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Journal of Animal Science - Animal Nutrition

Quantifying the effect of monensin dose on the rumen volatile fatty acid profile in high-grain-fed beef cattle1


This article in JAS

  1. Vol. 90 No. 8, p. 2717-2726
    Received: Feb 14, 2011
    Accepted: Feb 27, 2012
    Published: January 20, 2015

    2 Corresponding author(s):

  1. J. L. Ellis 2,
  2. J. Dijkstra*,
  3. A. Bannink,
  4. E. Kebreab§,
  5. S. E. Hook,
  6. S. Archibeque# and
  7. J. France
  1. *Animal Nutrition Group, Wageningen University, Wageningen 6708 WD, the Netherlands
    †Centre for Nutrition Modelling, Department of Animal and Poultry Science, University of Guelph, Guelph, ON N1G 2W1, Canada
    ‡Wageningen UR Livestock Research, Wageningen University Research Centre, Lelystad 8200 AB, the Netherlands
    §Department of Animal Science, University of California, Davis 95616; and
    #Animal Sciences, Colorado State University, Fort Collins 80523


Monensin is a common feed additive used in various countries, where 1 of the associated benefits for use in beef cattle is improved efficiency of energy metabolism by the rumen bacteria, the animal, or both. Modeling fermentation-altering supplements is of interest, and thus, it is the purpose of this paper to quantify the change in VFA profile caused by monensin dose in high-grain-fed beef cattle. The developmental database used for meta-analysis included 58 treatment means from 16 studies from the published literature, and the proportional change in molar acetate, propionate, and butyrate (mol/100 mol) as well as total VFA (mM) with monensin feeding dose (mg/kg DM, concentration in the feed) was evaluated using the MIXED procedure (SAS Inst. Inc., Cary, NC) with the study treated as a random effect. The mean monensin dose in the literature database was 30.9 ± 3.70 mg/kg DM and ranged from 0.0 to 88.0 mg/kg DM. Mean DMI was 7.8 ± 0.26 kg DM/d, mean concentrate proportion of the diet was 0.87 ± 0.01, and mean treatment period was 42 ± 5.6 d. Results produced the following equations: proportional change in acetate (mol/100 mol) = −0.0634 (± 0.0323) × monensin (mg/kg DM)/100 (P = 0.068), proportional change in propionate (mol/100 mol) = 0.260 (± 0.0735) × monensin (mg/kg DM)/100 (P = 0.003), and proportional change in butyrate (mol/100 mol) = −0.335 (± 0.0916) × monensin (mg/kg DM)/100 (P = 0.002). The change in total VFA was not significantly related to monensin dose (P = 0.93). The results presented here indicate that the shift in VFA profile may be dose dependent, with increasing propionate and decreasing acetate and butyrate proportions (mol/100 mol). These equations could be applied within mechanistic models of rumen fermentation to represent the effect of monensin dose on the VFA profile in high-grain-fed beef cattle.


Monensin (Elanco Animal Health, Ontario, Canada) is a common feed additive used in North America, Australia, and New Zealand, where the associated benefits for use in cattle include improved efficiency of energy and N metabolism by the rumen bacteria, the animal, or both, and retardation of digestive disorders resulting from abnormal rumen fermentation (Bergen and Bates, 1984). Positive effects on energetic efficiency are the result of increased propionate production in the rumen as a consequence of a general resistance to monensin of gram-negative bacteria that reduce succinate to propionate, whereas a reduction in population size and activity occurs in gram-positive bacteria groups (McGuffey et al., 2001). Protozoa and fungi tend to be inhibited (Bergen and Bates, 1984), and Gyulai and Baran (1988) showed that some strains are more sensitive to monensin than others, although the data for fungi are more sparse and contradictory than for rumen bacteria (McGuffey et al., 2001).

An increase in propionate production also results in a decrease in available substrate for methanogens, mainly hydrogen and formate, and reduces the amount of energy lost as methane (CH4; reviewed by Ellis et al., 2008), and there has been some interest in the potential use of monensin as a CH4 mitigation strategy (Beauchemin et al., 2009), although in vivo results are inconclusive.

