Understanding A Forage Analysis

Livestock are most productive when fed a ration balanced according to their nutrient needs. Unfortunately, many rations are balanced using average values for each feedstuff. These so-called "book values" often result in over- or under-feeding certain nutrients. More economical and better balanced diets can be formulated using nutrient concentrations determined from feed analysis.

Methods of Feed Testing

Once a feed sample has been collected properly, it can be analyzed for nutrients. Most commercial laboratories offer standard feed tests for forages, grains, or total mixed rations. Analyzing feeds for moisture, protein, and energy is recommended when designing diets for beef cattle. Typically, results are reported on an as-is and dry matter basis. Nutrients should always be balanced in a diet on a dry-matter basis because nutrient requirements for beef are reported on a dry-matter basis. After formulation on a dry-matter basis, values can be converted to an as-is basis, using the moisture content of the feed, to determine the actual amount of feed (as-is) that should be fed or delivered.

Sight, smell, and touch are useful, although frequently misleading, indicators of feed value. Stage of maturity at harvest, foreign material or pests, color, and leafiness can be detected visually and provide some limited information on the nutritional value of feed. Musty and foul odors can indicate lower quality due to deterioration in storage. Physical evaluations alone rarely are sufficient for predicting eventual animal performance, partly because of the lack of a good means of measuring such qualities as color and leafiness.

Nutrient analyses most commonly are done by chemically reacting or extracting important compounds in a laboratory and determining their amount in the feed. When representative feed samples are tested chemically, accurate predictions of animal performance usually can be made because the nutrient requirements also were determined using chemically tested feeds.

Near infrared reflectance (NIR) spectroscopy is a rapid, reliable, low-cost, computerized method to analyze feeds for their nutrient content. It uses near infrared light rather than chemicals to identify important compounds and measure their amount in a sample. Feeds can be analyzed in less than 15 minutes using NIR, compared to hours or days for chemical methods. This rapid turnaround and the resulting cost savings in labor make NIR an attractive method of analysis. When sending a sample in to be tested using NIR it is important to identify the type of feed/forage being submitted so as to make sure that the right feed library is used. This method will not accurately evaluate a full mineral profile of a sample. However, NIR does appear to fairly accurate determine calcium and phosphorus.

NIR does not do an adequate job of measuring the energy (TDN) content of the distillers grains that are feed byproducts from the ethanol industry. In an NIR analysis, TDN is estimated using ADF. ADF measures cell wall content of a feed. Distillers grains are high in fat; therefore, NIR will underestimate their energy content. NIR will adequately measure moisture, percent crude protein, calcium, and phosphorus in distillers grains.

Nutrients of primary concern in developing diets for beef cows are moisture content, percent crude protein, and energy (%TDN). Relative Feed Value (RFV) is important for dairy cattle because Relative Feed Value (RFV) and Relative Feed Quality (RFQ) are indicators of forage digestibility and therefore forage intake. For beef cattle, RFV and RFQ are not used in ration formulation.

Interpreting Test Results Important in Designing Diets for Beef Cows

Dry matter is the moisture-free content of the sample. Because moisture dilutes the concentration of nutrients but does not have a major influence on intake it is important to always balance and evaluate rations on a dry-matter basis.

Crude protein measures the proportion of nitrogen in a feedstuff multiplied by 6.25 and this includes both true protein and non-protein nitrogen. In ruminants, evaluation of the fraction that is degradable in the rumen, degradable intake protein (DIP), versus the rumen-undegradable fraction, undegradable intake protein (UIP; by pass protein), is also important. However, the rumen degradability of protein is not measured in most commercial labs. Therefore, it is recommended that rations be formulated using analyzed CP values and average values for DIP and UIP that can be found in the 1996 National Research Council Nutrient Requirements of Beef Cattle.

Nitrogen that has become chemically linked to carbohydrates and thus does not contribute to either DIP or UIP supply and is called Heat Damaged Protein or Insoluble Crude Protein (ICP). This linkage is mainly due to overheating when hay is baled or stacked with greater than 20% moisture, or when silage is harvested at less than 65% moisture. Feedstuffs with high ICP are often discolored and have distinctly sweet odors in many cases. When the ratio of ICP:CP is 0.1 or greater, meaning more than 10% of the CP is unavailable, the crude protein value is adjusted. Adjusted crude protein (ACP; see below) values should be used for ration formulation.

Adjusted Crude Protein (ACP) is the crude protein corrected for ICP. In most nutrient analysis reports, when ACP is greater than 10% of CP, the adjusted value is reported. This value should be used in formulating rations when ICP:CP is greater than 0.1.

Total Digestible Nutrients (TDN) is the sum of the digestible fiber, protein, lipid, and carbohydrate components of a feedstuff or diet. TDN is directly related to digestible energy and is often calculated based on ADF. TDN is useful for beef cow rations that are primarily forage. TDN values tend to under-predict the feeding value of concentrate relative to forage. The NIR analysis method does not estimate the energy (TDN) content to wet (WDGS) or dry (DDGS) distillers grains plus solubles very well. Our data suggests that suggest that WGDS and DDGS are 125% the energy value of corn in forage diets. Therefore if corn is 90% TDN on a dry matter basis then WDGS is 112.5% TDN (90% x 1.25)

Relative Feed Value (RFV) is a prediction of feeding value that combines estimated intake (NDF) and estimated digestibility (ADF) into a single index. RFV is used to evaluate legume hay. RFV is often used as a benchmark of quality when buying or selling alfalfa hay. RFV of feedstuffs other than alfalfa is not relevant. RFV is not used for ration formulation.

Sampling forages and understanding a forage analysis will result in supplementation strategies that meet the cows’ nutrient requirements, but also will impact profit potential of the enterprise. Over supplementation results in increased input costs without increases in animal performance.

By Dr. Rick Rasby, Professor of Animal Science
Animal Science, University of Nebraska - Lincoln
Institute of Agriculture & Natural Resources