Defining Forage Quality

Jun 26, 2023

Written by: Todd Steen, ProTrition Nutritionist
When producers contemplate forage quality, several different terms will come to mind – protein, fiber, total digestible nutrients (TDN), and mineral content, among others. For the ruminant animal, available nutrients associated with forage directly determine performance in terms of not only milk production or weight gain but also from an overall feed intake value. Typically, high-quality forage will provide more economically positive nutrients for the animal resulting in less high-cost ingredient purchases. Taking into account available energy content as well as overall dry matter intake becomes vital if forage is going to be counted on for high-performing animals. Thus, an essential point becomes digestibility so the animal can utilize these nutrients for its biological functions.  Excellent advances have been seen in how forage digestibility can be measured and predicted. 

Obtaining representative forage samples for assay greatly improves the strategic use of lower-cost forage while maintaining optimal production. Characterizing the fiber content has always been crucial for ration balancing. Fiber is broken down into portions consisting of neutral detergent fiber (NDF) and acid detergent fiber (ADF). The cell wall of plant structures (cellulose, hemicellulose, and lignin) is represented in NDF analysis and provides estimations of overall forage intake while the ADF portion (cellulose, lignin, and silica) very lightly describes indigestibility.  Values that are higher in NDF will represent lower overall intake, and values higher in ADF indicate material that has a poorer digestibility.  It can be postulated that the energy concentration of a given forage will be lower as the fiber portions increase. However, none of these parameters definitively describes fiber digestibility.
Remember, there are three points that are important for consideration of overall forage quality – genetics of the material, growing environment, and time of harvest. Since fiber digestibility is high in importance, plant breeders are developing varieties that significantly improve previous offerings of both corn and legume species. It is important to work with a seed representative for the evaluation of the best possible product. A huge factor that is not within control is how the environment influences quality. Hot, droughty weather profoundly impacts negatively on digestibility. Cool temperatures, particularly at night, reveal positive effects, perhaps explaining why first cutting and last cutting forage is often excellent material. The time of harvest extensively dictates digestibility factors. As the forage matures, digestibility is drastically reduced.

Taking into account these factors, laboratories are providing newer metrics such as total tract NDF digestibility (TTNDFD), NDF digestibility (NDFD), 30 hr. NDF digestibility (30), and undigested NDF (uNDF240). Using these values allows greater estimation of how much forage to include in rations for greater prediction of performances. Traditional ration balancing based solely on older ADF/NDF technology only partially describes ruminal digestibility. Using uNDF240 numbers allows for description of how long this material will stay within the digestive tract and could possibly predict the overall rumination of the animal which has a great impact on animal health.
By incorporating these parameters into diet evaluation, the highest digestible forage can be utilized, boosting dry matter intake while cutting feed costs. Your local Co-op can help in forage sampling and evaluating analysis for maximal use of forages.

For more content like this, check out the latest issue of The Cooperator.

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