Milking a dairy cow with a low somatic cell count (SCC) gives you the largest volume of high-quality milk with the greatest efficiency. A herd full of low-SCC cows would be what you want.
However, producers frequently voice concerns that if SCCs get too low more mastitis will occur. They usually mean clinical mastitis-cows get sick, and quarters get hot, hard and produce abnormal milk. What's unclear is the level they consider too low, and whether they're worried about individual cow SCCs or bulk tank counts.
To understand this issue better, let's break the issue down into three questions and answer them separately. The first concerns individual cows and the second bulk tank SCCs. Finally, let's consider whether you can have both low bulk tank SCCs and low clinical mastitis incidence.
Are lower SCC cows more likely to get clinical mastitis than higher SCC cows in the same herd?
Dutch and American researchers studied whether low-SCC cows in one herd were more likely to show clinical mastitis. They wanted to find out how cows that had clinical mastitis differed from those that had increased SCCs but, at the same time, showed no clinical mastitis signs.
The study herd milked 90 cows and had a bulk tank SCC test done about every two weeks. Over 12 years, only 10 of 288 tests were greater than 200,000 cells per millilitre. This showed a low incidence of subclinical mastitis (infection with no signs in the cow) during the study period. Yet clinical mastitis incidence was high. E. coli was the leading culprit, causing 43 per cent of the herd' s clinical cases.
When the researchers looked at factors common to clinically infected cows, they found characteristics identified in previous studies. For example, clinical mastitis was more likely to occur among cows in early lactation and among those with higher milk yield. As well, these cows were thinner, with body condition scores of 1, and had retained placentas or milk fever. Herdmates with no clinical mastitis had better body condition scores, at 3 to 3.75, and no retained placentas or milk fever.
However, the researchers also found that cows getting clinical mastitis had lower SCCs before the infections compared to unaffected herdmates. Furthermore, the lower the cow's SCC before infection, the more severe was the clinical mastitis.
This suggests some cows, with very low SCCs, had a lower immune efficiency and a reduced ability to contain bacterial infections. Some kinds of mastitis infections, such as E. coli, might have had a better chance to cause more severe cases of mastitis- severe enough that clinical signs occurred.
Cows were more likely to get clinical mastitis if they had SCCs less than 100,000. As cell counts decreased by 20,000, this effect got stronger. Animals with SCCs between 0 and 20,000 were most likely to get clinical mastitis. For a cow's individual SCC to be an important factor for clinical mastitis, it must be very low.
While these findings were true for this herd, they might not be the same in all herds. The study needs to be repeated.
Nonetheless, it does make us realize that in herds with bulk tank SCCs consistently below 200,000, mastitis can potentially pose different challenges than it would in higher SCC herds.
We should also note, however , that most of the low SCC cows in the study did not get mastitis. Besides SCCs, other factors also played a role in determining whether a cow got clinical mastitis. Some related to retained placentas and milk fever . Still others related to management and housing.
Do herds with low bulk tank SCCs have more clinical mastitis?
Bulk tank SCCs don't give you the udder health status of individual cows in your herd. A few high-SCC cows, for example could push your bulk tank count higher. That reduces the quality of milk in your tank but doesn't mean all your cows have mastitis. Similarly, a low bulk tank SCC tells you most of your cows have low counts, but it doesn't tell you how many have very low SCCs, nor how low they might be.
Herds with low bulk tank SCCs are likely to have more very low SCC cows than herds with higher bulk tank SCCs. But there's no guarantee that this is true in every low-SCC herd. Nor is there evidence that all low bulk tank SCC herds are at risk of having more clinical mastitis cases.
Nevertheless, if you're concerned about having low-SCC cows that might be predisposed to clinical mastitis, you have to manage your herd to protect these animals from infections. For example, you might have to consider a different management system and improved housing.
Another set of researchers has looked for management and facility factors that might be associated with herds having more cases of clinical mastitis. They studied the yearly rate of clinical mastitis in 171 herds and compared them on a variety of differing herd management, housing and hygiene factors.
Herds with more clinical mastitis had more cows with stepped-on teats, and that leaked milk, and did not disinfect the maternity area. These herds also used post-milking teat dip, and had lower bulk milk SCCs.
Teat injuries and leakers commonly occur because of stall and platform design. Not disinfecting the maternity area frequently occurs because the area is rarely empty-too many calving cows for too few boxstalls-or because it requires a lot of manual labour and time. Associating these factors with more clinical mastitis strongly suggests that the barn designs of the study herds did not provide environments safe and clean enough to prevent their cows from having more clinical mastitis.
Using post-milking teat dip appears to predispose some very low SCC herds to more clinical mastitis-in particular mastitis caused by E. coli. Current milking hygiene routines are specifically designed to prevent the spread of contagious mastitis bacteria like Stap. aureus. Some of these practices may not be as necessary for low-SCC herds that have eradicated contagious mastitis sources and can maintain that status. However, Ontario has few herds like this.
There could be different reasons for a herd having low bulk tank SCCs and many clinical mastitis cases at the same time. As we've discovered, these herds could have several very low- SCC cows predisposed to clinical mastitis. Another possibility is that the owner is careful to keep milk from clinical cows out of the bulk tank.
Can you reduce bulk tank SCCs and avoid clinical mastitis?
As you reduce a high bulk tank SCC from greater than 250,000, you change key characteristics of many of the cows you milk. Low-SCC herds have more low-SCC cows. These cows face different challenges than cows in high-SCC herds. As the proportion of low-SCC cows in your herd rises, so too will the chance that eventually there will be some more susceptible to clinical mastitis.
Some herds with low bulk tank SCCs have, and will continue to have, more clinical mastitis if they don't meet the needs of their low-SCC cows. However, many herds with low SCCs also have low clinical mastitis rates.
These producers understand and have made necessary improvements and changes. Most of these steps, like improving the hygiene of housing at calving time and keeping teats free from injury, make good common sense.
As the research we've cited shows, factors contributing to low-SCC cows getting clinical mastitis relate to the cow herself, housing, management and milking practices. Research needs to continue and define further the practices that will have the greatest impact on reducing clinical mastitis cases. .
Low somatic cell count: a risk factor for subsequent clinical mastitis in a dairy herd. Suriyasathaporn W, Y.H. Schukken, M. Nielen and A. Brand. I Dairy Sci. 83: 1248. June 2000.
Risk factors for clinical mastitis in a random sample of dairy herds from the southern part of the Netherlands. Ethers A.R W, D. Miltenburg, D. DeLange, A.P.P. Crauwels, H. W Barkema and Y.H. Schukken J Dairy Sci. 81:420. February 1998.
By Ann Godkin - Veterinary Scientist/OMAFRA
Government of Ontario - Ministry of Agriculture, Food and Rural Affairs website