J. Salinas-Chavira, J. Lenin, E. Ponce, U. Sanchez, N. Torrentera and R. A. Zinngrowth-performance, dietary energetics, and digestion of calf-fed Holstein steers

Comparative effects of virginiamycin supplementation on characteristics of

2009-09-11T07:21:48-07:00; 2009, 87:4101-4108.doi: 10.2527/jas.2009-1959 originally published onlineJ ANIM SCI 

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ABSTRACT: Two trials were conducted to evalu-ate the influence of virginiamycin supplementation on growth performance and digestive function. In trial 1, 144 Holstein steer calves (119 kg of initial BW) were used in a 340-d trial. Treatments were (DM basis) 1) control (no antibiotic); 2) 16 mg/kg of virginiamycin; 3) 22.5 mg/kg of virginiamycin; and 4) 28 mg/kg of monensin. Calves received a steam-flaked corn-based growing diet for the first 112 d and thereafter were fed a finishing diet from d 112 until slaughter. Steers were divided into 2 BW groups (lighter-half and heavier-half), and assigned within groups to 24 pens (6 steers/pen). Virginiamycin did not affect (P > 0.20) ADG, but increased 340-d G:F (linear; P = 0.02) and dietary NE (linear; P = 0.04). Improvements in dietary NE were most pronounced during the final 116 d of the feeding period (linear; P = 0.04) that comprised the hot summer months (June through September). Mon-ensin did not affect (P > 0.20) growth performance or dietary NE. Although average initial BW of the lighter-half and heavier-half BW groups differed by only 4 kg, the heavier BW group had greater HCW (13 kg, P = 0.02), LM area (5.4%, P = 0.01), ADG (4.3%, P = 0.02), G:F (2.3%, P = 0.08), and dietary NE (3.2%. P

= 0.01) than the lighter BW group. In trial 2, 4 Hol-stein steers (269 ± 3.5 kg) with cannulas in the rumen and proximal duodenum were used in a 4 × 4 Latin square design to evaluate treatment effects on diges-tion. Virginiamycin did not affect (P > 0.20) ruminal digestion of OM, NDF, starch, and N, and microbial efficiency, but tended to linearly increase (P = 0.09) ruminal N efficiency (nonammonia N flow to the small intestine/N intake). Likewise, virginiamycin did not af-fect (P > 0.20) postruminal or total tract digestion of OM, starch, NDF, and N. Compared with the control diet, monensin depressed total tract digestion of OM (4.2%, P = 0.03) and NDF (29.5%, P = 0.02). There were no treatment effects (P > 0.20) on ruminal pH, or ruminal lactate and total VFA molar concentration. Virginiamycin increased ruminal molar proportion of acetate (quadratic; P = 0.04) and estimated methane production (quadratic; P = 0.07), and decreased propi-onate molar proportion (quadratic; P = 0.09). Monensin did not affect (P > 0.20) ruminal VFA molar propor-tions or estimated methane production. We concluded that virginiamycin supplementation can enhance feed-lot growth-performance and dietary energetic efficiency of calf-fed Holstein steers.

Key words: cattle, Holstein, performance, virginiamycin

©2009 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2009. 87:4101–4108 doi:10.2527/jas.2009-1959

INTRODUCTION

Virginiamycin is an antimicrobial derived from Strep-tomyces virginiae that inhibits growth of gram-positive bacteria (Cocito, 1979). In a summary of 7 dose-response studies with feedlot cattle (Rogers et al., 1995), dietary

supplementation with 19 to 27 mg of virginiamycin/kg (DM basis) enhanced ADG (4.6%) and G:F (3.6%) and reduced (38%) the incidence of liver abscess. The effect on liver abscess incidence is due to direct inhibi-tory action on growth of Fusobacterium necrophorum and Actinomyces pyogenes (Nagaraja and Chengappa, 1998). The basis for improved growth-performance is less certain. Virginiamycin inhibits growth of ruminal lactic acid-producing bacteria, limiting ruminal lactate accumulation (Hedde et al., 1980; Nagaraja et al., 1987; Hynes et al., 1997; Clayton et al., 1999; Coe et al., 1999). Thus, virginiamycin might reduce risk of lac-tic acidosis and associated digestive dysfunctions after ruminal overload of rapidly fermentable carbohydrate

