Discussion of Strategies of Digestion
Instructions: Please answer the following question as thoughtfully, accurately and completely as you can.
1. Explain why hind gut fermentation is considered less efficient than pre gastric fermentation as a way to utilize feed resources.Some of the reasons include:
- Bacterial protein produced in the cecum does not supply amino acids to the host animal because they are not digested, but rather excreted in the feces (the large intestine does not have digestive enzymes). Similarly, vitamins produced by microbes are not absorbed if produced in the cecum (as opposed to the same vitamins produced in the rumen).
- There is less time for microbial extraction of energy in the cecum compared to the rumen because: (2.1) in general, the cecum is much smaller than the rumen. The smaller capacity means faster rate of passage (less time for microbial extraction of energy); and (2.2) the cecum does not "selectively" retain fiber in the way the rumen does;
- In animals that rely both on pregastric fermentation and hindgut fermentation (a cow), the cecum play a minor role in fiber fermentation because it "receives" a fiber that has already been thoroughly fermented in the rumen.
In short, the host (ruminant animal) cannot take advantage of cecal fermentation to the same extent that it can take advantage of the products of ruminal fermentation. Although VFA and ammonia produced in the cecum can be absorbed directly through the cecal wall, the cecum is not followed by gastric digestion of bacterial protein and absorption of amino acids and other products (e.g., vitamins) of cecal fermentation. These potentially useful nutrients are lost in the feces.
2. Explain the processes that allow ruminants to achieve higher cellulose digestion than any other animal species.
The strategy of host ruminant is to provide the microbial population of the rumen with an "ideal" environment to grow and ferment dietary fiber. For example:
- The rumen is anaerobic and is buffered with saliva (pH regulation) to maintain pH close to ideal for microbes that ferment fiber (i.e. neutral pH);
- The host provides rumen microbe is a “rich” continuous supply of nutrient and fiber carbohydrate (by reducing particle size and thus increase surface area available for fermentation);
- Removal of “bacterial waste:” The rumen wall absorbs VFAs and excess ammonia and the rumen allows for the passage of unfermented material to the lower digestive tract
Ruminants achieve higher cellulose digestion compared to any other species because of two main UNIQUE adaptations: (1) selective retention of fiber in the rumen and (2) the process of rumination itself.
- Rumination (cud-chewing) reduces fiber particle size as to expose greater surface area of the fiber for microbial fermentation. Thus, ruminants process dietary fiber more extensively than any other species of animals.
- The selective retention of fiber in the uniquely large rumen that allows for slowing down its passage and thus allowing for the time required by microbes to ferment the fiber. Cellulose fermentation takes time (many hours) and this process allows for microbes to extract as much energy as possible from the fiber before it leaves the rumen. The position of the rumen along the digestive tract has often been compared to a river (the digestive tract) flowing by a large lake (the rumen).
3. Explain why a cow might stop eating despite the fact that she is still “hungry” for more energy.
The major limitation of the ruminant’s mode of digestion is the inability to increase dry matter intake (on high fiber and thus “bulky” diets) in response to greater nutritional needs. As a result, rumen can be “physically full” especially if the animal consume a diet that is high in poor quality fiber that ferment slowly, yield little energy but stay in the rumen for a long period of time. In this situation, a cow may stop eating while still “hungry” because she cannot eat more despite the fact that she has not met her energy requirement yet. For example, a cow in early lactation may stop eating because her rumen is full, but she remains “hungry” because of unmet requirements for energy.
4. Describe the major differences in the anatomy of the digestive system of a horse compared to a cow and explain how those differences reveal different digestion strategies.
- The horse is a large herbivore classified as “hind gut fermenter” because its cecum and colon constitute more than 60% of the total volume of its digestive tract (210 liters). The cow on the other hand is a large ruminant herbivore. The rumen of a cow constitutes about 80% of its total volume capacity (360 liters).
- Thus the digestive system of a cow is adapted to extracting as much energy as possible from the fiber portion of roughage feeds. The mechanisms involved include selective retention of fiber in the rumen accompanied with rumination. This strategy limit the rate of passage through the digestive tract and thus the total amount of feed the cow can consume per day.
- In the case of the horse, the cecum and colon can accommodate a large amount of fiber, and thus the horse might benefit from the energy extracted by fermentation in its hind gut (but not benefit from the bacterial protein (or vitamins) synthesized during fermentation. This strategy entails a lower efficiency of energy extraction from the feed compared to a cow (because of lower amount of time allowable for fermentation), but does not put a restriction on the amount of feed consumed by the horse.
5. Based on what you know about the anatomy and physiology of the digestive system of a horse compared to a cow, predict in which digestive compartment starch will be digested and what product of starch digestion will be absorbed.
In horse starch will be digested primarily in the stomach and small intestine. The resulting glucose will be absorbed. In cow, starch will be fermented in the rumen leading to the formation of volatile fatty acids that are absorbed through the rumen wall.