Mastitis causes more than £160 million loss in milk production every year. Andrew Bradley, from Bristol University, explains how best to control the problem.
The dry period
Mastitis is an infection of the udder, most commonly caused by bacteria, which is why dry-period udder health management is so important.
There is a misconception that cows are not susceptible to new intramammary infections during the dry period.
Large numbers of new intramammary infections do occur during the dry period.
The cow is at her most susceptible in the early and late dry period and is relatively resistant to new infection in the mid dry period, as the diagram below shows.
The infections picked up in the late dry period are the ones most likely to impact on udder health in the next lactation.
Keratin plug
A second myth is that a natural keratin plug forms in the streak canal during the dry period, forming an effective and long-lasting seal against infection.
Research in New Zealand showed that more than half of quarters do not form a functional keratin plug by 10 days post drying-off and 5-10% of quarters never form a functional keratin plug.
Measuring the problem
Up to 70% of clinical mastitis occurring in the first month of lactation may be of dry-period origin (see diagram on opposite page).
In herds with individual cow somatic cell count records, an insight into the importance of the dry period can be gained by looking at the movement of somatic cell counts around the 200,000 cells/ml threshold.
The success of dry-period management can be quickly and easily assessed by calculating the proportion of cows with cell counts above 200,000 cells/ml at their first recording in lactation.
The target value should be less than 5%, herds with figures above this warrant further investigation of dry-period management.
A final option in non-recording herds is the use of the California Mastitis Test (CMT).
Used at four days post calving, and in the hands of a suitably trained operator, the test will detect more than 80% of infected quarters and can be a revelation to many producers.
Dry cow therapy
Dry cow therapy has two aims – cure and prevention of existing and new intramammary infections respectively.
In recorded herds, cows can easily be classified as “likely to be” infected or uninfected, using individual cow somatic cell counts.
It is then possible to select the product most appropriate for cure in the infected cows and prevention in uninfected cows.
Selection of dry cow therapy for the majority of cows should be based on prevention, rather than cure.
After all, most are uninfected at dry-off, so your product selection needs to reflect this.
Antibiotics provide good protection at the start of the dry period, but their efficacy may wane with time.
Care should be taken to choose products that are effective at the end of the dry period, which is also a very vulnerable time for mastitis infection.
Dry cow environment
Dry cows and their environment are often the most neglected on a unit.
This approach is usually driven by the misconception that dry cows are resistant to infection.
Another common mistake is to assume that dry cows at pasture are not at such a high risk of infection.
Unfortunately, dry cows are often poorly managed at pasture, particularly late in the season, and large numbers of new infections may occur (see diagram on opposite page).
All year round management of the environment is an essential component of dry period udder health management.
Minimising the impact of dry period infection
As milk yields have risen, it is increasingly important to address the yield of cows prior to drying off, rather than waiting until after drying off.
The rationale being that cows with a higher yield at drying off not only take longer to form a functional keratin plug but are also significantly more likely to acquire an infection during the dry period.
In a recent study each one-litre increase in the yield at drying off resulted in a 6% increase in the chance of a cow becoming infected during the dry period.
That equates to a doubling in the risk of a new infection between a cow dried off at 20 litres as opposed to 10 litres.
Minimising the impact of dry period infection
Steps can be taken to reduce the impact of dry period infection – by ensuring optimum nutrition, and in particular ensuring adequate Vitamin E and selenium levels and minimising negative energy balance at and around calving.
Lack of Vitamin E, selenium and negative energy balance have all been shown to have an adverse effect on the immune system and are likely to result in an increase in the proportion of sub-clinical infections becoming clinical.
A.J.Bradley@bristol.ac.uk