Mastitis in dairy cows: what do records tell us?

Find out how analysing data can help to guide your mastitis control plan.

To get the most out of mastitis records, they should be summarised and analysed at least every 3 months (more often, if practical).

From analysis of data comes understanding and from understanding comes effective action. Your milk recording organisation will provide useful reports, but different organisations may define mastitis measures slightly differently.  

Mastitis costs money, mainly due to the culling and replacement of heifers. A high culling rate for mastitis can hide some of the impact of mastitis on a farm; for example, bulk milk somatic cell counts (BMSCC) may be kept low by a high culling rate of cows with chronic mastitis.

Clinical mastitis records – the details

Clinical mastitis rate: number of clinical mastitis cases per 100 cows per year 

This is the number of quarter or cow cases recorded divided by the number of milking cows and multiplied by 100. If the result is more than 35 cases per 100 cows, or if it is higher than in the previous year, the farm should take action to improve mastitis control. If a quarter subsequently clears up and mastitis recurs 7 or more days later, this is treated as a recurrence of clinical mastitis.

Dry period origin clinical mastitis rate (in 12)

The proportion of cows affected with a new (first or ‘index’) case of clinical mastitis within 30 days of calving, out of every 12 cows that are ‘at risk’ (i.e., cows calved in the period that have been in milk for 30 days). If more than 1 in 12 (8%) had a new case of mastitis during the previous observation period, the farm should take action to improve mastitis control

Lactating period origin clinical mastitis rate (in 12)

The number of cows affected with a new (first or ‘index’) case of clinical mastitis after 30 days of lactation, out of every 12 cows that are ‘at risk’ (i.e., cows that are more than 30 days in milk). If more than 2 in 12 (16%) had a new case of mastitis during the previous observation period, the farm should take action to improve mastitis control.

Clinical mastitis cure rate (first case)

The proportion of new (first or ‘index’) cases that are cured as measured by non-recurrence of clinical mastitis and a reduction in somatic cell count.

Clinical mastitis recurrence rate

The total number of new cases is expressed as a proportion of the rate of all clinical mastitis cases to evaluate the impact of clinical mastitis recurrence. If this figure is high, your treatments are probably not very effective and/or you are dealing with chronic, difficult-to-cure infections. The national estimate is around 18%.

Heifer dry period origin clinical mastitis rate (in 12)

The number of parity one cows affected with a new (first or ‘index’) case of clinical mastitis within 30 days of calving, out of every 12 parity one cows that are ‘at risk’ (i.e., parity one cows calved in the period that are more than 30 days in milk).

Somatic cell count data – the details

Calculated bulk milk somatic cell count

The herd average SCC at each milk recording, calculated from individual cow SCCs and yields. If this is more than 150,000 cells/ml, or if it is increasing, the farm should take action to improve mastitis control. A general rule of thumb is for every 100,000 cells/ml increase in the herd bulk SCC, there is an 8–10% increase in the proportion of cows infected in the herd.

Percentage of cows in the herd with ‘chronic’ infection

The percentage of cows in the herd that have recorded an SCC over a threshold of 200,000 cells/ml more than once in the last three recordings.

Percentage of cows in the herd with an SCC of over 200,000 cells/ml

The percentage of cows in the herd with an SCC above a threshold of 200,000 cells/ml at the milk recording.

Lactation new intramammary infection (IMI) rate (%)

The rate of new infection, as measured by an increase in somatic cell count above a threshold of 200,000 cells/ml for lactating cows between successive milk recordings.

Dry period new intramammary infection (IMI) rate (%)

The rate of new infection, as measured by an increase in somatic cell count above a threshold of 200,000 cells/ml across the dry period (i.e., low cell count at drying-off but high cell count after calving). Those cows that were dried off with a low cell count and were below 200,000 cells/ml at first test after calving can be considered ‘protected’.

Apparent dry period cure rate (%)

The rate of cure as measured by a decrease in somatic cell count below a threshold of 200,000 cells/ml across the dry period (i.e., high cell count at drying-off but low cell count after calving).

Heifer dry period new intramammary infection (IMI) rate (%)

The rate of infection in first-calved cows, as measured by somatic cell count above a threshold of 200,000 cells/ml at first milk recording.

Key performance indicators for mastitis

Clinical mastitis data

*NMR 500: National Milk Records 500 Herd Study

Somatic cell count data

*NMR 500: National Milk Records 500 Herd Study
**NMR chronic somatic cell count (SCC) cows, in which the current and previous milk samples both had SCCs of 200,000 cells/ml milk or greater – update to 2019

When do your cows get mastitis? What does your data tell you?

Most of this data analysis is available in reports from your milk recording organisation, but there are some simple calculations you can do yourself.

Mastitis data calculations

What do changes in somatic cell count (SCC) over the dry period mean?

Looking at the changes in somatic cell counts of individual cows between the SCC at the end of one lactation and the SCC in the first 30 days of the next lactation can provide a lot of useful information on what is happening in the udder during the dry period. The best chance of curing mastitis is during the dry period. The other key aim is to protect cows from picking up infections during the dry period. Monitoring the changes can tell us how well management in the dry period is working.

Source of data used on this webpage: Fifth Report, GB Cattle Health & Welfare Group, November 2020 (PDF)