An animal’s nutrient requirements must be met if optimum performance is to be obtained. Thus, the first step in developing a feeding system is to properly identify energy, protein, mineral and vitamin requirements. The next step is to calculate the combination of feeds among those available that will economically meet these requirements,
Nutrients required for growth and or milk production are compared with nutrients available. Deficiencies are met by adding feeds containing high concentrations of deficient nutrients. Therefore, to accurately assess whether a feed will meet an animal’s requirements, accurate estimates of nutrients are necessary, whether feeds are homegrown or purchased.
Nutrient values for individual feeds vary considerably farm to farm and year to year, based on differences in variety, soil fertility, weather, date of harvest and harvesting and storage procedures. Table 1 presents average, minimum, maximum and standard deviation for several nutrients in selected New York feeds that were analyzed by the Dairy One Forage Lab. Shown at the top of the table are requirements for growing calves, finishing cattle and early lactation cows. These values can be compared with those for individual feeds to see which are deficient in various nutrients.
Looking at standard deviation of the nutrients, it is evident that corn grain is much more consistent than any of the mixed, mostly grass forages. As corn grain makes up about 50 percent of corn silage on a dry matter basis, it lands between grain and forage relative to consistency. Given that most cattle in the Northeast are not fed a high-grain diet, if producers are to economically meet nutrient requirements of their herds, then forage analysis is money and time well spent.
The challenge is to decide what is practical to test for. Nutrients of major concern include energy, protein, calcium and phosphorus. Vitamins and some trace elements are important but will not be covered in this article. In deciding what to test for, variability of the nutrient, cost of analysis and cost of supplementation must be considered.
Energy is the nutrient required in the greatest amount and is determined for maintenance, growth and milk production. I like to view energy as the gas pedal. More energy means more growth or more milk. Energy values are not directly measured but predicted by relationships using equations and relationships with other nutrients.
The primary source of energy is carbohydrates. Carbohydrates include sugar, starch, cellulose and hemicellulose. Cellulose and hemicellulose are contained in plant cell walls in a complex with lignin, an indigestible compound, whereas sugars and starches are found in cell contents. The cell walls are lowest in digestibility, and cell contents are highest. Thus, the higher the proportion of cell walls, the lower the energy value of the feed.
Total digestible nutrients (TDN): Although not perfect, for most situations in the Northeast TDN is the most common and easiest estimate of energy to use. In some labs, TDN represents the sum of digestible protein, digestible nonstructural carbohydrates, digestible NDF and 2.25 × the digestible fat. In others, it is a prediction formula using acid detergent fiber (ADF).
The following laboratory procedures are used to estimate usable energy content of a feed sample sent in for analysis.
- Neutral detergent fiber (NDF): This is the moderately digestible fiber contained in the cell walls and is what gives the plant rigidity. For feeds high in protein, starch and or ash, chemical procedures are used to remove these excess non-NDF nutrients and are listed as aNDF or aNDFOM. Due to the partial indigestibility of cell walls, the higher the NDF, the less cattle can and will eat. Therefore growth and or milk production goes down as the measure of NDF goes up.
- Acid detergent fiber (ADF): This is a measure of the poorly digestible cell walls, primarily lignin. As ADF goes up, the digestibility of feed goes down, which reduces the amount of energy available to the cattle. Again, less energy means less growth and or milk production.
- Fat: It is measured by ether extraction, which solubilizes other nutrients and is therefore reported as crude fat. It has 2.25 times as much energy as carbohydrates and is a valuable nutrient. However, most locally produced feeds contain less than 5 percent fat. Byproducts such as distillers’ grains and soybean meal are the exception and are high in fat.
Crude protein (CP): This nutrient is determined by laboratory analysis measuring the nitrogen content of the feed. The nitrogen value is converted to crude protein by multiplying by 6.25. If energy is analogous to a gas pedal, then I like to refer to CP as the oil in a car. Adding more oil will not make the car go faster, but if you don’t have the minimum requirement, eventually the car won’t go at all. Protein is composed of amino acids. In ruminants, a large supply of the protein requirement comes from microbes in the rumen.
- Degradable protein (RDP) consists of the soluble protein and proteins of intermediate ruminal degradability. The RDP is needed to meet the requirements for bacteria that ferment the forage and grain in the rumen. These bacteria are then digested in the small intestine to supply bacterial (i.e., microbial) protein to meet the animal’s requirements for maintenance and growth.The amount of RDP is one difference between dry hay and baleage. The fermentation process makes the protein more readily available to the rumen bugs (more RDP and less undegradable protein or RUP). For example, when baleage is the source of forage for growing cattle, the total CP of the diet will need to be higher compared with when dry hay (at the same CP and energy) is provided.
- Undegradable protein (RUP): This protein fraction is slow to degrade in the rumen and is passed into the small intestine for breakdown – also known as rumen escape or bypass protein. The RUP supplements the microbial protein (RDP) in meeting the animal’s requirement for maintenance and growth. Meeting RDP and RUP requirements is especially important for growing and finishing cattle.
- Acid detergent insoluble crude protein (ADICP): This is also known as heat damaged or unavailable protein. Heat damaged feeds will often appear dark brown or black. It is caused by heating during fermentation or drying. A portion of the protein reacts with carbohydrates to form an indigestible complex, rendering it unavailable for digestion.
Sampling Feeds for Analysis
Inaccurate sampling can lead to greater errors than using average feed composition tables. The sample must be representative of all feed in question. The important factors to consider are when and how to sample feeds for analysis. In most states, your local county agent has sample bags and information on obtaining feed analysis. Also, most reputable feed companies will analyze your feed. There are private laboratories that offer this service. After obtaining sample bags, the following guides can be used to obtain a representative sample.
- Bunker silos or piles: Take 15 or more handfuls from the face of the silo after it is opened and you are into well packed, good quality silage. Mix the samples in a clean pail, then place about 1 quart in a plastic bag and either freeze it or send immediately for analysis. It is desirable to sample several times during the feeding period, particularly if there is any great variation in plant maturity, variety or soil type.
- Dry grain sampling: Take a minimum of five grain samples with a grain probe if possible from various places in the bin or truck. Mix them in a clean pail, then place about 1 pint in a plastic bag, and seal and send in for analysis.
- Hay sampling: Take core samples from the end of a dozen or more bales taken from various places in the mow or stack. Mix samples together, then send in about 1 quart in a sealed plastic bag for analysis.
Calcium and phosphorus
Accurate methods are available for mineral analysis, and these are valuable as most rations need some supplemental minerals. The most important one, however, is the phosphorus content of silage and hay, since it is the most expensive to supplement; most rations, particularly high forage rations, are deficient in phosphorus. However, many forages will be adequate in phosphorus, and less expensive supplementation programs can be developed. Calcium can be cheaply supplemented with calcium carbonate (ground limestone), and it is therefore of no great concern to obtain an analysis to save supplemental calcium.
Whether you produce the majority of your own feed or purchase it, knowing the energy (fuel) and protein (oil) is critical to obtain the desired level of performance. Given the current low cattle prices, balancing a ration based on known values can keep your production costs in line.
Fox, D. G. The Meaning and Use of Feed Analysis. The Cornell Beef Production Reference Manual. Fact Sheet 1101.