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The high value of low heritability

Most of us misunderstand heritability. In simple terms, for any given trait, heritability tells us how much of the difference in actual performance is due to genetics, as opposed to management or the environment.

To better understand, think about two cows in two different herds. How much of the difference in their milk production is due to genetics? How much is due to management or environment? It turns out about 30% of the milk production difference is due to genetics, while 70% is due to management and environment. Therefore, milk has a heritability of 0.30.

What about pregnancy rates? Management and environment account for the 96% majority of variation between daughters. So the influence of genetics is minor, at just 4%. Thus, Daughter Pregnancy Rate (DPR) has a heritability of 0.04.

We commonly refer to the health traits like Productive Life (PL), DPR and Somatic Cell Score (SCS) as the lower heritability traits. Many producers believe that low heritability equates to less, or slower, genetic progress. However, in spite of lower heritability, it would be wrong to conclude that DPR, PL or SCS are insignificant as a result.

Perspective is important

In genetics, accuracy shows through when we evaluate results within one herd. In that herd, if we evaluate within a specific lactation group, and then within a specific time of freshening, we find a contemporary group. By evaluating within one contemporary group, we reduce the impact of management and environmental differences.

The overall heritability for health traits like DPR and PL is low. When we break our evaluations down into contemporary groups, that’s when we find the true genetic differences.

The proof is in the numbers

Take this real-life example from a 1,500-cow dairy with very good reproductive performance. We’ve separated out first lactation cows into four groups, based on their sire’s DPR. It’s clear to see that the high DPR sires create daughters that become pregnant more quickly than the daughters of low DPR sires.

Table 1# of cowsAverage Sire DPRActual preg rate
Top 25% - High DPR1742.327%
Bottom 25% - Low DPR137-1.120%

The same goes for Productive Life. Despite the low heritability at less than 9%, PL can make a real, noticeable difference in your herd.

This table compares how long the daughters of the industry’s best ten PL bulls and daughters of the industry’s bottom ten PL sires will last in a given herd. You can see that a higher percentage of high PL daughters, represented by the dark blue bars, remain in a herd than their low PL counterparts.

When you select for the lowly heritable PL, you will certainly create healthier, longer-living cows in your herd.

Focus on the economics

As a progressive dairy producer, don’t let confusion about heritability prevent you from using the right genetic tools to improve your herd. Health traits are economically important, and making improvement in these areas can have a huge impact on your bottom line.

Many traits have a high heritability, but no economic importance. In other words, we can make a lot of progress for these traits very quickly, but it will not make a more profitable cow.

A couple examples of high heritability traits are coat color and polled. Both of these traits have a heritability of 100 percent because they are completely controlled by genetics. However, even if we can make cows red or polled in one generation, what is the economic value of that?

By comparison, the economic value of more fertile cows that last longer because of fewer metabolic problems, fewer cases of mastitis, and less calving difficulty is clear to see. These genetic features make a more profitable production unit for each and every farm.

Selection secrets for healthier cows

When you set or reevaluate your genetic plan, take the following tips into account to maximize progress in the direction of your goals.

1. Define your goals

To set the right goals, first identify the most common reasons for culling in your herd. Is it reproduction, milk production, mastitis? This information gives you the basis for the genetic decisions you make going forward.

2. Choose your tools

Health traits offer dairy producers some powerful tools to help correct for low reproduction, metabolic problems, etc. Identify how important each of these trouble areas are to you. Place a proportionate emphasis on these traits when choosing the group of sires to use on your dairy.

3. Customize the solution

Industry standard selection indexes put different and continually changing weights on health traits. So don’t assume they reflect your individual goals and needs. Work with your trusted Alta advisor to make sure your genetic plan is customized to match your current situation and future goals.

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Proof terminology explained

The letters, numbers and acronyms on a proof sheet can be complicated. Here, we break down the meaning and explanation of the proof indexes, traits and terminology.
Selection indexes

Genetic selection indexes are set by national organizations or breed associations. Genetic indexes help dairy producers focus on a total approach to genetic improvement, rather than limiting progress by single trait selection.

