Monitoring New Infections in Dry Cows

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by Ashley Zondlak, MSU-CVM, Class of 2015 & Ron Erskine, MSU-CVM, Department of Large Animal Clinical Services

Somatic Cell Counts (SCC) are routinely used as an indicator of mastitis in dairy herds and thus can be an important part of a milk quality program. The most critical period during a cow’s lactation cycle to prevent new intramammary infections (IMI) occurs from dry off through the first two weeks after calving. Many infections that begin during this period will result in clinical mastitis and subclinical milk yield losses throughout lactation.A useful tool to monitor new infections during this period is the use of DHIA SCC records provided by PCDART ® (DRMS, Raleigh, NC).

The individual cow plot labeled “Prv Lct last test SCC score v TD01 SCC Score” provides a scattergram that compares the last test date before dry off SCC score to the first test date after calving (TD 01) SCC score. This plot includes all cows
(> 2 lactations) currently in the herd. Figure 1 is an example plot from a herd in Michigan. For our purposes, a cow with a linear SCC score of 4 or higher was considered to have an IMI while a cow with a score of below 4 was considered to be non-infected. The trend line was deleted and the crosshairs were then adjusted to the “4” mark on each axis. Cows with a score of less than 4 on their last test of their previous lactation are considered to be non-infected as they entered their dry period and are at risk for developing an IMI (the combined green and purple shaded area). From this group, cows that also had a TD 01 SCC score of 4 or greater (purple area only) were designated as new dry cow IMI.

Thus, the dry/fresh period new infection rate in this herd was calculated to be:
55 (new infections) = 20.1%
274 (total not infected at dry off)

So what is a reasonable herd goal for new infection rates during the dry period? Figure 2 is a distribution of new dry period infection rates from 122 herds throughout the Midwest. On average, 16.1% of non-infected cows developed an IMI during the dry period. It should be noted that the average 12 month weighted DHIA SCC for these herds was 166,000 cells/mL. The data was further reviewed to determine the relationship between individual farm SCC and dry cow new IMI. The results are displayed in Figure 3. As a farm’s dry cow IMI rate increases, the 12 month average SCC also rises. The coefficient of variability (R2) is 0.39, which in a broad sense means a herd’s dry cow IMI rate accounts for roughly 40% of the yearly average SCC. While many other factors contribute to herd SCC, this preliminary data illustrates the important connection between controlling dry IMI and improving milk quality.

Dry cow IMI should be monitored as part of a routine herd quality milk program. If a herd is not meeting goals for this key point, dry cow therapy products and infusion techniques, methods of decreasing cow milk production before dry off, dry and fresh cow housing, and milk bacteriology of clinical cases in early lactation should be reviewed.

Figure 1

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Figure 2

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Figure 3
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