Part 9: It’s all about the microbes
There are many supplements available to the cow-calf producer, and most producers utilize some sort of supplemental feeding when cattle are on a primarily forage-based diet. The choice of supplement should complement the ability of the cow to digest forage, and should not compete with forage digestion. Producers are often tempted to feed corn or other cereal grains when cattle on a forage-based diet (e.g. pasture or dry hay) are not gaining weight. However, feeding a high starch supplement in these situations can have disastrous consequences.
The population of microbes in the rumen adapts to the primary feedstuffs the cow is consuming. In general, there are two different types of microbes: 1.) microbes that digest high-fiber diets, such as cattle on pasture or hay, and 2.) microbes those that digest high starch diets, such as cattle on finishing diets consisting of cereal grains. When cattle on a poor-quality forage diet are supplemented protein, the microbial population in the rumen grows, increasing the capacity of the cow to digest forage. This creates more efficient digestion, and stimulates an increase in the intake of the poor-quality forage. Thus, the cow is getting more nutrients from each pound of hay consumed, and is able to consume more of the poor-quality forage as well. This is known as a positive associative effect of the protein supplement (Olson, 2015).
Conversely, a negative associative effect is seen when cattle on a forage-based diet are supplemented with corn or other high-starch cereal grains. The high starch content of the cereal grains does not support the growth and function of the microbes that digest fiber. The starch in these grains is rapidly fermented in the rumen to produce acids that lower the pH of the rumen very quickly. The more acidic environment kills the microbes that digest forage, decreasing the ability of the cow to digest the fiber in hay and other forages (Olson, 2015). This sudden drop in rumen pH can also lead to rumen ulcers, acidosis, sepsis, liver abscesses, laminitis, and death. High starch cereal grains are not a good supplement choice for cattle being fed low quality dry hay.
Supplements that are high in digestible fiber (e.g. dry distiller’s grains, soyhulls, cottonseed hulls, corn gluten feed, etc.) are good choices to use as supplements for cattle on forage-based diets. These supplements compliment the ability of the microbes in the rumen to digest forages, and do not alter the pH of the rumen. A good supplement option for cattle on a poor-quality forage is a protein source (e.g. soybean meal, cottonseed meal, non-protein nitrogen, distiller’s grains, etc.) along with a source of digestible fiber (e.g. dry distiller’s grains, soyhulls, cottonseed hulls, corn gluten feed, etc.). Distiller’s grains are often an excellent supplement choice for cattle on a poor-quality fiber diet, as they provide both a source of digestible fiber as well as protein usable by the microbes in the rumen. This combination of both protein and digestible fiber provides adequate protein for the microbes in the rumen to grow and function, while also providing a source of energy that is easily digested by the same population of microbes that digests forage, facilitating better utilization of the forage component of the diet. Producers should be cautious when using non-protein nitrogen-based supplements (e.g. “lick tubs”, liquid feed, some range cubes, etc.) these supplements contain urea. Urea is included in these supplements because under certain conditions, it can be used by the microbes in the rumen to build protein usable by themselves and the cow. This requires adequate energy, typically supplied by carbohydrates, in the diet. There are typically not enough carbohydrates in grass hay to facilitate this use of urea, so cattle get no benefit from the NPN supplement. As a result, producers often do not see any improvement in body condition, despite cattle consuming large amounts of the “lick tub” or liquid feed supplement.
The information in this article as well as additional information on protein supplements can be found at the following source:
Olsen, K.C., Cow Supplementation: Getting the Best Bang For your Buck. Proceedings: The Range Beef Cow Symposium XXIV. 2015. https://beef.unl.edu/documents/2016-RBCSymposium/RBCS-2015-Cow-Supplementation.pdf