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Animal Science Department, Cornell University, Ithaca, NY 14853
* Corresponding author (lot1{at}cornell.edu).
Received for publication September 23, 2002.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONTHE EFFECT OF IONOPHORES...EFFECTS OF IONOPHORES ON...EFFECTS OF IONOPHORES ON...ACCOUNTING FOR THE EFFECTS...SIMULATIONS OF THE EFFECT...FUTURE RESEARCH AND MODELINGCONCLUSIONSREFERENCES |
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Abbreviations: ADG, average daily gain • CNCPS, Cornell net carbohydrate and protein system • DMI, dry matter intake • ME, metabolizable energy • NEg, net energy for gain • NEm, net energy for maintenance • VFA, volatile fatty acids
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONTHE EFFECT OF IONOPHORES...EFFECTS OF IONOPHORES ON...EFFECTS OF IONOPHORES ON...ACCOUNTING FOR THE EFFECTS...SIMULATIONS OF THE EFFECT...FUTURE RESEARCH AND MODELINGCONCLUSIONSREFERENCES |
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Nutrients concentrate on livestock farms because more nutrients are imported as feed and fertilizer than are exported as products sold. Mass nutrient balances indicate that more than 60% of the N, P, and K imported per year as purchased feed remains on dairy farms regardless of their size (Klausner, 1993). In a study on a dairy farm in New York, 67 to 75% of the retained N (i.e., the surplus between inputs and products sold) was projected to escape into the off-farm environment (Hutson et al., 1998). About 10% of this excess N was predicted to leach into the ground water, with most of the rest being volatilized either as N2, N2O, NH3, or NOx compounds, some of which (NOx) can contribute to acid rain. Ammonia losses can represent as much as 70% of the N excreted by beef cattle in open feedlots (Council for Agricultural Science and Technology, 2002).
Implementation of comprehensive nutrient management plans on farms may improve efficiency of nutrient utilization, decrease imported nutrients, and nutrient loss to the environment while improving farm profitability (Klausner et al., 1998; Wang et al., 2000a, b). The major opportunity to reduce nutrient losses is through animal diet modification (Council for Agricultural Science and Technology, 2002). Absorbed protein (amino acids that are digested and absorbed in the small intestine) that is not synthesized into tissue or milk is excreted in the urine as urea N, which is converted to a volatile form (primarily NH3) and escapes to the environment. Therefore, the goal of the animal nutritionist in developing diets to reduce nutrients in manure is to accurately match dietary amount and sources of protein with animal requirements (Council for Agricultural Science and Technology, 2002; Klausner et al., 1998). Digestive and metabolic modifiers that improve N use efficiency in the rumen (e.g., ionophores; Bergen and Bates, 1984; Chen and Russell, 1989; Russell and Martin, 1984) or during metabolism (e.g., implants; Hancock et al., 1991) may reduce the amount of protein needed to meet animal requirements for either meat or milk production. An additional concern is to reduce the production of CH4 during ruminal fermentation, because of its role in the greenhouse effect (Moss, 1993).
Ionophores are molecules of various chemical structures that have the ability to entrap cations, usually Na+. They attach to the lipid bilayer of the cell membrane of ruminal gram-positive bacteria and protozoa (Chow et al., 1994), facilitating the net exchange of intracellular K+ for extracellular protons and Na+ across the membrane (Russell and Strobel, 1989). This forces gram-positive microorganisms to expel protons and Na+ at the expense of ATP (adenosine triphosphate), causing a depletion in the energy reserve, impaired cell division, and likely death of the microorganism (Russell and Strobel, 1989). The net effect is a change in the microbial ecosystem favoring those microorganisms, mostly gram-negative, that are not sensitive to the action of ionophores. Changes in fermentation dynamics in the rumen, when ionophores are added to the diet, improve the efficiency of energy capture and utilization of dietary N. Ionophores also have additional benefits in attenuating certain cattle digestive disorders (McGuffey et al., 2001), including bloat, an excess production of stable foam in the rumen, and acidosis, an accumulation of lactic acid and/or volatile fatty acids (VFA) in the rumen due to an increase of rapidly fermentable carbohydrates (grain) in the diet. It is generally recognized that the use of ionophores in cattle presents no hazard to human health arising from the potential to generate "resistant" foodborne bacteria, a topic reviewed in a separate review by Russell and Houlihan (2003). This is because ionophores are not used in human therapy due to their narrow therapeutic index, there is no genetic encoded resistance to their biophysical mechanism of action, and there is rapid cell death (Russell and Houlihan, 2003).
