Mastitis – Culling Decisions

Culling is a key strategy in managing mastitis on farm as it is the only way to eliminate some infections.  It is, however, expensive so it is important that culling decisions are made wisely and that other measures are taken to prevent new infections occurring, in order to achieve good mastitis control.

When making culling decisions we want to choose infected cows that have a low chance of curing over the dry period.

Antibiotic dry cow therapies do not cure all infections.  The following factors reduce the chance of an infected cow curing over the dry period:

  • Cows that have had repeated clinical cases of mastitis in one season. Cure rates are approximately 75% for first cases of mastitis, 45% for second cases and 12% for third cases (consider culling immediately after the third case or drying off the quarter if the same quarter is being affected).
  • Infections of long duration. Cows that had a high SCC on the last herd test last season and the first this season despite being treated with antibiotic DCT are likely to be chronically infected and have a low chance of curing.  Cows that have had high SCC only in this season are candidates for Antibiotic Dry Cow Therapy.
  • Staph aureus infection
  • Older cows

Subclinically infected cows have a bacterial infection (and usually a high SCC) but no clinical signs of mastitis.  A higher proportion of subclinical infections are contagious bacteria (as opposed to environmental) and pose a significant risk for the spread of infection within the herd.

Somatic cell count data from herd tests and clinical mastitis records are the main sources of information used to make culling decisions.  There are various options available for culturing milk samples from individual cows to identify the bacteria involved and/or sensitivity to different antibiotics.  These include laboratory testing through your vet, Mastatest on farm testing, DairySmart and Staph aureus testing via your herd test.  Taking sterile milk samples for bacterial culture can help justify a culling or treatment decision.  However, some Staph aureus infections do not reliably yield a positive culture result as the bacteria are good at hiding inside cells and emerging intermittently into the milk.  Conversely, a positive Staph aureus culture may still have a good chance of cure in a young animal with a relatively recent infection. 

SCC, clinical mastitis history and age should be considered alongside culture results.  Beware of making decisions from a single piece of information.  Without a comprehensive approach to mastitis control (including strict hygiene at teatseal/DCT insertion, milking management, early detection of mastitis and culling decisions) new cows will always be waiting to jump into the millionaire’s club.

Culling 1-2% of the herd each year for mastitis reasons is a realistic benchmark.  Whether it is economic to cull more than this will depend on the impact individual cows are having on the BTSCC, the consequent penalties or loss of incentives, the risk of mastitis spreading to other cows and the cost of replacements.

Talk to your vet if you would like help with making culling decisions for your herd.  Dairy Vets can provide expertise in all areas of mastitis management including investigating somatic cell count and clinical mastitis problems, milking management assessments, teat scoring, data analysis, milk culture, treatment decisions and staff training.

Johne’s Testing

Request a Johne’s test on your next herd test if you have not already done so!

It is that time in the season that is the most opportune moment to perform a whole herd screen Johne’s test to find positive cows.  Removing these cows now saves on the costs of wintering them and reduces the chances of infection in the biggest risk period during calving.

Cows infected with Johne’s generally do not perform well over the challenge of transition and calving, and it is often enough to tip them over the edge and they deteriorate clinically.  In one case study performed by LIC, a third of cows testing high positive had died on farm during the calving period.  Cows testing positive are more likely to leave the herd by dying on farm than going to the works, compared with cows testing negative.

Any cows which test positive, should be retested with a blood test to confirm the identity of that cow.  In a season where reproductive results have been strong, there may be more flexibility to remove cows from the herd which test positive.  It may well aid culling decisions, looking at SCC, udder conformation and mobility, if Johne’s status is known also.  It is also becoming common practice for buyers to request a Johnes test before purchasing cows.

LIC Johne’s Dashboard Development

LIC are currently developing a dashboard, via MINDA, to better display and manage Johne’s positive cows within the herd, based on historic and current Johne’s results on farm.  It is currently in a trial period at the moment, but from what we have seen it is looking to be useful tool to help visualise the extent of a JD problem on each farm. Particularly interesting is showing which cohorts have been most at risk to infection, and benchmarking compared with national average.


It is still in a prototype phase, but may well prove to be an important tool in seasons to come. Watch this space.

Below is a extract, taken from LIC’s website, about the importance of testing and showing how missing a year’s testing can cause Johne’s prevalence to resurge.

Prevalence of JD positive and high positive results in herds testing with LIC 2013-2023

* Note this is industry median data so may not predict what may happen on individual farms.

Key takeouts from this graph:

  • Farms that had six consecutive annual tests made the most progress in terms of reducing prevalence (3.3% to 1.3% over the five years since their first test).
  • Herds that missed two annual tests within this same period tended to have a rebound in prevalence (from their initial starting point, a median of 2.80% to their lowest median prevalence of 1.71% after three years to 1.96% after five years).
  • Median prevalence rebounded in herds that missed four annual tests to levels close to those identified when testing started (2.94% at year 0 to 2.04% after three years to 3.07% after five years).


Interpreting the results

The Johne’s ELISA is a very specific test – which means if a cow tests positive we know she is positive.  However, sensitivity is poor, which means if a cows tests negative, it does not necessarily mean she isn’t infected with Johne’s disease.  In any case, culling positive cows is the best way to make big leaps in managing Johne’s.

Below is a summary table explaining what each individual result means.

Testing and culling cows is of course only part of the solution. Management of the disease on farm will help mitigate spread. Below are some good resources available discussing management options, and further information on testing.



Dairy NZ Johne’s Disease Management –

Dairy NZ Johne’s Laboratory Testing –

LIC Johne’s Herd Test –

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