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North Carolina State University Return to main menu Rumen Microbes and Nutrient Management[1]
V. Fellner IntroductionThe dramatic changes occurring in the dairy industry are having an impact on the future direction of the U.S. dairy business. The number of dairy farms, with grade A and B permits, fell 37 percent between 1992 and 2000, but milk production rose 11.5 percent. The decline in total farm numbers did not lower milk production instead dairy farmers who remained in the business produced more milk from more cows. Current dynamics of a global economy and the incessant introduction of new technology have resulted in the development of adaptive approaches to increase the efficiency of milk production. The past several decades have seen a major shift in the direction of research in dairy science. Previous research focused on altering different aspects of dairy production systems. Such an integrated approach has its merits, however, the main thrust of dairy research today is to improve the efficiency of milk production. This is accomplished by manipulating fundamental biological processes regulating milk yield. The past 20 years have seen tremendous progress in the field of rumen microbiology enabling us to better understand the complex microbial interactions governing efficient nutrient utilization by ruminants. Renewed pressure, in part, due to regulation of nutrient output from farms has resulted in considerable research on minimizing nutrient wastage and maximizing nutrient use by the animal. A large proportion of dietary nutrients are made available to ruminants in the form of end-products of rumen fermentation, primarily short chain fatty acids (SCFA) and microbial protein. The removal of fermentation products from the rumen and the outflow of microbial biomass have a direct impact on the nutritional status of ruminants. Short chain fatty acids, are used by the host as a major source of metabolizable energy. Microbial protein is the major source of metabolizable amino acids for maintenance and milk synthesis. The efficiency, with which dairy cows convert dietary energy and protein into nutrients for milk synthesis varies considerably. Much of this variation is due to inefficiency that occurs in the rumen and appears to be related to the ruminal environment. Consequently, the ability to characterize and modulate SCFA and microbial protein synthesis is essential if we want to increase nutrient use towards desirable products such as milk, and decrease waste in the form of nitrogen excretion and methane. A key issue being addressed currently is the need to obtain more data on fermentation products including microbial protein synthesis to include in model systems designed to predict overall nutrient requirements of ruminants. The Rumen The uniqueness of ruminants was recognized perhaps as early as 400 B.C. when Aristotle compiled his 'Historia Animalium' textbook on animal husbandry. He recognized that ruminants have a stomach with four compartments and that it was different from the simple stomach of other animals. Scientific experimentation on the rumen, however, began in the mid to late 1700's by Réaumur, Stevens and Spallanzani. These workers demonstrated the disappearance of material from the rumen and also documented rate of passage of food through the animal's intestinal tract. In 1825, Tiedemann and Gmelin reported the presence of various gases in the rumen with particular reference to butyric acid. An important milestone in the study of rumen function was the development of a fistula by Flowers in 1844 and modified later by Toussaint (1875) and Colin (1886). Although, work done so far had suggested that the fiber was not degraded by the enzymes in the gastrointestinal tract it was in 1884 that Tappeiner established the importance of the rumen microflora as a digesting agent. During this same time Tappeiner and other investigators reported the fermentation of cellulose in the rumen and appearance of volatile fatty acids as well as the gases carbon dioxide, methane and hydrogen. It was not long before these investigators suggested that the fatty acids produced in the rumen might serve as an energy source for cattle. During the next 50 years other investigators accepted the fermentation hypothesis in the rumen. By mid 1900 methods for quantitative separation and identification of the volatile fatty acids were established by Barcroft and Elsden. It was during this same period that the cultivation of rumen microorganisms stimulated the interest in rumen function and further established the symbiotic relationship between the ruminant and rumen microbial population (Hungate, 1966). The last 50 years have resulted in extensive research on the rumen microbial system. Conditions in the Rumen The conditions in the rumen are not only complex but they are intermittent. The rate at which feed enters the rumen will be very different during grazing or meal feeding. Salivary flow rate is not steady and rumination activity that is not continuous will depend on the type of diet. The flow of substances into and out of the rumen may involve more than one pathway. Volatile fatty acids can leave the rumen via passage into the lower tract or they can be absorbed and partly metabolized in the epithelium. Urea may enter the rumen via saliva or directly from blood through the rumen epithelium. Despite all these variations there are certain generalizations that we can make regarding the rumen: 1. Temperature is usually maintained within the range of 38-41°C with 39°C used as a common mean temperature. 