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Differential responses to dietary cobalt in finishing steers fed corn-versus barley-based diets¹

Department of Animal Science and Interdepartmental Nutrition Program, North Carolina State University, Raleigh 27695-7621

	Abstract

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An experiment was conducted to determine the effects of dietary Co concentration on performance, carcass traits, and plasma, liver, and ruminal metabolites of steers fed corn- or barley-based diets. Sixty steers, initially averaging 316 kg, were stratified by BW and assigned randomly to treatments in a 2 x 3 factorial arrangement, with factors being a corn- or barley-based diet and supplemental Co added at 0, 0.05, or 0.15 mg/kg of DM. Control corn-and barley-based diets analyzed 0.04 and 0.02 mg of Co/kg of DM, respectively. Steers were fed individually using electronic Ca-lan gate feeders. Cobalt supplementation increased (P < 0.05) DMI and ADG over the total study. From d 85 to finish, Co supplementation increased (P < 0.05) ADG by steers fed corn- but not barley-based diets. The G:F was increased (P < 0.05) by Co supplementation during the first 84 d but not over the entire finishing period. Average daily gain and G:F were greater (P < 0.05) for corn- vs. barley-fed steers. Supplemental Co increased vitamin B₁₂ in plasma and liver (P < 0.05), and plasma vitamin B₁₂ was greater (P < 0.05) in steers fed corn-vs. barley-based diets. Cobalt supplementation increased (P < 0.05) ruminal fluid vitamin B₁₂ on d 84 in steers fed corn- but not barley-based diets. Folate was greater in plasma (P < 0.01) and liver (P < 0.05) of steers fed Co-supplemented diets. Increasing supplemental Co from 0.05 to 0.15 mg of Co/kg of DM increased (P < 0.05) liver folate in steers fed barley- but not corn-based diets. Supplemental Co decreased (P < 0.01) plasma methylmalonic acid concentration in steers. Increasing supplemental Co from 0.05 to 0.15 mg/kg of DM decreased plasma and ruminal succinate concentrations, and steers fed barley-based diets had greater (P < 0.05) plasma and ruminal succinate relative to those fed corn-based diets. Addition of supplemental Co to the basal diets increased (P < 0.01) plasma glucose concentrations of steers, and steers fed corn-based diets had greater plasma glucose than those fed barley-based diets. Steers supplemented with Co had greater ruminal propionate (P < 0.01) and lesser (P < 0.05) ruminal acetate and butyrate proportions than controls. Supplemental Co increased dressing percent (P < 0.10) and HCW (P < 0.01) at slaughter. These results indicate that feeding steers corn- or barley-based diets deficient in Co adversely affects performance and vitamin B₁₂ status.

Key Words: Steers • Cobalt • Vitamin B₁₂

	Introduction

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Ruminants require Co for ruminal microorganisms to synthesize vitamin B₁₂ (McDowell, 2000). In mammals, vitamin B₁₂ is an important cofactor for the enzymes methylmalonyl-Co A mutase and methionine synthase, which are important for gluconeogenesis and methionine synthesis (Banerjee and Chowdhury, 1999; Matthews, 1999). In ruminants, a Co deficiency results in a decrease in methylmalonyl-CoA mutase and methionine synthase activity (Kennedy et al., 1990), and alters lipid metabolism (Stangl et al., 1999a) and immune function (MacPherson et al., 1987). Recent studies have investigated the effects of dietary Co on performance and metabolism of finishing cattle fed corn-based diets (Tiffany et al., 2002, 2003). In those studies, when steers were fed a diet moderately Co-deficient (0.04 to 0.05 mg/kg of DM), intake, ADG, and G:F were decreased relative to Co-supplemented steers. In addition, decreased plasma and liver vitamin B₁₂, decreased ruminal fluid vitamin B₁₂ and propionate, and greater plasma methylmalonic acid concentrations were observed in steers fed low-Co diets.