Dynamic mechanistic models have been used increasingly to explore mitigation and feeding strategies and to aid in the development of future experiments (Benchaar et al., 2001; Bannink et al., 2010; Ellis et al., 2011, 2012), but to date, none have accounted for the use of non-nutritional supplements such as monensin. Because most high-grain-fed beef cattle trials currently being conducted at North American institutions include monensin in the diet, one either has to exclude these data from exercises of model development and challenge or has to discount the potential effect monensin will have on the VFA stoichiometry. It is therefore the purpose of this study to quantify the change in VFA profile with monensin dose for high-grain-fed beef cattle.


Animal Care and Use Committee approval was not obtained for this study because the data were obtained from existing literature.

Literature Database

The database used for this evaluation was constructed from a systematic literature search and, after application of various criteria for selection, comprised 58 treatment means from 16 studies spanning from 1976 to 2009. It included the studies of Ralston and Davidson (1976), Raun et al. (1976), Richardson et al. (1976), Utley et al. (1977), Heinemann et al. (1978), van Maanen et al. (1978), Thorton and Owens (1981), Zinn (1987), Morris et al. (1990), Clary et al. (1993), Zinn and Borques (1993), Zinn et al. (1994), Surber and Bowman (1998), Ives et al. (2002), Fandiño et al. (2008), and Meyer et al. (2009). The criteria for study selection were that the experiment involved beef cattle, that monensin was a treatment comparable against the appropriate control diet, that the basal diet was at least 80% grain, and that information was available on monensin dose (mg/d), DMI (kg/d), and ruminal acetate, propionate and butyrate expressed as a fraction of total VFA,or as mM. There was a preference for studies that also reported total VFA (tVFA, mM) so that the effect of monensin and tylosin (Elanco Animal Health) on tVFA could be evaluated, but Ralston and Davidson (1976), van Maanen et al. (1978), Zinn (1987), and Zinn and Borques (1993) did not report this information. Some studies included tylosin as an additive with monensin, while others included it with both monensin and control treatments. The mean monensin dose in the literature database was 30.9 ± 3.70 mg/kg DM and ranged from 0.0 to 88.0 mg/kg DM, mean DMI was 7.8 ± 0.26 kg DM/d, mean concentrate proportion of the diet was 0.87 ± 0.01, and mean treatment period was 42 ± 5.6 d. A summary of the database is presented in Table 1, and a summary of the VFA profile for each study is given in Table 2.

View Full Table | Close Full ViewTable 1.