Comparative effects of virginiamycin supplementation on characteristics of growth-performance, dietary energetics,

and digestion of calf-fed Holstein steers

J. Salinas-Chavira,1 J. Lenin,2 E. Ponce,2 U. Sanchez,2 N. Torrentera,2 and R. A. Zinn3

Department of Animal Science, University of California, Davis 95616

1 Current address: Universidad Autonoma de Tamaulipas, Ciudad Victoria, Tamaulipas, Mexico. CP. 87000.

2 Current address: Universidad Autonoma de Baja California, Mexicali, Baja California, Mexico.

3 Corresponding author: razinn@ucdavis.eduReceived March 11, 2009.Accepted August 27, 2009.

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(Owens et al., 1998). Virginiamycin might also enhance postruminal nutrient uptake. In swine, virginiamycin enhanced digestion of DM, N, and energy (Ravindran et al., 1984), ME (Vervaeke et al., 1979), and minerals (Ravindran et al., 1984; Agudelo et al., 2007). These benefits were attributable to prolonged intestinal reten-tion time (28 to 33%, Ravindran et al., 1984) and to re-duced intestinal bacterial evolution and corresponding organic acid production (Vervaeke et al., 1979).

The effect of virginiamycin on growth performance of calf-fed Holstein steers has not been reported. Unlike comparisons by Rogers et al. (1995) with beef breeds fed growing-finishing diets for 112 to 245 d, the calf-fed Holstein steer is typically fed a low-forage energy-dense diet continuously for more than 320 d. We investigated a dose response to virginiamycin on ruminal and total tract digestion, growth-performance, and dietary ener-getics of calf-fed Holstein steers continuously fed an energy-dense diet for 340 d.

MATERIALS AND METHODS

All procedures involving animal care and manage-ment were in accordance with and approved by the University of California, Davis, Animal Use and Care Committee.

Exp. 1

Animals and Diets. One hundred forty-four Hol-stein steer calves (119 ± 9 kg of BW) were used in a 340-d experiment to evaluate the influence of vir-giniamycin on growth performance. Calves originated from Tulare, CA, and were received at the University of California Desert Research Center, El Centro, on October 26, 2006. Upon arrival, steers were vaccinated for bovine rhinotracheitis-parainfluenza3 (TSV-2, Pfizer Animal Health, New York, NY), clostridials (Fortress 8, Pfizer Animal Health), and Pasteurella hemolytica (One Shot, Pfizer Animal Health), treated for parasites (Dectomax Injectable, Pfizer Animal Health), injected subcutaneously with 500,000 IU vitamin A (Vita-jec A&D 500, RXV Products, Porterville, CA), and 1,200 mg of oxytetracycline (Liquamycin LA-200, Pfizer Ani-mal Health), branded, and ear-tagged. Calves were di-vided into 2 BW groups, light and heavy, and randomly assigned within groups to 24 pens (6 steers/pen). Pens were 78 m2 with 33 m2 of overhead shade, automatic waterers, and fence-line feed bunks. Four dietary treat-ments were compared: control (no antibiotic); 2) 16 mg/kg of virginiamycin (V-Max, Phibro Animal Health, Ridgefield Park, NJ); 3) 22.5 mg/kg of virginiamycin; and 4) 28 mg/kg of monensin (Elanco Animal Health, Greenfield, IN). During the initial 112 d (early growing period) steers were fed a high-protein basal diet formu-lated to meet metabolizable AA requirements of a 200-kg steer gaining 1.5 kg/d (Table 1). For the remainder of the growing-finishing period (d 112 to 340), urea was the sole supplemental N source (Table 1). Diets were

prepared at weekly intervals and stored in plywood boxes located in front of each pen. Steers were allowed ad libitum access to their experimental diets. Fresh feed was provided twice daily. Steers were implanted with Synovex-C (Fort Dodge Animal Health, Fort Dodge, IA) on d 1 and reimplanted with Revalor-S (Intervet/Schering-Plough Animal Health, Millsboro, DE) on d 112 and 224.

