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ANIMAL PRODUCTION |
* Department of Animal and Food Sciences, Texas Tech University, Lubbock 79409-2141; and
Novus International Inc., St. Louis, MO 63304
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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0.006) with increasing HMTBA dose. No differences (P 
0.10) were noted for carcass characteristics, except for a tendency (P = 0.078) for a linear increase in the percentage of cattle grading USDA Choice with increasing HMTBA dose. In Exp. 2, 80 crossbred steers (initial BW = 450 ± 17 kg) in a randomized complete block design were assigned to a control (0% HMTBA) diet or to a diet in which the concentrations of HMTBA were gradually increased from 0.036 to 0.212% of DM over a 50-d period. The HMTBA-containing diet tended to decrease DMI (P = 0.132), but G:F (P = 0.319) for the overall feeding period, carcass measurements, and USDA quality grade (P 0.149) did not differ between treatments. In Exp. 3, continuous culture fermenters (n = 5/treatment) were used to determine the effects of HMTBA (control vs. 0.24% HMTBA) on microbial fermentation. No differences (P 0.31) were detected between treatments in ruminal OM digestibility, microbial N synthesis, pH, ammonia, molar proportions of VFA, or effluent concentration of selected long-chain fatty acids. These results suggest that HMTBA decreased DMI by feedlot steers fed a steam-flaked corn-based diet in a dose-dependent manner; however, gradually increasing the dose over time seemed to moderate effects on DMI. No major changes in microbial fermentation in continuous culture were observed with HMTBA at 0.24% of dietary DM, suggesting effects of HMTBA on DMI were not likely associated with changes in ruminal digestion or fermentation.
Key Words: continuous culture • feed intake • feedlot performance • 2-hydroxy-4-(methylthio)-butanoic acid
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Choline also functions as a methyl group donor via betaine, and ruminally protected choline has been reported to improve growth performance by feedlot cattle without negatively affecting carcass characteristics (Bryant et al., 1999; Bindel et al., 2000). The dietary requirement for S-containing AA for finishing beef cattle is likely met without supplementation when high-concentrate diets are fed (Titgemeyer and Merchen, 1990; Hussein and Berger, 1995). Nonetheless, even when Met is not a limiting AA, cattle might respond to supplemental Met via its role as a methyl group donor. Potential effects of Met in this role could result from changes in microbial lipid metabolism or host responses to additional methyl groups.
The development of Met supplements, such as DL-Met and 2-hydroxy-4-(methylthio)-butanoic acid (HMTBA), often termed methionine hydroxy analog, has played a major role in the advancement of AA nutrition of non-ruminant livestock species. In ruminants, free Met can be extensively degraded in the rumen; however, because of its chemical structure, HMTBA is capable of escaping ruminal degradation to some extent (Patterson and Kung, 1988). In addition, HMTBA can be produced less expensively than ruminally protected DL-Met. The use of HMTBA as a supplement for dairy cattle has been studied extensively, and it has increased milk yield and milk fat percent (Lundquist et al., 1983; NRC, 2001). Rodriguez et al. (2002) and Venable et al. (2005) reported improved performance of growing beef cattle fed forage-based diets supplemented with HMTBA, presumably as a result of effects on AA nutrition. Limited research has been conducted with HMTBA in high-grain diets for finishing beef cattle, for which, as noted previously, responses to supplying additional Met per se would be less likely than with growing cattle fed forage-based diets.
Our objective was to evaluate the effects of feeding HMTBA on performance and carcass characteristics of feedlot cattle. As a result of changes noted in feed intake, we also evaluated the effects of HMTBA on microbial digestion and fermentation in a continuous-culture system.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Experiment 1
Steers. Two groups of steers arrived at the Texas Tech University Burnett Center during January and February 2004 (170 crossbred British x Continental steers). Both groups were processed on arrival, which included: individual BW measurement (average = 270.8 kg); placement of a numbered ear tag; vaccination with a modified live virus vaccine (Titanium 5, Agri-Labs, Des Moines, IA) and a clostridial bacterin-toxoid (Vision 7 with SPUR, Intervet, Millsboro, DE); and treatment with moxidectin for parasites (Cydectin, Fort Dodge Animal Health, Overland Park, KS). Initially, steers were fed a 65% concentrate diet, which was subsequently switched to a 75% concentrate fed to limit ADG to approximately 0.9 kg/d for a 90-d period. Toward the end of this period, individual BW measurements were obtained, 160 steers were selected for use in the experiment based on uniformity of BW, and the steers were blocked in groups of 20 by BW. Within each block, steers were assigned randomly to pens (5 steers/ pen), after which treatments were assigned randomly to pens within each block. Pens had a concrete, partially slotted floor and were 2.9-m wide x 5.6-m deep with 2.4 m of linear bunk space. Steers were switched to an 85% concentrate diet and subsequently limit-fed, as described previously, for 7 d, at which time the steers in blocks 1 through 3 were implanted with Revalor S (Intervet) and those in blocks 4 through 8 were implanted with Ralgro (Schering-Plough Animal Health, Union, NJ). Approximately 5 d later, steers were weighed to begin the experiment. At this time, the diets were changed to the final 90% concentrate level (DM basis), and the treatments were initiated.
