When it comes to reducing mastitis on your dairy, you might adjust your milking procedure or step up the cleanliness of the cows' stalls. However, you may not have considered sire selection as a mastitis-control tool.
Selecting bulls with a low Predicted Transmitting Ability for Somatic Cell Score (PTA-SCS) hasn't received much attention since its introduction in 1994, notes George Shook, dairy geneticist at the University of Wisconsin. That may be due, in part, to the low emphasis placed on PTA-SCS in some genetic indexes, as well as the trait's low heritability compared to other traits. However, when managed properly, it can help you lower somatic cell counts and improve udder health. Use these strategies as your guide:
Recalculate an index
Consider using one of these indexes: Net Merit Index (NM$), Fluid Merit Index (FM$) or Cheese Merit Index (CM$). You can find them in the quarterly sire summary on page 58.
Choose whichever index you prefer. Then, ask your breeding company representative to recalculate the index so that it places more weight on PTA-SCS, says Gary Rogers, dairy geneticist at Penn State University. For example, PTA-SCS makes up 9 percent of the weight in the NM$ index. "For most producers, the weight (given to SCS in this index) is not too far off," Rogers says. However, if you'd like to boost the emphasis on PTA-SCS in this index, increase the weight from 9 percent to 15 percent, Rogers says.
In order to do this, you would need to decrease the weight given to the production and functional traits, like PTA milk and PTA feet and legs composite, by 6 percent.
Bottom line: Don't go overboard when placing more weight on PTA-SCS in the index, because that can eliminate good bulls from your list, Rogers explains. Don't over-emphasize PTA-SCS and ignore other important traits such as milk, fat and protein yield, feet and legs, udder composite and productive life.
And, keep the heritability - the portion of variation among animals that is due to genetics - of PTA-SCS in perspective. For example, the heritability of PTA-SCS is around 0.10, while milk production traits have a heritability of 0.30.
Choose low PTA-SCS bulls
For the second approach, use the NM$ index - or another index you like to use - to weed out sires with the highest somatic cell scores.
To create your list, avoid the 10 percent of bulls in the index with the highest somatic cell scores. For all active AI Holstein bulls, that means you would need to avoid those with a PTA-SCS of 3.3 and higher, unless these sires are superior for milk production traits, Shook says. For example, in the February genetic evaluation, two out of the 50 top-ranking NM$ bulls had PTA-SCS scores greater than 3.3. Avoiding these bulls gives you genetic selection for mastitis resistance, without impeding your selection for other key traits.
Next, use your list to compare the PTA-SCS of sires and determine which ones to use in your breeding program. "When you look at these PTAs, look at their differences," Shook says.
For example, compare 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 information, daughters of Bull "A" are expected to produce 633 pounds more milk and average 0.40 points less SCS than daughters of Bull "B." In other words, daughters of Bull "B" will tend to have a higher SCC and a greater incidence of mastitis than daughters of Bull "A." (Please see, "Low PTA-SCS sires reduce mastitis" above.)
Again, don't ignore production and type traits when selecting low PTA-SCS bulls for your breeding program. Likewise, keep in mind genetic defects, fertility, calving ease and semen cost.
Remember, genetic improvement doesn't happen overnight, so be consistent and use consecutive generations of low PTA-SCS sires to get the best results. Low PTA-SCS sires reduce mastitis
Research published in the October 2000 Journal of Dairy Science found that the incidence of clinical mastitis in daughters of sires with high PTA somatic cell scores was almost twice that of daughters sired by low PTA-SCS bulls.
To see how a bull's PTA-SCS influences his daughters' somatic cell counts and incidence of mastitis, consider the following two bulls. Bull "A" has a PTA-SCS of 2.79, while Bull "B" has a PTA-SCS of 3.19. Using the 0.40 PTA-SCS difference between these two bulls, you can determine the SCC and mastitis incidence for their daughters.
For example, if the SCC for daughters of Bull "A" is 300,000 cells per milliliter in a herd, multiply this number by the SCC ratio which corresponds with the 0.4 PTA-SCS difference (see table below) to arrive at expected SCC for daughters of Bull "B."
300,000 cells per milliliter
x 1.32 SCC ratio
= 396,000 cells per milliliter expected SCC for daughters of Bull "B"
To determine the risk of mastitis among Bull "B's" daughters, multiply the incidence of clinical mastitis of Bull "A's" daughters - 20 percent, for example - times the mastitis ratio listed in the table below.
x 1.25 clinical mastitis ratio
= 25 percent mastitis incidence for daughters of Bull "B"
"PTA-SCS does influence milk quality," says George Shook, dairy geneticist at the University of Wisconsin. "Include sire selection for somatic cell score on your milk-quality checklist."