Turning milk quality indicators into on-farm solutions

High bulk milk somatic cell counts can have many different farm-specific causes. By correctly calculating and interpreting key indicators for subclinical mastitis, these causes can be identified and addressed more effectively. A high rate of new intramammary infections (IMI) during lactation often points to issues with milking equipment, milking routine, or infection pressure in lactating cow housing. In contrast, a high proportion of cows calving with an elevated somatic cell count (SCC) usually indicates problems during the dry period, such as increased infection pressure, insufficient cure of existing infections, inadequate monitoring of high SCC cows during lactation, or antimicrobial resistance. When all indicators are considered together, a wide range of possible causes may emerge. A true milk quality detective uses these figures to pinpoint the most effective, farm-specific solutions. In the following article, we take a closer look at these indicators and illustrate their interpretation with practical on-farm examples.

1. Introduction

The solutions for achieving and maintaining good udder health on dairy farms are well known and are included in the National Mastitis Council’s “10-point mastitis prevention and control program”. Nevertheless, in practice it often remains difficult to achieve sustained improvements and to regularly monitor udder health on dairy farms. One reason for this is that we too often focus on the “solutions” rather than on the “problem.” When giving advice, we tend to correct every management deviation and adjust everything that does not align with guidelines, without sufficiently questioning whether these issues are actually related to the problem at hand. As a result, dairy farmers often feel overwhelmed and demotivated. Moreover, this approach frequently leads to frustration, as attention is commonly directed toward the most obvious causes – such as milking technique, the milking machine, or hygiene in lactating cow housing – while the real problem may lie with dry cows or pregnant heifers.

Although the range of solutions to improve udder health is similar across dairy farms, the underlying problem and its causes often differ from one farm to another. To truly succeed in improving udder health, it is essential to start by analysing the problem and to identify and propose the most effective, evidence-based solution for each farm-specific issue, grounded in facts and data.

Two dairy farms facing a high bulk milk somatic cell count (SCC) may therefore require very different solutions. On one farm, a high bulk milk SCC may be caused by a high proportion of new intramammary infections (IMI) during lactation, making the milking machine, milking routine, and housing of lactating cows the main areas of concern. On another farm, the elevated bulk milk SCC may result from too many cows calving with high SCC. In this situation, changes to the milking routine or lactating cow housing will not solve the problem, and the solution must instead be sought in dry cow management.

Farm-specific causes of mastitis problems can be identified using farm-specific data. Regular individual cow SCC measurements (i.e., every 4 to 6 weeks), combined with accurate recording of clinical mastitis cases, are therefore essential for improving and monitoring udder health on dairy farms. This article takes a closer look at the calculation of various key indicators that can help elucidate farm-specific causes of milk quality problems related to subclinical mastitis and guide the selection of the most appropriate solutions.

2. Milk quality monitoring indicators

To obtain a good overview of udder health on a dairy farm, it is preferable to analyze all available records from the previous year. In routine udder health monitoring based on milk recording data, emphasis should be placed on the analysis of the average herd milk SCC (see Section 2.1), on infection dynamics during lactation (see Section 2.2) and on infection dynamics over dry period (see Section 2.3). Still, it should be mentioned that on dairy farms with fewer than 200 lactating cows, it is often difficult to correctly interpret infection dynamics during the dry period at 4- to 6-week intervals (see Section 2.3) due to the limited number of cows calving.

2.1 Bulk milk somatic cell count

The bulk milk SCC is one of the main parameters used to monitor udder health on dairy farms. In most countries, it is determined by milk quality control laboratories and measured at least four times per month. From a somatic cell concentration of 200,000 cells/mL onwards, farmers are advised to take action, as this is a clear signal that a substantial proportion of cows have an IMI in at least one quarter.

In Europe, when the geometric mean of the results from the previous three months (12 measurements) repeatedly exceeds 400,000 cells/mL, penalty points may be applied, resulting in a reduced milk price or even suspension of milk collection. In some regions, financial penalties are imposed at even lower thresholds.

