Mature cattle are ruminants and utilize all four of their stomach compartments (rumen, reticulum, omasum, and abomasum) to allow for digestion of fibrous feeds. The digestive system of a newborn calf is very different from a mature animal. Initially, the digestive process is more similar to mono-gastrics (animals with one stomach). This allows maximum digestion of milk proteins, fats, and simple sugars. As the calf grows and is introduced to environmental organisms, grain, and forage the digestive system changes and evolves into a fully functioning ruminant.
As the proportion of the diet moves from milk to grain and/or forage many changes happen within the rumen. When the calf is newborn, the rumen is sterile, by day one a large concentration of bacteria is present. Most of these are aerobic (oxygen) bacteria and enter the rumen when a calf licks or swallows something from the environment. The rumen of mature animals is not inhabited by aerobic bacteria, these aerobic microbes can be thought of as temporary. Eventually they will be replaced by other microbes that can better compete in the rumen environment as the calf begins to consume dry feed.
The quantity and type of bacteria in the rumen changes as dry feed is consumed and available substrate for fermentation changes. In bottle fed calves it takes about 2 weeks of consuming grain to result in a dramatic shift in the microbe populations. Producers can stimulate this change in bacteria by providing fresh, clean water at all times from day one. Rumen microbes live in a water environment and a source of fresh water is important for their growth.
Rumen microbes will ferment carbohydrate and some of the protein in calf starter into volatile fatty acids (VFA). These acids are what stimulate rumen development and initiate the shift in rumen bacteria. In order to encourage starter intake and drive rumen development offer fresh, free choice starter to bottle fed calves by day three of age.
Transition to Concentrate Feeds
As a calf begins to eat more solid feed, the type of bacteria that dominate the rumen are influenced by the type of feed offered. The VFA’s butyrate and propionate are predominately produced by bacteria that digest starch. Bacteria that primarily digest fiber predominately produce the VFA acetate. Butyrate drives rumen development, as it is used by cells in the epithelial layer of the rumen wall for energy and growth.
Encouraging early intake of starter, results in starch fermentation, lowers rumen pH, and facilitates bacteria growth and production of butyrate. Butyrate stimulates the growth of rumen papillae, which increases the surface area of rumen that is available for absorbing nutrients. In early weaning systems it is necessary for calves to begin eating some grain each day by 2 weeks of age, to allow enough rumen development to occur before weaning at 5-6 weeks of age.
To ensure adequate rumen development, allow calves to consume grain for at least 21 days before initiating the weaning process. The weaning process can begin when the calf consumes 2.2 lbs. (1 kg.) per day for three consecutive days. Slowly transition off milk to allow calves to adjust to a concentrate diet.
Transition to Forage
The muscular layer of the rumen functions in the contraction and mixing of feed, and is developed with forage intake. Forage intake also stimulates rumination and saliva production. Typically, bottle fed calves in early weaning systems are not fed any forage, or only a restricted amount of chopped forage (≤ 5% of intake) until after weaning. This is because concentrate feeds stimulate papillae growth to a greater degree than forages in early life. Post-weaning calves should be offered hay, preferably high-quality grass hay.
But what about calves raised on the dam? Will they have sufficient rumen development when offered no grain prior to weaning? The answer is yes, forage/pasture intake alone will also result in normal papillae development, however, it takes longer and calls for the calf to be fed greater amounts of milk for longer periods of time for nourishment until they become a fully functioning ruminant.
Calves raised on the dam are typically weaned around 6-7 months of age. Weaned calves can be turned back onto pasture for several days post weaning or other programs my instead feed a forage-based diet with a supplement. Concentrate feeds high in digestible fiber and low in starch are good choices at weaning time. Some examples would be distiller’s grains, wheat midds, and soybean hulls, as they have been shown to provide adequate gains without the potential issues associated with starch-based concentrates.
Utilizing a mix of forage and concentrate post weaning will stimulate rumen capacity and development, and a healthy microbial population. Resulting in healthy calves and optimum performance post weaning.
Written by: Mariah Gull, M.S.