Somatic Cell Count, an indicator of milk quality

Somatic Cell Count (SCC) measurements taken from the bulk tank obviously describe the average herd count. This information cannot define the spread or degree of SCC variation within the herd; only individual testing of cows will determine whether there are, for instance, a significant number of cows with slightly higher-than-average counts or, conversely, a small number of cows with very large counts.

Generally, a threshold of 200,000 cells/ml of milk is used to decide whether a cow is infected. Some cows with few clinical symptoms of mastitis can have a SCC of 400,000 or higher, thus the importance of individual cow monitoring of cell counts is important to determine how many cows in the herd are affected and to what extent.

In smaller herds, the total effect of one or more cows with very high cell counts can be enormous. In the larger herd, milk from a sick cow is diluted and averaged down by the healthy animals. The bulk tanks on very large dairy units can accommodate mastitic cows in the herd without those cows affecting the overall milk quality rating, but for small to average-sized herds, knowing which cows are the 'culprits' enables action to be taken and decisions to be made over how to approach and deal with the problem. Milk recording schemes can calculate the contribution of a single high cell count cow to the herd's overall SCC, in percentage terms, demonstrating the potential improvements in total SSC if that cow's milk was to be not included in the bulk tank.

Where one or more cows with chronic symptoms of mastitis are present, they should be culled as they are a reservoir of infection for unaffected cows and are unlikely to be curable. Subclinically affected cows with high cell counts can be targeted for treatment, and their counts monitored over time to see if they recover or become chronic cases themselves.

In an ideal world, controlling cell counts in the dairy herd by following recommend practices would be simple, but unfortunately due to the innate variability of conditions on dairy farms in the UK and the multifactorial nature of mastitis incidence, the problem of high Somatic Cell Counts can be particularly perplexing for many herds that meet best practice recommendations but fail to significantly lower cell counts.

For these reasons, and because of the differing business aims of different dairy farmers and the differing contractual stipulations of their milk buyers, no single SCC target for all dairy herds may be appropriate or in many cases achievable. However, due to an increasing awareness of cow welfare and pressure to improve milk quality, it is obvious that most dairy farmers have to attempt to improve levels of both clinical and subclinical mastitis in their herds, and to benefit from the bonuses they can receive for producing milk with lower SCCs.

The generally-quoted aims for mastitis control and milk quality on UK dairy farms are:

• A mastitis incidence rate of no more than 30 cases per 100 cows per year.
• A mastitis persistence rate of no more than 20% of the herd affected per year.
• A mastitis re-occurrence rate of less than 10% of the total number of cases.
• A herd-average Somatic Cell Count below 150,000 cells/ml.
• An average Bactoscan result of below 5,000.

In general terms, rule of thumb is for every 100,000 cells/ml increase in the herd bulk SCC, there is an 8-10% increase in the proportion of cows infected in the herd. Persistently high individual cow SCC results can indicate chronic mastitis problems.