However, the bulk milk SCC shows only a moderate correlation with the average herd milk SCC (see below), particularly in herds with high bulk milk somatic cell counts. The average herd milk SCC represents the mean SCC of all individual cows. One important reason for this discrepancy is that farmers often withhold milk from problem cows from the bulk tank and use it, for example, to feed calves. As a result, relying solely on the bulk milk SCC can seriously underestimate the true level of IMI within the herd.

Therefore, when regular individual SCC measurements are available (milk recording every 4 to 6 weeks), the average herd milk SCC is the preferred parameter.

2.2 Infection dynamics during lactation

The average herd milk SCC is correlated with the percentage of cows showing an elevated SCC (typically ≥ 200,000 cells/mL) at milk recording. Cows with an elevated SCC have either acquired a new IMI (= newly infected) or have failed to cure an existing IMI (= chronically infected) since the previous milk recording.

As long as the number of newly infected cows per month exceeds the number of cows recovering from an infection (cure), the proportion of cows with elevated SCC – and consequently the herd SCC – will increase. Infection dynamics are therefore determined by the percentage of newly infected cows, the percentage of cured cows, and the percentage of chronically infected cows (Figures 1 and 2). Table 1 provides an overview of the calculations, targets, and alert thresholds for these indicators.

Figure 1. Graph illustrating a dairy farm with a high herd average somatic cell count (263,000 cells/mL) due to a high percentage of new intramammary infections (IMI) during lactation in combination with a low (spontaneous) cure rate (based on Keno™-M, MEXCELLENCE, Ghent, Belgium). Linear score: [Ln (SCC/100,000 cells/mL) / Ln 2] + 3.

Figure 2. Graph illustrating a dairy farm with a low herd average somatic cell count (168,000 cells/mL), despite a high percentage of new intramammary infections (IMI). Thanks to a high spontaneous cure rate among cows with elevated somatic cell counts at the previous milk recording, the herd average somatic cell count can be maintained below 200,000 cells/mL (based on Keno™-M, MEXCELLENCE BV, Ghent, Belgium). Linear score: [Ln (SCC/100,000 cells/mL) / Ln 2] + 3.

2.2.1 Percentage new infections during lactation

The percentage of new infections is calculated as the number of cows that experienced an increase in SCC from < 200,000 cells/mL at the previous milk recording to ≥ 200,000 cells/mL at the current milk recording, divided by the total number of lactating cows with at least two consecutive SCC measurements. The target value for the percentage of new IMI is ≤ 5% per month, and the alert threshold is ≤ 8% per month (Table 1).

Table 1. Definitions, target values, and alert thresholds for indicators related to infection dynamics during lactation.

2.2.2 Percentage cured infections during lactation

The cure rate is calculated as the number of cows that experienced a decrease in SCC from ≥ 200,000 cells/mL at the previous milk recording (cows “at risk” of cure) to < 200,000 cells/mL at the current milk recording, divided by the total number of cows with an elevated SCC (typically ≥ 200,000 cells/mL) at the previous milk recording.

A (spontaneous) cure rate of ≥ 40% per month should be targeted, which corresponds to an average infection duration of approximately 2.5 months. The alert threshold is 30%, corresponding to an average infection duration of approximately 3 months. Cows that do not cure (spontaneously) are considered chronically infected (Table 1). An insufficient cure of new high SCC cows is due to either an impaired immunity, an inappropriate treatment protocols for clinical mastitis or a lack of monitoring of subclinical mastitis (Table 2).T

Table 2. Risk factors for insufficient cure of new high SCC cows

2.2.3 Percentage chronic infections

The percentage of chronic infections is calculated as the number of cows with a SCC  ≥ 200,000 cells/mL at both the previous and the current milk recording, divided by the total number of lactating cows with at least two consecutive milk recordings. This definition is also used in the international scientific literature and forms the basis for the threshold values described below.

In some countries and regions, an infection is only considered chronic when cows have shown an elevated somatic cell count for at least three consecutive months. The target value for the percentage of chronic infections is ≤ 10%, with an upper alert threshold of 15% (Table 1).

