Milk quality and mastitis are primarily determined by management such as a clean environment and good milking procedures. Genetics also plays a role. Dairy farmers can buy semen from bulls that lower somatic cell counts (SCC). Since 1984, the source for USDA’s genetic evaluations for SCC has been the information collected through the DHI system. USDA found that on average about 12% of the variation in SCC between cows is determined by genetics. Genetic values for males and females in the “official” genetic evaluations are presented as PTA SCS. This stands for predicted transmitting ability (that what is transmitted to the offspring) and somatic cell score (SCS). The SCS is calculated from the SCC as SCS = log2(SCC/100,000) + 3. Equivalently, SCC = 2(SCS - 3) x 100,000. The figure shows the relationship between SCS and SCC on the left axis.

The genetic trait SCS is one of 12 traits included in the lifetime profit selection index Net Merit. The list of the top 20% of Holstein sires available in the US, as sorted by Net Merit in the August 2016 genetic evaluation (1,780 sires), shows that the lowest PTA SCS is 2.4 (SCC = 66,000) and the highest PTA SCS is 3.21 (SCC = 116,000). The average PTA SCS of the 1,780 sires is 2.80 (SCC = 87,000). To lower SCC by means of genetics, dairy farmers should look for sires with low PTA SCS. USDA says that “selection for lower SCS reduces the labor, discarded milk, antibiotic, and other health costs associated with clinical mastitis”. Lower PTA SCS may also lead to higher milk prices, depending on the quality premiums paid.

The economic value of one point greater PTA SCS per lactation in the Net Merit index was set at −$44 by USDA, which includes a lost quality premium of $24 plus $20 for labor, drugs, discarded milk, and milk shipments lost because of antibiotic residue. The loss of $44 per lactation is equal to a loss of $122 lifetime, which is assumed to be 2.8 lactations. As an example, suppose Sire A has a PTA SCS of 2.5 and Sire B has a PTA SCS of 3.0. Their daughters are expected to be 3.0 – 2.5 = 0.5 SCS different, which is valued at a difference of 0.5 x $44 = $22 per lactation and $61 lifetime in favor of the daughters of Sire A. The right axis in the figure shows what the expected change in profit per lifetime is for various SCS compared to an SCS of 3.

Emerging genomic testing technologies combined with new phenotypic data collection are also shaking up the way milk quality can be improved with genetics. Recently, Zoetis launched CLARIFIDE Plus, a commercial genomic test that gives dairy farmers the ability to genomically select animals based on different wellness traits. CLARIFIDE Plus allows farmers to identify and select animals with reduced genetic risk for six of the most common and costly health traits, including mastitis. Zoetis has reported that CLARIFIDE Plus can achieve genomic predictions at an early age with reliability values between 49% and 51% for the six health traits. Additionally, CRV also provides a commercial genomic test called HerdOptimizer that allows dairy farmers to genomically test young animals for multiple health traits, including clinical and subclinical mastitis. These genomic tests are based on farm recorded disease cases. Traditional and new genetic information should be part of any dairy farmer’s tool box to improve milk quality and reduce mastitis.