Forage and fiber in the diets of dairy cows is critical for their optimal health and milk production. How the fiber in forages ferments, digests and is metabolized is an ongoing subject of discussion and research.
It’s well-understood that fiber ferments more slowly compared to other nutrients such as starches and sugars, and its bulkiness is critical in creating a “mat” that’s necessary to maintain microbial life in the rumen.
Forages fed to dairy cows creates a challenge for nutritionists, however, since they are obligated to include forages in those diets to maintain proper rumen health, while at the same time formulating diets that contain high levels of metabolizable energy.
Forages, unfortunately, digest more slowly than other commodities in dairy diets, and their bulkiness limits dry matter intake. Two critical questions that need answering in order for nutritionists to accurately balance milk cow rations are: how much forage can a cow consume per day, and how much metabolizable energy can be expected from that forage?
The answer to those questions lies in both the amounts of digestible and undigestible fiber contained in any particular forage and the rates of digestion of those fiber fractions. Research has revealed that the amount of neutral detergent fiber (NDF) in forages limits total forage intake.
It is also now recognized that different fractions of the NDF are not uniformly digestible. Rather, NDF digests at varying rates – thus defined as fiber pools.
Effort is being expended to better understand the nutritional values of forages and predict how well or how poorly they perform in a diet. With increasing volatility of feed prices, more attention is being directed toward better utilization of forages in dairy diets.
Because of environmental concerns over the sustainability of animal agriculture and the use of grain and protein crops for animal feeds, the dairy industry must better understand the dynamics of forage digestibility.
The fiber in all forages is not uniformly digestible. The digestion of forages and forage fiber can range from relatively rapid rates, such as the cellulose found in the leafy portions of a plant, to the completely undigestible cellulose fractions and lignin.
For the purposes of modeling dairy cow diets, three pools appear to be relevant in evaluating NDF digestion – a fast pool and a slow pool, each which provide a certain amount of nutrition, and an undigestibile pool, which is excreted via manure providing no nutrition.
To complicate the research, the definition of fiber and its exact chemical characteristics have taken some time to pin down. Over the years, the laboratory assay of fiber and terminology as it applies to dairy diets has moved from crude fiber (CF) to acid detergent fiber (ADF) to neutral detergent fiber (NDF) in an effort to better understand the characteristics of the fiber found in forages.
Most recently, the consensus seems to have settled on aNDFom as the most accurate definition of fiber. The “a” indicates the testing process has included amylase to remove all starch and the “om” refers to organic matter indicating the NDF residue represents only organic matter with no residual ash (dirt, silica, minerals, etc.) that may have been present in the sample. What aNDFom then represents is various forms of cellulose that, depending on their molecular complexity, are rapidly or slowly fermented in the rumen along with the lignin, which is generally undigestible.
Research on identifying and understanding the dynamics of the different fiber pools have focused on the “fast” pool as being the aNDFom that digests in 30 hours, the “slow” pool as the aNDFom that digests between 30 and 120 hours, with the remainder being considered mostly as undigestible with a final time point being measured at 240 hours.
In a paper presented at the 2017 Cornell Nutrition Conference, educator Dr. Tom Tylutki eloquently described fiber as being “complicated.” The rates at which aNDFom digests for different forages is highly variable. The 30-hour pools differ significantly for grasses and legumes such as alfalfa. For dairy-quality alfalfa, a higher percentage of the aNDFom is digested in under 30 hours compared to grasses, due to a higher concentration of leaves. The aNDFom found in grasses tends to be in the slower pool of 30 to 120 hours, taking longer to digest.
According to Tylutki, the passage and digestion from the fast pool varies from 800 to over 1,600 grams of aNDFom digesta – a significantly wide range. Overpredicting or underpredicting the fiber digestibility can result in a poorly formulated diet, particularly when forage makes up a significant portion of a diet. Therefore, it’s critical to know what the 30-hour digestion of aNDFom is for all forages being fed to dairy cows in order to use them properly in a diet.
The amounts of aNDFom digestion in the slow pool and the uNDF240, on the other hand, are much less variable. However, these fractions of the NDF remain in the rumen for much longer periods of time, affecting the total turnover of feed in a diet.
Tylutki emphasized that, while the 30-hour digestible NDF value of a forage sample may appear to provide the most accurate prediction of the energy value of a forage, the total disappearance (i.e., degradation and passage rate) of the three pools are necessary in order to assess the nutritional value of a particular forage. The slow and undigestible fractions of NDF have an impact on rumen fill, dry matter intake and meal patterns of dairy cows.
The answer to the question of what is the maximum NDF that a cow can consume continues to be evasive. The amount of forage a cow can consume each day is heavily dependent upon the species of forage she is consuming and what combination of fast, slow and undigestible pools are associated with that forage.
Research has shown alfalfa and other legumes have an advantage by virtue of the fact that a larger portion of their NDF disappears more quickly in their fast pool, providing more energy in a diet. However, the physically effective NDF may be lacking in diets with high levels of alfalfa, which may cause depressed butterfat levels.
Grass hay, on the other hand, while having a fast pool that disappears more slowly compared to alfalfa, maintains a fiber mat for longer periods of time and ultimately digests a larger percentage of its total NDF. When balancing diets, the attributes of each species of forage must be taken into consideration.
Dairy farmers should recognize the reality that a single forage may not be adequate to meet the necessary nutritional requirements for a particular group of cows. The common practice of using only one silo or bunk out of convenience may be overfeeding a group of late-lactation cows or underfeeding a fresh group.
Due to the dynamics of fiber pools, different forages may be necessary for different production groups at any given time, and it may be necessary to have multiple silage silos or bunks or differing blocks of hay available to deliver different blends of forage to target groups.
The fundamentals of nutrition continue to dictate that milk production is driven by dry matter intakes, and forages continue to be the foundation of a dairy cow’s diet. However, as the dairy industry moves toward feeding higher proportions of forage, there must be a greater understanding of how forage fiber acts in the rumen and, for any given forage, how much energy can be expected from it.
The three-pool model of NDF digestibility appears to be helping nutritionists in determining the energy available from different forages based upon the differences in the fiber pools. 
ILLUSTRATION: Illustration by Kristen Phillips.
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John Hibma
- Consulting Ruminant Nutritionist
- South Windsor, Connecticut
- Email John Hibma
