The history of linear trait comparisons

 CONCEPTIONS    Dairy route newsletter                  Nov-Dec 2023

Prior to “linear” each breed had its own classification system.   Holsteins had “descriptive type” (and it is instructive that the seminal ancestors of modern Holsteins developed from this era) so was most affected by the change to “uniform type traits appraisal” as designed by scientists for four land grant universities that had on campus research herds.

USDA had an agenda in funding research into type traits: they wished to transform dairy feeding into using the mounds of corn rotting away in Kansas parking lots as a result of the federal grain crop incentive programs.     “Linear” defined the physique of the cow who would transform corn into milk  (instead of into weight gain, the traditional use of corn for steers, hogs and poultry).

Never having paid attention to “type” before, the small group of scientists involved made some mistakes that more skilled purebred breeders noted, but the industry momentum  (coming at a time when regional AI cooperatives were merging to compete with private national firms)  was focused on breeding young sires to be sampled by AI studs,  not on breeding exceptional cows.  
“Linear” was an easily-learned language even if it never considered the entire cow physique or concerned itself with all desired cow functions.    Milk at younger ages from corn was the focus.

It takes several generations before weaknesses of anything new in breeding show up.   For the first two-three generations, switching to linear had the same heterosis effect (“hybrid vigor”) as is promoted for crossbreeding.    Newly ranked sires were “outcross” to the previously ranking sire lines.    It was only in later generations that breeders’ “best” cow lines were seen to decline in functionality.    Geneticists explained all such observations as “examples of genetic trend” not acknowledging any intergenerational declines,  all evaluation formats transformed into “intra- generational” deviations  (in which half of all animals evaluated would always be “plus”).    

But within twenty years, the once-discredited practice of dairy crossbreeding came back with a vengeance, primarily in two types of dairies:  (1) larger forage-feed-based expansion systems;  (2) dairymen switching to grazing.   Both found their current cows lacked overall function to be profitable in their chosen systems.    Which crosses were most successful initially?   Those which quickly added substance, width and sturdiness (“round” qualities) back into highly angular cows.

Like all genetic ranking systems used to-date since 1970 in the marketing of AI sires, selection on “linear” after three generations gravitates into “single trait selection”.     Single-trait selection in any form promoted so far, has always led to the random increase in “inbreeding depression” (a misunderstanding of what should be termed “selection depression”).     While AI sire lines have become more narrow in their pedigrees, it is the constant selection pressure in favor of “linear angularity” traits that causes the physical or reproductive failure of typically short herdlife cows.

More dairymen seem to be selecting some linear traits on the “negative” side of the scale, thus hoping to overcome overall breeding trends.  Here again, lower trait heritabilities slow changes.