However, each farm is unique, with different situations and future plans. With that in mind, it’s important to understand what traits are included in each industry standard index. When you know what’s included, you can more effectively evaluate if the index truly matches your farm’s goals.

TPI = Total Performance Index
TPI is calculated by the Holstein Association USA (HA-USA) and includes the following trait weightings.

Image to show the weights on production, health and type for the TPI Index


21% Pounds of protein
17% Pounds of fat
8% Feed efficiency


13% Fertility index
-5% Somatic cell score
4% Productive life
3% Cow livability
2% Daughter calving ease
1% Daughter stillbirth


11% Udder composite
8% PTA type
6% Foot & leg composite
-1% Dairy form

NM$ = Net Merit Dollars

NM$ is a genetic index value calculated by the Council on Dairy Cattle Breeding (CDCB). It describes the expected lifetime profit per cow as compared to the base of the population born in 2010. Trait weightings are generally updated approximately every five years and include emphasis on the following traits. The current trait breakdown is in place as of April 2017. Please note that trait weights are rounded to the nearest percentage.

Image to show trait weights for production, health and conformation within Net Merit $.


24% Pounds of fat
18% Pounds of protein
-1% Pounds of milk


13% Productive life
7% Cow livability
7% Daughter pregnancy rate
-6% Somatic cell score
5% Calving ability
2% Cow conception rate
1% Heifer conception rate


7% Udder composite
6% Body weight composite
3% Foot & leg composite

CM$ = Cheese Merit Dollars

CM$ is an index calculated to account for milk sold to be made into cheese or other dairy products. The current CM$ index was adjusted in April 2017 and the following trait weights are considered. Please take note that trait weights shown have been rounded to the nearest percentage.

Image showing the trait breakdowns for production, health and type within the Cheese Merit dollars formula


22% Pounds of protein
20% Pounds of fat
-8% Pounds of milk

HEALTH = 37%

12% Productive life
-7% Somatic cell score
6% Cow livability
6% Daughter pregnancy rate
4% Calving ability
1% Cow conception rate
1% Heifer conception rate


6% Udder
-5% Body weight composite
2% Foot & leg


CDCB: Council on Dairy Cattle Breeding
Calculates production and health trait information for all breeds

MACE: Multiple-trait across country evaluation
Denotes that a bull’s proof evaluation includes daughter information from multiple countries

PTA: Predicted transmitting ability
The estimate of genetic superiority or inferiority for a given trait that an animal is predicted to transmit to its offspring. This value is based on the animal’s own records and the records of known relatives.

EFI: Effective future inbreeding
An estimate, based on pedigree, of the level of inbreeding that the progeny of a given animal will contribute in the population if mated at random

GFI: Genomic future inbreeding
Similar to EFI, an animal’s GFI als predicts the level of inbreeding he/she will contribute in the population if mated at random. Yet, GFI provides a more accurate prediction. It takes into account genomic test results and the actual genes an animal has.

aAa: an independent method for making mating decisions

DMS: a separate, independent method for making mating decisions



PTAM: Predicted transmitting ability for milk

PTAP: Predicted transmitting ability for protein

PTAF: Predicted transmitting ability for fat

PRel: the percent reliability of a sire’s production proof



PL: Productive Life
Measured as the total number of additional or fewer productive months that you can expect from a bull’s daughters over their lifetime. Cows receive credit for each month of lactation, with more credit given to the first months around peak production, and less credit given for months further out in lactation. More credit is also given for older cows than for younger animals.  

LIV: Cow livability
Measure of a cow’s ability to remain alive while in the milking herd.

SCS: Somatic cell score
The log score of somatic cells per milliliter.