Description of the database

Data source Monensin doses, mg/kg DM DMI1, kg/d Concentrate in diet, % BW, kg Tylosin, mg/kg DM Treatment period, d Basal diet, % DM
Ralston and Davidson (1976) 0.0, 5.3, 11.4, 23.9, 36.2 8.86 ± 0.254 100 386 0.0 152 83.0% steam rolled barley, 10.0% beet pulp, 5.0% molasses, 1.0% protein supplement, and 1.0% oyster shell flour
Raun et al. (1976) 0.0, 2.7, 5.5, 11, 22, 33, 44, 88 8.84 ± 0.284 97 363 0.0 28 Diet 1 (CA-40): 61.2% corn, 20.0% corn cobs, 3.1% alfalfa meal, 8.7% soybean meal, 5.0% molasses, and 2.0% vitamins and minerals
Diet 2 (CA-90): 70.0% corn, 10.0% corn cobs, 5.0% alfalfa meal, 8.0% soybean meal, 5.0% molasses, and 2.0% vitamins and minerals
Richardson et al. (1976) 0.0, 14.5, 79 6.70 ± 0.185 95 415 ± 4.0 0.0 10 to 30 CA-90 diet: 70.0% corn, 10.0% corn cobs, 5.0% alfalfa meal, 8.0% soybean meal, 5.0% molasses, and 2% vitamins and minerals
Utley et al. (1977) 0.0, 85 6.63 ± 0.735 100 266 ± 4.3 0.0 84 Diet 1: 74.0% ground shelled corn, 20.0% peanut hulls, 6.0% supplement
Diet 2: 75.8% acid-treated rolled corn, 18.6% peanut hulls, 5.6% supplement2
Heinemann et al. (1978) 0.0, 5.5, 11, 33 9.76 ± 0.207 82 446 ± 1.7 5.53 85 66.4% ground corn, 15.0% dehydrated beet pulp with molasses, 18.0% sun-cured alfalfa hay, and 0.6% vitamin/mineral
van Maanen et al. (1978) 0.0, 39 3.85 ± 0.000 80 155 to 253 0.0 35 64.0% cracked corn, 16.0% protein supplement, and 20.0% chopped alfalfa hay
Thorton and Owens (1981) 0.0, 55 4.10 ± 0.000 94 375 0.0 15 62.8% rolled corn grain, 10.0% soybean meal, 6.0% alfalfa meal, 5.0% molasses, 14.0% cottonseed hulls, and 2.2% vitamins and minerals
Zinn (1987) 0.0, 33 6.57 ± 0.427 80 295 ± 7.3 0.0, 114 14 50.6% corn, 1.0% barley, 20.0% whole cottonseed, 6.0% molasses 15.0% sudangrass, 5.0% alfalfa hay, and 2.4% vitamins and minerals
Morris et al. (1990) 0.0, 29 9.31 ± 0.052 95 512 ± 0.2 0.0, 113 18 76.9% steam-flaked milo, 5.0% ground alfalfa cubes, 5.0% cottonseed hulls, 1.5% yellow grease, 7.0% molasses, 1.0% premix, 1.0% urea, and 2.6% vitamins and minerals5
Zinn and Borques (1993) 0.0, 33 3.99 ± 0.000 88 180 0.0 14 74.9% steam-flaked corn, 6.0% alfalfa hay, 6.0% sudangrass hay, 4.0% yellow grease, 6.0% molasses, 1.1% urea, and 2.0% vitamins and minerals
Clary et al. (1993) 0.0, 25 10.9 ± 0.09 90 430 0.0, 103 19 78.0% rolled corn, 10.0% prairie hay, and 8.0% supplement
Diet 1: 0.0% tallow and 4.0% molasses
Diet 2: 4.0% tallow and 0.0% molasses
Clary et al. (1993) 0.0, 25 10.6 ± 0.02 90 430 0.0, 103 19
Zinn et al. (1994) 0.0, 28 4.82 ± 0.014 90 234 0.0 14 Diet 1: 79.8% steam-flaked corn, 2.5% alfalfa hay, 7.5% sudangrass hay, 3.0% yellow grease, 4.0% molasses, 1.1% urea, and 2.1% vitamins and minerals
Diet 2: 69.8% steam-flaked corn, 5.0% alfalfa hay, 15.0% sudangrass hay, 3.0% yellow grease, 4.0% molasses, 1.1% urea, and 2.1% vitamins and minerals
Zinn et al. (1994) 0.0, 28 4.83 ± 0.001 80 234 0.0 14
Surber and Bowman (1998) 0.0, 41.6 6.50 ± 0.011 80 to 87 430 112 21 Diet 1: 70.0% cracked corn, 20.5% grass hay, 3.8% soybean meal, 1.2% dry molasses, 1.0% urea, 0.2% canola oil, and 3.3% vitamins and minerals
Diet 2: 80.0% cracked barley, 12.7% grass hay, 2.1% soybean meal, 1.3% dry molasses, 1.0% urea, 0.2% canola oil, and 2.7% vitamins and minerals1
Ives et al. (2002) 0.0, 32 7.99 ± 0.230 90 345 0.0, 12.93 21 Diet 1: 72.1% rolled corn, 12.0% soybean meal, 10.0% alfalfa, 4.0% molasses, and 1.9% vitamins and minerals
Diet 2: 62.9% rolled corn, 30.0% wet corn gluten feed, 5.0% alfalfa, and 2.1% vitamins and minerals
Ives et al. (2002) 0.0, 32 8.47 ± 0.100 95 345 0.0, 11.73 21
Fandiño et al. (2008) 0.0, 31.3 7.55 ± 0.050 90 NA 0.0 24 27.9% ground barley grain, 25.6% ground corn grain, 10.9% soybean meal, 6.7% soybean hulls, 5.7% corn gluten feed, 5.7% tapioca, 7.2% sunflower meal, 10.0% barley straw, and 0.3% vitamins and minerals
Meyer et al. (2009) 0.0, 26.4 8.15 ± 0.450 93 512 0.0 28 66.0% high-moisture corn, 16.5% dry-rolled corn, 7.5% alfalfa hay, 5.0% molasses, and 5.0% supplement
1 Mean ± SEM.
2Volatile fatty acid results are the mean over these 2 diets.
3Tylosin (Elanco Animal Health, Ontario, Canada) was included in both the control and monensin (Elanco Animal Health) diets.
4Zero tylosin was included in the control diet, but tylosin was included in the monensin diet.
5As-fed basis.

