Can herd somatic cell count be too low?
As herds continue to reduce somatic cell counts and improve milk quality, is there a danger in SCC getting too low?
Dairy producers continue to do a great job with mastitis prevention and control, resulting in low herd somatic cell counts (SCC) that were once unheard of. The NorthStar Cooperative DHI 2012 Annual Performance Summary lists 60 herds with actual SCC under 100,000. The top two leaders in that list had a herd SCC of 31,000 and 35,000, levels that were unimaginable even a few years ago. Just five years ago, an annual average SCC of less than 100,000 was achieved by only 17 of the many herds tracked by NorthStar DHI. Of those herds, the lowest herd SCC was at 56,000.
As an educator with Michigan State University Extension, I encourage dairy producers to constantly improve milk quality and udder health. I have heard questions from dairy producers wondering will SCC levels continue to drop and if there is a lower limit that can be achieved. They have also wondered if it is wise to reduce SCC as low as the leaders and if there is any benefit to reducing SCC this low.
Somatic cells (SC) are primarily leukocytes that are part of the immune system of mammals designed to fight invading bacteria. According to Larry Fox of the College of Veterinary Medicine at Washington State University, the purpose of SC in milk is to provide protection to the mammary gland.
Some of those cells serve in a surveillance role seeking foreign invaders. When they find them, they recruit other cells. The first reaction is generally non-specific phagocytosis, or engulfing the bacteria. This is termed innate immunity. If the infection persists, the cells produce specific proteins so that phagocytosis is targeted at the particular pathogen. We call that acquired immunity.
When an infection occurs, there is an influx of immune cells from the blood into the milk and thus we see an increased SCC indicating an infection in the mammary gland. So if the number of SCs is low, is there enough ability to detect the invader and respond quickly or do we give up response time and have a greater incidence of clinical mastitis as a result?
Fox concluded based on a number of studies that the number of cells that are in milk at the time is not the primary factor associated with combating an infection, but rather the speed and number that can be mobilized to the gland. In addition, the ability to mount a specific, acquired immune response is critical. These factors are governed by the genetics of the cow. Therefore, low SCC does not leave the cow more vulnerable.
If low SCC is not a problem, is it an advantage? The answer would be “yes” if the cells that produce milk, the secretory cells, are impacted negatively in any way by higher SCC. Research tells us that that is exactly what happens.
Higher SCC is an indication that cells have been recruited from the bloodstream to the milk. It has been shown that the influx of leukocytes causes damage to milk secretory cells. In addition, phagocytosis and bacterial cell death both irritate milk secretory cells. The sensitivity of the secretory cells varies by development stage and it has been found that increased milk SCC is potentially more detrimental during the weeks following calving.
The inflammation associated with chronic or clinical mastitis can lead to blockage of the cell ducts by secretory cells so that the flow of milk is disrupted and reduced. Akers and Nichols in 2011 reviewed data that indicated there is a loss of 1.5 lbs. of milk per day for every doubling of SCC beginning at 12,500 cells/ml. This shows that milk loss begins at even very low SCC.
Therefore, we see that it is unlikely that there is a negative impact of low SCC on the ability of the cow to respond to infection and that there is benefit to low SCC in avoiding milk production loss.
The key is to manage cow udder health on the farm by lowering the exposure of the teats to bacteria in the environment and from other infected cows. Proper preparation for milking, proper milking procedures and a well-maintained milking system are very important.
More and more producers are showing us that lower SCC is not only achievable, but is advantageous. While there are many factors that impact milk production, it is worth noting that the average production of the 19 NorthStar DHI herds with a SCC equal or less than 75,000 was 26,735 lbs. of milk.
Monitor udder health not only by watching SCC, but also by culturing clinical cows to determine the problem pathogens and strive to keep clinical mastitis low. Dr. Pam Ruegg, DVM, recommends a Key Performance Indicator (KPI) for clinical mastitis of less than 25 new cases per 100 cows per year, and a recurrence rate of less than 30 percent of cows with recurrence of mastitis after 14 days post treatment. Therefore, it's a good idea to monitor and compare to your KPIs to set your sights on a low SCC.