2.2.4 Contagious versus environmental (opportunistic) pathogens

When interpreting mastitis problems at herd level, it is important to distinguish between the ecology and the epidemiology of mastitis pathogens. Although these concepts are closely related, they describe different aspects of pathogen behavior and have different implications for control strategies.

The ecology of a pathogen refers to its natural habitat and ability to survive and multiply in specific environments. Staphylococcus aureus and Streptococcus agalactiae are host-adapted mastitis pathogens: they rely on the cow and/or the udder for survival and replication. Their primary ecological niche is the mammary gland and teat skin. In contrast, Streptococcus uberis shows a more heterogeneous ecology. While many strains are predominantly environmental, other strains appear to be more host-adapted and capable of persisting within the udder.

The epidemiology of a pathogen describes how bacteria are transmitted within a herd and new infections occur. For Staphylococcus aureus and Streptococcus agalactiae, the epidemiology is clearly cow-to-cow transmission, primarily during milking. The combination of host-adapted ecology and cow-to-cow transmission means these pathogens are classified as contagious mastitis pathogens. Table 3 gives an overview of the fey factors contributing to a high bulk milk somatic cell count due to infections caused by contagious pathogens.

Table 3. Key factors contributing to a high bulk milk somatic cell count due to infections caused by contagious pathogens.

For Streptococcus uberis, however, the epidemiology is fundamentally different. Despite the fact that some Streptococcus uberis strains may be host-adapted and capable of causing persistent intramammary infections, transmission typically occurs opportunistically from the environment, rather than directly from cow to cow. As a result, the presence of persistent Streptococcus uberis infections on a farm does not necessarily imply that Streptococcus uberis should be considered a contagious pathogen. Instead, persistence reflects strain characteristics and host–pathogen interactions, not a change in transmission route.

Because contagious mastitis pathogens are well adapted to the cow and the mammary gland, they often cause chronic IMI. Chronically infected cows, in turn, act as a source of infection for herd mates. A high rate of new IMI caused by contagious pathogens suggests rapid transmission between cows during milking. In contrast, opportunistic mastitis pathogens, including Streptococcus uberis and Escherichia coli, do not require the cow or the udder to survive or multiply. The environment serves as the reservoir of infection. New IMI typically occur between milkings through the transfer of bacteria from the environment to the teats when cows lie down and the teats come into close contact with contaminated surfaces. Penetration of the teat canal may also occur due to reverse milk flow. In such cases, after removal of the milking unit, teats may be wet with milk or a drop of milk may remain at the teat end. During milking, bacteria and fecal particles can detach from the teat surface and be carried via milk to the teat orifice, or even be propelled into the teat canal. A high rate of new IMI caused by environmental pathogens suggests insufficient hygiene before, during, or after milking.

Bacteriological culture of milk samples collected from cows with clinical or subclinical mastitis remains the best method for identifying the bacterial cause of udder health problems on dairy farms. However, recent bacteriological results are often not available. As an initial step, a bulk tank milk sample can be collected for bacteriological culture or PCR analysis to determine whether contagious pathogens such as Streptococcus agalactiae, Staphylococcus aureus, or Mycoplasma spp. play a role in the problem. Occasionally, mastitis caused by Streptococcus uberis or Escherichia coli may also be detected in bulk tank milk samples. A limitation of bulk tank milk analysis, however, is that it does not allow differentiation between bacteria originating from undetected mastitis cases and contamination from the milking equipment (Table 4).

Table 4. Key factors contributing to a high bulk milk somatic cell count due to infections with environmental pathogens.

Given the differences in epidemiology between contagious mastitis and opportunistic mastitis pathogens, the likely cause of deteriorating udder health can also be inferred from the relationship between the percentage of newly infected cows and the percentage of chronically infected cows. A strong correlation between the percentage of new IMI and the percentage of chronic IMI strongly suggests the presence of contagious mastitis pathogens, as these bacteria spread easily from cow to cow (i.e., a high rate of new IMI) and are difficult to cure spontaneously (i.e., a high rate of chronic IMI) (Figure 3). A weak correlation between the percentage of new IMI and the percentage of chronic IMI is typical of farms experiencing environmental mastitis problems (Figure 4).

