Understanding Forage Quality Analysis
By Chris Jesse, Wyoming Department of Agriculture Senior Lab Scientist
For most livestock producers in Wyoming, the cost of purchasing or harvesting feed for livestock represents one of the largest annual operating costs. Feed quality varies widely for a variety of reasons so producers must maintain an optimum balance between feed costs and production.
One of the best ways to strike this balance is to have a laboratory analyze the forage/hay feedstuffs and use the results to formulate rations and/or any supplement needs for the livestock.
Interpretation of laboratory results may seem complicated but is made easier if you remember the following basics. For forage/hay feedstuffs, the items of primary interest are protein, fiber and total digestible nutrients (TDN) content. Each of these items carries its own significance in the overall forage picture.
Laboratories measure forage protein as crude protein. Crude protein is derived from the total nitrogen content of a feedstuff and is labeled as “crude” because the result combines both true protein and any non-protein nitrogen present. Crude protein is a simple estimate of the actual protein value.
Protein content depends not only on the type of feed or forage being fed but also the quality of that specific feed or forage.
Harvested forages such as grass hay, alfalfa, oat hay, etc. can have drastically different protein contents, even from different parts of the same field. Factors such as types of grasses present, maturity and stress affect protein and probably all other qualities of the forage.
In legumes, such as alfalfa or oats, protein may be the principal reason that forage is being fed. Protein requirements for animals vary significantly with factors such as type of animal being fed, body weight, growth state, season and desired gains. Growing, gaining or lactating animals require more protein than simply maintaining animals.
Fiber represents the portion of a forage cell structure or cell wall components that cannot be digested by the animal. Because fiber analysis approximates the non-digestible portion of the forage being fed, it plays a large factor in the overall quality of that feed.
ADF, or acid detergent fiber, is the most commonly reported fiber analysis in forages. ADF uses an acidic detergent to dissolve all but the toughest fractions of the forage cell structure, thereby approximating the digestibility of the forage in an animal’s stomach. The remaining fiber is the main content of the plant cell wall that can’t be digested.
Generally the higher the fiber, the lower the nutritional value and overall quality of the forage.
Another factor to consider is that as insoluble fiber content increases, the animal’s ability to consume the forage is affected. Forages with higher fiber content will result in lowered intake by the animal as digestibility decreases and the forage remains in the digestive tract longer. The animal will likely eat less before it feels full, and all these factors, in turn, decrease the animal’s productivity.
These facts emphasize the important effects of the forage cell wall composition on overall animal performance. Non-ruminant animals such as horses can compensate for poorer quality forage by eating more, as it passes through the digestive tract faster. But ruminants like beef, sheep and goats cannot alter intake to compensate for poorer quality forages.
Neutral Detergent Fiber, or NDF, is an analysis performed to determine the overall part of the forage material that is insoluble in a neutral, or non-acidic detergent.
Again, the higher the NDF, the less the animal will eat. Because of this, NDF can also be instrumental in determining rations.
NDF usually increases with advancing maturity of the forage. For the producer, this means striking a balance between forage quality and the overall amount of forage harvested by cutting the hay at an appropriate maturity.
Some producers mistakenly use only protein as the indicator of forage quality. While protein is an important nutrient component, fiber is equally important.
Total digestible nutrients
TDN represents the sum of the digestible fiber, protein, lipid and carbohydrate components of a feedstuff. TDN is often calculated from the ADF content of a forage. The higher the ADF content of a forage, the lower the TDN value will be, again illustrating the importance of the fiber content in the forage.
The TDN value, in conjunction with protein and ADF, is instrumental in determining overall quality and rations of the forage material when making feeding recommendations. TDN is primarily useful for determination of beef cow rations when feeding primarily forage.
Relative feed value
Some individuals raising or buying alfalfa and sometimes other forages, use a tool called Relative Feed Value (RFV). This is simply a calculation using ADF and NDF values to provide a prediction of feeding value for the forage tested.
RFV is used primarily to evaluate or compare alfalfa quality when buying or selling the hay. It provides the producer or buyer with a simple means of comparing the performance potential any given forage has with other available like forages.
RFV in forages varies greatly but again is solely related to fiber content and may be affected by factors such as variety and genetics, maturity at time of harvest and stress. RFV is meant only to be used for comparison and should not be used by itself to determine feeding rations.
For more information on forage testing, contact the Wyoming Department of Agriculture.