Large metabolic responses to Co supplementation have been observed in ruminants fed barley-based diets low in Co (Kennedy et al., 1990, 1991); however, Co requirements of feedlot cattle fed barley-based diets have not been evaluated. Fermentation of barley in the rumen is more rapid and extensive than that of corn (Herrera-Saldana et al., 1990) and feeding barley-based diets has resulted in lower ruminal pH immediately after feeding (Yang et al., 1997) and altered molar proportions of VFA (Franks et al., 1972; Zinn, 1993) relative to corn-based diets. Altered ruminal fermentation in ruminants fed barley- vs. corn-based diets may affect microbial production of vitamin B₁₂, and thereby, perhaps affect dietary Co requirements. The current study was conducted to compare responses in performance, vitamin B₁₂ status, and metabolic characteristics to increasing dietary Co in finishing steers fed barley- vs. corn-based diets.

	Materials and Methods

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General
Sixty Angus steers (316 kg initial BW) were used in this experiment. Care, handling, and sampling of the animals herein were approved by the North Carolina State University Animal Care and Use Committee before initiation of the experiment. Steers were purchased at feeder calf sales in North Carolina. After arrival, steers were ear-tagged, weighed, vaccinated with Bo-vashield 4 (Pfizer Animal Health, Exton, PA) and Vision 7 (Bayer Corp., Shawnee Mission, KS), and dewormed with Safe Guard (Hoechst Roussel Vet., Clinton, NJ). Steers were confined to fescue pasture, where they were supplemented with corn silage (2.0 kg of DM·steer¹·d^–1) until the beginning of the experiment. Steers were then weighed and allotted by BW and origin to one of five pens equipped with 12 individual Calan gate feeders (American Calan, Northwood, NH). Steers were housed in covered, slotted-floor pens (5 m x 10 m) for the duration of the experiment.

After adjusting to the Calan gate feeding system, steers were weighed on two consecutive days, implanted with Synovex-Plus (Fort Dodge Animal Health, Fort Dodge, IA), bled via jugular venipuncture, and assigned randomly within a pen to one of six treatments. Treatments were 1) corn-based diet with no supplemental Co; 2) corn-based diet supplemented with 0.05 mg of Co/kg of DM; 3) corn-based diet supplemented with 0.15 mg of Co/kg of DM; 4) barley-based diet with no supplemental Co; 5) barley-based diet supplemented with 0.05 mg of Co/kg of DM; and 6) barley-based diet supplemented with 0.15 mg of Co/kg of DM. Supplemental Co was provided from CoCO₃.

Ingredient and chemical composition of the basal diets is presented in Table 1. Diets were formulated to be similar in CP and ruminally degraded protein. The corn- and barley-based diets analyzed 0.04 and 0.02 mg of Co/kg of DM, respectively. Diets were formulated to meet or exceed nutrient requirements for finishing beef steers gaining 1.5 kg/d, with the exception of Co (NRC, 1996). Steers were fed once daily in quantities sufficient to provide ad libitum access to feed. Feed offerings and refusals were recorded daily.

On d 84, ruminal fluid was collected 2 h after feeding by stomach tube. Ruminal fluid was strained through four layers of cheesecloth, and 10.0 mL was added to a vial containing 2.0 mL of meta-phosphoric acid (25% wt/vol). The samples were placed on ice and transported to the laboratory, where they were frozen at –70°C until analysis for VFA and vitamin B₁₂.

An equal number of steers per treatment was slaughtered after receiving the dietary treatments for either 146 or 160 d. Steers were slaughtered at a commercial abattoir following an overnight fast. Liver samples were collected postmortem and immediately placed on dry ice for transport to the laboratory where they were frozen at –70°C until analysis of vitamin B₁₂ and folate.

Analytical Procedures
Feed samples for the analysis of Co were prepared using a microwave digestion (Mars 5; CEM Corp., Matthews, NC) procedure described by Gengelbach et al. (1994). Cobalt was determined by flameless atomic absorption spectrophotometry using a graphite furnace (GFA-6500; Shimadzu Scientific Instruments, Kyoto, Japan). Crude protein in diets was determined using the Kjeldahl N procedure (AOAC, 1999). Diets were analyzed for NDF and ADF using the method of Van-Soest et al. (1991) in a batch processor (Ankom Technology, Fairport, NY).