Estimation of Dietary NE. Energy gain (EG) was calculated by the equation: EG = ADG1.097 0.0557W0.75, where EG is the daily energy deposited (Mcal/d), and W is the mean shrunk BW (kg; NRC, 1984). Maintenance energy (EM) was calculated by the equation: EM = 0.084W0.75 (NRC, 1988). Dietary NEg was derived from NEm by the equation: NEg = 0.877 NEm − 0.41 (Zinn, 1987). Dry matter intake is related to energy requirements and dietary NEm according to the equation: DMI = EG/(0.877NEm − 0.41), and can be resolved for estimation of dietary NE by means of the quadratic formula: x = (−b − √b2 − 4ac)/2c, where x = NEm, a = −0.41 EM, b = 0.877 EM + 0.41 DMI + EG, and c = −0.877 DMI (Zinn and Shen, 1998).

Carcass Data. Hot carcass weights were obtained at time of slaughter. After carcasses chilled for 48 h, the following measurements were obtained: LM area (cm2) by direct grid reading of the LM at the 12th rib; sub-cutaneous fat (cm) over the LM at the 12th rib taken at a location 3/4 the lateral length from the chine bone end (adjusted by eye for unusual fat distribution); KPH as a percentage of HCW; marbling score (USDA, 1997; using 3.0 as minimum slight, 4.0 as minimum small, 5.0 as minimum modest, 6.0 as minimum moderate, etc.), and estimated retail yield of boneless, closely trimmed retail cuts from the round, loin, rib, and chuck (% of HCW; Murphey et al., 1960) = 52.56 − 1.95 × subcu-taneous fat − 1.06 × KPH + 0.106 × LM area − 0.018 × HCW.

Exp. 2

Animals and Sampling. Four Holstein steers (269 kg) with cannulas in the rumen (3.8 cm internal diameter) and proximal duodenum (Zinn and Plascenc-ia, 1993) were used in 4 × 4 Latin square experiment to study treatment effects on characteristics of diges-tion. Treatments were the same as those used in Exp. 1 (Table 1). All steers were fed the same basal diet with 0.40% chromic oxide added as a digesta marker. Dietary treatments (virginiamycin and monensin) were top-dressed on the feed allotment of an individual steer at time of feeding. Steers were maintained in individual pens (4 m2) with automatic waterers. Diets were fed at 0800 and 2000 h daily. To avoid the complications of feed refusals, DMI was restricted to 5.7 kg/d (2.1% of BW daily). This value is 97% of average observed DMI intake for steers in Exp. 1 (2.18% of BW daily). Experimental periods were 3 wk, with 10 d for dietary treatment adjustment, 4 d for collection, and 7 d of drug withdrawal (steers were fed only the basal diet

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with no added virginiamycin or monensin for 1 wk be-fore switching to new dietary treatment assignments of the subsequent experimental period). During collection, duodenal and fecal samples were taken twice daily as follows: d 1, 0750 and 1350 h; d 2, 0900 and 1500 h; d 3, 1050 and 1650 h; and d 4, 1200 and 1800 h. Indi-vidual samples consisted of approximately 700 mL of duodenal chime and 200 g (wet basis) of fecal material. Samples from each steer within each collection period were composited for analysis. During the final day of each collection period, ruminal samples were obtained from each steer via ruminal cannula 4 h after feeding. Ruminal fluid pH was determined on fresh samples. Samples were strained through 4 layers of cheesecloth. Two milliliters of freshly prepared 25% (wt/vol) meta-phosphoric acid was added to 8 mL of strained ruminal fluid. Samples were then centrifuged (17,000 × g for 10 min at 5°C), and supernatant fluid was stored at −20°C for VFA analysis (gas chromatography; Zinn, 1988), and L (+) lactic acid (Sigma procedure No. 826-UV, Sigma, St. Louis, MO). Upon completion of the experiment, ruminal fluid was obtained via the ruminal cannula from all steers and composited for isolation of ruminal bacteria by differential centrifugation (Bergen et al., 1968).

Sample Analysis and Calculations. Samples were subjected to all or part of the following analysis: DM (oven drying at 105°C until no further weight loss), ash, ammonia N, Kjeldahl N (AOAC, 1984), NDF (Go-ering and Van Soest, 1970; adjusted for insoluble ash), chromic oxide (Hill and Anderson, 1958), purines (Zinn and Owens, 1986), and starch (Zinn, 1990). Microbial OM and N leaving the abomasum were calculated using

purines as a microbial marker (Zinn and Owens, 1986). Organic matter fermented in the rumen was considered equal to OM intake minus the difference between the amount of total OM reaching the duodenum and mi-crobial OM reaching the duodenum. Feed N escape to the small intestine was considered equal to total N leav-ing the abomasum minus ammonia-N and microbial N and, thus, includes any endogenous additions. Meth-ane production (mol/mol glucose equivalent fermented) was estimated based on the theoretical fermentation balance for observed molar distribution of VFA (Wolin, 1960).

