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The dairy calf begins its life as a simple stomached animal, yet spends most of its life as a ruminant whose digestion depends largely on fermentation. The change from one digestive method to another is a process that is called rumen development. A dairy cow has a four-part stomach system consisting of the reticulum, rumen, omasum, and abomasum. The first two compartments make up one large fermentation vat, the third is an unusual looking organ that absorbs water and minerals from digesta leaving the rumen, and the fourth is the true stomach that functions like the stomach of monogastrics (including pigs and people). All four of these stomach compartments are present at birth; however, only the abomasum is fully developed and functional. The other compartments, most notably the reticulum and rumen, are essentially undeveloped in the neonate. The reticulum and rumen are sterile at birth, and it is often several weeks before a constant bacterial population is established that resembles the bacterial population of an adult ruminant.
When we think of feeding calves, the first thing that comes to mind is probably milk or milk replacer. Liquid feeds are the primary nutrient source for calves in the first weeks of life, and they bypass the reticulum and rumen via closure of the esophageal groove. The formation of the esophageal groove sends liquid feeds directly into the stomach compartment that will digest them best—the omasum followed quickly by the abomasum. When we offer nutrient-dense liquid feeds, they provide the needed nutrients for maintenance and growth of young calves. However, milk and milk replacer do not allow for much growth or any maturation of the reticulum and rumen as they are being bypassed. Feed, most notably dry feed, has to remain in the rumen in order to begin the rumen development process. Dry feed, such as calf starter (grain mixtures) or forage, will not pass through the esophageal groove, and thus flows from the esophagus into the reticulo-rumen where digestion begins.
The bacteria that colonize the rumen are obtained from the environment, other animals that the calf comes into contact with, and bacteria found on feeds. Milk often is one of the first sources of rumen bacteria.
When dry feed enters the rumen, it absorbs water that the calf has consumed. That, along with the anaerobic (absence of oxygen) environment of the rumen, provides a perfect place for bacteria to grow. As these bacteria grow and metabolize nutrients, they produce volatile fatty acids. The primary volatile fatty acids produced in the rumen are acetic, propionic, and butyric acids. This acid production lowers the pH of the rumen and establishes an even better environment for bacteria to continue their growth, especially for bacteria that digest starch and produce propionic and butyric acids. Calf starter feeds contain carbohydrates in the form of starch which is fermented by bacteria that produce propionic and butyric acids. When forages are digested, due to the different species of bacteria that digest fiber, the primary end product is acetic acid.
Acetic and propionic acids are absorbed through the rumen wall and are taken up by the blood and pass through the liver to be made into metabolites that can be used for energy sources by the calf. However, butyric acid is not absorbed through the rumen wall, and the cells of the rumen wall have an alternative metabolic process that allows butyric acid to be converted into an energy source for use by the cells in the rumen wall. Thus, butyric acid produced in the rumen primarily provides energy for growth of the rumen wall. Acetic and propionic acids provide energy for the entire calf, part of which is shared to the rumen wall, but overall compared to butyric acid, much less acetic and propionic acids are used to fuel rumen development.
Research has shown that once a significant amount of starter or grain is consumed by the calf each day (approximately 0.25 to 0.4 lb per day), it takes about 3 weeks to then develop the rumen to the point that this digestive organ by itself has an established microbial population and enough absorptive capacity to allow the calf to continue normal growth once milk or milk replacer is stopped (weaning). If liquid feeds are removed before rumen development has occurred, the calf will not grow and may even lose body weight for 1 to 3 weeks until the time that the rumen is developed.
Therefore, digestion of starch sources is a major component of rumen development, and calf raisers should provide feeding, housing, and management practices that encourage calf starter intake and thus rumen development. Many different studies in countries throughout the world have confirmed the feeding and management practices that inhibit calf starter intake. Classically, a poor housing environment that creates sick calves will reduce appetite and intake. Overfeeding milk or milk replacer (> 14% of body weight per day) reduces calves’ appetite for dry grain. Unpalatable, dusty, or moldy starters will also reduce calf intake. Free choice water is needed, as well as clean buckets for feeding both water and grain. Any time you notice 2-week-old calves that are not eating grain, stop and determine why they are not eating it. If they are not eating a half pound a day by 4 weeks of age, again, look for the cause.
Body weight gains from calf starter are always going to be cheaper gains than from milk, but both are needed in the young calf. Early weaning programs (35 days or less) require great attention to starter intake as the rumen will not be fully developed by the time milk feeding is reduced; however, with good management, these programs can be very successful. If high levels of milk are fed which restricts grain intake, it may still take 3 additional weeks of high grain intakes for rumen development to occur even if weaned at 8 to 10 weeks of age.
Any time we evaluate the cost of feeding and maintaining a dairy replacement animal, the preweaned calf is always found to be the most expensive per day (primarily labor and feed), while the first group after weaning is the very least expensive replacement animal. Thus, age at weaning and heifer economics go hand in hand. Obviously, weaning at a reasonable age is only part of the equation, as we want calves to continue to grow at all stages. Thus, rumen development is the key.
Calves are born with undeveloped rumens, yet they will spend the vast majority of their lives as ruminants. Our job is to allow calves to make the transition easily and in a timely manner so that they grow to be cost-effective forage consumers that are efficient and productive animals.
Professor of Dairy Science
Pennsylvania State University