Saturday, April 29, 2017

What the “genetic base change” really means

From the Jan Feb 2015 Dairy Route Letter

Every five years, AIPL recalculates the “genetic base” by the simple expedient of calculating testing averages off the most recent five years of cow birth dates and completed lactations.    Thus for this current cycle, the base period is 2010 (cows born 2010 or after with completed lactations) which is five years since the last base period (2005 through 2009 birth dates with completed lactations).

The concept of a “rolling” (annual, as Canada uses) or “stepwise” (five years, as USA uses) genetic base is that we need to compare “current” performance against “contemporaneous” standards.    Thus as long as herd averages in DHIA tested herds generally increase, the genetic “base” (average of recent yields) increases to keep pace with it.     If this were not done the argument is we would be printing evaluations with sires as high as +14,000 PTA Milk (ie, the first base period from the early 1960s found the average cow producing 12,000 pounds ME; today’s base average is more like 26,000 pounds ME).

The amount of change to “average” between 2005 and 2010

This latest base change produced an average gain in milk yield of:
Holstein = 382 pounds        Jersey = 327 pounds        Brown Swiss = 157 pounds
Equivalent gains in butterfat and protein yields of:
Holstein = 17 + 12              Jersey = 19 + 12               Brown Swiss = 6 + 6 pounds
Productive Life gains in months of lactation
Holstein = 1.0 months        Jersey = 0.8 months          Brown Swiss = 0.3 months.
Somatic Cell Score (logarithmic base)
Holstein = - 0.07                Jersey = 0.04                     Brown Swiss = - 0.02
Daughter Pregnancy rate
Holstein =  0.2                   Jersey =  0.0                      Brown Swiss =  0.0


Basically, if a given Holstein sire was + 382m +17bf +12pr and +1.0 PL 2.93 SCS +0.2 DPR “before” the base change, and he had no new daughters (born in the 2010 base period) his PTA values on the new base would now be +0 m +0 bf +0 pr  +0.0 PL 3.00 SCS +0.0 DPR.     If he had new daughters in the latest base period, they are part of the “current” base and his evaluation will basically be daughters vs contemporaries adjusted somewhat for whether these animals are in herds above or below the “base” average (ie, are the contemporaries providing the same level of competition as the base average?).

On the type side…

The Holstein Association is postulating the following base changes for type data:
0.99 Final Score    0.92 Udder Composite    0.78 Feet and Legs Composite
The Jersey and Brown Swiss Associations postulated 0.53 and 0.28 gains in final score.

Did all the classifiers in these breeds actually see type scores going up by a third (BS) to half (JE) to a full point?    I am guessing that in actuality there is an imputation based upon changes in linear scores for the “objective” measured traits, as the actual scoring procedures followed by type classifiers bases final scores in part on a standardized ratio of scores so that the population results conform to the “bell shape” distribution curve.     Ultimately, this just means “new” bulls have higher type ratings than “old” bulls (no matter how good in their era, Ivanhoe, Citation R, Chief, Bootmaker, Elevation, Astronaut, Valiant, Starbuck, Blackstar are all listed now as “minus” for type).

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