5 heritable traits to improve mastitis and cell counts

Udder health traits can have a profound effect on milk price and herd profitability. 

Genetics are among many factors that have helped improve udder health, such as better tools for early mastitis detection, milk price pressures for cleaner milk, and a stronger focus on mastitis control plans. 

“Genetics has unquestionably played a significant role, demonstrated by improvements in genetic indexes for both SCC [somatic cell count] and mastitis across the national herd,” says Marco Winters, head of animal genetics for AHDB.

See also: Why genetics are critical for improving dairy cow fertility

Since the early 2000s, the predicted transmitting ability (PTA) for both mastitis and SCC has markedly improved (see “Genetic trends for mastitis and cell counts”).

This has fed through to actual on-farm performance, illustrated in health data for udder traits registered with milk recording organisations and in various studies.

Progress in the national herd

NMR’s latest KPI report – in which 500 herds of Holstein Friesians are used as a representative cross-section of the national herd – demonstrates this well.

It shows that in 2023, 70% of herds kept average cell counts below 200,000 cells/ml, compared with only 44% in 2010.

Also in 2023, more than half (52%) of all cows in the sample completed their lactations without recording an SCC above 200,000 cells/ml.

This compares with 2010, when only 35% of cows avoided this high cell count figure.

Mastitis incidence similarly declined, and respondents within the 500-cow study saw an average of 22 cases for every 100 cows a year in 2023, down from 36 cases for every 100 cows in 2016.

“Improving a herd’s genetics has played a part in all of these gains, and breeding is usually a cheaper and more sustainable approach than changes to management,” Marco points out.

“Although improving genetics is never a quick fix, once embedded in a herd, genetic improvement will persist and accumulate over the generations.

“It also chimes with all of the farming industry’s efforts to work with, and not against, nature, as we are breeding animals that are innately easier to manage and require fewer antibiotics interventions,” he says.

Further gains

Despite the gains already achieved, Marco is confident there is scope for more progress:

“We know further improvements can be made from the choice of dairy sires available to producers, many of which have outstanding genetic indexes for cell counts and mastitis.”

He highlights five genetic traits that can help improve udder health and be considered within a broader breeding strategy.

1. Mastitis Index

Negative indexes should be chosen to reduce the incidence of the disease.

Expressed on a scale of -4 to +4, daughters of a poor index bull (+3) have about twice the chance of getting mastitis as daughters of sires with a favourable index (-3).

Using actual records from milk recording companies, this is seen as a mastitis incidence of 20% in +3 bulls’ daughters compared with 10% in -3 daughters, across all third calvers.

The same degree of difference can be seen in other lactation groups.

2. SCC Index

As with Mastitis Index, bulls with a negative score will reduce cell counts, but this index is expressed as a percentage, generally within the range of -40% to +40%.

For every 1% in a bull’s PTA for SCC, a change of 1% in his daughters’ SCC is predicted.

This means that daughters of a top SCC Index bull (-25%) are predicted to have 25% lower cell counts than those of a bull with an index of zero.

There is a strong link between SCC Index and Mastitis Index. However, a few bulls that will reduce SCC will not necessarily reduce cases of mastitis.

The Mastitis Index helps to identify these bulls.

3. Milking speed

Some producers are concerned that bulls with the best cell count scores may also slow down milking. However, correlations between ease of milking and udder health traits are low, at 0.09.

This was confirmed in a recent AHDB study (see “Impact of selection for a variety of genetic indexes on incidence of mastitis”), which showed the relationship between genetic index for milking speed and observed incidence of mastitis.

The evidence is clear that selection in favour of udder health has not slowed down milking.

However, producers naturally want to avoid extremes for this trait for management reasons, and if they have concerns, they can check the score for ease of milking of any bull being considered.

This is expressed on a scale of -3 (slow) to +3 (fast).

4. Udder conformation

This has been part of selection for many years and for a range of reasons.

AHDB assessed whether it has an effect on udder health and showed it is udder depth and fore udder attachment in particular that have an association with the incidence of mastitis.

However, correlations of 0.26 and 0.2, respectively, are considered reasonably low, particularly when compared with direct selection for Mastitis Index or SCC Index.

Selection for these showed a correlation of 0.78 and 0.7, respectively.

This means that, as expected, the direct udder health indexes have a far higher association with actual cases of mastitis observed in progeny than udder conformation traits.

For this reason, the advice is always to prioritise Mastitis Index and SCC Index as traits for genetic selection if seeking to improve udder health.

As with all genetic selection, it is always better to use the index for the trait you seek to improve, rather than a proxy.

5. Healthy Cow

(HC) was introduced in 2021 and is helping dairy producers identify the best bulls for improving all aspects of herd health.

However, within this composite index, 23% is assigned to udder health, meaning producers who refer to HC will not only improve overall health – and reduce the cost of poor health – but will specifically improve SCC and mastitis.

Such composite indexes are particularly valued by producers who favour simplified genetic selection, as HC is an index which gives an at-a-glance picture of a bull’s ability to transmit good overall health.