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Forage Management Guides
Self-Study Guide 6: Forage Utilization by Dairy Cows

There is considerable opportunity for improving the nutrition and health of the world’s population by increasing the productivity of our ruminant livestock. By utilizing more forages, dairy cattle can increase the acres available for human food production; at the same time, topsoil can be stabilized and soil fertility can be increased where sound dairy farming practices are followed. The most significant role of the ruminant in providing food for people is their ability to digest forage crops grown from the vast number of acres not suited for growing grains, vegetables, or other intensively managed crops. Their unique digestive system makes it possible to convert plant materials that are inedible for most nonruminants into human food. Compared to poultry and swine, the dairy cow is exceptional at using forages as raw products to produce milk and meat for human consumption.

Forage Options

Wide varieties of forages are grown in Arkansas and represent a diverse array of growth habits and quality characteristics. These forages are harvested and fed to dairy cattle in Arkansas in a variety of different ways; methods are dependent upon the topography of the land, soil condition, farm and herd size, and farmer preference. Forages can be offered to dairy cattle in pasture systems or as greenchop, long-stem hay, baled silage, or stored silage. Many Arkansas producers utilize more than one of these systems. Smaller herds are likely to utilize pasture extensively because it requires less capital outlay for equipment. Large herds are more likely to feed a blended, total mixed ration that relies heavily on precision-chopped silage or ground or shredded hay. The development of wrapping systems for large round bales enables dairyman to make quality baled silage with large round balers that are typically owned by dairy producers.

Forage Quality

A generation ago, feeding recommendations for dairy cows were largely based on crude protein, crude fiber, and total digestible nutrients (TDN), which was a measure of the energy content of feedstuffs. Forage testing services at the University of Arkansas still calculate TDN values for tested feedstuffs. However, many nutritionists have largely discarded these criteria over the last decade. Currently, more intense analysis of forages and feedstuffs are required to properly balance dairy rations; principal considerations involve more in-depth analysis of fiber components and protein.

Fiber Components

Fibrous or cell wall components of forages are primarily comprised of cellulose, hemicellulose and lignin and are the least digestible parts of the plant. Historically, plant fiber has been measured as crude fiber; however, the crude fiber assay did not completely recover certain portions of total plant fiber. Specifically, the crude fiber assay was not effective in recovering hemicellulose, the most digestible portion of plant fiber, or a portion of the lignin. Lignin serves to give plants structural integrity (especially in the stems), thereby helping them to remain erect and rigid, but it also increases in concentration as a plant matures and greatly restricts digestibility. Because of the limitations associated with crude fiber analysis, the neutral detergent fiber (NDF) assay was developed to estimate the concentration of cell wall components (total plant fiber) in the forage. This assay effectively recovers cellulose, hemicellulose, and lignin and is closely associated with the potential intake of a given forage. Acid detergent fiber (ADF) essentially recovers the same cell wall components as the NDF procedure, except for the hemicellulose, and is closely associated with digestibility. These values are critical inputs for nutritionists attempting to balance dairy rations for several reasons: 1) ADF (or sometimes NDF) is commonly used to estimate the energy content of forages; 2) NDF is negatively related to intake and is often used to estimate the potential intake of forages; and 3) critical levels of plant structural components are necessary in all dairy diets to maintain proper rumen function and long-term animal health. Let’s look at these points individually.

Energy Availability

For many years nutritionists have used TDN values to balance dairy diets for energy; however, actual experimental determinations of TDN on a routine basis are not practical. Therefore, these values are commonly estimated from equations based on quality indices that are easy to measure and are typically included in forage analysis packages offered by commercial or state laboratories. Usually separate equations are used to predict TDN values for different forage classes; this means different prediction equations are used for legumes, grasses, mixed hays, corn silage, etc. However, all equations used currently in Arkansas require ADF values to estimate TDN. Forages with high concentrations of ADF are normally poorly digested and have a low energy availability. For example, high-quality alfalfa harvested at early bloom has an ADF concentration of about 31 percent; in contrast, the expected ADF level for coastal bermudagrass hay harvested after 43 to 56 days of regrowth is about 43 percent. The energy contents of these forages, expressed as TDN, are 60 and 43 percent, respectively. Systems that use net energy of lactation to balance rations also often use ADF to estimate the energy content of feedstuffs. Regardless of the system used, ADF-based estimates of energy content are not always accurate, particularly when the forages evaluated are of extremely high (or low) quality, or when forages are grown under climatic conditions atypical for the area in which prediction equations were developed. It is important to note that crude protein content is not usually helpful in predicting energy availability of forages.