Statistical Design and Analysis. For calculat-ing steer performance in Exp. 1, interim and final BW were reduced 4% to account for digestive tract fill. Final shrunk LW was adjusted for HCW by dividing HCW by the decimal fraction of the average dressing percent-age (0.613). Pens were used as experimental units. The experimental data were analyzed as a completely ran-dom design experiment with a 2 × 4 factorial arrange-ment of treatments. There were no initial BW group × dietary treatment interactions (P > 0.20). There-fore, degrees of freedom for the interaction were pooled in residual error according to the following statistical model: Yij = µ + Wi + Tj + εij (Hicks, 1973), where µ is the common experimental effect, Wi represents initial BW group effect (df = 1), Tj represents dietary treat-ment effect (df = 3), and εij represents the residual error (df = 19). Treatments effects were tested using the following contrasts: 1) linear effect of virginiamycin level; 2) quadratic effect of virginiamycin level; and 3) control (nonmedicated) vs. monensin. Coefficients for polynomial contrasts (linear, quadratic effects of vir-

Table 1. Composition of the experimental diets fed to steers (Exp. 1 and 2)

Item Growing period (1 to 112 d)Growing-finishing period

(112 to 340 d, and Exp. 21)

Ingredient composition, %, DM basis Steam-flaked corn 68.40 77.75 Sudangrass hay 12.00 10.00 Canola meal 6.00 — Fish meal 2.50 — Yellow grease 4.00 3.50 Cane molasses 4.00 5.00 Limestone 1.55 1.60 Urea 1.00 1.20 Magnesium oxide 0.15 0.20 Trace mineral salt2 0.40 0.40 Dicalcium phosphate — 0.35Dietary treatment,3 g/T CON; VM, 30.1, VM, 43.60; MON, 153.6 CON; VM, 30.1, VM, 43.60; MON, 153.6Nutrient composition,4 DM basis NEm, Mcal/kg 2.22 2.26 NEg, Mcal/kg 1.55 1.59 CP, % 14.7 11.9 Ca, % 0.85 0.77 P, % 0.41 0.35

1Chromic oxide (0.40%) was added as digesta marker in Exp. 2.2Trace mineral salt contained: CoSO4, 0.68%; CuSO4, 1.04%; FeSO4, 3.57%; ZnO, 0.24%; MnSO4, 1.07%; KI, 0.052%; and NaCl, 92.96%.3Treatments: CON: control no antibiotic; VM: virginiamycin (V-Max, Phibro Animal Health, Ridgefield Park, NJ); MON: monensin (Rumensin

80, Elanco Animal Health, Greenfield, IN).4Based on tabular values for individual feed ingredients (NRC, 1996).

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giniamycin) with unequal spacing (0, 16, and 22.5 mg/kg of virginiamycin) were determined according to SAS Inst. Inc. (Cary, NC).

The effects of virginiamycin supplementation on characteristics of digestion in cattle were analyzed as a 4 × 4 Latin square. Treatments effects were tested using contrasts as indicated for Exp. 1.

RESULTS AND DISCUSSION

Treatment effects on feedlot performance and esti-mated NE value of the diet (Exp. 1) are shown in Table 2. There were no interactions (P > 0.20) between initial BW group and dietary treatments on growth perfor-mance. Across the 340-d feeding period, dietary treat-ments did not affect ADG or DMI (P > 0.20). However, virginiamycin increased (linear component; P = 0.02) G:F. Numerically, the increase in G:F due to virginia-mycin was 2.3 and 4.0% at the 16 and 22.5 mg/kg levels of supplementations, respectively. In a summary of 7 feedlot dose-response studies (Rogers et al., 1995) in-volving beef cattle, ADG and G:F increased with level of virginiamycin supplementation. Maximal responses in ADG (4.6%) and G:F (3.6%) were observed when

virginiamycin was supplemented at the rate of 19 to 27 mg/kg (DM basis).