The main objective of this paper is to summarize published information on the effects of adding ionophores to cattle diets on animal performance and the environment. A secondary objective is to describe how a mechanistic model can account for ionophore effects on rumen fermentation and animal performance. Two case studies are used to demonstrate how our model is applied to predict the effect of feeding monensin on cattle performance and nutrients excreted.
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THE EFFECT OF IONOPHORES ON THE EFFICIENCY OF FEED USE IN MEAT AND MILK PRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONTHE EFFECT OF IONOPHORES...EFFECTS OF IONOPHORES ON...EFFECTS OF IONOPHORES ON...ACCOUNTING FOR THE EFFECTS...SIMULATIONS OF THE EFFECT...FUTURE RESEARCH AND MODELINGCONCLUSIONSREFERENCES |
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Monensin was registered on 16 Dec. 1975 with the commercial name of Rumensin (Elanco Products Co., a division of Eli-Lilly, Indianapolis, IN) for improving feed efficiency in feedlot cattle. Feed conversion (feed to gain ratio) improves when monensin is added to the diet because of a more efficient ruminal fermentation, resulting from an increased proportion of propionate to acetate in the rumen, a concomitant depression in CH4 production, and an inhibition of degradation of dietary protein in the rumen (Raun, 1990). An improvement in feed conversion results in less feed resources being used for the same meat and milk production. These benefits have a direct effect on protecting the environment due to decreased feed usage and reduced manure excretion (less intake and higher digestibility).
Table 1 contains a summary of published reviews in which the effects of monensin on beef cattle performance have been evaluated. These data show a consistent improvement in feed efficiency due to the addition of monensin to the diet of beef cattle. In grain-based feedlot diets, feed efficiency has been improved by reducing dry matter intake (DMI) with little or no effect on average daily gain (ADG). In contrast, in pasture-based diets, ADG has increased when monensin is fed. Goodrich et al. (1984) summarized 228 experiments conducted before 1984 with beef cattle fed monensin in diets averaging 3 Mcal of metabolizable energy (ME) kg-1 (likely to have at least 5–7% forage). Raun (1990) summarized 37 experiments with beef cattle fed high-concentrate diets (average 15.7% forage) conducted from 1981 to 1990 in which the average concentration of monensin was 28 mg kg-1. Monensin increased ADG by 1.6 to 1.8%, decreased DMI by 4 to 6.4%, and improved feed conversion by 5.6 to 7.5% in growing cattle fed in feedlots. The net effect of monensin was to maintain animal performance while reducing feed intake.
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On the other hand, the use of monensin in diets containing unsaturated lipids, usually from vegetable sources (e.g., cottonseed, soybean), may have an additional beneficial effect because monensin decreases the lipolysis of such lipids to free fatty acids (FFA) (Van Nevel and Demeyer, 1995b), which are highly toxic to the ruminal microbes. Scarce information contrasting the effect of lipid from plant sources and ionophores on animal performance is available.
Results of studies with lactating and dry (nonlactating) dairy cows fed monensin have been variable in terms of DMI and milk production responses, probably due to differences in stage of lactation. A reduction in DMI is a consequence of animals eating to their energy requirement. When in positive energy balance (late lactation or dry cows), the inclusion of monensin in the diet may increase energy available per unit of feed consumed (Mcal kg-1), resulting in lower DMI. However, when cows are in negative energy balance (early lactation) the additional energy available due to monensin is used to improve performance and/or reduce body reserve losses.
