Understanding Forage-Based Nutrition and Deficits in Forage Quantity and Quality

October, 2015
Phillip Lancaster, UF/IFAS, Range Cattle REC, Ona

Beef cows require six classes of nutrients: carbohydrates, fats, protein, vitamins, minerals, and water. Forage is the primary and usually the least expensive source of these nutrients. Therefore, managers of beef cows should have 2 goals, 1) to manage forage to minimize supplemental feed needs while meeting nutrient requirements, and 2) to maximize forage utilization by the cow.

Carbohydrates and fats supply energy to cows. Cellulose or fiber is the primary carbohydrate found in forage species and the primary energy source for the cow. There is very little fat found in forage plants. Protein is composed of repeating units called amino acids, and it is these amino acids that are required by the cow. Rumen microbes digest the cellulose providing volatile fatty acids that the cow uses for energy. Rumen microbes also digest the plant proteins converting them into microbial protein, which has a very good amino acid profile that, for the most part, meets the amino acid requirements of grazing beef cows.

Vitamins are classified as water soluble or fat soluble. The water soluble vitamins are not a dietary requirement for beef cows in most cases, as rumen microbes make all of the B-vitamins for the cow, and the cow can make vitamin C. The fat soluble vitamins are typically not deficient in cows consuming green forage or hay, because forage species are generally high in beta-carotene and pro-vitamin D that are precursors for the cow to make vitamin A and D, respectively. Forage species are also high in the other fat soluble vitamins E and K. Mineral content of forages can vary greatly depending upon forage species, and soil texture and pH where the forage is grown.

Both the quantity and nutritive quality of forage can affect the ability of the cow to consume the required nutrients throughout the year. A large quantity of low quality forage can still lead to deficiency if the cow cannot consume enough of the forage. Similarly, a small quantity of high quality forage available to the cow can lead to nutrient deficiency because there is not enough of the forage available. For example, a cow requires 2 lb of crude protein per day. If the cow consumes 20 lb of low quality forage (5% protein), protein intake is 1 lb per day. If the cow consumes 10 lb of high quality forage (10% protein), protein intake is 1 lb per day. Thus, there has to be sufficient forage quantity of high enough nutritive value for the cow to consume the required amount of the nutrient.

Likewise, both the quantity and nutritive quality of forage changes throughout the year due to growing conditions and plant maturity. In the spring, temperatures begin to get warmer, day length increases, and fertilizer is typically applied to bahiagrass pastures, which all result in forage growth and increased forage quantity. This young forage is also of very high nutritive value, but there is usually not a large quantity available for the cow. Thus, the cow may not be able to consume enough of the high quality forage to meet nutrient requirements. In summer, rainfall increases, and temperature and day length are greatest of any time of year which leads to rapid forage growth. Rapid forage growth also leads to increasing plant maturity which decreases nutritive value. As forage plants mature, protein content decreases, and fiber content increases, but digestibility of fiber decreases due to increased lignification of the fiber. Lignin is a molecule that is virtually indigestible. In fall and winter, temperature, day length, and rainfall decrease reducing forage growth, and the forage plant puts nutrients into the roots preparing for regrowth in the spring. This decreases the nutritive value of the forage available to the cow. Therefore, forage quantity and quality are inversely related such that as quantity increases quality decreases because plant maturity increases.

Plant maturity also effects the ability of the cow to consume enough nutrients from the forage. Forage intake by the cow is limited by the passage of forage through the digestive tract. Forage with greater digestibility will pass through the digestive tract faster allowing the cow to consume more of the forage. As the forage plant matures, fiber content increases and fiber digestibility decreases which limits the amount of forage the cow can consume. The more mature forage has lower nutritive value and the cow cannot consume as much of the low quality forage compounding the effect on nutrient intake.

In addition to decreased fiber digestibility, more mature forage plants have decreased protein content. Protein intake is not only important for the cow, it is important for the rumen microbes to effectively digest forage fiber. Typically, forage protein content below 7% will decrease forage intake by the cow because the microbes are lacking protein necessary to digest forage fiber. Supplemental protein will be necessary to maximize forage intake and digestion. At 7% protein and above, forage fiber digestibility and forage intake are maximized.

Forage digestion is accomplished by microbes in the rumen which require a balance between energy (i.e., carbohydrates) and protein in order to grow and effectively digest forage. Too much energy without enough protein decreases fiber digestion by the microbes. The TDN:CP ratio is a measure of the balance of energy and protein in the diet. TDN is an abbreviation for total digestible nutrients which is a measure of the digestible energy in forage. CP is an abbreviation for crude protein which is a measure of total protein in forage. A TDN:CP ratio of 6 or less in the total diet is necessary to maximize forage digestion by rumen microbes. High quality forages that naturally have a TDN:CP ratio less than 6 will have the greatest forage digestion, but if having to purchase supplemental feed targeting a TDN:CP ratio of 6 in the total diet is the most economical.

Managing grazing and harvesting timing is the best method to improve forage quality. When the forage plant is grazed by the cow or mowed for hay, it causes the plant to put on new growth that has high nutritive value. Grazing or mowing more frequently will increase the nutritive value of the available forage to be harvested by the cow or for hay in the next grazing cycle or cutting. However, there is an optimum. Grazing or mowing too frequently will reduce plant growth and forage yield.

Forage testing is key to addressing potential deficits in forage nutritive value. Forage nutritive value changes throughout the year as we discussed earlier, but also changes every year even when measured at the same time of year. Forage nutritive value also varies both between pasture or hay field and within a pasture or hay field. This variation across time and location has to be taken into account when collecting forage samples for testing and determining the number of forage samples to test in order to get an accurate measurement of the forage available to the cow. A guide on how to collect forage samples is available through UF/IFAS Cooperative Extension Service (EDIS SS-AGR-63: Forage testing, https://edis.ifas.ufl.edu/aa192).

In summary, deficits in forage quantity are caused by poor growing conditions throughout the year and/or overgrazing. Deficits in forage nutritive quality are a result of environmental factors (temperature, day length, rainfall, fertilizer) affecting plant maturity and/or infrequent grazing or harvesting of forage to maintain young vegetative growth. Managing for improved forage quality will allow the cow to meet nutrient requirements from forage reducing the need for supplemental feed. Testing forage for nutritive value is essential to managing nutrition of the cow herd. This is especially important with winter forages, whether stockpiled limpograss or stored hay. Knowledge of forage nutritive value will allow managers to design supplements, when necessary, to meet cow nutrient requirements and balance energy and protein to maximize forage digestion by rumen microbes.


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