Genetic selection for flatter, more persistent lactation curves

 

Genetic selection for flatter, more persistent lactation curves.

Up to 1962 when USDA data shows the six measured breeds had national averages within 100 pounds of butterfat each per 305 day lactation (380 pounds for Milking Shorthorns, 480 pounds for Holsteins, every other breed in between) and the milk market basically priced on butterfat, there was little impact from the nearly 100% difference in the volume of milk each breed made.

This large difference in comparative milk yield mostly tracked cow frame size:  Holsteins, with twice the frame size (weight) of Jerseys, at that time produced nearly twice the volume of milk (but with less efficient rumens, barely 20 pounds more butterfat).   But a key difference, mostly ignored in the butterfat and pasture-based era was  the comparative lactation curves  between the smallest frame breeds (Jerseys and Guernseys) and the two largest framed breeds (Holsteins and Brown Swiss).    Ayrshires and Shorthorns fell in between, being breeds more favored for seasonal milk production in harsher climates, often selling to Grade B condenseries.

Jersey and Guernseys, the most “dairy” in behavior of all the breeds, but smallest in frame size, tended to have flatter, more persistent lactation curves, which would optimize butterfat yields when calved seasonally following the growth patterns of grasses in pasture:

 

 

Spring flush of grass / Weather warms, grass slows, corn grows / Fall rains, grass returns /  Snow
Calve early spring     (Store up spring hay)     Rebreed early summer    (Harvest silage)        Dry up

These breeds (especially Jerseys) bred back easier because they did not make fresh cow “peaks” so high that loss of body condition put them into negative energy states, while the smooth body type (and red-based hair/hide color) gave them more heat resistance.