Historically, mastitis experts have believed that milk production decreases in a linear fashion as the linear score of the somatic cell count of a cow increases. In fact, this was one of the main reasons for introduction of the linear score in DHIA systems. According to this relationship, milk production drops 1 to 1.5 pounds per day for each increase of one point in linear score. Each one point increase in linear score represents a doubling of raw SCC. Compeer and Zoetis did a study of the association between profitability and factors identified from farm records that suggested SCC had the greatest correlation with profitability of all factors considered, and that the amount of change in profitability is much greater than could be explained by increased milk production due to reduced linear scores. There has been a fair bit of discussion in the press regarding why this may be the case. There are probably a number of reasons, but part of it is the effect of first test SCC.
    In 2015, Kirkpatrick and Olson presented a paper at the NMC annual meeting that looked at the effects of a high first test or a recorded case of clinical mastitis in the first 60 days of lactation, and found that the reduction in milk production of either was significant, with the cost of high first test SCC being greater than the cost of clinical mastitis in the first 60 days. The study used records from 164,423 cows in western states. Cows were considered to have a high first test SCC if their SCC was over 200,000 or a linear score of more than 4.0. For clinical mastitis in the first 60 days, the milk loss was 1,007 pounds through the first 270 days of lactation. For a high first test SCC, milk loss was 1,583 pounds through the first 210 days of lactation. Using $18/cwt., this represented $181 and $285 lost value for first 60 day clinical mastitis and high first test SCC, respectively, and $129 and $203 lost income over feed costs for first 60 day clinical mastitis and high SCC, respectively. Cows with a high first test SCC were also 2.48 times more likely to have clinical mastitis in the first 60 days of lactation.
    This is not the whole story. The rates for removal from the herd by 60 DIM were 6.5 percent for cows with a high first test SCC and 2.2 percent for cows with a low first test SCC. High first test cows were three times more likely to be culled within the first 60 days than low first test cows, while cows with clinical mastitis in the first 60 days were only 1.89 times more likely to be culled. Finally, having either a high first test SCC or clinical mastitis in the first 60 days resulted in an increase of 18 days average median days to pregnancy.
    If we add milk loss per cow of $285 to additional costs of $34 for additional culling and reproductive losses per cow of $54 (my calculations) plus clinical mastitis cost per cow representing the increase in clinical mastitis due to high first test SCC ($32), we get a total loss for a high first test SCC of $405 per cow. The reproductive and culling costs are only estimates, but one can still conclude that a cow with a high first test SCC results in significant economic loss to the dairy. There are some additional costs to consider, too. For example, we know that once a cow has a case of clinical mastitis, she is more likely to get another case. There is also a cost of treatment and discarded milk for clinical mastitis to consider. The overall point here is that high SCC, even if we only look at the first test, is a real profit robber to the dairy and much greater than one would think using the old rule of 1.5 pounds of milk loss per point increase in linear score.
    Hopefully after reading this you are thinking, “Great. That’s why I always watch the percent of cows high on first test each month.” Or, “Boy, I really need to look at what percent of my cows are high each month.” If you use DC 305, the command of PLOT LGSCC=4 BY LGSCC\ZRY shows, among other things, the percent of fresh cows high on each test day. In DC 305, high cows are designated as hifresh. Farms should be able to keep this number below 15 percent on an annual basis. In our practice’s annual review of client records, we found an average of 17 percent hifresh, and a range of 7 percent to 36 percent for 2017. If you have lots of cows high on first test, do you know why? It is often suggested that the most common cause of early clinical mastitis or high first test SCC is infection during the dry period. However, personal experience from record analysis on many well managed dairies suggests this is not typically true. Most often, a high percent hifresh cows simply means there are too many chronically infected cows. These cows carry high SCCs from one lactation to the next. They are true profit robbers.
    Everyone is looking for more dollars from every milk check these days. Most of us probably significantly underestimate the true costs of mastitis. Improving udder health can yield significant returns. Looking at your herd’s first test SCC may help you develop a plan to harvest more of those dollars. For help, ask your herd veterinarian.
    Jim Bennett is a dairy veterinarian at Northern Valley Dairy Production Medicine Center in Plainview, Minn. He and his wife, Pam, have four children. Jim can be reached at bennettnvac@gmail.com with comments or questions.