Genetic Selection for Mastitis Resistance

Mastitis is mostly a management problem, but genetics can tilt the odds. Choosing sires and replacements with better udder health traits is a slow, steady way to reduce losses over the years.

Key points

Genetics lowers risk slowly but does not replace sound milking and housing.
Look at somatic cell score, udder conformation and clinical mastitis traits where reported.
Use your own case records to decide which cows should not produce replacements.
Balance mastitis resistance with production, fertility and feet.

What genetic selection can and cannot do

Mastitis is an infection of the udder, and most cases come from the environment, the milking routine and the cow's own defenses. Genetics affects how well a cow resists infection, but it does not remove the need for clean housing, good milking practice and prompt care.

Think of genetic selection as a long game. Each generation of heifers inherits a little more resistance, and that adds up. It will not fix a herd with poor teat prep or dirty bedding.

Think of it as one layer of protection among several. The others are cow comfort, hygiene, a good milking routine and fast, careful handling of any case that does show up.

Traits worth looking at

Genetic evaluations report several traits linked to udder health. The ones most often discussed are somatic cell score, clinical mastitis resistance where it is reported, and udder conformation such as udder depth, teat placement and attachment.

Somatic cell score is a measure of the cell count in milk and is associated with udder infection. Conformation traits matter because a deep, loosely attached udder is harder to keep clean and more exposed to injury.

Ask your genetics supplier or breed association which of these traits are available for the bulls you are considering and how they are calculated.

Somatic cell score
Clinical mastitis resistance, where reported
Udder depth and attachment
Teat length and placement
Milking speed

Building mastitis resistance into a selection index

Most farms do not pick on one trait alone. Overall selection indexes combine several traits into one ranking, and some of them already include udder health. Pushing too hard on milk yield alone can work against health traits, so look at the full picture.

Decide how much weight mastitis resistance should get in your own plan. A herd with a long history of high cell counts might weight it more heavily than one that is already doing well.

If your milk buyer pays premiums or applies penalties on somatic cell count, that gives you an extra reason to give udder health traits a real place in your index.

Using your own herd records

Genetic evaluations are averages across many herds. Your own records tell you which cows actually struggle. Keep a running list of clinical cases by cow, quarter and date, along with cell count test results from your milk testing.

Use that information to decide which cows should not be bred to produce replacements. Cows that repeatedly get sick, and their daughters, are poor choices for the heifer pen. Keep these notes simple enough that you will actually update them.

Be consistent about how you record cases. If three people write notes in three different ways, the records will be of little use when you sit down to make breeding choices.

Cow number and date of each case
Quarter affected
Test day cell count results
Outcome and whether the cow stayed in the herd

Management that has to go along with it

No sire list will make up for a poor milking routine. Follow your veterinarian's and milk quality advisor's guidance on teat prep, milking order, equipment checks and dry cow management. The National Mastitis Council publishes well-known guidelines you can use as a starting point.

Keep bedding clean and dry, give cows enough resting space and manage heat and crowding. Heifers should enter the milking string in good body condition and with clean udders.

Common mistakes

Farms sometimes chase a single trait and give up milk components, feet or fertility without noticing. Others expect quick results and drop the program after a couple of calf crops.

Another frequent mistake is ignoring udder shape because a bull ranks well for something else. Review each mating and ask whether the daughters will have udders that are easy to milk and keep clean.

Overweighting one trait
Skipping record keeping
Expecting fast change
Neglecting milking routine

Using genetic information alongside milk testing

Regular milk testing shows cell counts for each cow, which helps you spot cows that are likely to carry infection. Those cows should be discussed with your veterinarian rather than simply bred away.

Over several years, compare daughters of different sires in your herd. If one sire's daughters tend to have higher counts or more cases than others, take that into account when picking bulls again.

Review test day results with your milk quality advisor
Compare daughters by sire each year
Share findings with your genetics supplier
Reassess your index weighting once a year

When to get help: Work with your veterinarian and milk quality advisor on any mastitis problem, and follow label or vet-set withdrawal times for milk and meat from any treated cow exactly. Ask your milk buyer about their somatic cell standards.

Sources and further reading

Organization homepages are linked so you can search them for current guidance on this topic.

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