2. Rumen pH can range from around 7.0 on forage diets to as low as 4.6 on high-grain diets. 3. Mean redox potential is -350mv reflecting the strong reducing environment and the absence of oxygen. 4. Carbon dioxide and methane are the major gases present in the rumen. 5. The solid and liquid digesta leave the rumen at different rates. Acetate is found in the greatest concentration in the rumen followed by propionate and butyrate. Short chain fatty acids are passively absorbed through the rumen wall and the rate of absorption is dependent upon chain length, pH and concentration. At pH 6.5 or below longer chain fatty acids are absorbed at a higher rate, but at pH above 6.5 rates are similar. In general, undissociated acids are more rapidly absorbed, therefore as pH decreases absorption increases. Low pH also favors the absorption and production of lactic acid. In cases when large amounts of grain are fed lactic acid can accumulate and become toxic to the animal. Ammonia is readily absorbed and the rate of absorption is dependent on concentration and pH. It is rapidly absorbed at a higher pH and decreases as pH drops. Microbial Ecology The rumen contains one of the most diverse and dense microbial ecologies known in nature. A possible explanation for the diversity is the complex nature of the feed, which contains carbohydrate, proteins, fats, other organic compounds and minerals. In order to utilize these compounds organisms are either highly specialized (narrowly adapted) to compete for a few of the feeds or become widely adapted and are capable of using many nutrients. In addition to these categories there are microbes in the rumen that alter their metabolism depending upon the availability of nutrients. Another factor that selects for diversity is the ability of organisms to accomplish the most growth. Since growth is limited by the quantity of available food, efficiency at which food is transformed into cells will dictate survival. The conversion of carbohydrate to acetate, propionate, butyrate, carbon dioxide and methane results in a cell yield that is greater than other theoretical biochemical pathways. However, when nature or amount of carbohydrate is altered, heterolactic fermentation replaces the acetic-butyric type by microbes that are more efficient in the production of new cells. Given the various possible biochemical interactions, certain combinations of reactions produce more cells than do others and the conditions necessary for each may vary. The contents in the rumen are heterogeneous and consist primarily of a microbial suspension in free liquid, a solid mass of digesta, and a gas phase. Microbial populations of the rumen are characterized into those organisms free in the rumen fluid, those associated with feed particles and those associated with the rumen wall. Those associated with the solid particles have been further subdivided into those organisms firmly associated with particles and those more reversibly associated. In a fully functioning rumen, there is a dynamic equilibrium, as ruminal microbes adhering to and detaching from feed particles are constantly leaving or re-entering the fluid compartment. The rumen harbors a large number of different species of bacteria, fungi and protozoa. Metabolic interactions between these different populations are critical for their collective survival. Moreover, products of the metabolism of some species of microorganisms are sources of energy for the other species. These interactions regulate in a large part the concentrations and activities of individual species as well as the nature of the fermentation products. Ruminants — A Nutritional Challenge Unlike non-ruminants, formulating for precise nutrient requirements in ruminant rations is a challenge. Both ruminants and non-ruminants utilize nutrients in tissues but ruminants have another metabolic system - microbial metabolism in the rumen. This poses a nutritional challenge because nutrient requirements of the two systems are distinctly different. Maximizing ruminant productivity involves meeting the requirements for both systems. Inherent losses in energy and nitrogen associated with pre-gastric fermentation by microbes in the rumen make the digestion and subsequent absorption of dietary nutrients less complete in ruminants. The rumen is a complex microbial ecosystem and not easy to manipulate like an industrial fermentation. The primary research focus, from an evolutionary standpoint has been on making rumen microbes degrade grasses quicker and more extensively. In that respect, attempts have been made to modify feeds to make the cell wall more accessible to microbial enzymes. Rumen microorganisms are interdependent for supplies of carbon and nitrogen; improving the availability of carbon sources to increase digestibility would provide maximum benefit to overall fermentation if it were synchronized with the availability of nitrogen. New strains of genetically engineered bacteria that are more effective fiber digesters have been introduced into the rumen. Although temporal variations occur, the 'dynamic steady-state' of