View Full Table | Close Full ViewTable 2.

Mean VFA profile, by study, for the database1

Data source tVFA2, mM Acetate, mol/100 mol tVFA Propionate, mol/100 mol tVFA Butyrate, mol/100 mol tVFA
Ralston and Davidson (1976) 46.1 ± 0.76 48.4 ± 0.99 5.5 ± 0.32
Raun et al. (1976) 80.1 ± 0.91 48.4 ± 0.37 40.2 ± 0.44 7.4 ± 0.35
Richardson et al. (1976) 83.2 ± 2.57 51.0 ± 2.52 38.8 ± 3.52 5.7 ± 0.70
Utley et al. (1977) 101.0 ± 2.40 53.1 ± 0.15 33.4 ± 2.70 10.1 ± 2.00
Heinemann et al. (1978) 64.0 ± 1.81 60.9 ± 1.16 32.3 ± 1.70 6.8 ± 0.60
van Maanen et al. (1978) 64.7 ± 1.80 25.2 ± 2.75 10.2 ± 0.90
Thorton and Owens (1981) 132.9 ± 24.4 70.3 ± 4.00 17.3 ± 2.80 6.4 ± 0.15
Zinn (1987) 65.1 ± 0.15 24.0 ± 0.10 11.0 ± 0.25
Morris et al. (1990) 129.0 ± 2.15 46.5 ± 1.95 39.6 ± 0.35 9.9 ± 2.05
Zinn and Borques (1993) 49.3 ± 1.35 40.6 ± 3.05 10.2 ± 1.75
Clary et al. (1993) 111.4 ± 2.26 49.5 ± 2.14 37.4 ± 2.83 8.7 ± 0.13
Zinn et al. (1994) 123.0 ± 5.93 49.6 ± 1.86 39.0 ± 1.49 8.4 ± 0.57
Surber and Bowman (1998) 51.4 ± 0.69 57.5 ± 0.89 20.1 ± 1.24 15.8 ± 0.59
Ives et al. (2002) 101.7 ± 3.89 52.3 ± 1.39 26.7 ± 1.09 15.7 ± 0.38
Fandiño et al. (2008) 110.7 ± 1.90 54.9 ± 0.40 26.4 ± 1.15 12.6 ± 0.40
Meyer et al. (2009) 107.0 ± 2.25 50.5 ± 0.40 30.7 ± 2.15 12.1 ± 0.85
1Mean ± SEM.
2tVFA = total VFA.


The change in tVFA (mM) and the proportional change in acetate, propionate, and butyrate (in mol/100 mol tVFA) relative to the control diet were examined against the main effects of monensin dose (mg/kg DM) and tylosin dose (mg/kg DM) within the MIXED procedure (SAS Inst. Inc., Cary, NC). Mixed model analysis was chosen because the data were compiled from multiple studies, thereby making it necessary to consider analyzing not only the fixed effects of the dependent variables but also the random effect of study. This accounts for differences such as physiological status of the animals, experimental design, measurement methods, techniques, and varying laboratories (St-Pierre, 2001).

