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♦ Genetic Road Map: Which Direction are You Heading?
♦ Genetic Road Map: Which Direction are You Heading?

Genetic-Direction.jpg

You are already familiar with maps and the four cardinal directions. You also know the combination of north, south, east and west you use determines how fast you get from point A to point B. The purpose of a map is to help you determine the best route to take once you've weighed all the information that goes in to your decision. Generally, people choose the most efficient route rather than a route that winds around and may never get you to your destination. This is all common sense, right? Then let me ask, when it comes to genetic selection on your dairy do you choose the most efficient route or are you wandering aimlessly?

Here's a tip: most bull studs do not have the same breeding philosophy. Some studs focus mostly on the Lifetime Net Merit (LNM) index and fertility, others focus more on type, and others focus primarily on the price of their product. For this reason, if you only buy what each stud considers to be their "top" bulls, you likely fall into the category of wandering aimlessly with your genetic selection. Going back to Genetics 101, let's discuss why you should have a direction in mind when choosing the bulls you want to sire your future herd and how you can ramp up genetic progress in your herd.

Consider the equation for genetic progress (ΔG) below. Then we'll look more closely at it and our genetic road map. 
Genetic-Road-map-equ.jpg 

Heritability
Generally speaking, the lowly heritable traits can have the biggest economic implications on your selection program1. Take for instance DPR which was inadvertently selected against in the Holstein breed for decades. This lowly heritable trait slowly eroded away until the breed was a net negative2. In 2004, we began using this trait as a way to select for daughter fertility. The year 2009 marked the first time since 1993 that both Holstein cows and sires were net positive for DPR. The message here is that it takes multiple generations to improve or correct a lowly heritable trait while genetic progress through highly heritable traits can generally be realized more quickly.

Therefore, if you are overlooking lowly heritable traits and selecting mostly on high heritability traits then you may have a long road ahead of you to improve on things like cow fertility (DPR), longevity (PL) or mastitis resistance (Somatic Cell Score; SCS). The good news for producers using LNM is each of these low heritability traits is fixed into the LNM formula, so sires with negative DPR, PL or SCS are penalized accordingly.

Selection Intensity
You can make genetic progress through selection intensity by using a standardized set of criteria for sire selection. For example, if your breeding goal is to improve production but you also want to utilize high fertility sires for a conception advantage, you would make the most genetic progress by choosing bulls that meet both criteria. If instead you chose to use one group of bulls with high production and a second group of bulls with high fertility (and low production), the average genetic progress across the herd for production would be limited. Selection intensity can be magnified with the use of technologies like sexed semen, embryo transfer and in vitro fertilization.

A second way to increase selection intensity is through voluntary culling. If you have low value cows or heifers in the herd and are in a situation where you don't need to keep them, then you can make rapid genetic progress through culling. If you find this an interesting concept, you should consult with your trusted advisors about using low density genomic testing (i.e. 3K chip) to identify low-end animals or alternative methods of selecting cattle for culling (i.e. low relative value, low LNM of sire, chronically ill or problem breeder). Herds with excellent reproduction and high culling can make progress very quickly by removing low-end cows from the herd.

The Variance
The wild card in the equation is the σp or the phenotypic standard deviation. This plays a huge role in the amount of genetic progress made, yet you can't select for it. Instead, it's the impact the management scheme on your dairy or perhaps the location of your dairy has on genetic progress.

I have had a few discussions with producers who are good dairy managers but see little value in increasing selection intensity to improve their herd's genetic progress. Instead, they believe they can rely on variance or simply "manage" the genetic level of their herd. In other words, they try to use lower genetic merit bulls and good herd management to attain some level of genetic progress. My argument is that in this situation the genetic level of the animals still matters - selection intensity is still important even with good management. Think about this: generally the cattle with the best genetics (PTAs) for traits like Milk, DPR and SCS separate themselves from the rest of the herd in each of these categories. For example, the cows in the upper quartile for sire PTA Milk has a higher 305 ME, the upper quartile for DPR has fewer days open, and the upper quartile for SCS has lower somatic cell counts. This fact is shown on the ResultsTM reports, above, from a Genex member herd. The point is the top cattle are going to sort themselves out from the herd average, and in general the top cows in your herd will be the ones with the highest genetic levels.
Genetic-Road-map-results.jpg

Generation Interval
The final component of the equation is the generation interval or, in other words, the age of your herd. The youngest cattle on your farm should be the most elite genetically. To be even more exact, the youngest cattle on your farm whose dams are in their first lactation should be the most elite. This is important because the most rapid gains can be made through this heifer group. There are technologies available (i.e. sexed semen, embryo transfer, etc.) that can help amplify the genetics of these heifers in future generations.

Generation interval is important on the sire side too. It has been known for years that the most elite sires in the dairy industry are the younger bulls. However, bulls had to be identified as genetically elite through extensive progeny testing programs, and they were not available for widespread use until they were at least 5 years old. With the adoption of genomics, elite dairy sires can be identified at just a couple months of age and marketed before they are 2 years old. This means a 5 year old sire can have his first daughters evaluated at about the same time his grandson is beginning to be marketed. Do you think it is absurd to turn bulls around so quickly? Cattle genetics has actually lagged behind its poultry and swine counterparts for centuries because cattle have a longer gestation period and it takes cattle longer to reach maturity, both of which extend the generation interval. The combination of genomics, efficient reproduction and high voluntary culling can give dairy producers levels of genetic progress that were never available up to this point.

We took a tour through the equation for genetic progress and have arrived at our final destination. If you have not thought through your genetics program in a while, now is a great time to do so. Start with the sires you are purchasing. Make sure they have your selected criteria in common. If your situation allows for voluntarily culling, then take advantage of that option to improve your genetic progress. Lastly, decide if it is worthwhile for you to develop a strategy that gives you the most replacements from your best heifers. While A.I. has many benefits, the primary advantage is supposed to be the genetic gains that are made possible. Approach the genetics of your herd with a plan and take the most direct route to get where you want to go.

References:
1Cassell, B. Using heritability for genetic improvement. 2009. Virginia Cooperative Extension. Publication 404-084.
2AIPL Trends Report. http://aipl.arsusda.gov/eval/summary/trend.cfm?R_Menu=HO.d#StartBody. Accessed November 11, 2011.


December 2011


 
 
 
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