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a Source: Food and Agricultural Organization, 1999.

a Source: Food and Agricultural Organization, 1999.

Figure 1 The gradual increase in annual milk production in the United States (Panel A) has been accomplished with a declining number of cows having an increasing average milk production (Panel B).

steadily increases over a 6- to 8-week period and then slowly decreases for the rest of the lactation. Normally, the cow is bred again at 11-12 weeks after calving, and delivers her next calf some 40 weeks later. Thus, the cow is pregnant for the bulk of her lactation.

B. Organization of the Digestive Tract

The rumen is the first of the four preintestinal digestive chambers in ruminant animals and is physically proximate to the second chamber, the reticulum (Fig. 2). Because of their location and their similar function, the physiology and microbiology of the rumen and reticulum are usually considered together. At birth, the ruminant is essentially a monogastric animal having a functional abomasum that digests a liquid diet (colostrum and milk) high in protein (Van Soest, 1994). As solids and fiber are gradually introduced into the diet, the other three preintestinal chambers develop over a period of approximately 7 weeks. The rumen is a large organ (approximately 10 L in sheep and goats but up to 150 L in high-producing dairy cows) that together with the reticulum constitute about 85% of the stomach capacity and contains digesta having 10-12% of the animal's weight (Bryant, 1970). In the rumen, microbial fermentation converts feed components into a mixture of volatile fatty acids (VFAs)—acetate, propionate, and butyrate (For the sake of brevity, these and other organic acids will be referred to in this chapter as their anionic forms, although they are normally metabolized and transported across the cell membrane in their protonated (uncharged) form. An exception is made in the discussion of lactic acidosis (see IV.D.1), where the acid itself is

Figure 2 Schematic representation of the arrangement of the four preintestinal digestive chambers in the ruminant and illustrating the dominant size of the rumen.

responsible for the pathological condition.)—that are absorbed through the rumen wall for use by the animal as sources of energy and biosynthetic precursors. Thus, the ruminant animal cannot directly use carbohydrates for energy, and it is absolutely dependent upon its microflora to, in effect, predigest its food.

By virtue of its large size, the rumen has the function of slowing down the rate of passage of feed through the organ, which permits microbial digestion of essentially all of the nonstructural carbohydrate of the feed (starches and sugars) as well as over half of the more recalcitrant feed fiber (cellulose and hemicellu-loses) (Van Soest, 1994). Rumen contents, which contain 6-18% dry matter, are mixed by strong muscular movement and are periodically returned via the esophagus to the mouth for additional chewing (rumination). Despite this, the solids have a tendency to stratify, with some maintaining a suspension in the rumen liquor, some settling to the bottom of the rumen, and some being borne up by gas bubbles to form a floating mat at the liquid surface. Passage rates vary with intake, with the rates for solids averaging about twice of that for liquids. From several published experiments, mean retention times for the rumen liquid range from 8 to 24 h, whereas that of the particulate phase range from 14 to 52 h (Broderick et al., 1991). The consequence of these long retention times for solids is that ruminant animals can use fibrous feeds (forages and certain agricultural byproducts) that are not usable by humans and other monogastric animals, with the ultimate conversion of these feedstuffs to useful products.

In addition to VFAs, other products of the fermentation include microbial cells and fermentation gases. The microbial cells eventually pass through the omasum and into the abomasum (the acidic ''true stomach''), where the microbial cell protein is hydrolyzed to amino acids that are available for subsequent intestinal absorption. This microbial protein is a major contributor to the protein requirements of the animal, and it acts to counterbalance somewhat the considerable loss of feed protein that occurs as a result of microbial proteolysis and amino acid fermentation that occurs in the rumen (see Sec. IV.C.5).

Fermentation gases include primarily carbon dioxide (50-70%) and methane (30-40%). Rates of gas production immediately after a meal can exceed 30 L/h, and a typical cow may release 500 L of methane per day (Wolin, 1990). Although some gas is absorbed across the rumen wall and carried by the blood to the lungs for exhalation, most is eructated through the mouth.

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