Figure 3. Typical example of a contagious model. In this example, the percentage of newly infected cows per month and the percentage of chronically infected cows over six consecutive milk recordings are plotted against each other. There is a clear positive correlation between the percentage of new infections during lactation and the percentage of chronic infections. When the percentage of new infections during lactation increases, the percentage of chronic infections also increases, and vice versa. The R² value (= coefficient of determination) is 0.53, indicating that 53% of the variation in the monthly new infection rate is explained by variation in the chronic infection rate, and vice versa.

However, when effective measures have already been implemented on a dairy farm to prevent the spread of contagious mastitis pathogens during milking (Table 3), this reasoning may no longer apply. In such cases, a dairy farm facing contagious mastitis pathogens but no longer showing a strong correlation between new and chronic IMI may indicate that implemented measures – such as wearing gloves during milking, appropriate post-milking teat disinfection, cluster disinfection after milking infected cows, or mastitis vaccination – are effective.

Figure 4. Typical example of an environmental model. In this example, the percentage of newly infected cows per month and the percentage of chronically infected cows over six consecutive milk recordings are plotted against each other. There is a weak correlation between the percentage of new infections during lactation and the percentage of chronic infections. The rate of new infections during lactation appears to be independent of the rate of chronic infections, and vice versa. The R² value (= coefficient of determination) is 0.02, indicating that only 2% of the variation in the monthly new infection rate is explained by variation in the chronic infection rate, and vice versa.

2.3 Infection dynamics during dry period

Udder health problems related to dry cow management can be readily identified by calculating the percentage of freshly calved cows with an elevated somatic cell count, the percentage of new infections during the dry period, and the percentage of infections cured during the dry period. Table 5 provides an overview of the calculations, target values, and alert thresholds for these indicators.

Table 5. Definitions, target values, and alert thresholds for indicators related to infection dynamics over dry period.

2.3.1. Percentage of freshly calved cows with an elevated somatic cell count

The percentage of adult freshly calved cows with an elevated SCC is calculated as the number of adult cows with an SCC ≥ 200,000 cells/mL at the first milk recording after calving (maximum 42 days in milk), starting from 10 days in milk, divided by the total number of animals that had their first SCC measurement at that milk recording. A percentage of freshly calved cows with an elevated SCC exceeding 15% (target value) indicates that too many cows acquired a new IMI or failed to cure during the dry period (2). The upper alert threshold is ≤ 18% (Table 5).

2.3.2. Percentage of new infections during the dry period

A new IMI is considered to have occurred during the dry period when the SCC was < 200,000 cells/mL at the last milk recording before dry-off and ≥ 200,000 cells/mL at the first milk recording after calving (2). The percentage of new IMI during the dry period is calculated by dividing the number of cows with a SCC < 200,000 cells/mL at the last milk recording before dry-off and ≥ 200,000 cells/mL at the first milk recording after calving by the total number of cows with a somatic cell count < 200,000 cells/mL at the last milk recording before dry-off (i.e., cows “at risk” of becoming infected during the dry period) and that had a first milk recording after calving at that test. The target value for the percentage of new IMI during the dry period is ≤ 10%, and the upper alert threshold is ≤ 15% (Table 5).

2.3.3. Percentage of infections cured during the dry period

The percentage of IMI cured during the dry period is calculated by dividing the number of cows whose SCC was ≥ 200,000 cells/mL at the last milk recording before dry-off and < 200,000 cells/mL at the first milk recording after calving by the total number of cows with a somatic cell count ≥ 200,000 cells/mL before dry-off (i.e., cows “at risk” of cure during the dry period) that had a first milk recording after calving. The target value for the percentage of IMI cured during the dry period is ≥ 80%, and the lower alert threshold is ≥ 75%.

Animals with elevated SCC at dry-off, but not more than three times during the last three milk recordings, are not considered persistently infected and are expected to cure during the dry period, assuming they were treated with antibiotics at dry-off. If this is not the case, antimicrobial resistance or the presence of a mastitis pathogen known to be difficult to treat (e.g., Staphylococcus aureus, Klebsiella spp.) should be suspected. Bacteriological culture and antimicrobial susceptibility testing of the isolated pathogens can provide definitive answers.