Plasma, liver, and ruminal fluid vitamin B₁₂ and folate concentrations were determined using a competitive binding radioimmunoassay kit, in which nonspecific vitamin B₁₂-binding R-proteins were removed by affinity chromatography (ICN, Costa Mesa, CA). Before liver vitamin B₁₂ quantification, a tissue homogenate was prepared using a borate buffer (pH 9.2; Fisher Scientific, Suwanee, GA) and 1% bovine serum albumin (Sigma-Aldrich, St. Louis, MO), as described by Stangl et al. (1999a). Before the determination of vitamin B₁₂ concentration, ruminal fluid was centrifuged at 10,000 x g for 25 min and diluted with distilled H₂O until within the reference range of the assay.

Plasma samples for the determination of methylmalonic acid and succinate were prepared according to the procedures of McMurray et al. (1986) using a modified GLC method. The GLC (Model 5890 Series II; Hewlett-Packard, Palo Alto, CA) was equipped with a 25 m x 0.32 mm x 0.17 µm methyl siloxane column (Agilent Technologies, Wilmington, DE), on which 1.0 µL of acetyl chloride-butan-1-ol derivatized sample was injected. The oven temperature program used was 100°C initially, with a temperature increase of 10°C/min to 155°C for 2 min, followed by a temperature increase of 10°C/min to 215°C, where the temperature was held for 10 min to flush the column.

Ruminal fluid VFA concentrations were determined by GLC (Model 3800; Varian Instruments, Walnut Creek, CA) using a Nikol fused silica column, 15 m x 0.53 mm x 0.50 µm (Supelco, Bellefonte, PA). The oven temperature program used began with an initial temperature of 80°C, increased at 20°C/min to 140°C, which was held for 2 min. Temperature was then increased by 30°C/min to 175°C, where it was held for 1 min to flush the column. Plasma glucose was determined by a membrane-immobilized glucose oxidase enzyme coupled to an electrochemical sensor (Model 27 Industrial Analyzer; Yellow Springs Instrument Co., Inc., Yellow Springs, OH).

Data were analyzed statistically as a 2 x 3 factorial using the MIXED procedure of SAS (SAS Inst., Inc., Cary, NC). The model contained Co, grain source, pen, and Co x grain source interaction. Degrees of freedom for Co were partitioned into the following single degree of freedom contrasts: 1) control vs. 0.05 and 0.15 mg of Co/kg of DM; and 2) 0.05 vs. 0.15 mg of Co/kg of DM. Plasma variables were analyzed as repeated measures, with the model containing time and all possible interactions. Animal x pen nested within treatment was used as the error term. Initial values were used as a covariate when significant in the model. Data are presented as least squares means. One steer fed the corn-based diet without supplemental Co died after 1 wk (due to hypertension unrelated to treatment, as diagnosed by a veterinarian).

	Results and Discussion

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Performance
Cobalt addition to the diet increased (P < 0.01) ADG during the first 84 d and over the total finishing period (Table 2). Dry matter intake was increased (P < 0.05) by Co supplementation for the entire study. In agreement with these findings, Co supplementation of corn silage-based diets containing 0.08 or 0.07 mg of Co/kg of DM (Stangl et al., 1999a; Schwarz et al., 2000) or corn-based diets containing 0.04 or 0.05 mg of Co/kg of DM (Tiffany et al., 2002; 2003) increased gain and feed intake in finishing cattle. Gain:feed was increased (P < 0.05) by Co supplementation during the first 84 d, but not over the entire feeding period. This finding indicates that the ADG response to Co supplementation was largely due to increased feed intake rather than improved efficiency of gain. A major consequence of Co deficiency in ruminants is loss of appetite (Smith, 1997), which may be linked to a decrease in the rate of propionate clearance from blood (Marston et al., 1972).