Estimating Intake

Neutral detergent fiber is an extremely important quality index because it has an inverse relationship to intake. As discussed previously, the NDF assay measures the cell wall or structural components of the plant. Plant cells consist of a slowly-digestible cell wall that surrounds the highly-digestible cell contents or “solubles”. The cell contents contain most of the metabolic machinery of the plant; this includes mechanisms to harvest light energy, enzyme systems to trap and assimilate carbon dioxide into sugars, etc. For practical purposes, the cell contents are assumed to be completely and rapidly digestible and contain much of the protein and energy ultimately available to the cow after she consumes the plant. In contrast, the cell wall is slowly and incompletely digestible. As you might expect, the cell wall content, as measured by the NDF assay, is typically higher in stems than in the leaves; grasses frequently have higher NDF concentrations than legumes; warm season grasses usually have higher NDF concentrations than cool season grasses. The NDF content increases as all plants mature. These structural components of the plant constitute a considerable portion of the total plant dry matter and vary considerably, from about 30 to 80 percent, across different forage species (Table 1).

Table 1. Characteristics of forage NDF digestibility.

While mechanisms regulating intake are complex, the negative relationship between NDF concentration and intake of forages can largely be explained on a volume basis. The rumen of a dairy cow has a limited volume. Assuming the cow’s dietary demands for energy have not been met, she will continue to consume dry matter until the physical effects of “fill” limit intake. Research has estimated that dry matter intake is constrained by physical fill when a cow consumes 1.1 to 1.3 percent of her bodyweight each day in the form of NDF. Once this state is achieved, the only ways to alleviate “fill” are by passage to the lower digestive tract or by digestion. In the process of digestion, cell contents or solubles are usually digested quickly, particularly in legumes like alfalfa or red clover and in high-quality cool season grasses, thereby leaving the cell wall. At this point, a substantial portion of the plant has been digested on a dry weight basis; however, this is not true of plant volume, which is determined by cell wall content. Therefore, the rumen remains “full”. This can be visualized by considering a basket of eggs, where the basket represents the cow’s rumen and the eggs represent plant cells. We can simulate the early stages of digestion by poking a hole in both ends of each egg and draining the contents. If the empty eggshells or “cell walls” are placed back in the basket, the basket remains full, even though a great portion of the eggs or “plant cells” have been “digested” on a weight basis. It is important to note that the digestive properties of plant cell wall are not consistent across species and that they may change with plant maturity (Table 1). For instance, cell wall in alfalfa is digested at a much faster rate than in most grasses; cell wall in cool season grasses generally digests at a faster rate than in warm season grasses. Research results indicate strongly that cell wall digestion is slower in mature forages; however, some theories suggest that plant maturity limits the extent of cellulose digestion, but not the rate. Immature, thin-walled cells are not only more digestible than more mature, thicker-walled cells, but they are also more likely to collapse during rumination (cud chewing), thereby reducing plant volume. These factors reduce plant volume at different rates and partially account for differences in intake that are commonly associated with different forages.

Finally, it should be noted that the "fill" concept does not limit dry matter intake in all situations. For very energy-rich diets, intake is not necessarily limited by physical fill; in these cases, complex metabolic mechanisms within the cow may be the major factors controlling intake.

Minimum Fiber Required

Currently, dairy cows produce more milk than they did a generation ago; this requires diets that rely on higher proportions of concentrates to meet the extreme energy demands of high-producing cows. However, for several reasons, these energy demands can not simply be met by unlimited supplementation of concentrates. The most important reason not to excessively supplement with concentrates is health related; critical levels of NDF are necessary to maintain proper rumen function. When minimum fiber levels in the diet are not met, cows may show one or more of the following metabolic disorders: reduced total dry matter digestibility, reduced milk fat percentage, displaced abomasum, rumen parakeratosis, laminitis, acidosis or fat cow syndrome. Acidosis resulting from excessive supplementation of concentrates in diets containing poor-quality forages is a particularly common problem. In Arkansas, this problem is frequently related to the seasonal growth patterns and weather conditions that regularly cause dramatic reductions in pasture quality. Additionally, some of these metabolic disorders may also be observed in dairy cows grazing lush, very immature, spring pastures that have very low concentrations of plant fiber.