In several studies in the developmental database, tylosin was included either in all treatment diets (Heinemann et al., 1978; Surber and Bowman, 1998) or only in the monensin diet and excluded from control diets (Zinn, 1987; Morris et al., 1990; Clary et al., 1993; Ives et al., 2002). Tylosin is an antibiotic commonly included in high-grain diets, often with monensin, to reduce the occurrence of liver abscesses (e.g., Brown et al., 1973; Heinemann et al., 1978). In vitro work by Baldwin et al. (1982) showed that tylosin decreased tVFA concentration, and Nagaraja et al. (1987) reported that tylosin decreased the molar concentration of propionate. However, in vivo work done by Heinemann et al. (1978) showed no effect of tylosin on the VFA profile at similar doses to those evaluated here (11 mg/kg DM), and no interaction with monensin dose was evident. Nagaraja et al. (1999) also found, in vivo, that tylosin did not alter the concentration of tVFA in the rumen. In attempts to develop multiple regression equations including both monensin and tylosin dose, tylosin was nonsignificantly related to the change in acetate, propionate, butyrate and tVFA (P > 0.70) and was therefore excluded from the analysis.

Multivariate analysis, through introduction of the correlation of dependent variables (acetate, propionate, and butyrate) via the random effects statement as well as via the residual variances (repeated statement; Strathe et al., 2010), were tested but resulted in over-parameterization of the model and a lack of convergence or a non-positive R matrix. Acetate, propionate, and butyrate regressions were thus fitted simultaneously using a univariate approach. The statistical model waswhere Yij is the dependent variable, B1 is the overall fixed effect regression coefficient of Y on X, Xij is the value of the continuous predictor variable, bi is the random effect of study on the regression coefficient of Y on X, and eij is the residual error.

In this analysis, monensin and tylosin dose (independent variables) were centered around the mean, where, for example, monensin_new = monensin – monensin_mean. Centering the monensin or tylosin dose places the intercept at the mean of all the doses and therefore removes the correlation between slope(s) and intercept(s) in the regression model (van Landeghem et al., 2006). Without centering, both the mean value and the variation around that mean are involved in selecting model factors. Because this study is only interested in the change (slope) in VFA profile, this was an appropriate adjustment. The joint distribution of random effects was assumed to be multivariate normal, and the dual quasi-Newton technique was used for optimization with an adaptive Gaussian quadrature as the integration method. Analysis was performed with the assumption that variance distribution for the estimates followed a multivariate normal distribution.

Recently, Sauvant et al. (2008) highlighted the importance of considering additional sources of systematic variation in meta-analysis studies, and so this database was selected to minimize these sources. For example, the database was limited to beef cattle fed a diet of >80% concentrate, which severely cuts down on the number of available studies in the literature and, subsequently, the variation in diets. Monensin was expressed as milligrams per kilogram DM to exclude the interfering factor of DMI. This study was specifically interested in the relative change in VFA profile as monensin adjustment equations will be integrated with model-determined VFA yields, which would account for the effect of diet. Therefore, the effect of the control or basal value was removed, and equations were based on the proportional change in values relative to the control. This essentially removes any inter-study bias effect and examines only the slope.

Goodness of fit of the statistical model (inclusion or exclusion of random effects, variance or covariance structure selection) was evaluated using the Bayesian information criterion (BIC) fit statistic and the restricted log-likelihood function (−2LL; SAS Inst. Inc.), where smaller values indicate better fit, and significance of the fixed effect model parameters were tested against a P value of 0.05.


The relative amounts of VFA produced in the rumen are of particular interest in ruminant nutrition because the major VFA feed into important metabolic pathways in other organs. Propionate is a substrate for gluconeogenesis and is the main source of glucose for the ruminant, whereas non-glucogenic acetate and butyrate are precursors for long-chain fatty acid synthesis. The VFA profile, particularly the non-glucogenic to glucogenic VFA ratio, is associated with effects on end-product composition and energy balance in ruminants (Thomas and Martin, 1988; van Knegsel et al., 2007). The type of VFA formed in the rumen is also essential in mechanistic models that predict enteric methanogens (Alemu et al., 2011; Bannink et al., 2011; Mills et al., 2001) because propionate is a hydrogen sink whereas acetate and butyrate are hydrogen sources, and hydrogen is the major substrate for CH4 formation (Wolin, 1960). Methane represents an energy loss to the animal, where observations can range from 2% to 12% of GE intake (Johnson and Johnson, 1995), and it is also a greenhouse gas. Monensin, through its impact on the rumen VFA profile, has been questioned as a potential CH4 mitigation strategy. This paper focused on high-grain-fed feedlot cattle as within the beef industry it is the most likely sector targeted for CH4 mitigation. Feedlots contain large numbers of animals that are intensively managed and are typically fed ionophores.