Increasing supplemental Co from 0.05 to 0.15 mg/kg of DM did not significantly affect ADG, DMI, or G:F. In previous studies (Tiffany et al., 2002, 2003) with finishing cattle fed corn-based diets low in Co (0.04 to 0.05 mg of Co/kg of DM), increasing supplemental Co above 0.05 mg/kg did not significantly increase ADG or G:F. However, increasing supplemental Co from 0.05 or 0.10 to 1.0 mg of Co/kg of DM increased DMI in one study (Tiffany et al., 2003) but not in another study (Tiffany et al., 2002). Schwarz et al. (2000) evaluated10 dietary Co concentrations ranging from 0.07 to 0.69 mg of Co/kg of DM during a 280-d growing-finishing study with Simmental bulls. Cattle were fed corn silage ad libitum and 2.5 kg (DM) of concentrate per day in Schwarz et al. (2000). Using a broken line model, they estimated that 0.12 mg of Co/kg of DM was required for maximum ADG, and 0.16 to 0.18 mg of Co/kg of DM was required for maximum feed intake.

From d 85 until the end of the study, ADG and G:F were affected (P < 0.05) by a Co x grain interaction (Table 2). Cobalt supplementation increased (P < 0.05) ADG in steers fed corn-based diets but not in those fed barley-based diets. Gain:feed was not affected by Co supplementation in steers fed corn-based diets but was decreased (P < 0.05) by Co in steers fed barley-based diets. These responses were unexpected because the barley-based diet was lower (0.02 vs. 0.04 mg/kg of DM) in Co than was the corn-based diet. Average daily gain, ADFI, and G:F for the entire study were not affected by a Co x grain interaction.

Steers fed corn-based diets had greater ADG (P < 0.05), G:F (P < 0.01), and final BW than those fed barley-based diets. In contrast, Mathison and Engstrom (1995) reported similar performance among finishing steers fed barley vs. corn-based diets; however, results of other studies, in which corn was substituted for either rolled (Pritchard and Robbins, 1991) or unprocessed barley (Mandell et al., 1997), suggested that cattle consuming barley-based diets had lower intakes and ADG than those consuming corn-based diets. In the current study, NDF and ADF were greater in the barley-based diets relative to the corn-based diets (as % of DM, NDF = 33.2 vs. 24.5; ADF = 9.4 vs. 6.0, for barley and corn, respectively). Greater NDF and ADF in the barley-based diets would be expected to lower the ME of these diets and could explain the decreased ADG and G:F.

Vitamin B₁₂ and Folate
Plasma vitamin B₁₂ concentrations were affected by Co supplementation (P < 0.01), grain source (P < 0.05), and day of sampling (P < 0.01; Table 3). By d 56, and at all subsequent sampling times, steers receiving supplemental Co had greater (P < 0.05) plasma vitamin B₁₂ concentrations than did those consuming the unsupplemented control diets. Increasing supplemental Co from 0.05 to 0.15 mg/kg of DM did not increase plasma vitamin B₁₂ status at any sampling period. Steers consuming the corn-based diets had greater (P < 0.05) plasma vitamin B₁₂ concentrations from d 28 to 112 than steers fed the barley-based diets.

Steers receiving supplemental Co had greater (P < 0.05) liver vitamin B₁₂ concentrations at slaughter than those that consumed the unsupplemented control diets (Table 3). However, increasing supplemental Co from 0.05 to 0.15 mg/kg of DM did not significantly increase liver vitamin B₁₂. In contrast, increasing supplemental Co from 0.10 to 1.0 mg/kg of DM in finishing steers fed corn-based diets low in Co (0.04 to 0.05 mg/kg) increased liver vitamin B₁₂ concentrations (Tiffany et al., 2002, 2003). Based on liver vitamin B₁₂ concentrations at the end of a 280-d study with growing-finishing cattle, Stangl et al. (2000) estimated that 0.236 mg of Co/kg of DM was required for maximum liver vitamin B₁₂ concentrations. Liver vitamin B₁₂ concentrations were not significantly affected by grain source but tended (P = 0.13) to be less in steers fed barley-based diets (Table 3).

