The rumen is often described as the “powerhouse” of the cow and has the potential to provide up to three-quarters of the cow’s energy supply. A stable rumen environment is critical for the health and performance of ruminant animals and, as production demands and costs rise in the ruminant livestock industry, proper feeding has become even more crucial in achieving milk and meat production goals. Indeed, improving feed efficiency is known to increase income-over-feed cost (IOFC). Therefore, the goals for any feeding strategy to improve rumen efficiency should be to maintain a stable environment and supply sufficient, appropriate nutrients to the rumen microbes.
This stability promotes the growth and activity of fiber-digesting microbes key to the functioning of the rumen. The fiber-digesting bacteria are most active in a narrow, near-neutral pH range, so maintaining rumen pH within this range is important. Additionally, an anaerobic environment is required, not only for the proliferation of these fiber digesters, but also to reduce the growth and activity of less desirable microbes.
Volatile fatty acids (VFAs) are the primary energy source for ruminant animals, and their production is closely related to feed utilization efficiency. However, VFAs produced need to be absorbed across the rumen wall into the blood in order to be of any use. Another key effect of VFA absorption is the maintenance of stable, suitable rumen pH; thus, promoting conditions that increase VFA absorption will help reduce the risk of acidosis.
Rumen’s sway over immunity
The role of the rumen also goes beyond simply generating nutrients and energy. It plays a modulatory role in the animal’s immune defense.
For example, correctly balanced diets have been shown to reduce parasitic load and increase plasma immunoglobulins (antibodies). Conversely, poorly balanced diets often lead to dysbiosis, with a change in microbial population from beneficial bacteria to those of a more harmful nature, such as gram-negative bacteria. When the rumen wall is compromised, toxins from the latter may pass through the rumen wall causing inflammatory responses.
In addition, low ruminal pH can cause disruption to the rumen epithelium facilitating this passive transfer of these deleterious compounds. Not only can this have a direct negative effect on the animal’s health, but it also redirects nutrients and energy to the immune response and away from production.
Effective dietary strategies
Various dietary strategies have been investigated to try to maximize rumen health and function, ranging from methods of feeding to the use of certain ingredients. For example, restricted feeding has been found to be effective in heifers fed a high-forage diet. Holstein heifers of around 14.5 months of age and 340kg bodyweight were fed high-forage diets at either 1.25, 1.5, 1.75 or 2.0 percent of their bodyweight. These animals demonstrated a higher feed utilization efficiency until diets at 1.25 percent of their bodyweight.
Energy-dense diets are often fed to dairy cows during the transition phase in order to try to compensate for the reduced dry matter intake (DMI). Usually these diets are high in starch, and this presents a risk to rumen stability and function. Avoiding rapid pH drops and rumen dysbiosis is crucial, and animals should be allowed to adapt to starchy (grain) ingredients over the transition period. Additionally, diet digestibility may be increased by using ingredients low in degradable (fermentable) starch, as well as ensuring sufficient fiber in the ration. Forages with high neutral detergent fiber (NDF) digestibility may also be useful in this situation.
Additives aid in fiber digestion
Much work has gone into using additives and specific ingredients to enhance rumen function, such as buffers, yeasts, plant extracts, enzymes and unsaturated fatty acids. Buffers have been used in rations for many years and are aimed at preventing or counteracting drops in pH that would otherwise be deleterious to microbial activity; bicarbonates are probably the most common example. However, more "natural" preventative additives have been investigated, some of which are in common use today.
Yeasts and probiotics, such as Lactobacillus species, are often included in cattle diets to promote fiber digestion and stabilize the rumen environment. Yeast cells tend to associate with freshly ingested feed particles in the rumen presumably due to the oxygen attached to these particles. Yeasts then facilitate oxygen uptake thus removing the threat of an aerobic environment. Live yeasts also produce functional compounds, such as peptides, that can be used in rumen metabolism. Additionally, they facilitate lactate utilization by lactate-utilizing bacteria. Saccharomyces cerevisiae is the main yeast used in ruminant diets of which many strains have been developed.
Exogenous fibrolytic enzymes have also been used with encouraging results to enhance fiber-digestion, but more work is needed in this area before definitive effects are established.
Plant extracts have received much attention recently. The two main groups studied appear to be saponins and tannins. Both these groups of compounds have been demonstrated to reduce protein breakdown in the rumen, leaving more available for direct digestion and absorption in the small intestine. However, both appear to have negative effects on other aspects of rumen function that may lead to an increased risk of acidosis.
Essential oils have shown promise in some studies. However, their effects are often as a result of a negative impact on certain bacteria. The overall effect appears to be that of reducing protein and starch breakdown in the rumen, but their ultimate effect is influenced by the chemical nature of the individual oil. Highly unsaturated fatty acids are known to have a negative effect on rumen fiber fermentation. Despite this, they have proved effective in several studies, and there is interest regarding their ability to reduce enteric methane production.
Protein key ingredient
Energy is not the only consideration in rumen efficiency. Protein is the most expensive part of a ration and strategies to maximize nitrogen use efficiency can help to reduce diet cost. In terms of protein sources, microbial protein is regarded as one of the most cost effective, not least because of the similarity of its amino acid profile to that of milk and meat. It follows then, that ensuring a continual supply of nitrogen for the rumen microbes, together with an adequate supply of energy, will result in greater production of microbial protein as a result of greater rumen function efficiency.
Additionally, there is increasing recognition of the over-feeding of crude protein in diets. Practically speaking, for lactating dairy cow rations the consensus is to aim for about 100g of crude protein supplied for every liter of milk produced. Excess dietary protein has an energetic cost to the animal and this, coupled with the potentially negative impact on fertility, can impact production, animal health and, ultimately, the producer’s bottom line.
Rumen holds key to productivity
Diet obviously has a strong influence on rumen function, but there also appears to be considerable animal variation in rumen microbial communities, particularly those associated with the rumen wall. Additionally, animals seem to have differences in their capacity to mount an immune response, which may lead to altered energy and nutrient partitioning.
On the farm, there is little one can do to reduce individual animal variation, but in all cases, an efficiently functioning rumen is the key to animal health and productivity and, subsequently, the profitability of any ruminant operation. Many factors influence rumen function but diet is probably the factor that provides the greatest, practical opportunity for manipulation.
References available upon request.