Five picks for breeding to improve mastitis and cell counts

Big improvements have been seen in somatic cell count and mastitis rates, many of which are attributable to improved dairy cow genetics. But there’s lots more progress to be made, according to AHDB, who have introduced more breeding indexes to assist.

Dairy producers have done a remarkable job of breeding better-quality cattle over the past 20 years, and this has been seen through improvements in virtually every trait for which there’s a breeding index.

This is highly evident in udder health traits, which, as every producer is aware, can have a profound effect on milk price and the profitability of a herd.

And while many factors have been identified as contributing to improvements in somatic cell counts (SCC) and mastitis, improved genetics has unquestionably played a part.

Marco Winters, head of animal genetics for AHDB says:

“We know there are many contributors to improved udder health, ranging from better tools for early detection, milk price pressures which are forcing the removal of persistent offenders, and more attention being given to mastitis control plans.

“But genetics has unquestionably played a significant role, which has been demonstrated by improvements in genetic indexes for both SCC and mastitis across the national herd.”

Genetic trends for mastitis and cell counts, 2005 to 2024

Note: negative figures are desirable
Source: AHDB

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

The National Milk Record (NMR) latest Key Performance Indicator (KPI) report – in which 500 herds of Holstein Friesians are used as a representative cross-section of the national herd – demonstrates this well. This report 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 500-herd sample completed their lactations without recording an SCC above 200,000 cells/ml, compared to 2010, when only 35% of cows avoided this high cell count figure.

Mastitis incidence has similarly declined. Respondents in the 500-cow study saw an average of 22 cases per 100 cows per year in 2023, down from 36 cases per 100 cows in 2016.

Mr Winters says:

“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.

“And 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 which are innately easier to manage and require fewer antibiotic interventions.”

But despite the gains already achieved, Mr Winters is confident there is scope for more improvements.

He adds:

“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.”

Five genetic traits to improve udder health

Mr Winter highlights five genetic traits which can help improve udder health and these can be considered within a broader breeding strategy.

1. The first and perhaps most obvious is Mastitis Index, and negative indexes should be chosen to reduce the incidence of the disease. Expressed on a scale of about -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 in 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. A similar pattern exists for SCC Index, which is the other PTA that should be a priority when seeking improvements to udder health. 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 SCC PTA, 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, but a small number of bulls will reduce SCC but won’t necessarily reduce cases of mastitis. The Mastitis Index helps to identify these bulls.
3. Milking speed gets a mention as a trait for genetic selection as 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, confirmed at 0.09 in a recent AHDB study which showed the relationship between genetic index for milking speed and observed incidence of mastitis. The evidence is therefore 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 ease of milking score of any bull being considered. This is expressed on a scale of about -3 (slow) to +3 (fast).
4. Udder conformation has been part of selection for many years and for a range of reasons, but whether udder conformation has an effect on udder health was also assessed by AHDB. This shows it is udder depth and fore udder attachment in particular which have an association with the incidence of mastitis. However, correlations of 0.26 and 0.20 respectively are considered reasonably low, particularly when compared to direct selection for Mastitis or SCC Index. Selection for these showed a correlation of 0.78 and 0.70 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 and SCC Index as traits for genetic selection if seeking to improve udder health. As with all genetic selection, it’s 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. 

*Correlations

Correlations were calculated by AHDB as part of this analysis to identify the impact of selection for a variety of genetic indexes on the actual incidence of mastitis observed. The data was taken from AHDB’s genetic evaluations, incorporating mastitis data collected by the milk recording companies in the UK. Over 400 bulls with at least 200 daughters milking in the third lactation, and a minimum of 100 UK daughters type classified were scrutinised, and their genetic indexes correlated to the incidence of mastitis in their progeny during the first three lactations. This means over 450,000 cows were included in this study.