Ruminal fluid vitamin B₁₂ concentrations on d 84 were affected (P = 0.06) by a Co x grain interaction. In steers consuming the corn-based diet, ruminal fluid vitamin B₁₂ was increased (P < 0.05) by supplemental Co and was greater (P < 0.05) in steers supplemented with 0.15 mg of Co/kg of DM compared with those fed 0.05 mg of Co/kg of DM. This result agrees with previous research (Tiffany et al., 2002), in which Co supplementation increased ruminal fluid vitamin B₁₂ concentrations in finishing steers fed corn-based diets. Ruminal fluid vitamin B₁₂ concentrations were not increased by supplemental Co in steers fed the barley-based diets. When Co was supplemented at 0.15 mg/kg of DM, steers fed the barley-based diet had much lower (P < 0.05) ruminal fluid vitamin B₁₂ concentrations than those fed the corn-based diet. The lack of an increase in ruminal fluid vitamin B₁₂ concentrations with Co supplementation in barley-fed steers was unexpected; however, the lower ruminal vitamin B₁₂ concentrations in steers fed barley-based diets is consistent with the lower plasma vitamin B₁₂ concentrations observed in steers fed barley- vs. corn-based diets. These results suggest that ruminal fermentation in cattle fed barley-based diets may result in less vitamin B₁₂ production than in cattle fed corn-based diets.

Plasma folate concentrations were affected (P < 0.01) by dietary Co, grain source, day of sampling, and a grain x day of sampling interaction (Table 4). Steers supplemented with 0.05 or 0.15 mg of Co/kg of DM had greater (P < 0.05) plasma folate concentrations on d 84 and 112 than did control steers. However, plasma folate was not significantly affected by supplemental Co on the other days of sampling. Steers consuming the barley-based diet had lower (P < 0.01) plasma folate from d 28 to 112 than steers receiving the corn-based diet, but by d 140, the grain effect had diminished.

The role of folate during vitamin B₁₂ deficiency in ruminants is poorly understood. The methyl trap hypothesis, in which the conversion of 5-CH₃-H₄-folate to H₄-folate is impaired due to decreased methionine synthase activity, remains a plausible explanation (Stabler, 1999). Based on this hypothesis, folate would be trapped as the 5-methyl derivative and not be reoxidized to H₄-folate, thereby resulting in a functional folate deficiency (Shane and Stokstad, 1985). Cobalt supplementation of diets containing 0.07 to 0.08 mg of Co/kg of DM did not increase serum or plasma folate concentrations in cattle (Stangl et al., 1999b, 2000); however, in agreement with the current study, liver folate was increased by Co supplementation in those studies. Plasma folate concentrations were increased by Co supplementation in steers fed a basal diet that contained 0.05 mg of Co/kg of DM (Tiffany et al., 2002).

Ruminal Fluid VFA
Addition of supplemental Co to the diets decreased (P < 0.05) acetate and increased (P < 0.01) propionate molar proportions in ruminal fluid, resulting in a lower (P < 0.01) acetate:propionate ratio (Table 5). These findings are consistent with recent studies (Tiffany et al., 2002, 2003) with finishing steers, in which Co supplementation of high-concentrate diets, low in Co, increased molar proportion of propionate and decreased acetate:propionate ratio. Acetate:propionate ratio was affected by a Co x grain interaction (P = 0.07). Cobalt supplementation decreased acetate:propionate ratio to a greater extent in steers fed barley- than in corn-based diets. In ruminal microbes, the vitamin B₁₂-dependent enzyme methylmalonyl-CoA mutase catalyzes conversion of succinyl CoA to methylmalonyl-CoA (Nagaraja et al., 1997), during propionate production via the dicarboxylic pathway. The decreased molar proportion of propionate observed may reflect the lower ruminal vitamin B₁₂ concentrations observed in the unsupplemented control steers, and a resultant decrease in microbial methylmalonyl-CoA mutase activity.

