Development of the Rumen Epithelium and Meal Feeding of Calves
The development of the rumen is critical to successful weaning and good growth rates after weaning.
Factors Required for Rumen Development.
When the calf is first born, the rumen is sterile - there are no bacteria present. However, by one day of age, a large concentration of bacteria can be found - mostly aerobic (or oxygen-using) bacteria. Thereafter, the numbers and types of bacteria change as dry feed intake occurs and the substrate available for fermentation changes. The numbers of total bacteria (per ml of rumen fluid) do not change dramatically, but the types of bacteria change as the calf begins to consume dry feed.
Liquid in the Rumen.
To ferment substrate (grain or meal and hay), rumen bacteria must live in a water environment. Without sufficient water, bacteria cannot grow, and ruminal development is slowed.
Most of the water that enters the rumen comes from free water intake.
Milk or milk replacer does not constitute "free water". When milk or milk replacer is fed to calves, it by-passes the rumen and reticulum by the action of the esophageal groove. The esophageal groove is active in the calf until about 12 weeks of age. The groove closes in response to sucking, shunting milk past the reticulorumen and into the abomasum. Closure of the groove occurs whether calves are fed from buckets or bottles. Therefore, the feeding of milk replacer should not be construed as providing "enough water". Feeding water can increase body weight gain and starter intake.
Outflow of Material from the Rumen.
At birth, the rumen has little muscular activity, and few rumen contractions can be detected. Similarly, no regurgitation occurs in the first week or so of life. With increasing intake of dry feed, rumen contractions begin. When calves are fed milk, hay, and grain or meal from shortly after birth, normal rumen contractions can be measured as early as 3 weeks of age. However, when calves are fed only milk, normal rumen contractions may not be measurable for extended periods.
Calves will ruminate for increasing periods when dry feed (particularly hay) is fed.
Absorptive Ability of the Rumen Tissue.
The absorption of end-products of fermentation is an important part of ruminal development. The end-products of fermentation, particularly the volatile fatty acids (VFA; acetate, propionate, and butyrate) are absorbed into the rumen epithelium, the rumen must develop this absorbtive ability prior to weaning.
The rumen wall consists of two layers - the epithelial and muscular layers. Each layer has its own function and develops as a result of different stimuli. The muscle layer lies on the exterior of the rumen and provides support for the interior (epithelial layer). Its primary role is to contract to move the ruminal contents around in the rumen.
The epithelial layer is the absorptive layer of tissue that is inside the rumen and is in contact with the rumen contents. It is composed of a very thin film of tissue holding many small finger-like projections called papillae. These papillae provide the absorptive surface for the rumen.
At birth, the papillae are small and non-functional. Many researchers have evaluated the effect of various feeds on the development of the epithelial tissue in relation to size and number of papillae and their ability to absorb and metabolize VFA. The results indicate that the primary stimulus to development of the epithelium are VFA - particularly from meal or grain.
Availability and type of food.
We have seen that bacteria, liquid, rumen motility, and absorptive ability are established prior to rumen development, or develop rapidly when the calf begins to consume dry feed. Thus, the primary factor determining ruminal development is dry feed intake.
To promote early rumen development and allow early weaning, the key factor is early consumption of a diet to promote growth of the ruminal epithelium and ruminal motility. Because meal/grain provides fermentable carbohydrates that are fermented to propionate and butyrate, they are a good choice to ensure early rumen papilla development. On the other hand, the structural carbohydrate of forages such as hay tend to be less stimulatory to ruminal epithelial development but do promote rumen wall muscular development ,rumen contractions and rumen ‘capacity’. Early and aggressive intake of calf starter with access to hay and water are the keys to good ruminal development along with a high quality milk replacer. Offer starter meal from 3 days of age and keep it fresh, clean, and available. This will help provide the proper stimulation for rumen development and allow early weaning.