Treatments.
The 4 experimental diets consisted of a control (0% HMTBA) and diets with 0.069, 0.137, and 0.204% HMTBA (from ALIMET feed supplement, Novus International Inc., St. Louis, MO, which contains 88% HMTBA) that were designed to provide 5, 10, and 15 g/(steer·d) of HMTBA, respectively, based on an assumed DMI of 8.33 kg/d. Based on actual DMI for the overall experiment, HMTBA intake averaged 4.84, 9.45, and 13.75 g/(steer·d) for the 0.069, 0.137, and 0.204% diets, respectively. Ingredient and chemical composition data of the diets are shown in Table 1.
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. Steers were weighed (unshrunk; individual or pen basis as noted below) at 28-d intervals. At each weigh period, feed bunks were cleaned, orts were weighed, and a sample was taken and dried in a forced-air oven at 100°C for approximately 24 h. Average DMI by a pen was determined by subtracting the DM content of the refused feed at the end of the period from the quantity of DM delivered to the pen for the entire period. The corrected total of DM delivered was then divided by the number of animal-days to determine average DMI/steer in the pen. Steers were weighed individually in the morning before feeding (typically from 0600 to 0800 h) on d 0, 56, and just before shipment to slaughter. On d 56, steers in blocks 4 through 8 were reim-planted with Revalor S (Intervet). Individual BW measurements were obtained using a C & S single-animal squeeze chute (Garden City, KS) set on 4 Rice Lake Weighing Systems (Rice Lake, WI) load cells that was calibrated with 453.6 kg of certified weights before use. Steers were weighed on a pen basis using a platform scale (readability = ± 2.27 kg) on d 28, 84, and 112. The platform scale was calibrated with 453.6 kg of certified weights before each use.
Carcass Evaluation. Steers were shipped to slaughter on the basis of BW and visual evaluation to estimate external fat cover; thus, blocks were shipped to slaughter (Cargill, Plainview, TX) at different times. Steers in blocks 1 and 2 were shipped after 120 d on feed, whereas steers in blocks 3 through 6 were shipped after 140 d, and those in blocks 7 and 8 were shipped to slaughter after 154 d on feed. Personnel from the West Texas A&M University (WTAMU) Beef Carcass Research Center followed the steers to the slaughter facility to obtain carcass data. Measurements included HCW, LM area, marbling score, KPH, 12th-rib fat, and USDA calculated yield and quality grades. Because of an error in the packing plant, data were not available for all carcasses. Thus, complete data were available for 157, 155, 146, 155, 153, 154, 144, and 144 carcasses for HCW, dressing percent, marbling score, quality grade, 12th-rib fat, LM area, KPH, and yield grade.
Statistical Analyses. Performance data were analyzed as a randomized complete block using the MIXED procedure (SAS Inst. Inc., Cary, NC). Pen was the experimental unit, and block was a random effect. Carcass data (except USDA quality grade) were analyzed using the MIXED procedure of SAS with a model that included treatment as a fixed effect, and block and block x treatment as random effects. Percentages of carcasses grading USDA Choice or greater within a pen were analyzed as a binomial proportion using the GLIMMIX procedure of SAS. Linear and quadratic orthogonal contrasts for HMTBA dose were evaluated for all response variables.
