Choose sires with low somatic cell scores

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Choosing sires with a low Predicted Transmitting Ability for Somatic Cell Score (PTA-SCS) can help you reduce somatic cell counts, thereby improving milk quality on your dairy.

The use of genetic evaluations to reduce somatic cell counts may seem like an odd way to control mastitis on your dairy. However, there is a strong correlation between a sire’s PTA-SCS and the incidence of mastitis in his daughters, explains George Shook, dairy geneticist at the University of Wisconsin. And that makes choosing sires with a low PTA-SCS — a score of 3.2 or less for Holsteins, for example — a useful tool to help you lower your herd’s somatic cell count and reduce the incidence of clinical mastitis on your dairy.

To help you understand how PTA-SCS affects daughter performance, look at the PTA-Milk and PTA-SCS information for the following two bulls:




Bull PTA-Milk PTA-SCS
“A” 1,648 2.79
“B” 1,015 3.19
Difference 633 -0.40

According to the table, daughters of Bull “A” are expected to produce 633 pounds more milk and average 0.40 points less SCS than daughters of Bull “B”. Thus, daughters of Bull “B” are expected to have a higher average somatic cell count than daughters of Bull “A”.

For example, if the average SCC for daughters of Bull “A” in a particular herd is 300,000 cells per milliliter, then the average SCC for daughters of Bull “B” in the same herd is expected to be 396,000 cells per milliliter (300,000 x 1.32 SCC ratio = 396,000). The 1.32 SCC ratio is the multiplier that corresponds with the 0.40 PTA-SCS difference between the two bulls, as shown in the following table:

Multipliers that express the PTA-SCS difference for two bulls:



PTA-SCS Difference
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
SCC ratio 1.07 1.15 1.23 1.32 1.41 1.52 1.62 1.74 1.87 2.0
Clinical
mastitis ratio
1.06 1.12 1.18 1.25 1.32 1.40 1.48 1.57 1.66 1.75

Likewise, daughters of Bull “B” tend to have a higher incidence of clinical mastitis.

That becomes apparent when you calculate the incidence of clinical mastitis for daughters of these two bulls. For example, if the incidence of clinical mastitis in daughters of Bull “A” is 20 percent, the expected incidence in daughters of Bull “B” is 25 percent (20 percent x 1.25 clinical mastitis ratio = 25 percent). Again, the 1.25 clinical mastitis ratio is the multiplier that corresponds with the 0.40 PTA-SCS difference between the two bulls, as shown in the table above.

What’s more, if the difference in PTA-SCS between these two bulls is one point, the incidence of clinical mastitis in daughters of Bull “B” would be 75 percent higher than daughters of Bull “A”, Shook says.

“PTA-SCS does influence milk quality,” Shook says. “Include sire selection for somatic cell score on your milk quality checklist,” he adds. It can help you lower somatic cell counts and improve milk quality over the long run.
 


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