Why are critical levels of NDF necessary in the diet? Cell wall content, as well as forage particle size, are positively related to rumination time. The process of rumination reduces forage particle size and crushes cell walls; this also greatly increases the production of saliva, which has a substantial buffering effect in the rumen. This buffering effect helps to maintain the pH (relative acidity) of the rumen within the proper range to maintain desirable microbial populations (particularly those bacteria that digest cell walls), prevents damage to the rumen wall and liver, limits the incidence of the metabolic disorders mentioned previously, and maintains the long-term health of the cow. Sometimes additional buffers are added as dietary supplements. Even for the highest producing cows (with the greatest energy demand), nutritionists routinely recommend that forages make up at least 35 to 40 percent of the diet. Similarly, minimum levels of NDF concentration in the diet are also recommended. These guidelines typically range from 25 to 30 percent of the diet for high producing cows; at least 75 percent of this NDF should be of forage origin. If warm-season grasses like bermudagrass are the primary forage used, these NDF levels may have to be higher to accommodate the higher NDF concentrations typically found in these grasses. Long-stem hays require more rumination to reduce particle size than do finely chopped or ground forages; therefore, feeding these long-stem hays will result in increased saliva production and an increased buffering effect relative to finely processed forages.

A second reason that excessive supplementation with concentrates is a poor management choice is financial. Home-grown forages are generally the least expensive component of the diet and least-cost rations need to optimize their use to limit production costs. Clearly, when the energy demands of a cow are high, it is much easier to balance a least-cost ration with high-quality (high energy) forages than with poor-quality (low energy) forages. In fact, it is impossible to meet the energy demands of high-producing cows with poor-quality forages. In Table 2, three rations are presented that utilize different combinations of forages that vary substantially in energy content. In Ration 1, grass hay (52 percent TDN) is the only forage provided. Even after heavy (maximum) supplementation with concentrates (33.6 lbs of 16 percent dairy feed), the ration could only support 55 lbs of milk. In Ration 2, alfalfa pasture (61 percent TDN) was substituted, in part, for the grass hay. Although concentrate costs for Rations 1 and 2 were virtually identical, Ration 2 supported 10 additional lbs of milk. It is important to note that forage dry matter consumption was increased from 16.2 lbs (18.3 lbs x 0.885) in Ration 1 to 27.8 lbs [(10 lbs x 0.885) + (70 lbs x 0.27)] in Ration 2. Ration 3 supported the same 65 lbs of milk as Ration 2; however, the substitution of high-energy corn silage for some of the alfalfa reduced the need for concentrates from 31.7 to 19.2 lbs and increased the total forage dry matter intake to 31.0 lbs [(10 lbs x 0.885) + (40 lbs x 0.27) + (32.3 lbs x 0.35)]. Most importantly, please note that income over feed cost was maximized when dry matter intake from forages was at a maximum. This can’t be achieved when low-energy forages are used as the sole forage source.

Table 2. Example rations with high and low quality forages.

For many years, the protein needs of dairy cows were balanced strictly on the basis of crude protein. As production levels continue to increase, this simple measure has become inadequate. In the crude protein analysis, the total nitrogen concentration in the feedstuff is determined in an analytical laboratory, and this value is multiplied by 6.25. Therefore: Crude protein = % Nitrogen x 6.25.

The 6.25 factor was derived from the observation that forage proteins generally consist of about 16 percent nitrogen (100/16 = 6.25).