Ruiz et al. (2001) concluded that the addition of monensin improved milk production without affecting DMI and Ramanzin et al. (1997) concluded that monensin increased feed efficiency on average by 1.8% (DMI kg kg-1 of solids-corrected milk) due to a lower DMI (average across treatments of 5.6% DMI reduction) with no change in milk production. Based on several studies (Beckett et al., 1998; Lean and Wade, 1997; Moshen et al., 1981; Van der Werf et al., 1998), the National Research Council (2001) concluded that milk production is often increased as much as 3 kg d-1 for pasture-based diets when monensin is added to the diet of lactating cows. The results of Symanowski et al. (1999) and Wagner et al. (1999) represent a multiyear, multisite series of trials in which cattle were fed monensin throughout the productive cycle (two lactations), thus representing a steady state influence of monensin over time. Based on this information, the National Research Council (2001) concluded that the addition of monensin to the diets of dairy cows fed total mixed rations decreased DMI (
1 kg d-1), increased solids-corrected milk yield (1 kg d-1), increased efficiency of solids-corrected milk production, and resulted in less body condition loss during early lactation. Monensin-fed cattle had lower plasma non-esterified fatty acids (NEFA), indicating less fat mobilization during negative energy balance of early lactation. Further evidence of improved energy balance can be seen in reproductive responses. Tallam et al. (2002) concluded that cows fed a diet containing monensin ovulated for the first time postpartum five days earlier than controls, suggesting that nadir had been achieved earlier.
An evaluation of the effects of monensin on energy metabolism of the ruminant indicated that the ME value of the diet is increased due to increased dry matter digestibility and a shift in ruminal fermentation toward increased propionate production (Byers, 1980). This shift increased NEm supply by 7.2% with no effect on NEg, probably due to lower protein degradation rates at the tissue level because of increased propionic acid production when monensin was fed. Amino acids are spared from deamination and subsequent utilization for gluconeogenesis with more amino acids available to the animal (metabolic protein sparing). This conclusion was supported by the studies of Ørskov et al. (1979) in which monensin increased N retention in sheep when high levels of propionate were infused into the rumen.
There is also some indication that an ionophore in the diet decreases heat increment because propionate has a lower heat increment than acetate (Bergen and Bates, 1984; Ørskov et al., 1979), which would result in more available energy for production or less energy required for maintenance. These results would explain the reduction in monensin response with increasing ration ME value. As diet ME increases, more of the intake is available for production, which reduces the proportion of the diet used for maintenance. Byers (1980) concluded that monensin should increase feed efficiency by a higher percentage in diets where animals are fed closer to maintenance levels. This would at least partially explain the higher response to monensin in pasture-based diets (Table 1). Likewise, we could hypothesize that beef cattle may have a higher response than dairy cattle because their feed intake is closer to their maintenance requirement. In addition, diets with more grain shift the rumen microbial population toward more starch digesters, which produce a higher propionate to acetate ratio and less CH4, leaving less opportunity for monensin to improve fermentation efficiency. Since the main effect of monensin is to improve diet ME by increasing absorbed energy from the digestion of diet ingredients, the development of more mechanistic models may allocate the improved ME value to both NEm and NEg values of feeds.
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EFFECTS OF IONOPHORES ON METHANE PRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONTHE EFFECT OF IONOPHORES...EFFECTS OF IONOPHORES ON...EFFECTS OF IONOPHORES ON...ACCOUNTING FOR THE EFFECTS...SIMULATIONS OF THE EFFECT...FUTURE RESEARCH AND MODELINGCONCLUSIONSREFERENCES |
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In the rumen, hydrogen is produced during the anaerobic fermentation of glucose. This hydrogen can be used during the synthesis of VFA and microbial organic matter. The excess of hydrogen from NADH (nicotinamide adenine dinucleotide) is eliminated primarily by the formation of CH4 by methanogens, which are microorganisms from the Archea group that are normally found in the rumen ecosystem (Baker, 1999). The stoichiometric balance of VFA, CO2, and CH4 indicates that acetate and butyrate promote CH4 production whereas propionate formation conserves hydrogen, thereby reducing CH4 production (Wolin, 1960).
Decreasing the retention time of feed in the rumen may reduce CH4 production. Okine et al. (1989) indicated that a 30% decline in CH4 production was observed when the ruminal passage rate was increased by 50% or more. When expressed as a proportion of digestible energy, CH4 losses decreased 1.6 percentage units for each unit of increase in feed intake above the maintenance requirement (Johnson and Johnson, 1995). The addition of grain or soluble carbohydrates to the diet also changes the fermentation pattern in the rumen to a more competitive environment for the methanogens (Van Soest, 1994).