the flora seems highly resistant to changes. As a result the introduction and subsequent growth of exogenous bacterial strains with desirable biochemical properties is difficult if not impossible to sustain over long periods in the hostile rumen environment. Perhaps the greatest potential to manipulate ruminal fermentation has been realized with the use of dietary additives that alter microbial physiology to either enhance beneficial processes like fiber digestion, lactate fermentation and non-protein nitrogen use or minimize inefficient processes like methane production and ammonia absorption. Biochemical reactions in the rumen are highly interactive. Interfering with a reaction almost always results in a cascade of interrelated reactions, some of which may not necessarily be beneficial. For instance, reducing methane production invariably increases ruminal propionate, both of which are desirable from the whole animal perspective. On the other hand, lowering methane and increasing propionate at the cost of reduced acetate can limit the supply of ATP (energy), which is undesirable from a microbial perspective. Energetics of Rumen Fermentation Rumen microorganisms require strict anaerobic conditions for normal function. Consequently, a key feature of fermentation is the partial oxidation of substrates within the rumen. The energetics of microbial metabolism can be characterized into three main processes: (1) amount of organic matter fermented, (2) concentration of relative proportions of fermentation products produced and (3) amount and efficiency of microbial protein synthesis. Anaerobes conserve ATP in the form of a trans-membrane electrochemical gradient commonly referred to as the Proton Motive Force (PMF). However bacterial cell growth depends largely upon a membrane bound ATPase for the transfer of ATP from PMF. The rumen is a highly reduced environment and energy is often limiting. The survival of rumen microorganisms is dependent upon the efficiency with which ATP is produced, transferred and utilized during bacterial growth. Maintenance of normal fermentation within the rumen requires that the large amounts of reducing equivalents produced in the form of NADH must be re-oxidized. Microbial populations have evolved fermentation pathways that effectively lower the concentration of reducing equivalents in the rumen. The main products of these fermentation reactions are SCFA, CO2 and CH4. Acetate is the predominant SCFA found in the rumen and its formation is largely a function of the production of hydrogen from reduced cofactors. However, a high concentration of hydrogen gas is thermodynamically unfavorable and will inhibit further fermentation. Low hydrogen levels are maintained by methanogens resulting in greater hydrogen and consequently acetate production. Anaerobic conditions within the rumen result in most of the energy (ATP) in the fermented organic matter being retained in the products (SCFA and microbes) with some losses occurring in the form of CH4 and heat. The route with which reducing equivalents are disposed determines the availability of ATP. If the removal of hydrogen is coupled to acetate production, ATP yield is highest compared to ATP yields for butyrate and lactate. It is now believed that ATP yield from propionate can be comparable to that from acetate. If ATP production is uncoupled with microbial growth, excess SCFA production will be inversely related to microbial cell synthesis. This will have a major impact on the supply of the two most important sources of nutrients i.e. SCFA (energy source) and microbial biomass (protein supply). Factors Affecting Microbial Growth and Microbial Yield At any given time the rumen will contain variable levels of peptides, amino acids, ammonia, carbohydrates, polymers, isoacids, lipids, vitamins, and minerals. The levels of these substrates will vary with time after feeding and with the source and quantity of feed consumed. In order to optimize microbial growth a constant supply of nutrients is required. We know the major nutrients that are needed by microbial species in the rumen but we know little about the amounts and combinations needed to optimize microbial growth. Ammonia is the most important source of nitrogen for protein synthesis in the rumen. Ammonia concentration in the rumen can fluctuate between <1 mM (low protein roughage) and as high as 40 mM following feeding of rapidly degraded protein. Many microbes that use ammonia can also use amino acids or peptides. The use of ammonia is reduced in the presence of amino acids and peptides explaining the variability in ammonia concentrations in the rumen. Some studies suggest that larger peptides may be metabolized more rapidly than smaller peptides and that hydrophobicity reduces the rate of peptide hydrolysis. However, the combination of amino acids and peptides stimulate total microbial growth to a greater extent than either protein by itself. Microbial nitrogen requirements are dependent upon the type of carbohydrate being fermented in the rumen. Bacteria that ferment NSC prefer amino acids and peptides and those that ferment fiber rely primarily on ammonia as their nitrogen source. Carbohydrates are the major source of energy for rumen microbes. Studies suggest that following the uptake and hydrolysis of polysaccharides the resultant hexoses and pentoses are