The process of rumen fermentation can easily be manipulated to improve animal performance and/or to better suit a given type of production. Here we review strategies to achieve these targets under different feeding and management conditions.
With the increased ratio of roughages-to-concentrates in the diet, the ratio of acetic acid-to-propionic acid in the rumen will increase, and the contrary will happen when the roughages-to-concentrates ratio decreases. The changes in the molar ratio of acetate to the propionate also depend on the type of grain used in the concentrate part of the diet, and whether or not the grains are processed by grinding/pelleting (Table 1).
Such changes in VFA production may be useful when planning the feeding programmes for different classes of ruminants. For lactating cows, for example, it is better to have a higher level of acetic acid in the rumen by increasing the roughage-to-concentrate ratio because acetic acid is a basic precursor of milk fat synthesis. The increased milk fat is often associated with an increase in milk protein due to the high correlation (R2=40 or more) between the 2 elements.
In cases where the supply or balance of amino acids reaching the lower gut is limited, supplementing the diet with protein that is partially protected from degradation by physical or chemical methods can improve productivity in terms of meat, milk, wool, and/or hair production.
It should also be remembered that the use of physical treatment (heat processing) or chemical treatment (formaldehyde) may not necessarily lead to an improvement in animal performance. Some of the feed ingredients that are so treated may be poorly digested in the small intestine or may have a poor amino acid balance. Therefore, protection of such proteins may be a wasteful process unless followed by an examination of digestibility and amino acid profile.
The most practical way of utilising the protein protection concept is the selection of ingredients that have high rumen bypass values such as blood meal (80% bypass) and meat meal (65% bypass). This is particularly important when feeding lactating cows or growing cattle due to their high protein requirements.
It should be noted, though, that the extent of rumen degradation of dietary protein is not only a function of the dietary protein source but is also a function of the carbohydrate source in the diet. Highly soluble carbohydrate in corn silage, for example, is more effective in increasing the ammonia level in the rumen compared with the cellulosic carbohydrate in alfalfa hay. This is due to the availability of the substrates that enable rumen bacteria to degrade amino acids to ammonia.
Therefore, the regulation of rumen ammonia level is possible when selecting the right carbohydrate source with the right source of protein. In other words, with the high-soluble soybean meal in the diet, a low-fermentable carbohydrate source such as alfalfa hay should be used in the diet. By contrast, with low-soluble protein such as meat meal, a highly fermentable carbohydrate source should be used to achieve an optimal ammonia concentration in the rumen.
The addition of fat into the ruminant rations appears to be an efficient and easy way of reducing methane production and hence alleviating the negative effects of methane on energy efficiency. The effect of fats on methane production may, however, vary depending on the fat source (Table 2), and maybe attributed to the bio-hydrogenation of unsaturated fatty acids in the rumen, promotion of propionic acid production, and prevention of protozoa activity.
When feeding animals 5-6 times per day, there will be a stable pH in the rumen at levels ranging from about 5.5 to 5.8, but when feeding only 1-2 times per day, the pH value will, in this case, vary from about 5.1 to 7.1 within the same day. With a stable pH value in the rumen, digestibility of dietary fibre will increase due to the increased microbial activity in the rumen which results from the increased energy level needed for such an activity (the rumen ATP concentration is 2.5 times more under high-frequency feeding compared to the low-frequency feeding). Also, high-frequency feeding decreases the amount of ammonia produced in the rumen following digestion of protein, indicating low rates of degradable protein formation and high rates of non-degradable protein which is used for productive purposes.
The presence or absence of rumen protozoa is correlated with rumen fermentation characteristics and methane production. It is estimated that a single protozoan can ingest as many as 21,000 bacteria per hour. Therefore, the removal of rumen protozoa (defaunation) increases the bacterial population density, the efficiency of bacterial protein synthesis, and the rate of nitrogen flow to the duodenum, especially when the feed is deficient in protein relative to energy content. Though carbohydrate digestion of plant cell walls is reduced by defaunation, improving protein supply and livestock productivity together with a reduction in methane production may be more important. In terms of animal production, it was found that defaunated cattle grew at a rate 43% faster than the faunated cattle on the same intake when fed a low level of protein.
The following are methods commonly used for defaunation:
Various feed additives can be used in ruminant feeding programmes to manipulate rumen fermentation and direct it favourably. Examples of these additives include ionophores, buffers, antibiotics, enzymes, plant extracts, peppermint, and garlic leaves. Each of these additives has a specific role and mode of action in the rumen fermentation process and leads to improved animal performance.
References are available from the author upon request.