Ionophores decrease the production of CH4 due to a shift in the microbial population of the rumen from gram-positive to gram-negative bacteria. Bacteria that produce lactic, acetic, butyric, and formic acids and hydrogen as main end products are susceptible to ionophores whereas succinic and propionic acid-producing bacteria are resistant (Chen and Wolin, 1979). This causes a shift in fermentation end products to a higher propionate to acetate ratio. Rogers et al. (1997) indicated the decrease in acetate to propionate ratio ranged from 65 to 72% when monensin was added to the diet. After monensin was withdrawn, the ratio returned to preaddition values. However, Slyter et al. (1992) found no change in the ratio of VFA formed by pure cultures of several rumen bacteria when 2 µg mL-1 of monensin was added to the media. The average of six studies indicated that monensin can decrease CH4 emission by 25% (Van Nevel and Demeyer, 1995a). However, the inhibition of methanogenesis may not persist (Rumpler et al., 1986; Thornton and Owens, 1981) when monensin is withdrawn from the diet.
In a more comprehensive review, Van Nevel and Demeyer (1996) indicated that monensin clearly decreased CH4 production both in vitro by 32.1 ± 2.8% (n = 40) and in vivo by 20.6 ± 2.8% (n = 9); the extent of the inhibition was related to dose and type of ration. However, the inhibition of CH4 production when ionophores were used in long-term in vivo studies has not been consistent. Some findings (Johnson et al., 1994a; Johnson and Johnson, 1995; Rumpler et al., 1986; Sauer et al., 1998) have indicated that the decrease in CH4 production has not persisted. After short periods of time (up to 30 d) levels of CH4 production returned to pre-ionophore supplementation levels, probably due to an ability of the ruminal microflora to adapt to the ionophore (Johnson and Johnson, 1995). The shift in VFA profile (higher percentage of propionate) was maintained for long periods of time (Mbanzamihigo et al., 1995; Richardson et al., 1976; Rogers et al., 1997), indicating that the uncoupling effect of ionophores on CH4 production and VFA profile is not in agreement with the stoichiometrical relationship of the ruminal fermentation (Wolin, 1960), as long as no other means to eliminate the excess of H+ in the rumen is provided (Hino and Asanuma, 2003). On the other hand, several other studies (Davies et al., 1982; Mbanzamihigo et al., 1995, 1996; Rogers et al., 1997) have shown a long-term (40–240 d) effect of monensin on CH4 depression with no evidence of microbial adaptation.
Possible explanations for these contrasting findings are the use of different methods to assess CH4 production (Johnson and Johnson, 1995). Some studies that reported a short-term effect of ionophore on CH4 production used respiration chambers (Kinsman et al., 1995; Rumpler et al., 1986; Sauer et al., 1998) to collect CH4 from the rumen and hindgut compartments. This suggested that some CH4 may have been produced in the hindgut (Siciliano Jones and Murphy, 1989) when high levels of grain were fed, which compensated for the depression of CH4 in the rumen. Another explanation may be the proportion of concentrate to forage, partially supporting the previous explanation. However, Mbanzamihigo et al. (1996) reported that inhibition of methanogenesis in the rumen cannot be compensated by CH4 production in the hindgut due to a shift in digestion from rumen to hindgut in animals supplemented with monensin. This is probably because of the difference in size and anatomy of these compartments (rumen and hindgut), different passage rates (turnover is greater in the hindgut), and a possible residual effect of the ionophore in the hindgut.
Other studies have used sulfur hexafluoride (SF6) gas as a tracer to assess CH4 and CO2 production (Johnson et al., 1994a,b). Even though the average daily CH4 emissions predicted from this technique (11.6 ± 0.7 L h-1) were similar to those measured using open circuit respiration calorimetry (12.9 ± 0.7 L h-1) (Johnson et al., 1994b), variation among animals has been larger in the SF6 (11.7%) than in the calorimetry technique (0.1%) (Boadi et al., 2001), which is in agreement with several other reports (Lassey et al., 1997; Leuning et al., 1999; Ulyatt et al., 1999), suggesting that more animals per treatment and sufficient collection days per animal are required to minimize day-to-day variation and to detect treatment differences in experiments.
Even if the adaptation of the ruminal microflora to ionophores exists, the depression in CH4 production is likely to always occur due to the decrease in DMI. O'Kelly and Spiers (1992) reported that the decrease in DMI was solely responsible for 55% of the depression in CH4 production.
