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Forage quality from the perspective of 1-billion bacterial cells

Nutrition with John McKinnon

It is a time of year when feedlot operators and cow-calf producers are implementing their winter feeding programs. Forage, whether it is fed as hay, stockpiled forage (i.e. barley swaths or standing corn) or silage will play a big role in the vast majority of these operations. For feedlot operations, good-quality hay is often used to start calves on feed, while silage is a critical source of both energy and protein in growing rations and to a lesser extent in finishing programs. Cow-calf operators are even more reliant on their forage supply as it is a critical source of nutrients for wintering cows and calves.

The ability of cattle to use forage as a source of nutrients is due to the nature of their digestive tract, specifically the rumen. The rumen is often referred to as a large fermentation vat that houses a vast array of bacteria, protozoa and fungi that collectively ferment the feed the animal consumes. When I say a vast array, I am actually understanding reality! It is often stated that one teaspoon of rumen fluid will contain in excess of one billion bacterial cells. These bacteria, and to a lesser extent, the protozoa and fungi are critical to the animal’s ability to utilize forage.

Energy in forages is tied up to a large extent in the cell wall of the plant, particularly the cellulose and hemicellulose components. Both of these cell wall constituents are indigestible due to their complex chemical makeup (i.e. the animal lacks the right enzymes in the gut to digest). In contrast, in cereal grains the energy is contained in starch, which is readily available to the animal. In forages, a further complicating factor is the fact that as a plant matures, lignin becomes an important component of the cell wall and can bind cellulose and hemicellulose, further reducing their digestibility.

It is the complex matrix of cellulose, hemicellulose and lignin in plant material that makes the rumen and specifically the bacteria so valuable in ruminant production systems. The rumen bacteria possess the necessary enzymes to ferment cellulose and hemicellulose, releasing the energy contained within their chemical bonds. This energy is released as volatile fatty acids which are absorbed across the rumen wall and are then utilized by the animal for its maintenance and productive needs. Rumen bacteria can also ferment starch and in fact are very efficient at it. However, as stated above, rumen fermentation of starch in cereal grains is not necessary, as the animal has the ability to digest starch.

To this point, you might be thinking this is a nice review of rumen biology but are beginning to wonder how it relates to your feeding program. For beef producers, forage quality primarily relates to its energy content. Yes, protein is important, but your major focus when producing or buying forage should be on energy content. The energy content of a forage, particularly hay, will depend on a number of factors that influence the relative proportions of cellulose, hemicellulose and lignin in the plant cell wall. These include the grass/legume ratio, plant maturity at harvest, environmental conditions over the growing season and method of forage preservation. Perhaps most important is the stage of maturity at harvest. More mature plants will have higher levels of cellulose, lignin and hemicellulose. In particular, the negative influence of lignin on plant cell wall digestibility increases as the plant matures, lowering the relative energy value of that forage. Silage is somewhat different in that as the plant matures, these components of the cell wall increase to a point and then plateau or even decrease. In contrast, starch content increases as the plant matures and is positivity related to the energy content of the silage.

The energy value of a forage can be determined by having your feed tested. Labs use a number of tests that reflect the relative proportions of the plant cell wall in that sample. Typically, they measure fibre content, particularly acid (ADF) and neutral detergent fibre (NDF). The NDF content reflects the amount of cellulose, hemicellulose and lignin in the sample. The NDF content of a good-quality grass hay can range from 50 to 55 per cent (DM basis) while good-quality barley silage will range from 45 to 50 per cent NDF. Very high levels of forage NDF will limit the intake of that forage. Acid detergent fibre content measures the amount of cellulose and lignin in the sample and is a measure of digestibility. The higher the ADF, the lower the digestibility and the lower the energy content of the forage. In fact many labs predict the energy value of a sample from its ADF content. Good-quality grass hay will have an ADF value ranging from 35 to 40 per cent while barley straw can exceed 50 per cent ADF. In silages, direct measurement of starch content can be used to help determine its energy value.

The bottom line of this discussion is that your forage supply is one of your largest feed investments. If you truly want to know the feeding value of the forage you are purchasing or planning to feed, there is no substitute for a feed test!

About the author

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John McKinnon is a beef cattle nutritionist at the University of Saskatchewan.

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