Experiment 2
Steers. Eighty British x Continental steers that were consuming a 65% concentrate diet ad libitum were weighed (average BW = 431.4 kg), switched to a 77.5% concentrate diet, and subsequently switched to an 85% concentrate diet 5 d later. The steers were assigned to 8 BW blocks (10 steers/block) and then were assigned randomly to 1 of 2 treatments, with treatments assigned randomly to the 2 pens within each block (5 steers/pen). Steers were moved to their assigned pens (same pens as in Exp. 1) and implanted with Revalor S (Intervet). Seven days later, all steers were switched to a 90% concentrate diet, and after an additional 2 d, the steers were weighed individually to begin the experiment.
Treatments.
Treatment diets were a standard 90% concentrate (DM basis), steam-flaked corn (SFC)- based finishing diet without HMTBA (control) and an HMTBA-containing diet, in which the concentrations of HMTBA were increased with time on feed. The initial concentration of HMTBA was set to supply approximately 2.5 g/steer daily for the first 7 d, with a switch to 5 g/steer daily thereafter through d 28 of the feeding period. At each subsequent week, the concentration of HMTBA was increased to supply an additional 2.5 g/ (steer·d) until a final concentration designed to supply approximately 15 g/(steer·d) was achieved on d 50 of the study. This process resulted in the following concentrations of HMTBA in the dietary DM: 0.036% for d 0 to 7; 0.072% for d 8 to 28; 0.107% for d 29 to 35; 0.142% for d 36 to 42; 0.178% for d 43 to 49; and 0.212% from d 50 through the end of the feeding period. As in Exp. 1, the HMTBA was added to the diet as a liquid premix. Ingredient composition data for the 2 diets used in the study are shown in Table 2. In an effort to decrease the potential negative effects of HMTBA on DMI observed in Exp. 1, a special supplement was used with the HMTBA treatment, which had ammonium sulfate removed to compensate for the increased S added to the diet by HMTBA (Table 2).
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. Individual initial and final BW measurements (unshrunk) were collected with the equipment described for Exp. 1, and pen weights were measured on d 28, 56, 84, and 112 (some blocks of steers had already been shipped on d 84 and 112). Steers in blocks 1, 2, and 3 were shipped after 131 d on trial, whereas those in blocks 4 and 5 were shipped after 103 d, and steers in blocks 6, 7, and 8 were shipped after 69 d on feed. Carcass Evaluation. Personnel from the Texas Tech University Meat Lab and WTAMU Beef Carcass Research Center obtained carcass data (same measurements as in Exp. 1) at the slaughter facility described for Exp. 1.
Statistical Analyses. Pen performance and carcass data were analyzed as a randomized complete block using the MIXED procedure of SAS, with the proportion of steers grading USDA Choice or greater analyzed using the GLIMMIX procedure of SAS.
Experiment 3
Preparation of the Apparatus.
Ten 1-L, continuous culture flasks (Bellco Glass, Vineland, NJ) were used to determine the effects of the addition of the Ca salt of HMTBA (MFP, Novus International Inc.) on fermentation and microbial growth in a continuous culture system similar to that described by Slyter and Putnam (1967). The buffer used was McDougall’s artificial saliva (McDougall, 1948), which was infused via a Carter-Monostat 12/6 (Barnant Inc., Barrington, IL) peristaltic cassette pump. Flow rate for each flask was adjusted so that approximately 50 mL/h were infused. Each flask contained a magnetic stir rod that was turned by an individual stir plate located beneath the flask, and a vented tube was attached to the flask to allow gas to escape. An overflow port allowed effluent to flow into glass 2.5-L bottles that were located in an ice bath beneath the fermenters.
Ingredient composition of the 2 diets is shown in Table 2. Treatments were included in a standard 90% concentrate (DM basis), SFC-based finishing diet that was formulated to be similar to the diets fed in Exp. 1 and 2. The control diet contained 0% HMTBA, and the experimental diet was formulated to provide 0.24% HMTBA on a DM basis. Rather than using the liquid form of HMTBA as described in Exp. 2 and 3, the dry Ca salt of HMTBA (85% purity) was used because of ease of handling. The SFC used in these diets was air-dried to facilitate storage and handling. Zinn and Barrajas (1997) reported that air-drying did not alter the feeding value of SFC. The treatment diets were mixed by tumbling for approximately 2 to 3 min in 18.9-L plastic buckets and then ground to pass a 2-mm screen in a Wiley mill before being added to the fermenters. Treatments were applied randomly to the flasks (5 fermenters/ treatment), and flask was the experimental unit.