Even though the findings are conflicting, the data generally indicate that the widespread use of ionophores in cattle diets may decrease the amount of CH4 per unit of product (meat, milk, fiber, etc.), reducing the emission of CH4 to the atmosphere from cattle production, either by changing the fermentation pattern in the rumen or by decreasing feed intake. Other alternatives such as those discussed by Hino and Asanuma (2003) may be used in combination with ionophores to more efficiently mitigate the CH4 production by ruminants.
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EFFECTS OF IONOPHORES ON PROTEIN METABOLISM IN THE RUMEN |
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TOPNOTESABSTRACTINTRODUCTIONTHE EFFECT OF IONOPHORES...EFFECTS OF IONOPHORES ON...EFFECTS OF IONOPHORES ON...ACCOUNTING FOR THE EFFECTS...SIMULATIONS OF THE EFFECT...FUTURE RESEARCH AND MODELINGCONCLUSIONSREFERENCES |
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-amino-N and peptides. Russell et al. (1988) identified species of bacteria that had 18 to 39 times higher NH3 producing ability than previously known species in the rumen. These bacteria [classified as gram-negative strains of Peptostreptococcus and Clostridium species; Paster et al. (1993)] required an amino acid source for growth and were sensitive to monensin. Other studies have shown a 10-fold decrease in ruminal bacteria that utilize amino acids and peptides as their primarily source of N for growth when monensin was given to holstein cows (Yang and Russell, 1993). These findings suggest that a greater proportion of dietary true protein (i.e., amino acids) may escape the rumen (ruminal protein sparing) when monensin is added to the diet (Faulkner et al., 1985; Muntifering et al., 1981). Other studies have reported mixed results regarding protein sparing in the rumen (Rogers et al., 1997) but total tract N digestibility was consistently increased. On average, the addition of monensin increased N digestibility by 3.5 percentage units in cattle (Spears, 1990). Muntifering et al. (1980) have reported an increase in the amount of N that was retained from 18.6 to 23.6 g d-1 (26.9%), which represented an increase of 18.1% as percentage of the N intake (20.4 vs. 24.1%; respectively) and 12.2% as percentage of the absorbed N (36.2 vs. 40.6%; respectively). The increase in crude protein (CP) digestibility was 4.4% (56.5 vs. 59%; Muntifering et al., 1980). Because the digestibility of dietary protein is usually greater than bacterial protein (Van Soest, 1994), the increase in retained N may be explained by the greater amount of dietary protein escaping the rumen compared with bacterial protein. Rogers et al. (1997) found that monensin decreased the population of some species of protozoa (Entodinium spp. and Enoplopastron spp.) in the rumen, which is in agreement with the findings of Poos et al. (1979) and Hino (1981); however, an adaptation of protozoal activity to monensin was observed in a long-term treatment (240 d) (Rogers et al., 1997). The increase in flow of bacterial N to the duodenum was also observed by Rogers et al. (1997). These findings suggest that despite the reduction in microbial protein as shown in in vitro trials, a lower predation of bacteria by protozoa would increase the bacteria yield and improve fermentation in the rumen in vivo.
Based on the modifications in N metabolism caused by ionophores at the rumen (Chen and Russell, 1991; Krause and Russell, 1996) and tissue (Muntifering et al., 1981; Spears, 1990) levels, it is likely that ionophores may indirectly contribute to decreased N release into the environment because of a lower amount of N excreted when the diet contains an ionophore. Some studies have suggested that monensin may decrease N lost in the urine (Beede et al., 1986; Flachowsky and Richter, 1991) while others have found an increase in urinary N (Vijchulata et al., 1980). Because ionophores may increase the amount of N (i.e., amino acids) at the tissue level, the amount of N excreted via urine is likely to increase if the amino acid profile is inadequate to support the production level allowable by the energy supply. Therefore, the strategic use of ionophores to increase N retention and minimize N excretion has to be coupled with an adequate and correct nutrition program specific for each situation. We were not aware of any experiment that has been conducted to further investigate this hypothesis and to additionally consider the effect of ionophores on the fate of N once it is eliminated via manure and subsequently applied to land.