Consistent with improvements in G:F, virginiamycin supplementation increased estimated dietary NEm and NEg (linear effect; P = 0.04; Table 2). Improvements in estimated dietary NE were apparent during the second 112-d growing phase (linear effect; P = 0.06) and the last 115-d finishing phase (linear effect; P = 0.04). The finishing phase comprised hot summer months of June through September (40°C average daily maximum tem-perature, 67% average daily humidity). Overall (340 d), virginiamycin supplementation at the rate of 22.5 mg/kg increased estimated dietary NEm and NEg by 3.8 and 4.9%, respectively. Likewise, Rogers et al. (1995) observed that virginiamycin supplementation at levels ≥19 mg/kg increased estimated NEm and NEg values of finishing diets fed to beef cattle by 3.6 and 3.8%, respectively. The basis for these improvements in ener-getic efficiency is not certain. As will be shown later, effects of virginiamycin supplementation on OM diges-tion were not appreciable. However, based on in vitro continuous culture fermentation in conjunction with in vivo intestinal passage studies in swine, Vervaeke et al. (1979) observed that virginiamycin supplementation re-

Table 2. Treatments effects on growth performance responses in feedlot cattle during growing-finishing period (d 1 to 340, Exp. 1)

Item

Dietary treatment1 P-value

BW group

P-value SDCON

VM, mg/kgMON, mg/kg VM effect2 MON

vs. CON16 22.5 28 Lin Quad Light Heavy

Pen replications 6 6 6 6 12 12BW,3 kg Initial 121.0 118.1 119.8 117.1 117.0 121.0 1.5 Final 585.8 587.5 600.2 587.5 0.27 0.40 0.88 578.4 602.1 0.006 19.0ADG, kg/d d 1 to 112 1.46 1.48 1.51 1.50 0.27 0.63 0.38 1.48 1.49 0.65 0.08 d 113 to 224 1.54 1.55 1.52 1.53 0.88 0.63 0.80 1.55 1.52 0.37 0.10 d 225 to 340 1.12 1.12 1.21 1.14 0.26 0.34 0.75 1.05 1.24 0.001 0.12 d 1 to 340 1.37 1.38 1.41 1.38 0.21 0.52 0.61 1.36 1.42 0.02 0.06DMI, kg/d d 1 to 112 5.35 5.46 5.58 5.46 0.08 0.58 0.37 5.39 5.54 0.09 0.21 d 113 to 224 8.72 8.51 8.41 8.55 0.13 0.97 0.42 8.61 8.49 0.41 0.34 d 225 to 340 9.18 9.10 9.11 9.26 0.69 0.86 0.70 8.93 9.40 0.005 0.36 d 1 to 340 7.77 7.70 7.72 7.78 0.63 0.79 0.94 7.66 7.83 0.06 0.21G:F d 1 to 112 0.272 0.270 0.271 0.274 0.82 0.86 0.77 0.274 0.269 0.27 0.010 d 113 to 224 0.177 0.183 0.181 0.178 0.16 0.41 0.67 0.180 0.179 0.51 0.006 d 225 to 340 0.121 0.123 0.132 0.123 0.13 0.29 0.82 0.118 0.132 0.004 0.011 d 1 to 340 0.176 0.180 0.183 0.178 0.02 0.46 0.49 0.177 0.181 0.08 0.005Dietary NE, Mcal/kg Maintenance 2.11 2.13 2.19 2.12 0.04 0.30 0.84 2.11 2.17 0.01 0.055 Gain 1.44 1.46 1.51 1.45 0.04 0.30 0.84 1.44 1.49 0.01 0.048Observed/expected NE Maintenance 0.94 0.95 0.97 0.94 0.04 0.30 0.83 0.93 0.96 0.01 0.024 Gain 0.92 0.93 0.96 0.92 0.04 0.30 0.83 0.92 0.95 0.01 0.031

1Treatments: CON: control no antibiotic; VM: virginiamycin (V-Max, Phibro Animal Health, Ridgefield Park, NJ); MON: monensin (Ru-mensin 80, Elanco Animal Health, Greenfield, IN).

2Lin = linear; Quad = quadratic.3Initial and final BW reduced 4% to account for fill.