readily fermented and support microbial growth with similar efficiencies. However, due to differences in the rate of hydrolysis of different oligosaccharides total protein yield per unit of time may vary. Some bacteria, particularly the cellulolytic species, may require the isoacids in order to meet their amino acid requirements. Isoacids are produced via the deamination of dietary amino acids as well as from the growth and lysis of other microbes. Ruminal pH has a major impact on fermentation of microbial growth. When sources of readily fermentable carbohydrates are added to forage diets fiber digestion may be impaired. Depression in digestibility due to a drop in pH is not equal for all nutrients. While fibrolytic and proteolytic activities are severely depressed, fermentation of starches and sugars remain very high. Liquid and solid turnover rates in the rumen are not the same and vary with intake. Since microbes are present in all compartments within the rumen their survival will depend on their ability to reproduce at a rate equal to or greater than the turnover of that compartment. Attachment to feed particles is necessary for survival of microbes that have a slow rate of growth, such as protozoa and fiber digesters. Both total yield and efficiency of microbial yield are influenced by liquid and solid turnover in the rumen. Microbial yield is important because this is an index of the amount of microbial protein made available to the cow each day. Since microbial protein can supply well over 50% of the requirements of a lactating cow, the practical significance of maximizing microbial yield is obvious. Microbial efficiency is important because it is part of the calculation of yield. Intake, rumen turnover and microbial efficiency are all positively related. An increase in intake will increase ruminal turnover, resulting in an increase in microbial growth. If organic matter digestion in the rumen is reduced it may have a negative impact on the efficiency of microbial growth. Approach to Manipulating Ruminal Dynamics Carbohydrate fermentation Carbohydrate is the major component of ruminant diets and it differs widely in the rate and extent of fermentability in the rumen. In forage based diets the cell wall polysaccharides (structural carbohydrates) are the primary source of energy whereas in cereal based diets the storage polysaccharides (starch and fructosans) provide most of the energy requirements. Microbes convert both the cell wall and storage polysaccharides to five and six carbon sugars. These sugars are rapidly fermented into SCFA and can provide up to 70% of the energy requirements of the cow. Dietary ingredients have a significant impact on the proportion of acetate, propionate and butyrate produced in the rumen. Diets of dairy cattle are often supplemented with fermentable carbohydrate to meet the energy demands associated with higher milk production. Starch may be digested by either microbial or host enzymes and shifting the site of starch digestion has been the focus of intense research. The efficiency with which the three main SCFA are utilized has been suggested to be similar but theoretic estimations indicate starch digestion in the small intestine to be energetically more efficient. In contrast, performance data from several experiments show that starch fermented extensively in the rumen results in higher milk production. This suggests that increasing ruminal propionate is more beneficial in increasing the capture of fermentation energy since it reduces carbons that would be lost in methane. There may be limits to the use of starch in the lower tract and its fermentation in the rumen and maximizing starch use requires a clear understanding of those parameters. There is increasing evidence to suggest that the source of starch may result in a variable response. Feeding corn over barley has shown to alter ruminal fermentation. Barley is more rapidly fermented in the rumen and supports a higher SCFA production compared to corn. This implies that a greater proportion of carbon from barley would be made available to the cow in the form of propionate in contrast to reduced fermentation of corn in the rumen and greater capture of corn carbon as glucose in the lower tract. We know very little about how these fermentation schemes impact the energy status in the rumen and subsequent outflow of microbial protein. Nitrogen metabolism Microbial cells and dietary nitrogen that escapes ruminal degradation are the major sources of protein and amino acid requirements of ruminants. Although plant materials that comprise the bulk of ruminant feeds are composed of a vast array of nitrogenous compounds, most of the nitrogen contained in the forages and cereals fed to ruminants is protein. Compounds that are not true protein, but contain nitrogen are non-protein nitrogen (NPN) and include nucleic acids, nitrates and supplemental urea. Enzymatic activity in the rumen converts dietary protein into amino acids, which are in turn deaminated to ammonia and various carbon skeleton compounds (organic acids). This affects the composition of dietary protein that escapes the rumen as well as the microbial protein fraction. This process appears to be wasteful since the animal requires some amino acids for its own use. However, some plant proteins may be very indigestible by the host enzymes and