Traits and Indexes

Production predicted transmitting abilities (PTAs) show the potential for traits such as milk, fat, protein and persistency to be passed on to the next generation. These PTAs allow selections that produce cows to suit your milking contract.

Genetic indexes for production were the first to be introduced and have played an essential part in the recent increases in milk production.

All male and female dairy cattle are assigned a genetic index for these production traits:

• Milk (kg)
• Fat (kg)
• Fat (%)
• Protein (kg)
• Protein (%)

The range of values for a PTA will vary from trait to trait. For example, a milk (kg) PTA may exceed 1,000 kg, whereas a fat (kg) PTA is unlikely to be more than 50 kg.

Bulls’ persistency PTAs give you an indication of their daughters’ ability to maintain production throughout lactation.

Persistency PTAs are expressed as a percentage of yield at 280 days compared with yield at 60 days. These vary depending on breed, but for Holstein bulls, they commonly fall within the range of 57% (low persistency) to 67% (high persistency).

A higher persistency PTA generally reflects a flatter lactation curve, which is believed to result in fewer metabolic problems. This may result in better health and fertility. However, there is not yet any UK research to confirm the genetic link between lactation shape and health traits.

We recommend only using persistency PTAs for a secondary screening of bulls.

See HealthyCow for a sub-index incorporating health, welfare, and fitness PTAs

Health, welfare, and fitness traits have become more important, not simply because they are 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 a 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.

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 SCC, 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 are 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 is 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.

The Mastitis Index is published alongside the SCC Index and 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 is 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 criterion.

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 is 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 SCC 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 is 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 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.

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. They 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.

LA 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 is expressed on a scale 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. Genotyped Holstein females 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 Adv has small but favourable relationships with all traits currently in the UK breeding indexes – £PLI, £SCI and £ACI. Selecting bulls with positive TB Adv 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.

Innovate UK funded establishment of a data asset that allowed accurate gBVs to be delivered for Holsteins.

See EnviroCow for a sub-index incorporating production, lifespan, and feed efficiency.

Management and calving PTAs allow a streamlined and undisrupted milking routine and have increased in importance as farm dynamics have changed. PTAs such as temperament, ease of milking, maintenance, and calving ease can make a difference to the management and efficiency of your farm.

The Maintenance PTA is published as a stand-alone trait that estimates the feed efficiency differences of animals and allows farmers to select bulls accordingly.

The cost of maintaining a cow is related to its weight, so a cow weighing 600 kg will have a lower feed requirement than a cow weighing 700 kg, even if they give the same amount of milk.

As we do not routinely weigh dairy cattle, we have studied the traits most closely related to the cow’s weight. These are stature, chest width, body depth and angularity, and they are used to indicate the costs of maintenance.

Figures for Maintenance PTA are expressed on a scale of roughly +50 kg to -50 kg, giving an indication of the average difference in weight expected in a bull’s daughters. Negative figures are desirable, as they will help producers to breed cattle with lower feed costs.

The Maintenance PTA of individual bulls should be used as part of a broader breeding strategy. 

Feed Advantage helps dairy producers identify bulls with the greatest tendency to transmit good feed conversion on to their daughters. It is expressed as a PTA in kilograms of dry matter intake saved during each lactation.

The index’s launch represents the culmination of over 30 years of research and data collection from the award-winning Langhill herd in Dumfries. Undertaken by Scotland’s Rural College (SRUC), the studies have measured the Langhill cows’ dry matter intake throughout their entire lives.

Adjustments are made for the size of the animal, as a smaller cow requires less feed than a larger cow giving the same level of milk production. The most efficient cows consume as much as 400kg less in one lactation compared with the least efficient cows, meaning that for the same level of production there is substantial scope for cutting feed use.

Animals that calve easily and milk calmly help reduce labour time.
Calving Ease (CE) indexes are expressed on a scale of -3% to +3%, with a breed average of zero. Positive figures predict easier-than-average calvings and negative figures predict more difficult calvings.