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tarded small intestinal microbial metabolic activity and consequent lactic acid and VFA production, thereby enhancing glucose and AA uptake and dietary NE in swine. Presumably, this effect is augmented during a period of stress when intestinal passage rate is reduced (Warren et al., 1974).

Compared with control (nonmedicated diet), mon-ensin supplementation did not affect ADG (P = 0.61), DMI (P = 0.94), G:F (P = 0.48), or estimated dietary NE (P = 0.84). Growth-performance responses to mon-ensin supplementation of feedlot cattle have been incon-sistent. In a 228-trial summary, Goodrich et al. (1984) observed that monensin supplementation tended to in-crease ADG and G:F by an average of 1.6 and 8.0%, respectively. However, the coefficient of variation for those measures were extremely high (531 and 86.7%, respectively), with responses tending to decrease with increasing dietary energy density, and when NPN was the principal source of supplemental N (as was the case in the present study; Table 1). In feedlot cattle fed fin-ishing diets based on steam-flaked corn comparable with that of the present study (Zinn and Borques, 1993; Zinn et al., 1994; Depenbusch et al., 2008), monensin supplementation did not enhance ADG or G:F.

Although average initial BW of the light-half and heavy-half BW groups differed by only 4 kg, initial BW grouping had a marked effect growth performance. Across the 340-d feeding period, heavy-group steers had greater ADG (4.4%; P = 0.02), DMI (2%; P = 0.06), and G:F (1.7%, P = 0.08) than light-group steers. This effect was especially pronounced during the final 115-d finishing phase, where heavy-group steers had marked-ly greater ADG (18%, P = 0.001), and G:F (12%, P = 0.004) than light-group steers. The overall improvement in G:F was largely due to an apparent enhancement (P = 0.01) in dietary NEm (2.9%) and NEg (3.8%). It has been clearly demonstrated (Hicks et al., 1990; Zinn et al., 2008) that initial BW of cattle when first placed into the feedlot is an important determinant of DMI

and ADG. In the present study, ADG increased by 0.012 kg/kg of initial BW (r2 = 0.13; P = 0.01).

Treatment effects on carcass characteristics of feedlot steers (Exp. 1) are shown in Table 3. Hot carcass weight, dressing percentage, fat thickness, KPH, quality grade, and retail yield were not affected (P > 0.20) by dietary treatments. Virginiamycin supplementation tended to increase (linear effect; P = 0.15) LM area, likely the indirect effect of virginiamycin on final BW (LM area increases by an average of 0.185 cm2/kg change in BW gain; Block et al., 2001). With respect to initial BW grouping, heavy-group steers had greater HCW (3.7%; P = 0.02) and correspondingly larger LM area (5.5%, P = 0.01) than light-group steers.

Dietary treatment effects on characteristics of ruminal and total tract digestion (Exp. 2) are shown in Tables 4 and 5. Virginiamycin supplementation did not affect (P > 0.20) ruminal digestion of OM, NDF, starch, feed N, and microbial efficiency (g of microbial N per kg of OM fermented), but tended to decrease (linear component, P = 0.10) ruminal N efficiency (nonammonia N flow to the small intestine/N intake). Likewise, virginiamy-cin supplementation did not affect postruminal or total tract digestion of OM, starch, NDF, and N.

The effects of virginiamycin supplementation on char-acteristics of digestion in cattle have not been previous-ly reported. In swine, virginiamycin supplementation enhanced digestion of DM, N, and energy (Vervaeke et al., 1979; Ravindran et al., 1984). These benefits were attributable to prolonged intestinal retention time (28 to 33%; Ravindran et al., 1984), and reduced intestinal bacterial evolution and corresponding organic acid pro-duction (Vervaeke et al., 1979).

