Feeding a dairy cow is first about maintaining a healthy rumen. If the rumen isn’t working efficiently, the cow will neither be feeling her best or able to produce the milk we expect from her. What makes a rumen healthy are the billions of microbes – bacteria, protozoa, fungi and yeasts – whose job it is to ferment the feedstuffs that a cow consumes in her diet. Collectively, we call them “bugs.” The rumen bugs also have their own life cycle and nutritional requirements. As they expire they become a major source of the cow’s metabolizable protein.

The ruminant species that includes not only dairy cows, but goats, sheep, deer, moose, llamas and alpacas, has been created for the purpose of using, for much of their diet, plant material that monogastric species such as swine and poultry as well as humans cannot digest. The rumen is nature’s wonderful way of allowing ruminants to use forages that are high in cellulose such as pasture grasses, corn stalks and even weeds that can be converted into food and fiber. It’s the way undomesticated animals like deer, elk, moose and bison can survive in the wild. It’s the way exotic beasts of burden like camels, alpacas and llamas can survive in harsh environments. This all happens because of a process called fermentation and it’s the bugs in the rumen that make it happen.

Fermentation is the bioconversion of complex carbohydrates into smaller molecular units. Fermentation also occurs during the baking of bread and the brewing of beer. The end products of rumen fermentation are called volatile fatty acids (VFA), which are absorbed in the cow’s small intestine and are converted to glucose in the liver. The rumen can ferment fibrous cellulose as well as nonfiber carbohydrates (NFC) such as grains and other commodity byproducts.

For modern dairy cows, the rumen must function continuously and consistently around the clock for them to produce the many gallons of milk they do. Any disruption in feed supply or a drastic change in diet will disrupt the work of the rumen bugs. When reduced in number for any reason, cow health and milk production suffers.

Excessive levels of the nonfiber carbohydrates (NFC), while having the ability to increase metabolizable energy in a cow’s diet, can disrupt the delicate pH balance in the rumen, which affects the bugs’ ability to ferment feed at their optimal ability. The nonfiber carbs such as starch and sugars are more easily broken apart by the bugs and when there’s excessive amounts of those carbs being fermented, more acid is produced than can leave the rumen, and the rumen environment becomes increasingly more acidic. The bugs can’t function as well as the acidity increases and other microbes take over, which create an even more potent acid – lactic acid. It’s the accumulation of lactic acid that causes rumen acidosis.

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Proper balancing of feed rations is necessary for creating and supporting a healthy microbial population in the rumen. Keeping the pH level in a rumen between 6.0 and 6.5 is the generally accepted range to support good fermentation in the rumen. Diets excessively high in NFC will produce acids that will drive pH below 6.0, which creates a hostile and even toxic environment for the fiber-digesting bacteria. Continued low rumen pH levels will eventually result in clinical acidosis for the cow and a rapid decline in milk components, milk production and health.

The challenge for ruminant scientists and dairy farmers is to feed those carbohydrates that are soluble and quickly fermented while providing adequate fiber to keep rumen microbes in the rumen long enough to accomplish fermentation. In the modern dairy industry acute and subacute rumen acidosis are second only to mastitis as the most prevalent metabolic diseases in dairy cows. It’s generally believed today that most commercial dairy farms in the U.S. – and especially those on diets containing high levels of corn – experience some level of acidosis in their cows.

Complex carbohydrates such as cellulose, which are most abundant in forages, are the key to keeping a rumen happy, healthy and balanced. It’s recommended that the level of forage in dairy cow diets does not drop below 30 percent of total dry matter consumed. In an effort to balance carbohydrate fractions and avoiding an acidotic rumen, nutritionists suggest keeping nonfiber carbohydrates balanced between 32 percent and 38 percent of dry matter intakes when feeding higher levels of grains like corn and barley or byproducts such as wheat midds, hominy and distillers grains. When rations contain high quality hay, haylage, brown mid-rib corn or sugar beet or citrus pulp, and the effective fiber is adequate, the level of NFC can be increased to 38 percent to 42 percent of total dry matter intake.

The high producing cows in any dairy herd require much higher levels of fermentation in the rumen to meet energy needs and microbial efficiencies to sustain their milk production. Any large variation in rumen pH throughout the day will have the greatest impact on the fresh cows.

The economics of dairy farming have necessitated the increase of milk production per cow for many dairy farmers to remain afloat financially. To get more milk out of a cow means increasing her plane of nutrition and making sure that microbial fermentation is optimal throughout the day. The greater the number of microbes in a rumen, the more fermentation that can be accomplished. The more microbes working in the rumen also means more microbial protein that will be available for proper amino acid nutrition. The more nutrients a cow can absorb every day, the more milk she can produce.

Read more: Optimizing the Rumen