Health, welfare and fertility PTAs

There are a number of PTAs (predicted transmitting abilities) concerned with health, welfare and fertility, including important aspects such as SCC and mastitis. Find out more about them and how they’re calculated.

These traits have become more important in recent years, not simply because they’re needed to sustain milk production over several lactations, but also due to growing demand from consumers for high-health-and-welfare produce.

Breeding for these traits makes it possible to produce cows with an inherently higher chance of experiencing better health and welfare or that can better fight off infection.

Health traits may have a lower heritability than some other traits, but including them in your breeding decisions will have a positive cumulative effect on your herd.

HealthyCow is a sub-index of the £PLI and should be used as a secondary filter when selecting bulls. 

This ensures these bulls have profitable daughters due to improved health genetics instead of just production or efficiency.

The HealthyCow index contains the following traits:

• Lifespan
• Calf survival
• Fertility index
• Somatic cell count
• Mastitis
• Functional type (Feet and Legs and Mammary)
• Lameness
• Calving ease (Direct and Maternal)

It has been demonstrated that progeny from the top-ranking HealthyCow index show a higher health status throughout their longer lifetime.

The HealthyCow index enables farmers to find bulls that will breed even more fertile, healthier cows that stay in the herd for longer.

Somatic cell count (SCC) PTAs are expressed as a percentage and generally fall within the range +40% to -40%. For every 1% in a bull’s SCC PTA, a change of 1% is predicted in his daughters’ SCC.

Negative PTAs are desirable for traits such as somatic cell count, mastitis and maintenance. For example, daughters of a bull with a -10% SCC are expected to have cell counts 10% lower than daughters of a bull with a SCC PTA of zero.

Mastitis PTAs indicate a bull’s ability to transmit mastitis resistance to his daughters. They’re calculated from actual cases of mastitis, and data comes directly from on-farm records via the milk-recording organisations National Milk Records (NMR) and the Cattle Information Service (CIS).

The Mastitis PTA is expressed as a percentage, on a scale from -5% to +5%, with negative figures desirable. This means that for every 1% decrease in a bull’s Mastitis PTA, there will be a corresponding 1% decrease in the proportion of his daughters expected to get mastitis.

Although there’s a strong link between SCC Index and reduced mastitis cases, a small number of bulls will reduce SCC but not necessarily reduce mastitis cases. The Mastitis Index helps to identify these bulls.

Not only is the Mastitis Index published alongside the SCC Index, it has also been incorporated into the Profitable Lifetime Index (£PLI).

The PTA for fertility (known as the Fertility Index, or FI) predicts female fertility and is based mostly on a combination of calving interval and non-return rates. A bull with an above-average FI will breed cows with improved calving intervals and better non-return rates.

As a rough guide, every point increase in FI, for example from +4 to +5, will decrease the calving interval by 0.6 days and improve non-return rates by 0.25%.

 For example, the daughters of a +15 bull are expected to have about a nine-day shorter calving interval than daughters of an average bull whose FI is zero.

 FI scores generally fall within the range of -15 to +15.

 Information used in the FI calculation includes:

• Calving interval
• Non-return rate at 56 days
• Body condition score
• Milk yield at around the time of insemination (110 days)
• Days from calving to first insemination
• Number of inseminations needed to get a cow in calf

The first two on this list are by far the most important, although early in a bull’s life it’s necessary to use more of the others as predictors of fertility, as well as genomic and parent information.

In an average herd, using bulls with the best FI can reduce calving interval by around seven days in one generation. These benefits accumulate over generations.

FI should be used as part of a wider selection policy and not as a primary selection criteria.

Lifespan PTAs predict reduced or increased daughter survival and are expressed as extra days of life. They generally fall within the range of -250 days to +250 days, with an average of zero and positive indexes being desirable.

Daughters of a +90 Lifespan Index bull are predicted to live, on average, three months longer than daughters of a sire whose index is zero. As with all UK genetic indexes, zero represents the breed average.

