Selection indexes are great tools. The two main U.S. genetic indexes, Total Performance Index (TPI) and Net Merit (NM$), have played a large role in the genetic improvement of the Holstein breed throughout the years.
Indexes are important selection tools because they combine all significant traits into one. They get us away from setting minimum criteria for particular traits, which allows us to make progress for many different traits all at once.
TPI and NM$ explained
When asked, a majority of the producers who utilize these indexes cannot explain the makeup of TPI or NM$. An easy way to think of TPI and NM$ is to group together the traits that make up each formula into three main categories – production, health and conformation, or type.
The current NM$ formula places the most emphasis on health traits followed by production and then conformation. TPI is more production-focused with relatively equal weights on health and conformation.
TPI has typically been a production and type index throughout the years. It even stood for Type-Production Index at one point but has recently been increasing the focus on health traits.
NM$ started out primarily as a production index. As recently as 2005, the index still had 55 percent of its weight on production traits. Many producers still perceive NM$ to be driven by production traits, when in reality the current formula is focused more on health than on milk and components.
Changes over time
To develop any national index is a challenging task. The goal of these indexes is to provide an easy way for the average farmer to rank sires (and cows) available for selection. We must realize that any index is subject to change over time and – rightfully so – as production and market prices change.
Since economic conditions and prices for milk, beef and feed have shifted once again, changes have been proposed for a new version of the NM$ formula. A new TPI index has already been finalized and will take effect starting in December.
Both NM$ and TPI will take emphasis away from conformation and add it to the production traits. Within TPI, those conformation points will be put on a new index called feed efficiency, which is largely driven by extra pounds of fat and protein.
NM$ will also make a more drastic shift to move several percentage points away from health over to production. This means NM$ will revert back to a more production-focused index.
It is important to keep in mind that with these adjustments, bulls you have been selecting from atop various index lists will certainly re-rank with the next proof round.
The effect of various traits on sire rank
Since the new TPI formula has already been finalized, we’ll dig deeper into the differences various traits make toward a bull’s final TPI. Tables 1 and 2 demonstrate these changes. The additional points on PTAP and PTAF come from the weight on feed efficiency.
Fertility index is the other addition to the new TPI formula, which is a combination of DPR, heifer conception rate (HCR) and cow conception rate (CCR).
To see how selection index changes can impact a bull’s rank on an industry list, let’s look closer at two different daughter-proven sires. These are the differences we will see in TPI from now to December, given no change in daughter information.
You can see in Table 3 that because extra emphasis will be placed on DPR, HCR and CCR, and reduced emphasis on PTAT, UDC and PL, a bull like Fixman will gain nearly 50 points.
On the other hand, a bull like Porter will drop in rank because of the index modification. While he excels in PTAT, UDC and PL, less emphasis will be given to those traits. Porter’s new TPI will drop him to a ranking lower than Fixman, a bull he currently outranks by more than 100 TPI points.
These two bulls remain exactly the same bull before and after the index change, yet they rank very differently from the current to new indexes.
If you’ve set your own genetic plan to put emphasis on only the traits that matter to you, these bulls will not change rank. If you want high production and components, Fixman will still be a great fit for you. And if conformation is what you’re after, Porter will still fit the bill.
How important is rank?
If you evaluate rankings more closely on the top current active TPI and NM$ lists, you’ll see that a meager 40 TPI points separate the 100th-ranked from the 200th-ranked bull – less than the number of points that a bull like Fixman or Porter could gain or lose because of the index change.
Just 31 NM$ separate the 100th best genomic sire for Net Merit from the 200th, and as you go further down the rankings, even fewer points separate a larger number of bulls.
Do those 40 TPI points or 31 NM$ really signify that the 100th-ranked bull is that much better for your breeding program than the 200th bull on the list?
We generally associate a higher rank to mean a “better” bull, but “better” is also a subjective and relative term. When selecting sires to use in your breeding program, it’s important to know what a bull’s rank on these lists really means. Keep in mind that just because a bull ranks high on TPI or NM$ this proof round or the next doesn’t mean that his traits match your breeding goals.
If you are a producer who never culls cows because of issues with udders or feet and legs, then any index points you gain from the conformation traits are of little to no value to you. While the fluctuating weights of these industry-standard indexes might be the right weights for the average dairy, the real question you need to ask yourself is if these are the right weights for you.
Make the right selection decisions
The various combinations on different trait weightings are numerous, and the way different traits affect a bull’s rank on various index lists all make one point evident: There is no such thing as a “one-size-fits-all index.”
So as changes are implemented and you evaluate sires to utilize on your farm, keep in mind:
1. Changes are coming to the TPI and NM$ indexes starting in December, so be sure you know what you are selecting for if you choose to utilize an industry-standard index.
2. A bull’s TPI and NM$ rank will change disproportionately with even a small increase or decrease in individual traits because of the pending index changes and because so many bulls are being genomic tested.
3. The best selection index is the genetic plan you develop with your A.I. representative. When you customize your plan to put weights on the traits that matter to you, you’ll continually make progress toward your goals – even when industry indexes change. PD
Nate Zwald is the U.S. sales manager with Alta Genetics. He can be reached by email .
PHOTO: We must realize that any index is subject to change over time – and rightfully so – as production and market prices change. Photo by PD staff.
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Chrissy Meyer
- Marketing Editor
- Alta Genetics
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