USDA evaluations for PTASCS, sire analysts have a more direct measure of mastitis in a bull's daughters. Using these new data will help ensure that highly unfavorable bulls are not used to sire future generations of dairy cattle.

PTASCS for the most frequent sires of top-400 Holstein TPI bulls

Sire	Number of sons	PTASCS	Reliability
A	68	3.12	99%
B	51	3.62	99%
C	46	3.17	97%
D	38	3.29	97%
E	35	3.28	91%
F	26	3.15	99%
G	25	3.38	98%
H	25	3.73	96%
I	17	3.50	99%
J	10	3.43	97%

Source: University of Minnesota

This fact sheet is intended as a nontechnical primer on USDA's sire summaries for PTASCS. For a more detailed discussion of PTASCS, cotact the Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350. Ask for a reprint of Genetic Evaluation of Somatic Cell Scores for United States Dairy Cattle.