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ACCOUNTING FOR THE EFFECTS OF IONOPHORES IN DIET FORMULATION |
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TOPNOTESABSTRACTINTRODUCTIONTHE EFFECT OF IONOPHORES...EFFECTS OF IONOPHORES ON...EFFECTS OF IONOPHORES ON...ACCOUNTING FOR THE EFFECTS...SIMULATIONS OF THE EFFECT...FUTURE RESEARCH AND MODELINGCONCLUSIONSREFERENCES |
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Fox and Black (1984) developed adjustment factors for DMI and apparent feed NEm and NEg values when monensin is fed, based on DMI, ADG, and feed conversion (feed to gain ratio, F/G) reported for the original 19 trial summary of feedlot studies with monensin (Elanco, 1978). The adjustments for the inclusion of monensin in the diet were as follows: DMI was reduced by 6% for monensin in the diet at 22 mg kg-1 and by 10% for monensin at 33 mg kg-1. Feed NEm and NEg were increased by 6% when monensin was fed at 22 mg kg-1 and by 11% for monensin at 33 mg kg-1.
Subsequently, these adjustments have been refined based on later studies with larger and more complete databases. In initial experiments, molar percentages of propionic acid were increased 5.6% and acetic and butyric acids were decreased 4 and 1.1%, respectively, when monensin was added to the diet. This shift in ruminal fermentation resulted in 3.1% more energy recovered from hexose (Raun, 1990). This improvement in fermentation efficiency would result in 5.7% more ME available and 5.5% less feed required for gain. However, the ME of the diet should be increased by 14.9% to match the predicted and observed ADG, which is considerably larger than the 3.1% based on ruminal VFA change (Raun, 1990). This suggests that the in vivo actual increase of propionic acid production may be two to four times greater than the propionic acid molar percentage concentration change, which would result in fermentation efficiency improvement accounting for all of the improvement in feed efficiency (Prange et al., 1978; Richardson et al., 1976; Van Maanen et al., 1978). Based on heats of combustion of hexose and CH4, monensin-supplemented diets increased fermentation efficiency by 6.3% in holstein steers weighing 290 kg (Armentano and Young, 1983).
We developed adjustments to DMI and feed NE values when monensin is added to the diet of growing cattle based on the analysis of Byers (1980), and on the summaries of research data by Goodrich et al. (1984) and Raun (1990). Calculations indicated that the diet NEm value would have to be increased 20% to predict the experimental results summarized by Goodrich et al. (1984), in which the diets averaged 32 mg kg-1 monensin (DMI reduction of 6.4% and improvement in ADG of 1.6% and feed efficiency by 7.5%). Our calculations indicated that the diet NEm would have to be increased by 12% to predict the experimental results summarized by Raun (1990) in which the diets averaged 28 mg kg-1 in the dry matter (DMI reduced 4%, ADG was increased 1.8%, and F/G was improved 5.6%).
Using these adjustments, the data shown in Table 2 were obtained with simulations of two different levels of concentrate in the diet. Based on our simulations, the National Research Council (1996)( 2000) recommended that diet NEm be increased by 12% to account for the effect of any ionophore and DMI be reduced 4% when monensin is fed at 28 to 33 mg kg-1.
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The CNCPS model does not include any adjustments for ionophores for lactating dairy cattle since this is not an approved use in the USA (McGuffey et al., 2001). In the CNCPS model (Fox et al., 2003), the effects of ionophores are accounted for in formulating diets for growing cattle by using National Research Council (1996)( 2000) recommendations. The following computational procedures are applied when an ionophore is added to the diet that does not have supplemental animal fat added: (i) DMI is predicted with the National Research Council (1996)(2000) equations, using diet NEm unadjusted for ionophores, then is reduced 4% when monensin is fed at 28 to 33 mg kg-1; (ii) diet NEm used to compute feed required for maintenance is increased 12% if an ionophore is fed; (iii) peptide uptake rate is reduced one-third if monensin is fed; (iv) the concentration of nutrients needed in the DMI adjusted for ionophores to support the energy allowable ADG or milk production is determined; and (v) the nutrients excreted are predicted.
Lana et al. (1997) evaluated the CNCPS model containing these adjustments and found that monensin increased the efficiencies of feed and nitrogen utilization in all treatments. Ruiz et al. (2001) evaluated this model in lactating dairy cows and reported that monensin increased milk production by 1.85 kg d-1, decreased fecal N by 13.6% (21.1 g d-1), and increased urinary N by 6% (9.1 g d-1) with an overall manure N reduction of 4% (12 g d-1).