Compared with the control (nonmedicated treat-ment) diet, monensin supplementation depressed to-tal tract digestion of OM (4.2%, P = 0.03) and NDF (29.5%, P = 0.02). The decrease in OM digestion was largely attributable to a tendency (P = 0.15) for de-creased ruminal OM digestion. Monensin supplementa-

Table 3. Treatments effects on carcass characteristics of feedlot steers

Item

Dietary treatment1 P-value

BW group

P-value SDCON

VM, mg/kgMON, mg/kg VM effect2

MON vs. CON16 22.5 28 Lin Quad Light Heavy

HCW, kg 359 360 367 362 0.34 0.49 0.63 355 368 0.02 13Dressing percentage 61.2 61.2 61.2 61.6 0.92 0.87 0.24 61.4 61.2 0.31 0.63Fat thickness, cm 0.77 0.79 0.96 0.81 0.17 0.27 0.76 0.83 0.85 0.81 0.20KPH, % 2.29 2.37 2.43 2.35 0.17 0.87 0.53 2.33 2.39 0.39 0.18LM area, cm2 75.5 76.3 79.0 78.3 0.16 0.38 0.20 75.2 79.3 0.01 3.7Marbling score3 5.51 5.13 5.62 5.96 0.98 0.18 0.24 5.50 5.59 0.80 0.65Retail yield, % 49.8 49.8 49.6 50.0 0.56 0.62 0.72 49.7 49.9 0.64 0.67

1Treatments: CON: control no antibiotic; VM: virginiamycin (V-Max, Phibro Animal Health, Ridgefield Park, NJ); MON: monensin (Rumensin 80, Elanco Animal Health, Greenfield, IN).

2Lin = linear; Quad = quadratic.3Using 3.0 as minimum slight, 4.0 as minimum small, 5.0 as moderate, and 6 as slightly abundant (USDA, 1997).

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tion did not affect ruminal (P = 0.56), postruminal (P = 0.15), or total tract N (P = 0.18) digestion. Although decreased ruminal OM digestion has been a character-istic response to monensin supplementation (Poos et al., 1979; Zinn and Borques, 1993; Zinn et al., 1994; Surber and Bowman, 1998), the effects of monensin on total tract OM digestion have been much less con-sistent. In some cases (Zinn, 1987, 1988; Morris et al., 1990; Zinn et al., 1994), monensin supplementation did not affect total tract OM digestion. In other cases mon-ensin supplementation depressed (Simpson, 1978; Poos et al., 1979) or increased (Wedegaertner and Johnson, 1983) total tract OM digestion.

There were no treatment effects (P > 0.20) on rumi-nal pH and lactic acid concentrations (Table 5). Ru-minal lactic acid concentration was only 14.9 mg/dL, accounting for only 1.4% of total organic acids (molar basis). Variation in ruminal pH (CV = 7.9) was as-sociated with ruminal organic acids (mM): pH = 7.61 − 0.0146 × ruminal organic acids, mM (r2 = 0.76; P < 0.001).

There were no treatment effects (P = 0.62) on ru-minal VFA molar concentration. Consistent with in vitro fermentation studies of Nagaraja et al. (1987), virginiamycin supplementation increased ruminal mo-lar proportion of acetate (quadratic effect; P = 0.04)

Table 4. Influence of virginiamycin and monensin on characteristics of ruminal and total tract digestion (Exp. 2)

Item

Dietary treatment1 P-value

SDCON

VM, mg/kgMON, mg/kg VM effect2

MON vs. CON16 22.5 28 Lin Quad

Steer replications 4 4 4 4Intake, g/d DM3 5,723 5,723 5,723 5,723 OM 5,400 5,400 5,400 5,400 NDF 725 725 725 725 N 99 99 99 99 Starch 3,328 3,328 3,328 3,328Flow to the duodenum, g/d OM 2,879 2,873 2,880 3,126 0.99 0.96 0.16 217 NDF 582 631 563 610 0.96 0.33 0.68 93 Starch 745 763 756 889 0.88 0.91 0.16 127 N 116.7 110.6 113.2 115.4 0.10 0.14 0.58 3.3 Microbial N 91.1 86.8 86.0 86.8 0.26 0.88 0.35 6.2 Nonammonia N 113.3 107.4 110.0 112.0 0.10 0.14 0.58 3.2 Feed N 22.2 20.6 24.0 25.3 0.84 0.54 0.56 7.0Ruminal digestion, % OM 63.6 62.9 63.5 58.2 0.76 0.99 0.15 4.6 NDF 19.6 13.1 22.3 16.0 0.97 0.33 0.70 12.8 Starch 77.6 77.0 77.2 73.3 0.87 0.91 0.16 3.8 Feed N 77.6 79.2 75.8 74.5 0.84 0.54 0.56 7.1Microbial efficiency4 26.7 25.6 26.0 28.0 0.52 0.66 0.40 2.0Protein efficiency5 1.14 1.08 1.11 1.13 0.10 0.13 0.56 0.03Fecal excretion, g/d OM 1,076 1,072 1,101 1,258 0.76 0.70 0.03 87 NDF 468 464 489 544 0.51 0.39 0.02 32 Starch 79 84 97 107 0.34 0.59 0.13 22 N 29.3 28.7 27.6 31.0 0.23 0.60 0.18 1.6Postruminal digestion, % of flow to duodenum OM 62.5 63.0 62.0 60.0 0.82 0.71 0.21 2.9 NDF 17.0 27.0 12.2 9.3 0.90 0.15 0.44 12.7 Starch 89.5 89.1 89.0 89.0 0.68 0.86 0.83 2.8 N 75.0 74.0 76.0 73.0 0.64 0.21 0.15 1.5Total tract digestion, % OM 80.1 80.2 80.0 76.7 0.76 0.69 0.03 1.6 NDF 35.4 36.1 33.0 25.0 0.52 0.38 0.02 4.5 Starch 97.6 97.5 97.1 96.8 0.33 0.57 0.13 0.7 N 70.4 71.0 72.1 69.0 0.23 0.60 0.18 1.7