The mean propionate molar proportion was relatively high (31.3 ± 1.87 mol/100 mol tVFA) and that of acetate was low (54.5 ± 1.70 mol/100 mol tVFA; means for control treatments). This is expected as, in general, the low-fiber and high-starch content of high-concentrate diets, from stoichiometric principles, is known to result in a high-propionate-producing rumen profile (Bannink et al., 2008). Relative changes in molar proportions of acetate, propionate, and butyrate (mol/100 mol tVFA) were related to monensin dose in the diet. The relationships developed in the present study modify model-determined proportions of individual VFA (mol/100 mol tVFA) as follows:where the default VFA proportion (acetate, propionate, or butyrate; mol/100 mol tVFA) is the proportion of VFA determined as a result of the VFA stoichiometry represented in a given model and the new calculated VFA proportion (mol/100 mol tVFA) is related to the monensin dose via a proportional change. Proportional changes were significant for propionate (P = 0.003) and butyrate (P = 0.002) but not for acetate (P = 0.068). The joint BIC value was 1291 and 2LL was 1277. The root-mean-square prediction error (RMSPE) values (percent of observed mean; Bibby and Toutenburg, 1977) for the 3 regressions tested back against the developmental database were 5.2%, 9.4%, and 17.0% for acetate, propionate, and butyrate, respectively, and the error was mostly random (93.9%, 91.8%, and 96.5%, respectively). Concordance correlation coefficient (CCC; Lin, 1989) values were 0.063, 0.327, and 0.357, respectively, with R values (indicator of precision) of 0.122, 0.412, and 0.488 and Cb values (indicator of accuracy) of 0.512, 0.792, and 0.732, respectively. A low R value for the non-significant acetate equation causes the CCC value to be poor compared to analysis using RMSPE, which does not pick up on this error (Ellis et al., 2010). Regressions of the proportional changes in Eq. [2] through [4] are illustrated in Figure 1.

Figure 1.
Figure 1.

Proportional change in observed rumen molar (top) acetate, (middle) propionate, and (bottom) butyrate, measured in moles/100 moles of total VFA, vs. monensin dose (mg/kg DM), where the dotted lines represent individual study (Table 1) regression lines and the solid lines represent the overall best fit regression (Eq. [2], [3], and [4]).


It is evident from Figure 1 that the majority of trials tested monensin doses in the range of 0.0 to 40.0 mg/kg DM. As a result, some caution should be used in applying these equations to greater doses, although the slope of the best fit regression appears similar to the slope of the trials that tested doses greater than 40.0 mg/kg DM (Figure 1).

Across studies, the molar proportion of propionate consistently increased with monensin dose, whereas the most variation in regression slopes occurred with the molar proportion of butyrate and the least variation occurred with the change in molar proportion of acetate (Figure 1). The reason for high variation in the butyrate slope between studies is unknown but might be due to it being the smallest fraction of the three VFA examined, resulting in proportionally more error in measurement. Henderson et al. (1981) examined the in vitro effect of monensin on bacteria and found differences in the amount of inhibition between 2 strains of Butyrivibrio bacteria in the presence of monensin. Differences in the Butyrivibrio rumen bacteria communities between studies may also contribute to the increased variation in the change in butyrate results seen here. Furthermore, protozoa are known to quickly metabolize sugars (e.g., see Dijkstra, 1994) and produce relatively large amounts of butyrate. Studies have suggested that the sensitivity of protozoa to monensin might contribute to the initial decrease in CH4 as protozoa and methanogens interact closely in the rumen (Richardson et al., 1978; Hino, 1981; Habib and Leng, 1987). However, this effect may subside due to the adaptation of the protozoal population to monensin over time (Johnson and Johnson, 1995; Tedeschi et al., 2003; Guan et al., 2006). The net result is that a decrease in CH4 production might be of greater magnitude initially (see the review by McGuffey et al., 2001). Therefore, variation between studies in the length of the treatment period (varied from 14 to 152 d) could lead to variation in the phase of the response of protozoa and therefore heightened variation in the response of butyrate to monensin dose.

