What is Breeding Worth (BW)?
Updated: Feb 17, 2020
The Economic Values (EV) that are used to calculate Breeding Worth (BW) are about to be updated (so EVERYONE'S BW's will change!). We thought it would be a great opportunity to dive a little deeper into what BW actually is, how it is estimated and what it can be used for.
If you don't want to use BW (or breeding values; BV) that's cool with us too. At The AgriSciencer, we are not here to tell you what to do (or not do!). We are simply here to make science more accessible.
Breeding Worth (BW) is the index used to rank dairy animals on their ability to meet the national breeding objective
A cow's performance is made up of her genetics and her environment - BW is one way to measure her genetics
As we get more information about an animal, their ancestors and their progeny, we become more confident that their estimated genetic merit is closer to their "true" genetic merit
So what actually is Breeding Worth?
Breeding Worth (or BW for short) is the index used to rank dairy animals on their ability to meet the national breeding objective - "to breed dairy cows that are efficient converters of feed into profit".
BW is a combination of Breeding Values (BV) and Economic Values (EV) for traits that are deemed to be relevant to breed efficient and profitable dairy cows.
New Zealand Animal Evaluation Limited (NZAEL) manages the National Breeding Objective for New Zealand dairy cattle, so decide which traits are to be included in BW.
Currently, there are 8 traits that are included in BW, all with slightly different weightings.
Somatic Cell Score
Body Condition Score
As at the time of writing, 62% of BW is made up of "Productive Efficiency" - protein, fat, volume and liveweight and the remaining 38% is made up of "Robustness" - somatic cell score, fertility, residual survival and body condition score (source-DairyNZ).
Breeding values (BV) are the best available estimates of a cow or bull's genetic merit for a trait (what genetics they have that can be passed on to their babies).
Economic values (EV) are the estimates of the dollar value of each trait to a dairy farmer in NZ.
Some EV are positive (i.e. protein and fat) and some EV are negative (i.e. milk volume and liveweight).
EVs that are positive indicate that selecting animals with higher BVs for those traits is expected to generate more profit for a farmer.
EVs that are negative indicate that selecting animals with higher BVs for those traits is expected to generate less profit for a farmer.
Each of the 8 traits have a BV and an EV and these are multiplied together then all summed up to create an animal's BW, as below:
BW = (Protein BV x Protein EV) + (Fat BV x Fat EV) + (...other traits...) + (BCS BV x BCS EV)
BVs are calculated as the difference in genetics between the 'Genetic Base Cow' and the cow in question.
The 'Genetic Base Cow' has BVs of zero as the reference point for all other cows to be compared against. A cow with a fat BV of +6 means that her daughters are expected to produce 3kg (half of +6kg) more fat than the 'Genetic Base Cow'. A cow with a fat BV of -6 means that her daughters are expected to produce 3kg (half of -6kg) less fat than the 'Genetic Base Cow'.
A cow's performance is made up of her genetics and her environment.
BW is the best estimate of genetics, so is only part of the story of what that animal's performance will be. Importantly, out of environment and genetics, genetics is the only thing you can select for when breeding stock. You can't select for the animal's future environment, but you can select for the best estimate of their genetics - BVs and BW is one tool to estimate an animal's genetics.
Why did my BW change?
Economic values were updated
Breeding values were updated
Breeding value calculations were changed
1. Economic Values were updated
To remain current, EVs are updated in February of each year. This reflects changes in milk prices, cull cow values etc.
Each time the EVs are updated, there is a shift in BW for most animals.
This is a reflection of how much the dollar value of the trait has changed, not how much an animal's genetics has changed.
2. Breeding Values were updated
Breeding values and hence BW are updated regularly as new information is added to the Dairy Industry Good Animal Database (DIGAD). Each update is called an 'Animal Evaluation Run' or 'AE run' where all of the data on all of the animals in DIGAD is run through the complicated statistical models to generate a BV for each animal, for each trait.
