by Tamara Scully
Dairy cows are ruminants, evolved to eat grass, which is composed primarily of carbohydrates. Microbes present in the rumen ferment carbs and break them down into volatile fatty acids (VFAs), the main source of energy for cows. These microbes also break down rumen-degradable proteins and non-protein nitrogen into amino acids and ammonia, which are used to build additional microbial proteins. The microbes themselves are digested and utilized in the abomasum as a source of protein for the cow, along with protein from the diet. Fats are primarily digested in the small intestine; the small intestine and abomasum handle proteins not digested in the rumen as well.
The importance of the rumen – which holds about 42 gallons in the mature dairy cow – in optimizing cow energy has led to research on how best to develop a healthy rumen in dairy calves.
“We want to be able to utilize that as best as possible to convert cellulosic material into high-quality milk,” Christopher Gambino, assistant professor of animal science at Delaware Valley University, said of the rumen.
Rumen in Calves
It might make sense that those same forages which a rumen is best able to digest would be the ones needed to prime a dairy cow for optimal rumen development, but calves aren’t born with functioning rumens. Instead, they’re classified as pre-ruminants.
The calf rumen is very small, and the abomasum is twice its size, composing about 60% of the calf stomach. An esophageal groove exists, which creates a channel for fluid to bypass the rumen and flow into the abomasum when the calf suckles. If milk were to go into the undeveloped rumen, it would curdle, causing digestive upset.
The rumen will develop, increasing in size after about three months, and the abomasum will shrink as the calf matures. As the rumen matures, its muscles need to develop to promote motility; a microbial population must become established; and papillae (the projections in rumen lining that work to absorb VFAs into the bloodstream) must grow, covering as much surface area as possible.
Rumen musculature allows fermentation to occur by moving feed around, facilitating digestion, putting the rumen microbes in contact with the feed and optimizing the adsorption of the VFAs, Gambino explained. The characteristics of the rumen that are developed during this critical time persist throughout the cow’s lifespan.
Research from Cornell, among other studies, has shown that development of the papillae is enhanced by dry feed in the diet, and that feeding grain can stimulate rumen development faster than forages alone, ultimately increasing rumen capacity and papillae surface area. Milk-only diets, or milk and pasture forages, don’t do as much to help the rumen form.
“Without adequate feedstuffs to stimulate fermentation in the rumen, then that rumen will not develop properly,” Gambino said.
Volatile Fatty Acids
During the 1950s, it was demonstrated that it isn’t simply bulk feed that stimulated the rumen to develop, but that metabolites of fermentation were needed to best develop the rumen. Researchers looked at the role of VFAs, measuring changes in the percentage of the salt forms of the primary rumen VFAS – acetic, propiaonic and butyric acids – present in the rumen across various diets.
Acetic acid is the most prevalent VFA in the rumen when on a forage-based diet. It is the least acidic of the three. Propionic acid increases as the grain ration is increased. Butyric acid is the most prevalent acid when concentrate-based diets are fed, and is the most acidic.
It was hypothesized that the greater acidity level in the rumen influences the development of the papillae, which then leads to better adsorption of VFAs and increased energy available to meet the animal’s own needs and to produce milk. The rumen development influences the cow throughout its lifespan.
New research at Virginia Tech shows the calf diet, and its influence on rumen development, has to do with genes related to the adsorption of VFAs. Different feeds can turn these genes on or off.
The different VFAs provide different forms of energy efficiency for the cow, based on the glucose available. Proprionate is almost completely converted to glucose in the liver, and the glucose moves into the mammary glands, where it directly goes into lactose or into glycerol. Acetate travels into tissues, and when it enters the mammary gland is converted into short-chained fatty acids – the milk fats. Butyrate is metabolized into ketones, which move into the bloodstream, where some will enter the mammary gland. These are converted into milk fatty acids, and some long-chained fatty acids as well.
“Propionic acid is higher efficiency in energy conversion than acetic and butyric acid,” Gambino said, and changing the diet can increase its prevalence. “This is just to show you why the fatty acids are so important, and why manipulating the diet to change the bacteria in the rumen is so important: because of the impact VFAs can have on our milk production.”
Importance of Starter Diets
The rumen microbiota is being established during the post-natal to weaning period. As the microbes colonize the rumen, it transitions to its mature functioning state. Manipulating the calf diet can change the bacteria in the rumen, which changes fatty acid production. The changes in VFA levels impact milk production.
The importance of us manipulating the diet when that calf is post-natal and at the weaning point is that the diet has a huge impact on the establishment of rumen microbiota, Gambino said.
Energy-dense, fermentable carbs with increased particle sizes to stimulate muscle development and enhance rumen motility are thought to be the best feeds for rumen development. Although the exact processes that occur during this development period aren’t known, there isn’t a forage-based starter diet that is readily fermentable which might provide the same rumen development advantages as does a grain-based one, Gambino said.
Yet the interest in grazing cattle is rapidly increasing, driven by a grass-fed milk market and an interest in low-input farming. Gambino and certified organic dairy farmer Matt Bomgardner of Pennsylvania are applying for a grant to study whether a novel forage-based starter diet to enhance rumen development can be found. They will also compare a variety of transitional and certified organic grain-based starter diets with an all-liquid starter to determine which diet is best “to transition the pre-ruminant calf to a fully formed ruminant animal and get increased milk in the bulk tank,” Gambino said.
For those grass-based farmers who question why, based on the research, a dairy calf doesn’t seem able to develop rumen adequately on its natural milk and forage diet, Gambino has a theory: We’ve changed dairy cow genetics over time by feeding grains, and through breeding selection, making a grain-based diet necessary to stimulate the calf rumen for optimal growth.
“I think that genetics play a huge role,” Gambino said. “I do think environment and diet play enormous roles as well, in terms of establishing the microbial population, which is probably one of the most important aspects.”