For herds enrolled in herd testing, most would have their first herd test completed and the information in place to make an accurate assessment of chronic subclinical infections and the infection status of the herd. It is worth spending a little time looking through this information to obtain the maximum value. 

Firstly, it is the best way of gauging the chronically sub-clinically infected animals in the herd. By combining the previous seasons individual cow cell count information (ICCC)and dry cow treatment records it is easy to see which animals were treated with DCT that were also over 150,000 cells/ml at the last herd test (or 120,000 cells/ml for heifers). Now examine the first herd test from this season. If an animal remains infected, based on ICCC, and has no history of clinical mastitis this season, then it is likely that the animal was not cured by the DCT during the dry period and is at risk of being chronically infected. This is not cast in stone of course – there is the possibility that the cow was cured during the dry period and then has become a new subclinical infection between calving and the first herd test. However, identifying these animals is important in understanding the size of the group within the herd that are likely to be chronically infected and therefore at risk of having a lower cure rate to DCT at the end of this season. 
 
Secondly, the number of animals over the ICCC threshold for sub-clinical infection can be used to gauge the size of the infected group overall. This herd test information is only a snap-shot in time in that it represents only two milkings. However, this can be compared to the herd bulk milk cell count (BMCC) to reality check the size of the subclinical group in the herd. Recall that a useful rule of thumb is that for every increase in 100,000 cells/ml BMCC, there are approximately an additional 10% of infected cows in the herd. Hence, with a BMCC of 100,000 there are around 10% sub-clinically infected while at 300,000 this would be 30%. 
 
Lastly, the ICCC of the heifers should be viewed carefully against clinical mastitis records. If a heifer has no record of clinical mastitis since calving, yet has an ICCC over 120,000, this indicates a reasonable risk of a new subclinical infection being established. This information, combined with the number of clinical mastitis cases in heifers, can be combined to indicate the new infection risk in the herd. This is because we assume that heifers, on the point of calving, have a sterile udder and are uninfected animals. If the ICCC indicates a subclinical infection then this should be assumed to be a new infection during lactation. Understanding the size of the new infection rate in a herd is useful in assessing the overall mastitis risk. 
 
So, the first herd test ICCC information is most useful in three ways and sets us up for accurately assessing mastitis dynamics within a herd as lactation progresses.