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SIMULATIONS OF THE EFFECT OF MONENSIN IN REDUCING RESOURCE USE AND NITROGEN INPUTS TO THE ENVIRONMENT |
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TOPNOTESABSTRACTINTRODUCTIONTHE EFFECT OF IONOPHORES...EFFECTS OF IONOPHORES ON...EFFECTS OF IONOPHORES ON...ACCOUNTING FOR THE EFFECTS...SIMULATIONS OF THE EFFECT...FUTURE RESEARCH AND MODELINGCONCLUSIONSREFERENCES |
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Dairy Farm Case Study
In approved experiments, diet concentrations of monensin for lactating dairy cattle (10–15 mg kg-1) are at least twice lower than used in beef feedlot situations (25–33 mg kg-1) (Feed Additive Compendium, 2002); therefore, the 12% adjustment to diet NEm probably represents an overcorrection for typical lactating dairy cow diets. Since we have not modeled the effects of feeding monensin to dairy cattle, we used data summarized from the trials of Ramanzin et al. (1997), Symanowski et al. (1999), and Ruiz et al. (2001) to predict the effect of feeding monensin to lactating dairy cattle (Table 4). On average, monensin decreased DMI by 3% and milk fat by 1.2%, and increased milk production by 2.6%, milk protein by 1.3%, and milk production efficiency (kg kg-1) by 5.7%. The increases in milk fat and milk protein represent lower milk fat and protein concentrations, but due to the 2.6% higher milk volume, total fat and protein production is increased. When these values were extrapolated to all milking cows in the United States (approximately 9210000 cattle; USDA, 2001) feed intake would be reduced by approximately 2.126 Tg yr-1 and milk production increased by 2.311 Tg yr-1. Typical lactating dairy cow diets contain 18% crude protein. Assuming dietary crude protein contains 16% N, the 3% reduction in feed intake (Table 4) represents a 61.2 Gg yr-1 reduction in N intake (2.126 Tg feed yr-1 x 0.18 g protein g-1 feed x 0.16 g N g-1 protein = 61.23 Gg N yr-1). Assuming milk protein contains 15.7% N and milk protein is augmented by 11 g d-1 when using monensin (Table 4), the increase in milk N production would be 5.81 Gg yr-1 (11 g protein d-1 x 0.157 g N g-1 protein x 365 d yr-1 x 9210000 cows = 5.806 Gg N yr-1). Therefore, the inclusion of monensin may decrease N excretion by 67 Gg yr-1. Fox et al. (2002) and Wang et al. (1999) reported that ground and surface water nutrient levels increase in direct proportion to the amount of manure (and manure nutrients) applied to the land. Since less nutrients are fed (and excreted) when monensin is fed to lactating dairy cattle, nutrient loading rates would decline, thus aiding in the better utilization of remaining nutrients.
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FUTURE RESEARCH AND MODELING |
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TOPNOTESABSTRACTINTRODUCTIONTHE EFFECT OF IONOPHORES...EFFECTS OF IONOPHORES ON...EFFECTS OF IONOPHORES ON...ACCOUNTING FOR THE EFFECTS...SIMULATIONS OF THE EFFECT...FUTURE RESEARCH AND MODELINGCONCLUSIONSREFERENCES |
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A more mechanistic model can be used to compute CH4 production in each unique production situation, given the VFA production profile (Wolin, 1960) as shown by Benchaar et al. (1998). A simpler approach is to compute CH4 production empirically based on diet carbohydrate fractions such as neutral detergent soluble and neutral detergent fiber compounds. Johnson and Ward (1996) compared CH4 production using three empirical equations; the equation using sugars and starch could account for 82% of the variation in CH4 production. Level of intake can also be used to discount CH4 production (Blaxter and Clapperton, 1965). Once CH4 is estimated, the equivalent energy can be added or subtracted from the digestible energy; the heat of combustion of CH4 is 0.211 Mcal mol-1 (Benchaar et al., 1998), leading to an adjusted digestible energy for CH4. This approach is more mechanistic than using an invariable 12% to account for increases in diet NEm due to CH4 reduction caused by ionophores.