DPR: Daughter pregnancy rate
Daughter Pregnancy Rate is defined as the percentage of non-pregnant cows that become pregnant during each 21-day period. A DPR of ‘1.0’ implies that daughters from this bull are 1% more likely to become pregnant during that estrus cycle than a bull with an evaluation of zero. Each increase of 1% in PTA DPR equals a decrease of 4 days in PTA days open.

HCR: Heifer conception rate
A virgin heifer’s ability to conceive – defined as the percentage of inseminated heifers that become pregnant at each service. An HCR of 1.0 implies that daughters of this bull are 1% more likely to become pregnant as a heifer than daughters of a bull with an evaluation of 0.0

CCR: Cow conception rate
A lactating cow’s ability to conceive – defined as the percentage of inseminated cows that become pregnant at each service. A bull’s CCR of 1.0 implies that daughters of this bull are 1% more likely to become pregnant during that lactation than daughters of a bull with an evaluation of 0.0.

MAST: expected resistance of an animal’s offspring to clinical mastitis
Daughters of a bull with a MAST value of +1.0 are expected to have 1% fewer cases of mastitis than the average herdmate.

METR: expected resistance of an animal’s offspring to metritis
Daughters of a bull with a METR value of +1.0 are expected to have 1% fewer recorded cases of metritis than the average herdmate.

KET: expected resistance of an animal’s offspring to ketosis
Daughters of a bull with a KET value of +1.0 are expected to have 1% fewer recorded cases of ketosis than the average herdmate.

DA: expected resistance of an animal’s offspring to displaced abomasum
Daughters of a bull with a DA value of +1.0 are expected to have 1% fewer recorded cases of displaced abomasum than the average herdmate.

MFEV: expected resistance of an animal’s offspring to milk fever (hypocalcemia)
Daughters of a bull with a MFEV value of +1.0 are expected to have 1% fewer recorded cases of milk fever than the average herdmate.

RP: expected resistance of an animal’s offspring to retained placenta
Daughters of a bull with a RP value of +1.0 are expected to have 1% fewer recorded cases of retained placenta than the average herdmate.

HRel: the reliability percentage for a sire’s health traits



SCE: Sire calving ease
The percentage of bull’s calves born that are considered difficult in first lactation animals. Difficult births include those coded as a score of 3, 4 or 5 on a scale of 1-5.

DCE: Daughter calving ease
The percentage of a bull’s daughters who have difficult births during their first calving. Difficult calvings are those coded as a 3, 4 or 5 on a scale of 1-5.

SSB: Sire stillbirth
The percentage of a bull’s offspring that are born dead to first lactation animals.

DSB: Daughter stillbirth
The percentage of a bull’s daughters who give birth to a dead calf in their first lactation.



PTAT, UDC and FLC are all calculated by the Holstein Association USA.

PTAT: Predicted transmitting for type – referring to the total conformation of an animal

UDC: Udder composite index; comprised of the following linear trait weights:
19% Rear udder height
17% Udder depth
-17% Stature
6% Rear udder width
13% Fore udder attachment
7% Udder Cleft
4% Rear teat optimum
4% Teat length optimum
3% Front teat placement

FLC: Foot and leg composite index; comprised of the following trait weights:
58% foot and leg classification score
18% rear legs rear view
-17% stature
8% foot angle

TRel = the percent reliability for a sire’s conformation/type proof



PO: observed polled
PC: genomic tested as heterozygous polled; means 50% of offspring are expected to be observed as polled
PP: genomic tested as homozygous polled; means that 100% of offspring are expected to be observed as polled

RC: carries the recessive gene for red coat color
DR: carries a dominant gene for red coat color


These codes, or symbols representing the code, will only show up on a proof sheet if an animal is a carrier or test positive for one of the following. The acronyms denoting that an animal is tested free of a recessive will only show up on its pedigree.

