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Back to the basics: Understanding genetic traits and indices

Mandy Brazil for Progressive Dairyman Published on 18 April 2017
Genetic traits and indices

Genetic theories are complicated. Choosing a package of genetics to create more profitable cows doesn’t need to be. The dairy genetics industry has done a nice job creating an overwhelming number of traits from which dairy producers can select. In fact, there are over 70 selection traits. Nobody has time for that.

Consider where your expenses add up at and where your income comes in from. Advanced genetic progress does not have to be any more complex than this. One of the best things a herd manager can do is use a single economic index alongside several trait selection criteria thresholds.

Select traits which correlate to saving expense or generating income. While all traits certainly serve a purpose, not all traits will be as pertinent to one dairy operation’s needs as it is to another.

Economic indices and Net Merit Dollars

Everything in moderation is a bit of advice for new year’s resolutions – and genetics. It is important to maintain sensible balance on traits so one trait does not become too extreme and others are not forgotten. The easiest way to achieve this is to use an economic index which will put a weighted average on important traits.

Several relevant indices in the industry are available depending on your milk market, facilities and operational costs. If none of those seem quite right, you may have the option to work with your genetics supplier to develop a customized selection index.

Net Merit Dollars (NM$) is the most popular economic index for commercial dairy producers. NM$ predicts how profitable a cow will be over its lifetime compared to the average U.S. cow. It is expressed in dollars and is applicable for most milk markets. The index incorporates 16 different production, component, health, fertility and calving traits.

The weightings on the different traits are based on how economically important they are to the average dairy’s operating profit. Economists with the USDA optimize the index for maximum profit based on estimations for future milk prices, component value, cull cow prices, feed costs, replacement rearing expenses and health incident treatment.

Back to the basics on PTAs for production, female fertility and body size

Most traits are Predicted Transmitting Abilities, or PTAs. These values represent how good or bad the genetics are that an animal will pass down to the next generation. A PTA number is standardized for each breed. The formulas for PTAs incorporate genomic information, family performance information and any progeny information.

The PTA calculations also factor in the amount of heritability a trait has. The end result number is an estimate of the actual amount of gain or loss which can be noticed in a single generation. Regardless of whether the trait is low- or high-heritability, the PTA is the difference which can be expected.

PTAs for production

PTA for milk, or PTAM, does exactly what it sounds like: predicts how positively or poorly a sire’s genetics will improve milk production. PTAM represents how much yield per mature lactation a sire’s daughter will have compared to the current breed average.

The value is expressed in pounds. For example, a bull with +1000 PTAM is expected to have daughters who milk 500 pounds more per lactation than a bull with +500 PTAM. At 20 percent, PTAM is one of the highest-heritability traits, and a herd can be impacted quickly with production increases through focused PTAM selection.

PTA for fat (PTAF) and PTA for protein (PTAP) have the same concept. Calculated in pounds, these predict the increases per mature lactation a bull’s daughter will have compared to the breed average production yield.

PTA Daughter Pregnancy Rate

Daughter Pregnancy Rate (DPR) correlates to pregnancy rate, the percentage of eligible open cows which become pregnant during a 21-day period. For female fertility, management and environment account for 96 percent of the variation in days open and pregnancy rates. The influence of genetics on pregnancy rate is 4 percent for the trait DPR.

Although this is one of the lower-heritability traits, herds with excellent reproductive management see strong expression for DPR in their herd. Even with excellent management, genetic potential will still eventually cap a cow’s performance. Maximizing this trait will maximize reproductive efficiency over a cow’s lifetime.

An increase of +1.0 DPR corresponds to one percentage unit better pregnancy rate. DPR also correlates to a decrease of four days open. For example, a bull with +2.0 DPR will have daughters which breed back eight days sooner than daughters from a bull with +0.0 DPR.

PTA Body Size Composite

Ruminant animals must consume 1.97 percent of their bodyweight just for maintenance. Therefore, the larger cows we create genetically, the more feed we will need to give them. Larger animals will not as efficiently convert feed into profit (milk) because they require more body maintenance for their size.

Beyond feed intake efficiency, cow comfort is not typically optimized for large cows in most facilities.

One trait to consider in sire selection is Body Size Composite (BSC). This value is calculated to reduce the entire mass of the cow. One point of BSC is equivalent to 40 pounds of Holstein mature bodyweight. For example, a bull with +2.0 BSC is predicted to have daughters 80 pounds heavier at mature bodyweight than breed average.

With more dry matter intake for body maintenance and without an increase in yield, some dairies find that a greater BSC equals a less profitable cow.

What does this mean to your operation?

Genetics are truly a balancing act. However, they do not need to be a complicated one. Do not get too caught up in the individual traits, but rather work on identifying the key traits which can create a positive impact on your profit.

Work with a trusted genetics consultant to create a consistent genetic plan; this will ensure any sires brought into your herd align with your key focus areas.  end mark

Mandy Brazil
  • Mandy Brazil

  • North American Dairy Genetic Services Specialist
  • ABS Global
  • Email Mandy Brazil

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