Two major problems exist with the use of the 6.25 factor to estimate crude protein from nitrogen content: (1) all forage nitrogen is not protein nitrogen; and (2) all forage proteins are not comprised of 16 percent nitrogen. In most plants, true protein nitrogen accounts for about 60 to 80 percent of the total plant nitrogen; the balance consists primarily of soluble nonprotein nitrogen (NPN). Several factors may affect the proportion of true protein found in forages. Making quality silage from precision-chopped forages or, more recently, from baled moist forage wrapped in plastic will greatly increase the proportion of NPN at the expense of true protein; fermented silages can easily have NPN concentrations that account for >50 percent of all plant nitrogen. In well-made silages, this occurs primarily as a result of the proteolytic activity of plant enzymes during wilting or the early stages of fermentation and is unavoidable. Another factor that can affect NPN concentrations in grasses is nitrogen fertilization. Higher plant nitrate levels are associated with increased nitrogen fertilization; in addition, increased pools of amino acids can cause the proportion of NPN to increase relative to those in moderately-fertilized or unfertilized grasses. Fertilization may also reduce sugar content in grasses, thereby creating nutritional problems; these could include poor nitrogen utilization by the cow and magnesium (grass) tetany. It should be noted that NPN values are often calculated when crude protein is determined analytically. Although considerably less important, it should also be noted that the 6.25 multiplication factor is not necessarily a good approximation for all forages; for instance, the actual experimentally determined factor for alfalfa (6.33) is close to the commonly used value, but others are less accurate (corn leaves = 6.94).

Another important topic that has received considerable attention in the last decade is the concept of ruminally degradable and nondegradable (bypass) proteins. Forages not only contain different amounts of crude protein (Table 3), but these proteins vary in their natural resistance to degradation in the rumen. As discussed previously, the retention time for forages in the rumen is partially dependent on the fiber characteristics of the forage. The interaction of these factors largely determines the quantity of nondegraded forage protein that passes to the lower digestive tract. Generally, high-quality forages have high concentrations of crude protein that offer comparatively little resistance to degradation in the rumen; these forages (alfalfa, red clover, immature cool season grasses, etc.) frequently have low proportions (10 to 30 percent) of bypass protein. In contrast, low-quality forages (especially mature warm season grasses) have low concentrations of crude protein that offer high resistance to degradation; these forages typically have high proportions of bypass protein (> 40 percent). This reduced degradation occurs despite the longer rumen retention times associated with these coarse forages.

Forages also have wide ranges of nitrogen that are immediately soluble in the rumen; this fraction is largely comprised of the NPN discussed previously and is generally (but not necessarily always) available immediately to the microbial flora. Experimentally, this immediately soluble fraction is often calculated by determining the portion of plant nitrogen that is soluble in water. For instance, alfalfa has a large immediately soluble fraction (perhaps 30 to 40 percent of the total forage nitrogen) and the remaining potentially digestible nitrogen degrades at a rapid fractional rate (about 10 to 20 percent per hour). In contrast, warm season grasses like switchgrass or eastern gamagrass have relatively low proportions of total plant nitrogen that are immediately soluble (15 to 30 percent); this is also coupled with slow fractional degradation rates (2 to 5 percent per hour) for the remaining potentially digestible forage nitrogen.

There are several reasons why these factors are important. Protein nutrition in the rumen is complex. Unlike monogastrics, protein requirements for ruminants are met from two sources: (1) dietary protein that bypasses or escapes microbial degradation in the rumen; and (2) synthesized protein that is harvested from the microbial flora that proliferate in the rumen and then pass to the lower digestive tract. Research has shown that crude protein content is of limited value in ration formulation for high-producing cows unless these factors are also considered.

Three goals exist in meeting the protein needs of lactating cows:

1. minimize losses of dietary protein as a result of inefficient microbial fermentation in the rumen

2. maximize the quantity of protein supplied to the small intestine for absorption

3. improve the quality of the protein absorbed in the lower digestive tract.

When cows consume forages (or concentrates) with protein that is degraded quickly by the rumen microbial population or that has high concentrations of NPN, nitrogen-containing compounds are rapidly broken down to ammonia. This ammonia can be utilized by the microbial flora in the rumen to yield microbial protein that is available to the cow in the lower digestive tract. It is desirable to maximize the synthesis of microbial protein; however, rumen ammonia levels can exceed the capacity of the microbial flora to synthesize proteins. The capacity of rumen microbes to synthesize protein is primarily set by available energy. Under conditions where rumen ammonia concentrations exceed the capacity of the microbial population to utilize it, the ammonia is absorbed across the rumen wall, transported in the blood to the liver, converted to urea, and excreted in the urine. These processes are inefficient and result in a costly loss of both protein and energy to the cow. This partially explains why the performance of dairy cows on diets adequately balanced for crude protein can sometimes be disappointing. Dairy producers can monitor milk urea nitrogen (MUN) through the Dairy Herd Improvement Association (DHIA) or other commercial labs. Analysis of MUN can provide an estimate of blood urea nitrogen levels, which are an indicator of rumen ammonia levels. Some producers have decreased feed costs by several thousand dollars by utilizing MUNs to reduce excessive ammonia levels in the rumen.