these proteins often have a low content of the essential amino acids. Since the microbes synthesize protein from ammonia and other suitable carbon skeletons they contribute to the nutritional metabolism of the animal. Nucleic acids (5 to 10% of dietary N) are fermented rapidly and the nitrates are converted to nitrite, which is fermented to ammonia. The most common strategy is to increase the escape of high quality dietary protein by minimizing proteolysis, peptidolysis and deamination. It is well accepted however, that increasing the undegradable intake protein fraction must not be at the expense of lowering the degradable intake protein in the diet. Optimal protein supply to the animal depends on adequate degradable protein to maximize capture of organic matter in microbial biomass. Synchronizing the rate of nitrogen hydrolysis with the rate of energy release will increase the rate of assimilation of ammonia by microbes and maximize nitrogen use by the animal. Lipid metabolism Basal rations fed to high producing lactating cows typically contain 2 to 3% fat. It is common to increase the energy density of diets fed to early lactating cows by including additional fat. However, fats tend to have a negative effect on microbial metabolism and it is recommended not to include supplemental fat in amounts greater than 3 to 4% of the diet dry matter. Lipids in natural feeds consist primarily of triglycerides and glycolipids. Supplemental fats can be either of vegetable origin (oils and oil seeds) or animal origin (tallow, grease and fish oils) or they can be a blend of the two (blended fats). Rapid hydrolysis of the acyl ester bonds of lipids releases the glycerol from the fatty acids. Glycerol is fermented but the unesterified fatty acids are adsorbed onto particulate matter (feed or microbial cells) and are not fermented or degraded any further. Anaerobic conditions in the rumen are unfavorable for oxidation of fatty acids. Consequently, unesterified fatty acids are either incorporated into microbial lipids or are hydrogenated by the microbes. Given the caloric benefits of additional fat and the limitations on intake during peak production, the inclusion of higher levels of fat in the diet are extremely beneficial. Much of the research is aimed at manipulation of lipid metabolism in the rumen to minimize the anti-microbial effects of fatty acids and to ameliorate disruption of ruminal fermentation. Efforts to increase flow of unsaturated fatty acids stimulated by human health concerns about saturated fat have resulted in another aspect of manipulation of dietary lipid metabolism by rumen microbes. Controlling biohydrogenation to affect absorption of selected fatty acids has been shown to improve the nutritional qualities of milk. Since unsaturated fatty acids are more detrimental to fermentation than the saturated fats, biohydrogenation seems to be a defense mechanism of the microbes. The process of biohydrogenation also competes for metabolic hydrogen, which is used as mentioned earlier, in the reduction of CO2 and production of propionate. Lipolysis is a prerequisite for biohydrogenation to proceed because the isomerase that catalyzes the initial step to form the trans-11 isomer is not functional unless the fatty acid has a free carboxyl group. Therefore, the main objective in manipulating lipid metabolism is to minimize lipolysis. In that respect, the feeding of protected lipids is a common practice. Ruminal Disorders and Microbial Transformations In addition to using dietary nutrients for maintenance, growth and production microbes are instrumental in the transformation of other dietary compounds. These transformations may in some instances exert toxic effects on the animal and in other cases result in a beneficial response. Lactic acidosis is a common metabolic condition observed when ruminants are overfed large amounts of grain or other rapidly fermented carbohydrates. Fermentation of readily degraded carbohydrate increases the production of acid in the rumen particularly lactic acid which is a stronger acid and responsible for the onset of ruminal dysfunction. Streptococcus bovis is a major lactate-producing bacterium and is tolerant to low pH. Organisms like megasphaera elsdenii and the selenomonads are major lactate utilizers and are inhibited by low pH. The change in microbial populations can be very rapid and occur within a 24-hour period. Several pathological changes in animals are associated with the onset of ruminal lactic acidosis and some of these changes are directly related to changes in the microbial population in the rumen. Excessive grain intakes increase the concentrations of endotoxins in the rumen due presumably to the disintegration of the gram-negative bacterial cells. Adapting the rumen to increasing concentrations of grain can reduce the incidence of acidosis. Gradual adaptation allows the simultaneous growth of both lactate-producing and lactate-utilizing microbial populations. Addition of anti-microbial agents such as ionophores is also effective in limiting ruminal dysfunction associated with the proliferation of lactic-acid producing bacteria with high-grain diets. When gas produced in the rumen accumulates it can result in ruminal distention causing bloat. Severe cases of bloat impair circulation and may lead to death of the animal. Bloat is commonly associated