BY: Brachyspina
TY: Tested free of brachyspina

BL: BLADS, or Bovine leukocyte adhesion deficiency
TL: Tested free of BLADS

CV: CVM or Complex vertebral malformation
TV: Tested free of CVM

DP: DUMPS, or Deficiency of the uridine monophosphate synthase
TD: Tested free of DUMPS

MF: Mulefoot
TM: Tested free of mulefoot

HH1, HH2, HH3, HH4, HH5: Holstein haplotypes that negatively affect fertility
HCD: Holstein haplotype for cholesterol deficiency

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The genetic guide to healthier cows

Industry buzz has been booming about new genetic programs that promise to create healthier cows.

That means it’s the perfect time to revisit the impact that selection for Productive Life within your genetic plan can have on the overall health and longevity of your cows.


The Productive Life (PL) number that appears for any given sire on your proof sheet is defined as the number of months longer (or shorter) that his daughters will be productive in your herd as compared to herdmates. If a bull is 7.0 for PL, his daughters within a given herd are predicted to live seven months longer than the average cow in that herd. If a bull is -2.0 for PL, his daughters are predicted to live about two months less than the average cow in that given herd.

PL is often associated with old cows. However, if you think about which cows live longest in your herd, it would be those that have no troubles calving, fewer incidences of mastitis, no respiratory issues, fewer hoof problems, and so on.

Four-event cows

In reality, genetic selection for PL doesn’t just mean more old cows; it predicts which cows are toughest, healthiest and easiest to manage. We call those the four-event cows. If you look at a cow card on your herd management software program, a four-event cow has only four events listed throughout her lactation: 1-fresh; 2-bred; 3-confirmed pregnant; and 4-dry.

If those four events are the only major things a cow experiences throughout her lactation, chances are she’s trouble-free, making you money, and will stick around for several lactations.

Any time an event takes place, such as milk fever, a displaced abomasum, retained placenta, mastitis, pneumonia, or any other disruption to the normal progression of a cow’s lactation, milk is lost. In addition to lost milk production, vet and treatment costs add to the dollars lost.

The proof is in the numbers

Selection for Productive Life propels you toward the goal of a herd full of four-event cows. Since the actual measure of PL is not calculated until after a cow leaves the herd, we can use other ways to see if higher PL bulls actually create healthier and more trouble-free cows.

# of cowsSire PLAborts‘Do Not Breed’SoldDiedMastitisRPDAKetosisPneumoniaMetritisInjuryLame
Top 50%: High PL478>3.515121136331155118
Bottom 50%: Low PL502<3.6709024152309612153762930

Table 1 breaks down the events within a real 2,400-cow Holstein herd on all first lactation animals with known sire ID’s. Based only on the animal’s parent average or Productive Life, this shows the extreme difference in health events between cows with a high PL pedigree versus those with a low PL pedigree

These are real numbers, recorded on this farm’s herd management software program. Keep in mind, management is consistent throughout the herd, and no preferential treatment is provided for any given cows.

As the table clearly illustrates, far fewer of the high PL cows had issues after calving and throughout their lactation. Fewer cows from high PL group were coded as ‘do not breeds’ (DNB) and therefore, fewer of the high PL cows died or were sold. This means more cows from within that high PL group claimed the title of trouble-free, four-event cows.

On your dairy, how much does a displaced abomasum decrease a cow’s profitability over her lactation? How much of your milk check is sacrificed with every case of mastitis? How many dollars are lost for every lame cow or case of pneumonia? If you put a dollar value to the lost production and treatment cost associated with each extra health event experienced by the group of low PL cows it adds up significantly.

Want healthier cows? Let Productive Life get you there

While environment, cow comfort and overall management practices all play an integral role in the health of any given herd, genetic selection can also aid your quest for a herd of healthy, trouble-free cows. To do that, keep these points in mind.

  1. Genetic selection for PL will help you create longer living cows.
  2. Despite new genetic programs promising added immunity or greater health during a cow’s transition period, PL remains the standard for breeding tougher, healthier cows with fewer issues throughout their lactations.
  3. Include selection for PL as part of your customized genetic plan in order to build your herd of the profitable, four-event cows.
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