Cattle 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 overfeeding or underfeeding certain nutrients. This is especially true for forages, which can vary considerably in nutritive value. 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.
Once a feed sample has been collected properly (see NebGuide 331, Sampling Feeds for Analysis, PDF), 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 are rarely sufficient for predicting eventual animal performance.
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 of the animal were 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 the correct feed library can be referenced. NIR should not be used for sample types that do not have an equation (library). This sometimes happens with different byproduct feeds that may vary widely in sample type and are not often analyzed . It also commonly happens with forage types not commonly analyzed by the laboratory. A good laboratory should recognize the limitations of their equations and use wet chemistry where a sample falls outside the range of samples included in equations. However, the only way for them to know that this should be done is if the sample is properly identified. NIR will not accurately evaluate a full mineral profile of a sample; however, NIR does appear to fairly accurately determine calcium and phosphorus.
Nutrients of primary concern in developing diets for beef cows are moisture content, percent crude protein, and energy (%TDN).
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, which measures the proportion of nitrogen in a feedstuff multiplied by 6.25, includes both true protein and non-protein nitrogen. In ruminants, evaluation of the fraction that is degradable in the rumen, called rumen degradable protein (RDP) or degradable intake protein (DIP), versus the rumen-undegradable fraction (by pass protein), called rumen undegradable protein (RUP) or undegradable intake protein (UIP) 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 RUP (UIP).
Nitrogen that has become chemically linked to carbohydrates and thus does not contribute to either RDP or RUP supply 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 which is a measure of the cell wall content. 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 predicted energy value of distillers (WDGS, MDGS, and DDGS) is significantly underestimated by laboratory analysis (often being reported as 83-85%). In addition to highly digestible fiber distillers grains are high in both bypass protein (RUP) and fat; therefore, the prediction equations used will underestimate their energy content. Our data (2015 Nebraska Beef Report pages 34-35) suggest that DGS are 136% of the feed value of corn in forage-based diets.
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.