with the formation of a stable froth. The formation of stable froth is a combination of gas bubbles as a result of microbial activities and factors associated with plant materials in the food. Pasture bloat is often observed in animals grazing on legume pasture. In this instance, soluble plant proteins seem to contribute in the formation and stabilization of the froth. Rapid degradation of legumes and slow rate of ruminal clearance are contributing factors to the frothing process. In contrast, feed lot bloat is characterized by the formation of slime by amylolytic bacteria in the rumen resulting in stable foam. Rumen microbes can destroy naturally occurring toxins in feeds, which have been related to adaptive changes in the ruminal microbial populations. Some transformations however can result in the formation of toxic materials in the rumen. One such example is the reduction of nitrate to nitrite in the rumen. Under normal conditions nitrite can be converted to ammonia but it may also accumulate when diets contain high levels of nitrate. Excess nitrite is absorbed and combines with blood hemoglobin to form met-hemoglobin, which impairs the oxygen-carrying capacity of red blood cells. Readily fermentable carbohydrates, lactate and hydrogen are sources of electrons for the reduction reactions and facilitate nitrite production. If the physiological capacity of the microbes to carry on the reduction reactions is not exceeded, nitrate and nitrite reductions increase simultaneously due to an adaptation of the rumen microorganisms to those diets. Maximizing Nutrient Flow from the Rumen A large proportion of the nutrients used by the ruminant is the end product of rumen fermentation, primarily the short chain fatty acids and microbial proteins. Consequently, it is imperative to manage the rumen to optimize microbial growth and fermentation. There are several indicators of optimum fermentation and the total flow of microbial protein to the lower tract is an important criterion. As discussed above, diet digestibility and microbial efficiency affect daily microbial protein yield. Most rumen microbes use carbohydrates mainly for energy, and growth will be proportional to the amount of carbohydrate fermented. Carbohydrate sources include structural (SC) and non-structural (NSC) carbohydrates. Structural carbohydrates include the fiber components cellulose, hemicellulose and lignin associated with the cell wall fractions. Sugars, starches, pectins, ß-glucans, and gums are referred to as the NSC. The NSC is more digestible than the SC and therefore the total carbohydrate digested is positively related to the proportion of NSC in the diet. However, digestion of both fractions varies due to differences in rate of ruminal degradation among feed sources. Synchronization of rates of digestion among sources of NSC and SC is important to provide a continuous supply of available carbohydrates. On high grain diets ruminally available protein can be more limiting than available energy. It has been shown that increasing ruminally degradable protein did not have a major influence on carbohydrate digestion but markedly increased microbial efficiency. There is little known about the importance of the sources of protein required to optimize microbial growth. This includes both requirements for NPN, amino acids and peptide as well as appropriate rate of protein degradability needed to synchronize and carbohydrate availability. Feeding Management Strategies Feeding strategies that optimize rumen function also maximize milk production and milk component yield. Although fat, protein and lactose increase in proportion to milk volume, milk composition changes are minimal. Maximizing feed intake is critical to minimize the negative energy balance during early lactation. Increasing energy intake will increase milk protein content. High producing cows should eat 3.5 to 4.0 percent of their body weight daily as dry matter. On high grain diets increasing the feeding frequency of forage can increase the milk fat levels; during hot weather it will keep feed fresh and palatable. It is important not to drop below a 40 to 60 forage to concentrate ratio. Other dietary factors also need to be adhered to in order to maintain optimal ruminal pH. A drop in pH will increase propionic acid production, reduce fiber digestion and lower milk fat. Therefore feeding concentrates requires appropriate forage to concentrate ratio and non-fiber carbohydrate levels. Appropriate NFC levels can improve both milk fat and protein while overfeeding often leads to milk fat depression. Fibrous by products such as soybean hulls can replace starchy grain and reduce the severity of milk fat depression in rations high in non-fiber carbohydrate. Type of grain and method of processing can alter site and extent of starch digestion. In general, ground, rolled, heated, steam-flaked, or pelletized grain increases starch digestibility in the rumen. Flaking compared to rolling seems to improve milk production and milk protein yield but it tends to lower milk fat percentage without affecting milk fat yield. These effects are attributed to increased total tract starch digestibility, increased recycling of urea and increased microbial protein flow. Due to differences in the rate and extent of fermentation it is important to match carbohydrate and protein sources