While this study took a linear approach to the analysis of monensin dose on VFA profile, it is possible that the slope may be non-linear, particularly at high doses (Figure 1). However, without more high-dose studies, it is not possible to develop a reliable nonlinear equation to describe the data. Analysis of the residuals (predicted minus observed values) in Figure 2 shows that error in prediction is not systematically related to the observed value, DMI, or the acetate, propionate, and butyrate molar proportion. For the residual vs. butyrate (mol/100 mol) plot, there is a seemingly negative slope, but this is largely influenced by a single datum point. Removal of this datum point reduces the slope from −2.07 to −0.02. In this paper it was assumed that the change in VFA molar proportion due to monensin addition was related to the observed VFA molar proportion. In this approach, the magnitude of the change is assumed to depend on the actual VFA molar proportion (relatively small change if VFA molar proportion is small and vice versa), rather than fixed and independent of the VFA molar proportion. If in real life the correction should not depend on the basal VFA molar proportion, the residuals would have shown a consistent pattern of over- and under-prediction at low and high VFA molar proportions. The absence of consistent patterns indicates that a correction on a relative basis is valid. The residuals are also not systematically related to the treatment period length (data not shown).

Figure 2.
Figure 2.

Residual (predicted minus observed) of the proportional change in rumen molar proportion of (left) acetate, (middle) propionate, and (right) butyrate, measured in moles/100 moles of total VFA, vs. (top) equation-predicted proportional change (Eq. [2] through [4]), (middle) DMI (kg/d), and (bottom) molar proportion of acetate, propionate, or butyrate (mol/100 mol of total VFA).


Because Eq. [2] through [4] were developed independently and yet are related when expressed as moles/100 moles, the post-adjustment sum of moles/100 moles acetate, propionate, and butyrate is not necessarily identical to the pre-adjustment total. For example, a diet that produces a VFA profile that is 55 mol acetate/100 mol tVFA, 35 mol propionate/100 mol tVFA, and 8 mol butyrate/100 mol tVFA (sums to 98 mol/100 mol tVFA), with a 40 mg/kg DM monensin dose, becomes 53.6 mol acetate/100 mol tVFA, 38.6 mol propionate/100 mol tVFA, and 6.93 mol butyrate/100 mol tVFA (sums to 99.2 mol/100 mol tVFA). This has to be corrected by scaling total acetate, propionate, and butyrate (mol/100 mol tVFA) back to 98 mol/100 mol tVFA when it is also presumed that the fraction of other VFA remains the same.

Other minor VFA (e.g., valerate, isovalerate, and isobutyrate) were not consistently reported in the database studies, and the studies reporting these VFA were not numerous enough to allow meta-analysis. Total VFA was not reported in studies by Ralston and Davidson (1976), van Maanen et al. (1978), Zinn (1987), and Zinn and Borques (1993). Analysis of the effect of monensin dose on the change in tVFA for the remaining studies (using the same MIXED model analysis described above but replacing individual VFA proportions with tVFA) was not significant: The change in tVFA (mM), which equals (0.0007 ± 0.06041) × monensin dose (mg/kg DM), where P = 0.99, is illustrated in Figure 3. Aside from 1 study that observed a 31% decrease in tVFA (mM) with a 55 mg/kg DM monensin treatment (Thorton and Owens, 1981), this is in agreement with other publications reporting no change in tVFA with monensin feeding (Guan et al., 2006). The relationship remained non-significant whether the Thorton and Owens (1981) study was left in or removed, as the equation became the change in tVFA (mM) = 0.0034 (± 0.0368) × monensin dose (mg/kg DM), where P = 0.93 when it was excluded.

Figure 3.
Figure 3.

Proportional change in rumen total VFA (tVFA, measured in mM) vs. monensin dose (mg/kg DM), where the dotted lines represent individual studies and the solid line represents the overall best fit regression (Table 1).


