Milk by Anne Mendelson

several breeds are more common than ones made up of single-breed registered stock. Many a “grade” Holstein-Friesian herd owes its fine milkfat and SNF records to a strategic admixture of Jersey genes. But higher and higher volume per cow certainly is a universal (though not exclusive) goal.
    Merely breeding dairy cows generation after generation for skinny contours and brimming milk pails would not have brought daily record yields from the neighborhood of 56 pounds (28 quarts) to 152.5 pounds (76 quarts, for Ellen) or roughly 186 pounds (93 quarts, for Lucinda) between the mid-nineteenth and late twentieth century. The early “scientific” breeders worked before it was possible to apply Mendelian genetics to the cows or chemical analysis to the milk. Not until these tools were mastered and coupled with artificial insemination (starting in 1938) and advanced record-keeping techniques did breeding for actual genotype heat up. There seemed almost no limit to yields once farmers could identify heritable milking qualities in the daughters of a particular sire who could then impregnate hundreds or thousands of other cows. But breeding by itself was only half the story behind today’s super-high-producing cows. The other half wasfeeding, which has been revolutionized as drastically as applied cattle genetics by modern information technology.
BREED, FEED, AND EXCEED
    The unimproved cows of yesteryear suckled their calves while eating what cows were made to digest:grass. Like human mothers, some produced more milk and some less. The higher producers might eat more to compensate for the greater amounts of energy and nutrients being channeled into their milk, but the energy-balance system was largely stable and self-regulating. Genetic selection for abnormally high yields complicated the picture. It meant that the “best” cows were always in a sort of metabolic race to outrun “negative energy balance,” with capacity for intake being pitted against capacity for output.
    Calves left to their own devices start experimentally nibbling grass within days of birth and are at least partly weaned in a few months. Suckling a calf places lesser demands on the mother’s body than being completely milked out in a milking parlor twice (or even three times) a day for a year or more at a time. A little arithmetic shows that the 1,750-pound Ellen was directing close to 8.75 percent of her own weight a day into that 152.5-pound milk output. Few cows can eat and drink fast enough to keep up with such losses.
    Now, a ruminant’s stomach does not resemble a human stomach. It is a series of chambers that postpone the business of digesting food by gastric juices in the fourth stomach, or abomasum, until the cellulose of grasses has first been partly broken down by trillions of microbes in the gigantic fermenting-tank system formed by the rumen and reticulum (the first two stomachs) and ground fine by the many-leaved walls of the omasum (third stomach). Even the role of the mouth is different, for what the animal eats gets two chewings, one perfunctory, the other a more thorough “rumination” of a bolus of cud regurgitated from the rumen. Ruminants live in delicate symbiosis with their bacterial and protozoal guests, a multispecies population swimming in a reticulo-ruminal soup replenished by gallons of saliva per hour. As well as breaking down fiber, the microbes begin the work of elaborating the particular proteins and fatty acids that the host animal needs.
    For millennia after cattle were domesticated, different sorts of grass (or hay) continued to be virtually their whole diet. The resident ruminal bacteria need it to maintain a stable balance of different microbial organisms. Altering the diet kills off some species while encouraging others, until the entire chemistry of the rumen may be thrown out of whack. This does the cow no good but, depending on what you’re feeding her, may make her give more milk. For generations it has been