Have you looked at a forage lab analysis lately? On it you’ll find numerous fiber values that go well beyond the days of crude fiber. Recently at the Four-State Nutrition Conference in Dubuque, Iowa, Dr. John Goeser, Rock River Laboratories, Inc., and Dr. Ev Thomas, Oak Point Agronomics, Ltd., clarified some of the fiber terminology and measurements we evaluate today.
    The most common fiber measurements in dairy nutrition is neutral detergent fiber (NDF). To measure NDF, feed laboratories utilize an amylase enzyme, along with neutral detergent, to wash away starch, sugar, protein, fat and other non-fiber compounds. The remaining residue is NDF, which consists of other less digestible nutrient compounds, such as acid detergent fiber (ADF), lignin and ash. While ADF and lignin still appear on most laboratory reports, they have been replaced by a more applicable term referred to as undigestible NDF, or uNDF. This uNDF is computed at different digestive time intervals, with the longest being 240 hours and expressed as uNDF240. Feed never stays in the rumen for 240 hours, but this extensive digestion is necessary to find out how much fiber is available to digest in the rumen.
    NDF digestibility (NDFd) has been reported by most commercial forage labs for several years. Because of feed retention time in the rumen, most nutritionists have migrated to either 24- or 30-hour time points. Recently, nutritionists have recognized that rumen retention time depends on many factors, so using one time point to estimate digestion is not suitable. Instead, we measure digestion at multiple points in time and then calculate the NDF digestion rate (NDF kd) to help us estimate fiber digestion for different animal development stages at different intakes. This value also appears on your forage analysis report.
    Fiber is important for both physical and energetic aspects. We feed adequate fiber to maintain sound rumen function and metabolism. The development of a solid rumen mat is essential to allow time for rumen microbes to digest valuable nutrients. The physical particle length provides the scratch factor necessary to promote cud chewing, resulting in the production of enzymes and buffering compounds necessary for digestion and rumen health.
    Starch and fiber both contain similar calorie content per pound, yet we understand the energy available to the cows varies greatly between these two nutrients. With fiber, the available energy is reduced because of the lignin and cell wall linkages, which prevents cellulose from being broken down into more digestible complexes. Goeser shared results from 2014, indicating the average total-tract fiber digestion rate in lactating cows is 40-50 percent, whereas the average total-tract starch digestion rate is more than 90 percent.
    Not all fiber is created equal. On a given farm, we may have alfalfa haylage, corn silage and a fibrous by-product, such as beet pulp, all with the same NDF level on paper. Yet we know the NDF kd will be significantly different for all three. This also explains the excitement surrounding low lignin alfalfa, and the continued interest in brown mid-rib (BMR) corn silage. Researchers have shown that every 1-pound increase in digestible fiber equates to 3-4 pounds of milk.
    Keep in mind the fundamental role fiber plays with rumen health, along with the opportunity to provide digestible nutrients. While laboratory techniques and terminology continue to evolve, the digestive system of the cow has not changed. Fiber will always be her friend.
    Barry Visser is a nutritionist for Vita Plus.