What does the statistical “heritability” of each PTA trait really mean?

Dairymen have a right to be confused about the so-called “health and fitness” traits, which faced a great deal of resistance from breeders and AI stud personnel who were trained to believe “genetics” was about PRODUCTION and TYPE.     Select for PTA milk and you get more milk: Select for PTA type and you get sounder type.     PTA Milk would make milk checks, and PTA Type would lower the herd turnover rates, reducing replacement costs.     Then along came “management traits” (so called by nay-sayers) to confuse the genetic selection and the index ranking calculations.    

The statistical measures of “heritability” indicate how powerful a selection response you get from each trait.     Traits are not equal:   “h2” for milk, butterfat and protein yield (pounds) is estimated 25%- 30%.

“h2” for butterfat % and protein % (density) is estimated 55%- 60% (twice that of yield volume).    You can feed for “more milk” but you have to breed for a “higher milk price” from bf% and pr%  selection.

In other words, the higher the “h2” [heritability%] of the trait, the less you can effect change by feeding and cow comfort and better milking procedures.      Linear type traits have a similar pattern of as high as 40% h2 for stature, 25%  to 35% on udder traits, generally 10% to 20% on feet and legs and frame traits.   

By comparison, the “health” and “fitness” traits (SCS = Somatic Cell score, DPR = daughter Pregnancy rate, PL = Productive life, LIV = livability, plus the calving ease and stillbirth PTAs) began life with a lower scale of “heritability” (5% to 20%).    Thus, in the opinion of those pedigree/type and index/milk breeders, these should remain secondary considerations.

Heritabilities vary with the calculation and the geography

Our experience with New Zealand genetics among grazing-based dairymen indicates that “heritability”  is not fixed in stone: it is an accumulation of the consistency of selection within a cow population.   In a seasonal breeding system, “fertility” is the key genetic trait, and multiple generations of selection in its favor appears to improve the heritability (NZ genetics sets “h2” of calving interval at twice the level we have observed in the USA).    

The data from our friends’ herd would certainly suggest a clear linear result between DPR selection and resulting days open—and it should, as this is the basis of the DPR calculation anyway.     We were told for decades that, if cows are to milk more, we must expect conception rates to be lower:  however, once dairymen quit accepting that line, and forced geneticists to study the data closer, they found some cattle are just a bit more fertile than others, and this could be tracked by family lines.

Body condition ability:  a key “environmental” variant in the search for fertility

Inside the dairy genetics mainstream, “body condition” is something for nutritionists to manage.   It has not been considered “genetic”, except as the linear type system (devised in the 1970s to identify young cows who would respond to more grain fed with more milk produced) actually preferred the cow that would delay body conditioning.     In other words, to 1960-70s geneticists, cows who gained weight if fed more grain were undesirable cows.  Today, with higher forage utilization, these genes are needed.

In the “aAa” (Weeks Analysis) system, we recognize what all other biologists recognize, that ability to maintain healthy body condition (a key indicator of positive nutrient energy utilization) is heritable.   It is not hard to figure out that always mating cows to be more angular will inhibit expression of + DPR.