It is also important to realize that only 60 to 70 percent of the nitrogen in rumen bacteria is in the form of true protein; the remaining nitrogen exists in forms that are largely unavailable to the animal. Therefore, some additional efficiency is lost during the conversion of dietary protein to microbial protein. Unfortunately, most high-quality forages (alfalfa, alfalfa silage, immature cool-season grasses, etc.) contribute to these problems via rapid protein degradation rates, high soluble nitrogen content, or both. Sometimes efficiency can be improved by providing rapidly fermentable carbohydrate; this may increase the available energy in the rumen when ammonia levels are high, thereby increasing the potential for microbial protein synthesis and subsequent delivery of this protein to the lower digestive tract. In the last two decades, researchers have spent considerable resources in an attempt to characterize protein degradability in various forages and concentrates. Other work has evaluated processing schemes that could potentially limit the rate and extents of protein degradation in the rumen after these high-quality feedstuffs are consumed. Many of these processing methods involve heat treatment. Heat treatment can denature proteins, thereby making them more resistant to degradation in the rumen. However, their denaturation can also result in elevated levels of heat-damaged protein, which is ultimately unavailable to the cow.

Dairy nutritionists have determined that about 35 percent of the crude protein in the entire diet should bypass the rumen undegraded in order for production to be maximized. The reasons for this need are complex; generally, when this requirement is met, it is assumed that more, higher-quality protein is ultimately available to the cow in the lower digestive track for increased milk production. As discussed previously, high-quality forages are generally deficient in providing bypass protein. Unfortunately, this deficiency can not really be corrected by management, and supplementation with concentrates high in bypass protein may be necessary. Growth factors like climate, water-stress, fertilization, etc. may all affect bypass protein values of forages, but not enough to alter recommended management practices. There is a strong tendency for bypass protein values to increase in grasses as they mature, but these benefits are offset by lower crude protein concentrations, decreased energy content, increased fiber components, and poorer intake and digestibility. Forages harvested as hay often have higher levels of bypass protein than corresponding standing forages. Furthermore, moderate spontaneous heating during silage fermentation or in a hay bale may improve bypass protein values of forages. A recent study in Kansas reported that bypass protein levels for alfalfa hay increased from about 20 to 28 percent when substantial spontaneous heating occurred during bale storage; however such heating can’t be controlled in practical situations and benefits were offset by increases in heat-damaged protein, which is ultimately unavailable to the cow. Inducing spontaneous heating by baling hay before it is adequately dried is not a recommended management practice. Most warm season grasses have substantially higher levels of bypass protein than cool season grasses or legumes; however, these benefits are also offset by higher NDF concentrations.

A final word of caution is advised regarding the interpretation of Table 3. Estimates of bypass protein are calculated on a percentage basis for each forage species. This provides no information about the actual quantity of forage protein escaping the rumen; estimates of quantity are a function of both bypass and crude protein contents. For example, a forage with 60 percent bypass, but only 2 percent crude protein content would provide only a small quantity of bypass protein to the lower digestive tract.

Table 3. Characteristics of forage nitrogen digestibility used by Arkansas Cooperative Extension Service to balance dairy rations.

Summary

1. Crude fiber analysis has been replaced by ADF and NDF procedures.

2. Acid detergent fiber (ADF) is frequently used to predict energy content of forages and is related to digestibility.

3. Neutral detergent fiber (NDF) is a measure of plant cell wall components and is closely related to intake.

4. Critical levels of plant fiber are necessary for proper rumen function.

5. Crude protein alone is not an adequate basis for balancing protein requirements of high-producing dairy cattle.

6. High quality forages are high in energy, but usually have rapidly degradable protein.

7. Poor quality forages are low in energy, but usually have slowly degradable protein.

For more information about forage management, contact your county Extension office or refer to one of our publications.

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University of Arkansas
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Last Date Modified 11/20/2008
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