to ensure proper fermentation patterns. Providing adequate levels of fiber is also critical to stimulate rumination, saliva production and maintenance of normal milk fat and protein composition. Forage should comprise no less than 40 to 50 percent of the total ration dry matter or should be included at no less than 1.4 percent of body weight. Minimum acid detergent fiber (ADF) and neutral detergent fiber (NDF) levels should be 20 and 26 percent, respectively. At least 75 percent of the NDF should come from a forage source. The recommended level of UIP ranges from 32 to 39 percent of crude protein. Soluble protein should be between 30 to 32 percent of total protein or about half of the DIP level. It is essential to meet requirements for both crude protein and rumen undegradable protein. Diets with a protein percent between 9 and 17, containing no supplemental fat, have shown to increase milk protein by 0.02 percentage unit with each 1-percent increase in dietary protein. Dairy Systems and Manure Nutrients Successful waste management depends on accounting for nutrients in the diet, manure, and land application. The use of animal waste as a fertilizer has been known for several years. Under intensive livestock operations quantity of manure nutrients may exceed the capacity of the land. Good waste management should emphasize utilization of manure rather than its disposal. This would reduce the need for commercial fertilizers and minimize contamination of surface and ground water. Several states have outlined best management practices (BMPs) to maximize economic benefit and avoid environmental problems. Sustainable agriculture involves maintaining high yields of agricultural product while maintaining high levels of ecological efficiencies. The two minerals of major concern seem to be nitrogen and phosphorus. Nitrogen can readily move through soils and contaminate ground water leading to excess nitrate in drinking water. Phosphorus binds tightly to soil and therefore has a lower potential to contaminate ground water. In some areas there is also concern regarding other minerals such as copper and zinc. The growing concern for the environment has led to increased animal dietary nutritional manipulation to find solutions to these issues. Nitrogen, in excess of animal requirements, will be excreted in the urine or feces. A major portion of nitrogen losses is associated with inefficiencies of digestion and absorption. Therefore, dietary manipulation will influence nitrogen excretion. For lactating ruminants, 30 to 35% of feed nitrogen is captured in milk and the remaining 65-70% is lost as indigestible protein from feed or microbes in the feces and as urea in urine. Microbial fermentation of nitrogen in urine and feces results in the production of ammonia gas, which has a strong odor and may be harmful to humans and animals. The production of ammonia gas may be reduced by reducing the amount of nitrogen excreted or by proper waster management. Excretion of nitrogen is dependent upon dietary nitrogen, productive capacity of the animal and the energy relationship between productive capacity and amount of nitrogen in feed. Phosphorus occurs mainly in the form of phytic acid, which must be broken down before it can be utilized. Ruminants can use the phosphorus from phytic acid due to microbial degradation in the rumen. Managing nutrients on a dairy operation is difficult due to the inability to accurately predict animal requirements. The first step to meet the requirements of a nutrient is to use accurate feed composition values in formulating ration. Values reported in the NRC can be quite different from actual laboratory analysis. The next step is to formulate diets based upon the grams of nutrient fed compared to the grams required. Nutrient concentrations may be higher or lower depending upon dry matter intake. For nitrogen, requirements need to be considered with regards to bacterial and animal requirements. For dairy cows one needs to maximize rumen microbial growth to supply the cow with energy and protein and then supplement with dietary amino acids. Matching microbial requirements with animal requirements will allow for adequate protein in the diet and lower nitrogen excretion. Nutrient Management Plan Animal agriculture has been implicated as a source of nutrients found in groundwater and the atmosphere. A wide range of regulations have been adopted to change farm management in response to concerns about these nutrient losses. Regulations usually rely on the voluntary adoption of farm practices or development of nutrient management plans by farmers that include explicit criteria for environmental protection. The definition of nutrient management can vary in scope and detail but often includes sustaining an increase in agricultural production while protecting the environment. Nutrient balance should prevent the application of nutrients at rates that will exceed the capacity of the soil and planned crops. Plants contain high-energy carbon, minerals and other nutrients required by animals. Animals rely on the energy of plants to grow and maintain themselves. Excretion of unavailable, unused, and recycled minerals by animals are in turn reused by plants to fix more energy for animal use. Mineral excretion in nature is critical to the process of energy capture by plants and animal