There are two CE indexes that together give a complete picture of a bull’s "calving performance":

• Direct Calving Ease (dCE%) – this predicts the ease with which a calf by that sire will be born
• Maternal Calving Ease (mCE%) – this predicts the ease with which a daughter of that sire will give birth

You should avoid negative dCE% when breeding maiden heifers, as these are predicted to give a higher proportion of difficult calvings. However, dCE% should not be ignored in older cow matings, where it is wise to avoid bulls which are likely to produce very difficult calvings.

Long-term selection for dCE% without any regard to mCE% could set up problems for the future, so you should also consider maternal calving ease.

It is recommended to select primarily for Profitable Lifetime Index (£PLI) and use the CE indexes – as well as other fitness traits – as secondary criteria. Both dCE and mCE are incorporated into the £PLI, £SCI and £ACI.

The sex of a calf and lactation number affect gestation length, so are used when calculating Gestation Length PTAs. The use of sexed semen does not affect the prediction ability of this model.

The heritability of gestation length is 43%, indicating that genetics plays a large role in reducing gestation length.

Gestation length is expressed in days and ranges from +7 days (longer) to -7 days (shorter). For every one-point change in a bull’s Gestation Length PTA, a change of one day of gestation length for the bull’s calf is predicted.

For example, a bull with a -5 gestation length is expected to have his calves born five days earlier compared with a 0 PTA bull.
Gestation Length is published as a stand-alone trait.

Although rearing and handling are key to shaping a cow’s temperament, the clear genetic component to this trait has led to the calculation of genetic indexes for temperament in the parlour. This PTA is expressed on a standardised scale of -3 to +3, with the highest indexes indicating the predicted transmission of a placid temperament.

Genetic indexes for ease of milking are also expressed on a -3 to +3 scale. Lower minus figures represent hard-milking cows, while high positive figures predict very fast milking and daughters potentially running milk.

The Dairy Carcase Index (DCI) has been developed primarily for producers sending dairy youngstock to the beef supply chain or rearing their own beef.

It is calculated from weight, age and carcase information from most major abattoirs in the UK, and is mainly based on average daily carcase gain and carcase conformation.

DCI is expressed on a scale of -5% (bad) to +5% (excellent), and an increase in the index predicts improved carcase conformation and average daily carcase gain in a bull’s progeny.

Type traits can play an important role in determining a cow’s suitability for sustained milk production. A cow’s durability is affected by 17 traits, which are split into three groups, as outlined below.

Type evaluations are undertaken on first-lactation cows by the breed societies, each of which has its own breed standards. The data is used to calculate genetic evaluations for males and females. As with individual production and health indexes, type indexes should not be compared across breeds.

The 17 linear type traits, which describe the physical attributes that make up how a cow looks, are measured on a linear scale. These are generally presented on a bar chart, which provides a useful at-a-glance assessment of a bull’s predicted breeding pattern.

While it is generally considered desirable for bars to be on the right-hand side of the chart, they do not necessarily need to be on the extreme right, depending on your breeding priorities. For example, extreme stature improvement is no longer desirable for most producers.

Two obvious exceptions are rear legs viewed from the side and teat length, each with approximately mid-score optimums. The intermediate score of zero represents the breed average for all linear traits. Linear type traits are scored on a scale of -3 to +3.

The two composite traits – Feet and Legs, and Udder – also appear on a bull’s bar chart, but unlike the 17 linear traits above, these are described in terms of desirability rather than degree. Based on the type evaluator’s observations, they are also scored on a scale of -3 to +3, with the highest scores indicating the best overall quality.

Type merit (TM) predicts a bull’s ability to transmit overall type. It is based on a combination of his daughters’ scores for Feet and Legs, Udder and body conformation, in order of importance. Again, it is scored on a scale of -3 to +3.