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.

Return Interval Analysis and Non-Returns


The non return rate (NNR) is the percentage of inseminations that occur early in the AB period where the cow did not return to heat within the maximum normal estrous cycle period (24 days). Cows that do not return to heat are either pregnant or phantom ( a non return that is not pregnant).

NRR can be an accurate estimation of conception rate (CR) if heat detection is accurate.

A low NNR (<64%) may be an early warning sign of poor conception rate. This could be due to a number of factors including inaccurate heat detection, poor insemination practice, disease, low body condition at calving or excessive loss of condition, a high number of late calvers or high number of treated non-cyclers.

Return interval analysis

A return interval analysis looks at the spread of returns to heat for cows that have been mated in the early AB period.

A cow will normally cycle every 18-24 days if she does not become pregnant. Reasons for short (<18 days) or longer (>24 days) returns include: inaccurate heat detection (missed or invented heats) and early embryonic loss.

By monitoring the NNR and return interval analysis for your herd from about 4 weeks after the planned start of mating there are some actions that can be taken to reduce the impact of poor heat detection and conception rate on the final mating result if a problem is detected. These may include early pregnancy testing, hormonal intervention, additional heat detection aides, changes to insemination practice or altering the length of mating period. Actions to improve results for next year can also be started early.

Please get in contact if you would like to do some early scanning to identify and treat phantom cows. We can scan cows from 35 days – phantom cows can get enrolled in a CIDR programme. The other advantage of early scanning is to determine an accurate conception rate for those cows.

Managing Calf Scours

Towards the end of August scouring calves become more common. It is likely that this is because the pathogen load in the calf pens has risen at the same time that colostral antibodies are declining.

There is a risk period between about 1 and 4 weeks of age when antibodies from colostrum ingested in the first day (passive immunity) has declined but that from exposure to pathogens (active immunity) is not yet optimal.


If only a few calves are scouring, it is worth isolating these in a sick pen. If more become sick, it is best to leave the calves where they are. One teaspoon of rotavirus scour can infect thousands of calves so if the cause is infectious, it is likely all calves in the pen have already been exposed. It is still a good idea to put calves that are worse affected in a small pen together as they can be treated and monitored better than in a big group. If left together ad lib electrolytes will often be drunken by the healthy calves with the ones that need it more missing out.

Avoid overcrowding pens. If pens must be reused, consider replacing or topping up the bedding. Keep hay feeders high so hay stays clean and calves are not eating off the ground. Clean water troughs daily. This will help reduce the spread of disease and encourage water intake.


Electrolytes and feeding:

. Scours cause loss of fluids and body salts leading to dehydration. Electrolytes are the main treatment for scouring calves. Below is a treatment protocol for calves with mild to moderate scours.

Scouring calves should continue to be fed milk if they are willing to drink it. Weight loss will be less than if fed electrolytes alone and recovery faster.

Warm milk will encourage sick calves to drink.

It is recommended that scouring calves are fed milk twice a day with electrolytes in between.

Unwell, scouring calves are less likely to want to drink one larger feed and will receive more nutrition if fed twice.

It is best not to tube milk to scouring calves – if a calf refuses to drink milk, replace that milk feed with an extra electrolyte one. If they are still refusing to drink after 48 hours, consult your vet.

Tubing electrolytes is fine, and often necessary.

Use a high quality electrolyte such as Enerlect.



Scouring calves often feel miserable. Rheumocam (1ml/40kg under the skin) reduces fever and inflammation and will help get the calf back drinking again.

Talk to your vet if you are having scours in your calves. They can provide further advice on managing the outbreak and test samples to see what pathogen/s are involved. This will help determine if additional treatments are needed and will help with strategies for prevention in the future.

The risk period makes preventing scours challenging, however colostrum management in the first 12 hours of life is still the number one factor in preventing scours or reducing the severity of illness.

Calves that have a higher initial peak in colostral antibodies will take longer to decline and will be less severely affected if infected. Ensure every calf receives 4-6L of 22% + brix tested colostrum within 6-12 hours of birth. Organise calf blood testing with your vet annually to check that colostrum managements is working on your farm.


Multimin has New Zealand data to show that calves injected with 1ml at birth have a 50% reduction in the risk of disease and death until weaning. Optiguard is a pathogen binder that may help prevent and minimize the severity of scours. Vaccinating with Rotavec or Scourguard will boost antibody levels to Rotavirus, Coronavirus and E.coli in colostrum. Continuing to feed fresh or preserved transition milk for as long as possible, especially from vaccinated cows will help ‘mop up’ pathogens in the gut as it passes through.

Vaccinating to prevent calf scours

Rotavirus is the most commonly diagnosed cause of calf scours in New Zealand. 
Outbreaks of scours can cause a huge amount of stress at a busy time of the year and there can be high rates of sickness and death.  The lining of the gut can be severely damaged with life-long effects on health and productivity. Shed preparation and hygiene practices are important to reduce the viral load in the environment however there is little that can be done to prevent exposure to Rotavirus.  It is shed by adult cattle and can survive in the environment between seasons.  Rotavirus is extremely infectious and opens the calf up to infection with other organisms such as Cryptosporidium, coronavirus and Ecoli K99.  The pressure of our seasonal calving system creates a high-risk environment. For these reasons, the greatest protection comes from practices that boost the calf’s own immunity.  Rotavirus vaccination is a valuable tool to help achieve this.  Rotavirus vaccines produce protection against Bovine Rotavirus, bovine coronavirus and Ecoli K99. Cows are vaccinated between 12 and 3 weeks before they are due to calve to ensure the optimal level of antibodies are present in colostrum at the first milking.
There are 2 ways in which Rotavirus vaccines works to protect calves from scours:

1. Boosting Rotavirus, coronavirus and Ecoli K99 antibodies in the calf via passive transfer of immunity.  Good colostrum management is essential to ensure that the benefits of Rotavirus vaccination are fully realized.  Calves must be fed 4-6 L of top quality (22+% brix reading) colostrum from vaccinated cows within 12 hours of birth (ideally 6).  Blood sampling calves to check for adequacy of passive transfer can be valuable to assess how well your system is working.

2.  When calves are fed colostrum from vaccinated cows, antibodies bind Rotavirus organisms as they pass through the gut.  Feeding stored colostrum and transition milk (from the first 8 milkings), for as long as possible, will maximise the length of time calves are protected for.  Ensure colostrum is preserved and stored correctly.
When Rotavirus vaccination is implemented alongside excellent colostrum management, morbidity and mortality from calf scours can be greatly reduced. 

There are two commonly used vaccines available: Rotavec is a one dose product and Scourguard is a two dose (but can be used to boost a Rotavec vaccinated cow). Talk to your vet to plan Rotavirus vaccination for your herd.