Significant Changes in Forestomach Size and Function Occur with Dietary Changes in Early Life

At birth the forestomach is about equal in size to the abomasum in both lambs and calves, a stark contrast to the normal adult proportions, in which the forestomach accounts for more than 90% of the total stomach volume.

In cattle the period of forestomach development is arbitrarily divided into the nonrunιinant period, from birth to 3 weeks, and the transitional period, from 3 to 8 weeks. Approximate adult distribution of stomach proportions is achieved usually by 8 weeks, if calves have access to solid feeds. Calves can be seen eating grain and forage as early as 2 weeks of age and fre­quently ruminate by 3 weeks, indicating considerable fore­stomach development by this time. Withholding solid feed dramatically reduces the rate of rumen development. In calves that are given diets of only milk or milk substitute (“replacer”), forestomach development remains rudimentary for 14 to 15 weeks or longer.

Development of forestomach epithelium parallels the general development of the organ. At birth the epithelium is thin, with small or nonexistent papillae. Exposure of the epithelium to VFAs appears to stimulate papillary develop­ment and general organ development as well. Highly digestible feeds, such as concentrates, result in the greatest VFA pro­duction and fastest epithelial development. Some dietary forage may aid in muscular development of the fore­stomachs, but calves and lambs in the transitional period should receive most of their solid feed as grain because their energy needs are high compared with their ability to ferment forages.

The forestomach is sterile at birth but is quickly colonized by environmental bacteria, mostly facultative organisms. As bacterial fermentation proceeds in the anaerobic confines of the forestomach, the electromotive force decreases; the typical reductive environment of the rumen is created by bacterial action. This environment creates conditions neces­sary for the growth and establishment of the strict anaerobes. The development of forestomach bacterial flora occurs independently of any special inoculation process, and indeed, it is impossible to prevent it from occurring except by raising calves under gnotobiotic conditions. Protozoal inoculation, in contrast to bacterial inoculation, seems to require some exposure to other cattle; calves raised in com­plete isolation do not develop protozoal fauna. It appears that aerosol spread of protozoa can occur because no direct physical contact among cattle is necessary to establish a protozoal fauna.