1Treatments: CON: control no antibiotic; VM: virginiamycin (V-Max, Phibro Animal Health, Ridgefield Park, NJ); MON: monensin (Rumensin 80, Elanco Animal Health, Greenfield, IN).

2Lin = linear; Quad = quadratic.3DMI was restricted to 2.1% of BW daily (97% of average intake in Exp. 1; Table 2).4Microbial N, g/kg of OM fermented.5Nonammonia N flow to the small intestine as a fraction of N intake.

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and estimated methane production (quadratic effect; P = 0.07), and decreased the ruminal molar proportion of propionate (quadratic effect; P = 0.09); these effects were maximal at the 16 mg/kg level of virginiamycin supplementation. Consistent with numerous studies (Givens et al., 1981; Zinn, 1987; Galyean et al., 1992; Zinn et al., 1994), monensin supplementation did not affect (P > 0.20) ruminal VFA molar proportions or estimated methane production.

We conclude that supplementation of calf-fed Holstein steers with 22.5 mg/kg of virginiamycin may enhance G:F and apparent dietary NE. The effects of virgin-iamycin supplementation on characteristics of ruminal fermentation and total tract digestion are small and do not account for improvements in growth performance.

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Table 5. Influence of virginiamycin and monensin on ruminal pH and VFA molar proportions (Exp. 2)1

Item

Dietary treatment2 P-value

SDCON

VM, mg/kgMON, mg/kg VM effect3

MON vs. CON16 22.5 28 Lin Quad

Ruminal pH 6.00 6.03 5.99 5.68 0.98 0.90 0.29 0.40Total VFA, mM 113 102 116 125 0.98 0.41 0.51 23Ruminal VFA, mol/100 mol Acetate 39.4 43.5 42.0 40.0 0.04 0.04 0.64 1.5 Propionate 48.2 43.2 46.1 49.8 0.15 0.09 0.43 2.7 Isobutyrate 0.7 0.9 0.6 0.6 0.75 0.13 0.50 0.3 Butyrate 8.1 9.1 8.5 6.7 0.66 0.57 0.37 1.9 Isovalerate 1.4 1.4 1.1 0.9 0.27 0.50 0.11 0.4 Valerate 2.1 1.8 2.2 1.9 0.96 0.61 0.82 0.8Acetate/propionate 0.8 1.0 1.0 0.8 0.05 0.15 0.97 9.9Lactic acid, mg/dL 14.0 15.7 13.7 16.3 0.94 0.57 0.54 5.1Methane4 0.22 0.30 0.26 0.21 0.12 0.07 0.58 0.04

1Dry matter intake was restricted to 5.723 kg/d (2.1% of BW daily).2Treatments: CON: control no antibiotic; VM: virginiamycin (V-Max, Phibro Animal Health, Ridgefield Park, NJ); MON: monensin (Rumensin

80, Elanco Animal Health, Greenfield, IN).3Lin = linear; Quad = quadratic.4Methane production (mol/mol of glucose equivalent fermented) was estimated based on the theoretical fermentation balance for observed molar

distribution of VFA (Wolin, 1960).

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