Molar proportion of butyrate in ruminal fluid was affected by a Co x grain interaction (P < 0.05; Table 5). Cobalt supplementation decreased (P < 0.01) molar proportion of butyrate in steers fed barley, but it did not significantly affect butyrate in those fed corn-based diets. When Co was supplemented at 0 or 0.05 mg/kg of DM, steers fed barley-based diets had greater (P < 0.05) molar proportions of butyrate than those fed corn-based diets; however, molar proportion of butyrate was similar for the grain sources when Co was supplemented at 0.15 mg/kg of DM. Molar proportions of iso-butyrate and valerate were decreased (P < 0.05) by Co supplementation. Isobutyrate and isovalerate were greater (P < 0.05), whereas valerate molar proportion was less (P < 0.05) in steers fed barley-based diets than in those fed given corn-based diets (Table 5).

Methylmalonic Acid, Succinate, and Glucose
Plasma methylmalonic acid concentrations were not affected by day of sampling or by a Co x day of sampling interaction (Table 6). Cobalt addition to the control diet decreased (P < 0.01) plasma methylmalonic acid concentrations and increasing supplemental Co from 0.05 to 0.15 mg/kg of DM further decreased (P < 0.01) plasma methylmalonic acid regardless of grain source. Cobalt deficiency results in increased plasma methylmalonic acid concentrations due to decreased activity of methylmalonyl-CoA mutase, a vitamin B₁₂-dependent enzyme (Kennedy et al., 1990). In previous studies (Tiffany et al., 2002, 2003), Co supplementation of low-Co diets decreased plasma methylmalonic acid concentrations in finishing steers, and increasing supplemental Co from 0.05 to 0.10 mg/kg of DM further decreased methylmalonic acid concentrations. Diagnostic criteria for the determination of vitamin B₁₂ deficiency based on plasma methylmalonic acid (and vitamin B₁₂) have been proposed (O’Harte et al., 1989; Paterson and MacPherson, 1990). Results of the current study also suggest that plasma methylmalonic acid is a good indicator of vitamin B₁₂ status, and, thus, clinical and subclinical Co deficiency. However, the methylmalonic acid concentrations of 2 to 4 µmol/L suggested as indicative of subclinical Co deficiency in cattle fed forage-based diets (Paterson and MacPherson, 1990) may be too low for cattle fed high-concentrate diets. In our study, plasma methylmalonic acid averaged 2.31 µmol/L in steers supplemented with 0.15 mg of Co/kg of DM, a concentration that seemed to provide adequate Co.

Plasma succinate was affected by day of sampling (P < 0.01) but not by Co or by Co x day of sampling or Co x grain interactions (Table 6). Steers supplemented with 0.05 mg of Co/kg of DM had greater (P < 0.10) ruminal fluid succinate concentrations than did those supplemented with 0.15 mg of Co/kg of DM; however, ruminal succinate in steers fed the control diets did not differ from those supplemented with 0.05 mg of Co/kg of DM. In contrast to the current study, Tiffany et al. (2002) found greater plasma succinate concentrations in steers fed a low-Co diet compared with Co-supplemented steers. The discrepancy between that study and the current one may relate to the degree of Co deficiency. In the current study, control steers were fed the low-Co diet for a shorter time. Steers in the current study were less Co-deficient at the end of the study based on plasma and liver vitamin B₁₂ and plasma methylmalonic acid concentrations than in the previous study (Tiffany et al., 2002).

Steers fed barley-based diets had greater (P < 0.05) plasma and ruminal succinate concentrations than those fed corn-based diets (Table 6). These findings are consistent with the lower ruminal vitamin B₁₂ concentrations observed in barley-fed steers relative to corn-fed steers (Table 3). Decreased ruminal vitamin B₁₂ production in barley-fed steers, relative to those corn-fed, may have decreased microbial methylmalonyl-CoA mutase activity, resulting in accumulation of ruminal succinate and decreased ruminal proportions of propionate. Kennedy et al. (1991) reported much higher ruminal and plasma succinate concentrations in sheep fed a barley-based diet low in Co (0.004 mg/kg of DM) compared with those supplemented with 1.0 mg of Co/kg of DM.

