YOU ARE HERE….On The Somatic Cell Count Map

By Ronald Erskine, DVM, PhD and Rhyannon Moore, DVM

Bulk tank somatic cell counts (BTSCC) or DHI average SCC are the most common numbers that are used to measure mastitis in a dairy herd. However, individual cow SCC are more timely and in some cases, accurate than herd averages. Therefore, dairy producers, veterinarians, employees, and anyone else on a farm who has a stake in quality milk should be aware of the trends for key individual cow SCC values beyond herd average SCC. In fact, depending on the proportion of cows in a herd that are treated for clinical mastitis, milked in three quarters, culled, or dried off, the prevalence (or level) of subclinical mastitis may be underestimated if evaluated by herd average SCC alone. For more on why individual SCC information is an important tool for a mastitis control program, please read Are Herd Somatic Cell Counts The Best Measure?

Useful Measures of Individual Cow SCC

Four useful measures of individual cow SCC are: 1) the percent of milking cows with a linear SCC score (LSCC) of 4 or greater (or ≥ 200,000 cells/mL), 2) the monthly new infection rate, 3) the proportion of cows that had a new infection between the last test date of the previous lactation and the first test date of the current lactation, or simplified, “dry cow infection rate”, and 4) the percent of cows that have a LSCC of ≥ 4 on the first test date (TD 01) for the current lactation. All of these values can be readily attained from standard DHI reports.

The proportion of milking cows with a LSCC of ≥ 4 is highly correlated to herd average SCC; the higher the proportion of cows with an elevated SCC (who are likely to be infected), the higher the herd average SCC. However, the percent of infected cows does not tend to fluctuate from month to month as much as herd average SCC; this is because a small number of high SCC cows account for a disproportionately high percentage of the herd average SCC (see web link address cited above). Thus, tracking the percent of cows with subclinical mastitis (LSCC ≥ 4) is a better measure of subclinical mastitis in a dairy herd than herd average SCC. Likewise, tracking the percent of cows that develop subclinical mastitis or “new infections” each month (LSCC below 4 the previous test date and ≥ 4 on the current test date) can be an excellent tool to monitor mastitis in response to changes in management such as bedding, equipment maintenance, milking protocols, etc.

Monitoring changes in LSCC and subclinical mastitis over the dry/fresh period was described in the article Monitoring New Infections” and was found to be strongly associated with herd average SCC. Herds with lower new infection rates for dry cows have lower average SCC. This is important because many times, management of dry and transition cows is overlooked relative to mastitis and milk quality.

The portion of cows that start lactation without subclinical mastitis (TD 01 < 4) is also an important measure of subclinical mastitis in a dairy herd. If cows begin their lactation with an infection and if the infection is not cured, either spontaneously (by the immune system) or by treatment with antibiotics, the long term losses for the rest of the lactation from lost production, potential treatments, and reduced fertility can be substantial. In a study of 164,000 cow records from 22 Western dairy herds, cows that had a high first test date SCC (LSCC ≥ 4) were three times more likely to be culled by 60 days in milk, three times more likely to have clinical mastitis, 15 more days open to pregnancy, and nearly 1,600 lbs of reduced milk through the first 210 days of lactation compared to cows that had a TD 01 LSCC < 4 (Kirkpatrick 2015). So, where does your herd stand compared to other herds for these four individual cow measures of mastitis? The figures below are the distribution of DHI (PCDART) SCC information from 138 herds during May and June, 2015; 131 herds were from Michigan and 7 herds from Indiana, Ohio, and Pennsylvania. The median (50 percent of the herds above and below), as well as the 25th and 75th percentile was calculated for each of these key indicators. The average test date SCC for the herds was 158,000 cells/mL, the median SCC was 135,000 cells/mL. Thus, the overall milk quality in the herds was very good. Nonetheless, there was a wide range between herds with respect to the four individual cow SCC values listed above. If you have the SCC option available through DHI, it might be time to see where your herd stands on the SCC “map”. More importantly, use these distributions to track trends for mastitis in both milking and dry/fresh cows in your herd and track trends that result from changes in management protocols. References: Kirkpatrick, M. 2015. Somatic Cell Counts at First Test: More than a Number. 54th Ann. Proc. Natl. Mastitis Council, Memphis, TN. pp. 53-56. QMA_8.20_Graph 1QMA_8.20_Graph 2QMA_8.20_Graph 3QMA_8.20_Graph 4

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