growth. Concentrating animals in an area will tend to accumulate nutrients. Excess nutrients on a farm can be reduced by decreasing the imports of nutrients in excess of the potential crop utilization in the area. This can be done by limiting the nutrients in animal diets or by limiting the number of animals that are supported by external inputs so that nutrient balance goals for land application of manure are achieved. This approach can however, affect the competitiveness of individual farms by limiting their potential income and making it difficult for agribusiness's to survive. In order to stay competitive animal agriculture is being reclassified in terms of animal feeding operations (AFOs). A Comprehensive Nutrient Management Plan (CNMP) is a component that is unique to animal feeding operations. A CNMP is a grouping of conservation practices and management activities which, when combined into a system, will help to ensure that both production and natural resource goals are achieved. The goal of a CNMP as described in the Unified National Strategy for animal feeding operations is to minimize the adverse impacts of AFOs on water quality and public health. To accomplish this goal will require a significant increase in the intensity and comprehensiveness of technical assistance provided to producers. Some practices that reduce non-point source pollution are costly to implement. However, nutrient management usually requires a meager investment and may generate measurable returns via reduced nutrient purchases. There are many environmental benefits to following a nutrient management plan. Applying nitrogen during the time of crop uptake reduces the possibility of it running off or leaching into groundwater. Reducing excess phosphorus applications can result in improved surface water quality when phosphorus is the limiting nutrient in freshwater systems. This may also prevent aquatic plants from overpopulating a lake or stream. Rations play a great role in livestock nutrient output. Ration balancing models can assist producers in improving dietary efficiency to reduce nutrient output. Nutrient analysis of the feed, including forage grown on the farm, can provide the information needed to reduce purchased feeds and the input of some nutrients. Cornell's Net Carbohydrate and Protein System for Adequately Meeting the Requirements of Cattle model has successfully reduced off-farm feed costs, raised milk production, and lowered nutrient concentrations in manure. Several possibilities exist for a comprehensive manure management strategy. A key component of a nutrient management plan is to maximize nutrient recycling from animals to crops and back to animals again. Meeting crop nutrient requirements with manure is a key step in reducing commercial fertilizer purchases and may improve profitability. However, crop nutrient requirements as well as manure nutrient values must be known when substituting manure for commercial fertilizer. Nutrient concentrations in manure can vary a great deal from herd to herd. There are general guidelines to follow when substituting manure nutrients for fertilizer. Unless the manure is injected or immediately incorporated, the availability of nitrogen in the form of ammonium decreases daily. Another manure strategy may be to export all the manure nutrients to another farm. Installing treatment systems that convert the manure in to innocuous byproducts, nitrogen gas, water, or composted solids for sale might also be considered. Summary We have made tremendous progress in the area of rumen microbiology in the past several years. There is still much that we do not know about the microbial ecology that exists within the rumen. Molecular techniques to identify specific bacterial strains will undoubtedly contribute to our knowledge on the complex network of interrelated biochemical reactions in the rumen. With the current interest in this area across several scientific disciplines it is only a matter of time before we will have reliable models that will allow us to predict bacterial responses to dietary manipulations. Increasing awareness of the public to production agriculture, growing environmental concerns resulting in strict guidelines on nutrient disposal and the changing nature of the dairy business are factors that will continue to challenge ruminant nutritionists and microbiologists to better understand the biochemical processes that underlie rumen energetics. We know that biological reactions have built-in inefficiencies. With the state of current knowledge and the direction of future research in the area of microbial physiology the potential to lower inherent inefficient pathways and improve rumen function is enormous. Nutrients will accumulate where animals are concentrated because of the biology of animals. The growing awareness of the public is now generating concerns over the environmental consequences. The challenge for nutrient management consultants, extension educators, and producer advisors is to work with clients to develop integrated business management systems that will support the environment. These systems should include the strategic goals of agriculture. As consultants, and educators we need to provide essential assurance to farmers, agribusinesses, and other stakeholders about management outcomes.
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