Guide to using Johne’s tests to cull in dairy herds

Culling Johne’s-positive cows is a key area of Johne’s disease control and often an intervention that farmers struggle to get right.

There is no universal template for making culling decisions about Johne’s and an appropriate culling strategy must be applied to the farm’s individual situation.

This is according to vet Peter Orpin, director of Park Vet Group and co-developer of Myhealthyherd, who stresses that a “test and cull” approach on its own is not sufficient to control Johne’s.

We outline the steps you should take.

Step one: Understand the disease

Before you consider culling, it’s important to understand how the Johne’s bacteria – known as Mycobacterium avium, subspecies paratuberculosis (and often abbreviated to MAP) – works to produce Johne’s.

See also: Search 4 common Johne’s mistakes and how to avoid them on dairy units

In its clinical form, Johne’s presents as thin cows with diarrhoea, but a herd can have high disease prevalence without any clinical signs of diarrhoea.

“Managing Johne’s is really about preventing bacteria spreading from infectious or shedding cows to susceptible calves,” says Mr Orpin. “Culling is a tool used to remove the most infectious animals from the herd.”

The MAP bacteria are controlled by the immune system of the animal in the early years. Eventually, a combination of stress and challenges to the cow – such as calving – triggers the latently infected animal to lose control of the disease and to start to shed bacteria into the environment though faeces.

This progresses over some months to the animal losing weight and production and, ultimately, in developing clinical Johne’s.

Culling cows is therefore a key part of the Johne’s management plan. Blood, faeces and milk tests can provide information about the most economic time to cull the cow. 

Step two: Develop a comprehensive Johne’s management plan

Before you consider culling Johne’s disease out of your herd, it is important to look at a comprehensive control plan with your Johne’s-trained vet, which will centre on the following areas:

• A typical management plan for controlling MAP and Johne’s should involve preventing spread of infection from an infected animal to a calf in maternity areas, via milk and colostrum or via manure contamination of pastures and feed for older cattle.
• A comprehensive approach, using culling to remove infectious animals and effective herd management and segregation to protect young calves from infection, is vital, as more than 80% of Johne’s infection occurs in the first two months of life.
• The plan needs to be sufficiently detailed to be effective and the whole farm team have to implement the plan every day for several years for the programme to be effective.

Step three: Deciding what cows you should cull

Culling must be used as part of an effective management plan set up by your vet, explains Mr Orpin. 

The aim of the programme is to remove infected and infectious cows at the most cost-effective time for the farm.

This is to be informed by milk test results, which can either be expressed as a decimal – for example, 0.6 (Cattle Information Service) – or as a whole number – 60 (National Milk Records and Quality Milk Management Services) – which relates to the antibody presence in the milk. 

The standard advice for herds using the quarterly milk testing programmes would be:

• Cull the red cows (0.6/60 or more for two consecutive tests or any cow above 1.0/100 once) before calving
• Ensure amber cows (one result at 0.3-0.6/30-60) are clearly marked with tags
• Segregate amber cows from green cows at drying-off until re-entry into the milking herd

Mr Orpin stresses that culling approaches vary according to disease prevalence, system and farm economics, and typically involve three decisions when analysing Johne’s test results.

1. Cull the most severely affected, “heavy shedding” cows within three to six months
2. Do not serve red cows – cull at the end of lactation or within 12 months
3. Serve any positive cows to terminal sires – cull within 18 months

However, he warns that each farm needs to define its own culling criteria based on detailed advice provided by their own vet.

For farms with MAP prevalence (15-20% of the herd), Mr Orpin says it can be a financial blow if large numbers of cows need to be culled.

Therefore, each farm requires a tailored culling policy, which is influenced by the following areas:

• Availability of replacement heifers How many replacement heifers are coming through the system to replace outgoing cows? Often the number of replacements may constrain the culling policy for Johne’s.
• Culling pressures Other pressing demands for cow removal, such as infertility and lameness, may influence the number available for Johne’s removal.
• Johne’s disease prevalence The Johne’s management plan is often dependent on the prevalence. If the MAP prevalence is less than 3%, farmers can afford to be more aggressive with their culling and adopt an improved farm management and test and cull strategy, whereas with higher prevalence it may be more economic to cull out the most severely affected animals and segregate or manage the other infected cows (improved farm management and test and manage). Your Johne’s vet will help you choose the correct strategy for your herd.
• Replacement policy Are heifers being bought in? Are all heifers home-bred? Are a portion of pedigree heifers sold? Is it a flying herd? Can you afford to repopulate with bought-in stock?
• Cull value Often the testing programme can be used to remove animals well before they show clinical signs, thus optimising the cull value for the cow. Alternatively, you may be able to secure another calving from cows just starting on the Johne’s journey, and a certain proportion of cows can be bred to terminal sire.