For younger animals, or those with their first progeny being herd tested for the first time, there can be fairly large changes to their BVs and hence BW.
Older animals and those with high reliabilities are unlikely to have their BW change much - more on this later on in this post.
3. Breeding Value calculations were changed
NZAEL reviews calculations for BVs and sometimes identifies issues in that the calculations are not as accurate at estimating genetic merit as they could be.
Sometimes this results in background calculations for BVs being changed to be better at estimating genetic merit.
Examples from the last few years include changes to Fertility BV and BCS BV (more here).
What is BW used for?
The BW of an animal is a measure of their expected ability to breed profitable replacement heifers. It is not their ability to be profitable themselves.
This means that BW should be used to assist in making breeding decisions (hence why 'breeding' is in the name!).
Now what makes BW pretty unique (and pretty cool!) compared with other genetic evaluation systems is that it is calculated across all breeds. This means that a Jersey cow with a BW of $140 is directly comparable to a Friesian cow that also has a BW $140.
So for farms that have a large mix of breeds and crossbreeds, it is nice and simple to select what bulls to use.
With genetics, half is passed on from dad and half is passed on from mum. This means when selecting bulls, take BW and divide by 2 for the expected profit ($) of their daughters per unit of feed.
Lets run through some fun examples!
The BW is the best current estimate of an animal's genetic potential.
With each Animal Evaluation run, as more data is added (daughters, mum, grandma, aunties, sisters etc.) we get to know more about each animal and we can get closer to their 'true' BW.
If we remember from our Calving Difficulty post, we are trying to get the closest estimate of an animal's 'true" breeding value (BW in this example). We can never know the true BVs, only estimates of them, even with high accuracy/reliability of an animal's estimated BVs there is always a chance that they will change as more records are added.
For example, say we have 2 bulls, one has a BW of $120 (purple) and the other has a BW of $125 (blue). The tip of each peak in the graph below shows the estimated BW that has the highest chance of being the 'true' BW.
As we get more data, there is the possibility BVs (and hence BW) could change. As we can see in the graph, there is a small possibility that these 2 bulls may actually have the same 'true' BW (the overlapping area).
Remember a bull (or cow) only passes on HALF of their BW to their offspring.
When a calf is born, she (or he) gets half of Dad's BW and half of Mum's BW.
Lets say we used the $125 bull above in blue and mated him to our top cow Gertrude who has a BW of $100.
This calf (yay it's a heifer! We can call her Sally), when she is born will have a BW of $112.50 --> ($62.50 from Dad + $50 from Mum).
Up until Sally begins her own lactation, her BW is made up entirely of data from ancestry records (mum, dad, aunties, cousins, sisters etc.).
This means the reliability is pretty low for Sally's BW, which can be visualised in the graph below by how low and flat the blue shaded area is. Our best estimate of her 'true' BW is $112.50, however, we are 95% confident it is in between $100 and $125 (a $25 range).
Once she gets her first herd test, LWT, mating, calving data into DIGAD, her own records begin to play a part in her BW calculation.
Because she has her own records in there, we become more confident that we are getting closer to the 'true' genetic merit of Sally.
Once Sally's daughters (progeny) begin lactating, we become even more confident that we are getting closer to her 'true' genetic merit.
By the time Sally is 7 years old and has 3 daughters lactating, her BW is now $111, calculated from ancestry records, Sally's own records and her progeny's records.
The more progeny records we have on an animal, the greater the reliability is. As we can see in the graph below, the purple shading (representing Sally's BW as a cow) is narrower than the blue shading (representing her BW as a calf).
This is why bulls tend to have higher reliabilities than cows, because they have more daughters (bulls can have waaaay more daughters than a cow can!).
This also explains why the bulls used by breeding companies (on the RAS list) generally don't change their BW very much, but cows in the herd do tend to change their BW.
We hope you found this post useful. If you would like to know more, leave us a comment or send us a message on our Facebook page (@agrisciencer) or in the comments below.