How do I read sire ratings for Somatic Cell Score ?

The DHIA systems around the USA offer Somatic Cell testing within the usual milk volume and component % data collection. Of course, you may be among the many who are concluding a once a month reading on Somatic Cell is not helping you much to control mastitis flare-ups, one of the key occurrences affecting milk quality bonus payments.

However, from a genetic selection standpoint, there is some heritability to the SCS trait score. Latest estimates put the h2 of Somatic Cell Score at 15%, right in the middle of the range over linear type and production traits. Over time, you can build a better herd profile by paying some attention to this in sire data.

What is a “somatic cell score”?

A “somatic” cell is any cell with a nucleus. The testing is designed to estimate the current level of spent white blood cells in the milk (evidence of the body fighting an infection somewhere in a cow’s system). The “Coulter” counters used do have trouble telling the difference between an infection cell and other nuclei, such as a shed skin cell (extraordinarily high butterfat% will affect the SCC measure adversely by such cells sticking to the fat particles in the milk).

Note that SCC does not specifically mean just “mastitis” infections. ANY infection in the cow’s body which draws a white blood cell response from the immune system will raise SCC levels. In fact, the biggest spikes in SCC often come when cows are fighting heel warts or foot rot.

How do I read the data ?

The sire PTA values are standardized around 3.00 as being “average”. A better bull will be under that 3.00 level, a lesser bull will be over that 3.00 level. It is best to assume that values between 2.95 and 3.05 are essentially “average” – you will have trouble detecting any change from sires in that range of SCS. Note: The lowest SCS bulls (down around 2.40) often have a reputation for “slow” milking speed, and the highest SCS bulls (up around 3.50) may be that way because their daughters “leak” milk prior to milking. Integrity of the teat sphincters has an impact (good or bad) on both milking speed and milk letdown.

Current examples of Holstein sires with exceptional SCS values

566 HO 1345 Claytop G-Plus EVER RED SCS 2.61 and this is in spite of being +.21% b’fat. Milking Speed is rated 103 (3% better than average) . His aAa of 456 indicates him a source of “Strong” substance, “Smooth” body condition ability, “Style” durable bones. He looks pretty good on the Zoetis “Wellness Trait” DNA measurements. He is A2A2 for Beta Casein.

566 HO 1335 E-Lane Escobar LAYNE RED SCS 2.59 +.18 b’fat. Milking Speed 100. He is also another A2A2 Beta Casein source and is rated 110 by Zoetis for Mastitis Resistance.