Nagaraja et al. (1997) examined the average change in acetate, propionate, and butyrate concentration (in mM, mol/100 mol tVFA and mol/d) for the 70% and 50% roughage diets originally reported by Prange et al. (1978) and Rogers and Davis (1982). In that summary Nagaraja et al. (1997) illustrated a 6% decrease in acetate, a 29% increase in propionate, and a 14% decrease in butyrate (mol/100 mol tVFA) on the 70% roughage diet and a 4% decrease in acetate, a 15% increase in propionate, and a 25% decrease in butyrate (mol/100 mol tVFA) on the 50% roughage diet, with monensin doses of 33 g/kg DM. When a 33 mg/kg DM monensin dose is assumed here, with the equations developed in this study, based on diets with an average of 13% roughage, equations predict a 2.1% decrease in acetate, an 8.6% increase in propionate, and an 11.1% decrease in butyrate. Aside from the large change in butyrate reported for the 50% forage diet, these results suggest a greater impact on the VFA profile for diets at greater forage % diets. However, the studies of Prange et al. (1978) and Rogers and Davis (1982) also had treatment periods of 21 d compared with the slightly greater 28-d median value for this study, which may have some minor impact on the magnitude of the results (e.g., because of the longevity of protozoa effects). Also, results here are based on a higher number of studies and may therefore be more reliable. However, this observation also agrees with the results of Thorton and Owens (1981), where both the magnitude of change in the VFA profile and CH4 emissions were amplified for high-forage diets at the same monensin dose.

In agreement with the dose-dependent results presented here, Beauchemin et al. (2008), in a summary of the literature, found that the effect of monensin on CH4 may be dose dependent. In that review, doses of less than 15 mg/kg DM monensin had no effect on CH4 production (g/d or g/kg DMI) in dairy cows, doses of less than 20 mg/kg DM either had no effect or reduced total CH4 (g/d but not g/kg DMI) in dairy cows, and greater doses (24 to 35 mg/kg DM) reduced CH4 production (g/d by 4% to 10% and g/kg DMI by 3% to 8%) in both beef cattle and dairy cows, with short-term decreases of up to 30% for both high- and low-forage diets. The equations developed here may represent part of the longer-term impact of monensin on the rumen fermentation pattern, as opposed to transient modifications in the rumen (e.g., effects on protozoa). Changes in the VFA profile appear to remain constant and persist over time (Guan et al., 2006).

In terms of other ionophores used in beef cattle production, monensin and lasalocid (Elanco Animal Health) are licensed in the United States and Canada, laidlomycin propionate is licensed only in the United States, and salinomycin (Elanco Animal Health) is licensed only in Canada (NRC, 2000). Martineau et al. (2007) found no significant differences in acetate, propionate, or butyrate (mol/100 mol tVFA) between lasalocid and monensin treatments, and neither did Clary et al. (1993). However, although Morris et al. (1990) found no differences between lasalocid and monensin in terms of acetate and propionate, butyrate and valerate were significantly decreased with monensin treatment (mol/100 mol). Domescik and Martin (1999) concluded that laidlomycin propionate altered fermentation by mixed ruminal microorganisms in a manner similar to monensin but that monensin seemed to be a more potent inhibitor. Galyean et al. (1992) also found no difference in the VFA profile (mol/100 mol tVFA) between laidlomycin propionate and monensin treatments. However, the VFA profile for the ionophore treatments compared to the control treatment were also non-significantly different. Although no direct comparison with monensin was made, Merchen and Berger (1985) found the VFA profile shifted in a linear manner toward more propionate and less acetate and butyrate (mol/100 mol tVFA) with an increasing dose of salinomycin. The NRC (2000) groups all ionophores together in terms of their effects, and so it is possible that the equations developed here for monensin could also apply to other ionophores or that similar equations could be developed if the dose-response amounts differ between ionophores (Domescik and Martin, 1999).

In summary, this paper provides equations to quantify the change in VFA profile with monensin feeding in high-grain-fed beef cattle. The results of this meta-analysis indicate that the changes in VFA profile are dose dependent and that monensin should not be ignored when considering beef cattle data in modeling exercises on rumen fermentation profiles. No change in tVFA was evident. The equations produced can be used to adjust VFA stoichiometry within a model of rumen fermentation that may provide more accurate estimates of VFA production for beef cattle being fed monensin.




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