On dairy farms with a high bulk milk SCC, insufficient cure during the dry period is often primarily attributable to cows that already had elevated SCC at multiple milk recordings during the previous lactation. More intensive monitoring of cows showing increases in SCC during lactation will therefore generally result in improved cure rates during the dry period in the medium term. Although no specific research has yet been conducted on this topic, similar to infections during lactation, the likelihood of cure during the dry period likely decreases as the duration of infection increases, regardless of the pathogen involved or the treatment applied.

It is also important to recognize that SCC is only an indicator of IMI. A low apparent cure rate during the dry period, calculated based on somatic cell counts before dry-off and after calving, may also be caused by an excessively high rate of new infections during the dry period. In such cases, cows with elevated SCC before dry-off may cure during the dry period but become re-infected during the same dry period, resulting in calving again with an elevated SCC. Because cure is determined based on SCC, this situation may lead to the incorrect conclusion that cows failed to cure.

On farms where the annual cure rate during the dry period is consistently too low, it is therefore preferable to collect milk samples for bacteriological analysis at dry-off and again from 7 days after calving. This allows determination of whether the post-calving increase in somatic cell count is caused by the same pathogen responsible for the elevated somatic cell count at dry-off. Particular attention should be paid to farms where the percentage of new infections during the dry period is excessively high.

Figure 5 illustrates infection dynamics during the dry period in a herd where most cows that failed to cure during the dry period were already chronically infected (i.e., somatic cell count ≥ 200,000 cells/mL at least four times during the last milk recordings before dry-off) at the end of the previous lactation. By making timely decisions for cows with elevated SCC in such herds, the number of animals with at least four consecutive high SCC recordings in the months before dry-off will decrease, and the cure rate after the dry period will naturally improve. Although the likelihood of spontaneous cure in cows with elevated SCC depends on many factors (cow age, number of infected quarters, individual SCC level, pathogen, etc.), it can be stated that for cows with elevated SCC for three consecutive months, it is advisable to collect milk samples for bacteriological analysis. Based on the results, an informed decision can then be made on whether to wait, treat, or cull the cow. Delaying action further is rarely beneficial, as the longer cows remain infected, the lower the likelihood of cure.

Figure 5. Infection dynamics during the dry period in a herd where most cows that failed to cure were already chronically infected (SCC ≥ 200,000 cells/mL at least at the last four milk recordings before dry-off) at the end of the previous lactation (based on Keno™-M, MEXCELLENCE BV, Ghent, Belgium). Linear cell score: [Ln (SCC/100,000 cells/mL) / Ln 2] + 3.

Figure 6 illustrates infection dynamics during the dry period in a herd where too many cows acquired a new IMI (> 10%) and where cows failed to cure adequately, despite not being chronically infected at dry-off. In such herds, it is advisable to collect milk samples for bacteriological analysis from all cows with elevated somatic cell counts at the last milk recording before dry-off in the following months and to resample these cows from 7 days after calving. The low cure rate during the dry period, as determined by SCC before dry-off and after calving, is suspected to be due to re-infection during the dry period, a suspicion supported by the high rate of new infections during the dry period.

Figure 6. Infection dynamics during the dry period in a herd where too many cows acquired a new intramammary infection (> 10%) and where cows failed to cure adequately, despite not yet being chronically infected at dry-off (based on Keno™-M, MEXCELLENCE, Ghent, Belgium). Linear score: [Ln (SCC/100,000 cells/mL) / Ln 2] + 3.

3. Conclusions

A thorough understanding of how key milk quality indicators are calculated and interpreted is essential for developing effective, farm-specific action plans and identifying the most appropriate solutions. When decisions are based on objective data rather than assumptions, advisory efforts become more focused, efficient, and credible. This targeted approach not only increases the likelihood of improving udder health and milk quality, but also helps avoid unnecessary interventions and frustration on farm. Ultimately, clear insight into the underlying causes of milk quality problems is what enables dairy farmers to feel confident, engaged, and motivated to implement evidence-based recommendations in daily practice.

4. References

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