The Lifespan Index is calculated:

• From actual daughter survival, when that information is available
• When it’s not, it’s either calculated:
  • From the animal’s own genotype, if it has a genomic index, or
  • From predictive traits, such as type traits (Feet and Legs and Udders) and Somatic Cell Count Index, all of which are correlated with lifespan
• Where necessary, information on ancestors’ lifespan is also included, but this (and all other predictors) will diminish in importance as the animal acquires progeny lifespan information of its own

An important feature of Lifespan PTAs is that they predict involuntary rather than voluntary culling. As there’s such a strong relationship between milk production and survival (because low producers are generally culled earlier), Lifespan PTAs are corrected for milk production. This ensures the PTA is more a measure of daughters’ ability to survive than of their failure to produce milk (which, of course, would be apparent from their production PTA).

The Calf Survival (CS) PTA shows that calves of some sires are more likely to survive between tagging and 10 months of age than those sired by other bulls.

This PTA was developed using records from the British Cattle Movement Service (BCMS) and enables the selection of bulls with above-average calf survival.

The typical range for CS PTAs is from -6% (bad) to +6% (excellent), which indicates a 12% difference in the probability of survival between the progeny of the worst and best bulls.

Calves by a bull with a +5% CS PTA are 5% more likely to survive than those by a bull with a CS PTA of zero. They’re 10% more likely to survive than those by a bull with a CS PTA of -5%.

The heritability of CS has been found to be around 5%, which will enable breeders who continually select bulls with improving calf survival genetics to achieve incremental improvements with each new generation of calves.

CS is published as a stand-alone trait but also incorporated into the Profitable Lifetime Index (£PLI), Spring Calving Index (SCI) and Autumn Calving Index (£ACI).

Lameness Advantage (LA) PTAs can be used to reduce cases of lameness in the herd and are calculated using information about actual lameness incidents, which comes directly from on-farm lameness records via NMR and CIS.

This information is combined with existing data for locomotion and feet and legs, together with bone quality scores and digital dermatitis records from breed society type classification services, as recorded at the National Bovine Data Centre (NBDC).

LA PTAs are expressed as a percentage and range from -5% (bad) to +5% (excellent). Every 1% change in a bull’s LA predicts a change of 1% of daughters becoming lame per lactation. For example, a bull with a +5% LA is expected to have 5% fewer cases of lameness in his daughters per lactation than a bull with an LA of zero.

Lameness Advantage is published as a stand-alone trait and has been incorporated into the UK national breeding indexes, the Profitable Lifetime Index (£PLI), Spring Calving Index (£SCI) and Autumn Calving Index (£ACI).

Digital dermatitis data has been collected as part of the dairy breed society classification process for many years and has been used as part of the Lameness Advantage calculation since 2018.

However, many producers are keen to know which bulls specifically transmit better resistance to digital dermatitis to their daughters, so it was also made available as a stand-alone index in 2020.

The Digital Dermatitis Index (DD) is expressed on a scale from -2% to +2%, with positive figures being desirable. Daughters of a bull with a +2% DD Index are expected to have 2% fewer cases of digital dermatitis than those of a bull whose DD Index is zero.

TB Advantage (TB Adv) helps dairy farmers breed cattle with improved resistance to bovine tuberculosis (bTB). It’s expressed on a scale that runs from -3% to +3%, where positive scores are desirable. For every +1 point in the index, 1% fewer daughters are expected to become infected during a TB breakdown.

The index is available for bulls with sufficient daughters milking in the UK, or Holstein and Friesian bulls which have had their genotype measured. Holstein females which have been genotyped will also be given a TB Advantage rating.

The average reliability of TB Adv is 65% for bulls with UK daughters and 45% for those with a genomic index only. Although the reliability of genomic predictions for the TB Adv is currently less than for some other indexes, it can still be used as part of a dairy herd’s breeding strategy and has shown to be valuable in predicting future performance.

TB Advantage has small but favourable relationships with all traits currently in the UK breeding indexes – £PLI, £SCI and £ACI. Selecting bulls with positive TB Advantage will therefore, on average, have no detrimental effect on any other trait. However, farmers should look at each bull on a case-by-case basis, as any individual could have weaknesses that should be avoided for a particular herd.

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