Although the majority of CH4 production occurs in the rumen, a large amount may be produced in the hindgut if starch escaping from the rumen is increased by feeding ground or pelleted diets. Increases in VFA production in the hindgut were observed with high grain content diets (Siciliano Jones and Murphy, 1989), which indirectly increased CH4 production in the hindgut. Immig (1996) reported that 28 to 592 mmol d-1 of CH4 may be produced in the hindgut, accounting for 6 to 14% of total CH4 production. Ionophores can equally control CH4 production in the hindgut (Mbanzamihigo et al., 1996) via acetate to propionate ratio change; therefore, a hindgut model is an essential component to adequately estimate ruminant CH4 production and minimize it.
The mechanisms controlling DMI are complex and not fully understood. The decrease in DMI associated with feeding an ionophore may be more multifaceted than currently modeled by the National Research Council (2000). It is well documented that ionophores increase the proportion of propionate to acetate. This may partially explain the reduction in DMI, since ruminal infusion of propionate reduced intake more than acetate infusion in mid-lactation holstein dairy cows (Sheperd and Combs, 1998). Additionally, Anil and Forbes (1980)( 1988) demonstrated that infusion of propionate into the portal vein had a noticeable depression effect on feed intake. In ruminants, in response to nutritional and hormonal regulation, gluconeogenesis from propionate is the major source of glucose (Danfær et al., 1995) depending on the availability of propionate. The increase in glucose in the blood may in part explain animal satiety and consequent reduction in intake in nonruminants; however, apparently the infusion of glucose either into the portal or jugular veins produced no effect on feed intake for ruminants (Van Soest, 1994). Reduction in intake has also been observed when ruminal osmolality increases (Forbes et al., 1992; Villalba and Provenza, 1996).
Another factor that governs intake of forage diets is the physical distension of the rumen (Mbanya et al., 1993); the degree of rumen fill is affected by the rate feed is degraded and escapes the rumen (Clark and Armentano, 1997). There has been some indication that ionophores decrease fluid passage rate, as determined in both in vivo (Lemenager et al., 1978; Pordomingo et al., 1999) and in vitro methods (Stanier and Davies, 1981), but results have been mixed in grazing animals (Branine and Galyean, 1990; Ward et al., 1990a,b).
Based on the discussion above, it is clear that a large amount of data has been accumulated regarding the effects of ionophores, more specifically monensin, in animal diets on performance and ruminal fermentation pattern; however, little information regarding ionophores and animal metabolism is available. Data modeling and simulation when formulating diets can be used to direct future research and to apply current knowledge to better account for the effects of ionophores and other compounds on CH4 and N production and potential losses to the environment.
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TOPNOTESABSTRACTINTRODUCTIONTHE EFFECT OF IONOPHORES...EFFECTS OF IONOPHORES ON...EFFECTS OF IONOPHORES ON...ACCOUNTING FOR THE EFFECTS...SIMULATIONS OF THE EFFECT...FUTURE RESEARCH AND MODELINGCONCLUSIONSREFERENCES |
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Our simulations indicated that if ionophores were withdrawn from all beef cattle diets in the USA, N excretion to the environment would be increased by 9.8 Gg yr-1. Feed intake would increase by 1.38 Tg yr-1 of dry matter, which corresponds to an increase in 135000 to 140000 ha of corn field for grain production. Additionally, if monensin was fed to lactating dairy cows, a reduction in 67 Gg yr-1 of N was estimated by our simulations. Because a high proportion of N from urine is volatilized, reductions in total manure N excreted would benefit air quality in areas with high concentrations of livestock.
The inclusion of ionophores in cattle diets to reduce CH4 emission to the atmosphere, improve protein retention and efficiency (thus reducing N excretion), and decrease feed resources consumed while increasing or maintaining the cattle performance (meat and milk) is consistent with the development of more sustainable and environmentally friendly cattle production systems.
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TOPNOTESABSTRACTINTRODUCTIONTHE EFFECT OF IONOPHORES...EFFECTS OF IONOPHORES ON...EFFECTS OF IONOPHORES ON...ACCOUNTING FOR THE EFFECTS...SIMULATIONS OF THE EFFECT...FUTURE RESEARCH AND MODELINGCONCLUSIONSREFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONTHE EFFECT OF IONOPHORES...EFFECTS OF IONOPHORES ON...EFFECTS OF IONOPHORES ON...ACCOUNTING FOR THE EFFECTS...SIMULATIONS OF THE EFFECT...FUTURE RESEARCH AND MODELINGCONCLUSIONSREFERENCES |
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