Plasma glucose increased (P < 0.01) in response to supplemental Co (P < 0.01; Table 6). The slightly lower plasma glucose concentrations in steers fed the low-Co diets is consistent with the findings of recent studies (Tiffany et al., 2002, 2003). The lower plasma glucose observed in control steers may relate to the lower molar proportions of ruminal propionate observed in those animals. In addition, the lower plasma vitamin B₁₂ concentrations observed in the control steers may have decreased conversion of methylmalonyl-CoA to succinyl-CoA in the liver due to decreased methylmalonyl-CoA mutase activity, thereby limiting the availability of gluconeogenic precursors. Plasma glucose was lower (P < 0.01) in steers fed barley-based diets than in those fed corn-based diets (Table 6). This result is consistent with the lower molar proportions of ruminal propionate in steers fed barley-based diets, limiting the concentration of propionate available for postabsorptive metabolism and gluconeogenesis.

Carcass Characteristics
Steers receiving supplemental Co had greater HCW (P < 0.01) and dressing percent (P < 0.10) than did control steers (Table 7). Previous studies (Tiffany et al., 2002, 2003) have reported greater HCW when supplemental Co was added to Co-deficient diets. Other carcass measurements were not affected by dietary Co.

In conclusion, based on improved ADG and DMI, increased plasma and liver vitamin B₁₂ and folate, and decreased plasma methylmalonic acid concentrations in response to supplemental Co, the control diets (0.02 or 0.04 mg of Co/kg of DM) used in the current study were deficient in Co. Steer performance did not differ significantly among steers supplemented with 0.05 or 0.15 mg of Co/kg of DM. Numerical increases in plasma and liver vitamin B₁₂ that occurred when supplemental Co was increased from 0.05 to 0.15 were also nonsignificant; however, the lower plasma methylmalonic acid concentration observed in steers supplemented with 0.15 mg of Co/kg of DM suggests that methylmalonyl-CoA mutase (a vitamin B₁₂-dependent enzyme) limited conversion of methylmalonyl-CoA to succinyl CoA in steers receiving 0.05 mg of supplemental Co/kg of DM.

Numerous metabolic characteristics affected by Co deficiency were altered to a greater degree in steers fed barley- vs. corn-based diets. Interpretation of data is complicated by the fact that the control barley diet was lower in Co (0.02 vs. 0.04 mg/kg of DM) than the control corn diet. The highest concentration (0.15 mg) of supplemental Co evaluated in our study eliminated differences among barley- and corn-based diets in some me-tabolites altered by Co deficiency, such as liver folate and ruminal proportion of butyrate. The much higher ruminal acetate:propionate ratio observed in steers fed barley- vs. corn-based diets was also partially alleviated by Co supplementation; however, plasma vitamin B₁₂ concentrations were lower in steers fed barley-based diets, regardless of dietary Co. Based on ruminal samples collected on d 84 of the study, ruminal microorganisms in steers fed barley diets were unable to increase vitamin B₁₂ production in response to increased Co supply. The greater ruminal and plasma succinate concentrations noted in steers fed barley- vs. corn-based diets and the lack of a Co x grain source interaction is consistent with this hypothesis. It is possible that cattle fed high-barley diets will benefit from an exogenous supply of vitamin B₁₂ to meet microbial requirements for vitamin B₁₂-dependent enzymes. Additional research is needed to further define Co requirements of finishing cattle especially those fed high-barley diets.

	Footnotes

 
¹ Use of trade names in this publication does not imply endorsement by the North Carolina Agric. Res. Service or criticism of similar products not mentioned.

² Current address: Burkmann Feeds, Greeneville, TN 37744.

³ Correspondence: Box 7621 (phone: 919-515-4008; fax: 919-515-4463; e-mail: Jerry_Spears{at}ncsu.edu).

Received for publication February 4, 2005. Accepted for publication July 19, 2005.

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