Collection of Ruminal Inoculum. On the afternoon of the day before beginning the experiment, ruminal inoculum was obtained from 2 ruminally cannulated Jersey steers (approximately 3 yr of age; BW approximately 450 kg) fed a SFC-based, 75% concentrate diet. A sufficient quantity of fluid to inoculate all 10 fermenters was collected approximately 4 h after feeding and strained through 4 layers of cheesecloth into an insulated container. After returning to the laboratory, the ruminal fluid was bubbled vigorously with CO2. At this time, a pH measurement (Accumet Basic pH meter, Fisher Scientific, Hampton, NH) was recorded as the beginning pH for all flasks.
Sampling Procedures. One liter of inoculum was added in random order to each fermenter. The flasks were immersed in a water bath equipped with an Isotemp 2100 immersion circulating heater pump (Fisher Scientific, Hampton, NH) that maintained the temperature at 39°C. After all flasks had been filled, a glass pipette was used to transfer 25-mL aliquots of inoculum from flask to flask, which was done to further ensure that the inoculum was equalized among flasks. The lids of the fermenters were fastened, the peristaltic pump was activated, and 10 g (as-fed basis) of the assigned treatment diets was added to each flask. Thereafter, 20 g (as-fed basis) of the corresponding diet was added to each flask at 0900 and 2100 h daily.
Aliquots (12.5-mL) of ruminal fluid were collected before feed was added to the flasks at 0900, 0930, 1000, 1100 h, and before feed was added at 2100 h daily. The pH of these samples was determined, 500 µL of a 25% (vol/vol) sulfuric acid solution were added to stop fermentation, and samples were frozen for later analyses. In addition, at 0800 h each daily, the 2.5-L effluent bottles were emptied, and the contents were measured volumetrically to calculate the flow rate for each fermenter. On the morning of d 5 through 7 after the initial inoculation, the entire quantity of effluent was collected and frozen, which corresponded to a 24-h collection of effluent for d 4 through 6. On the morning of d 7, after the sample had been removed from the fermenter, the entire contents of the fermenter were retained for further analysis and preserved with the addition of 100 mL of a 10% formalin (vol/vol) solution and refrigeration, as described by Merchen and Satter (1983).
Sample Analyses.
The 12.5-mL aliquots taken from the fermenters 5 times daily were thawed, and the heavier particles were allowed to settle to the bottom of the sample container. A 2-mL aliquot was removed from the clear fraction of the fluid sample and centrifuged at 10,000 x g for 5 min. The clear supernatant fluid from this centrifugation was retained and frozen. The centrifuged samples collected at 2 h after feeding were thawed, treated with meta-phosphoric acid, and sent to North Dakota State Univ. (J. S. Caton, Fargo, ND) for VFA analysis by gas chromatography using the procedures of Goetsch and Galyean (1983). An aliquot at each sampling time also was thawed and used for ammonia analysis using a phenol-hypochlorite method (Broderick and Kang, 1980).
The contents from the 2.5-L effluent bottles from d 4 through 6 were weighed, placed in aluminum pans, and then allowed to dry in a forced-air oven at approximately 30°C for 2 d, after which the temperature was set to approximately 60°C for another 2 d to allow for complete drying. The change in weight was used to calculate the DM content of the effluent from the fermenters. These dried samples were then ground to pass a 2-mm screen in a Wiley mill, composited over days within fermenter, and stored frozen in plastic containers. Ash content of the effluent samples was obtained by placing approximately 1 g of dried sample in a 500°C muffle furnace for 2 h. Nitrogen content of the effluent was measured with a combustion analyzer (Leco FP-2000, Leco Inc., St. Joseph, MI). Ammonia concentration of the liquid effluent was analyzed using the phenol-hypochlorite method described previously for the fermenter samples.
Fatty acids in the fermenter effluent samples were extracted by a chloroform/methanol method adapted from Yong and Watkins (2001), and fatty acid methyl esters were quantified by gas chromatography with using an Agilent 6890 Series Gas Chromatograph (Agilent Technologies, Santa Rosa, CA) equipped with a Hewlett-Packard 7683 auto injector (Hewlett-Packard, Palo Alto, CA) and a HP-88 capillary column (100-m x 0.25-mm i.d. with a 0.2-µm film thickness; Agilent Technologies). The analysis involved a programmed run with a split/splitless injection system (split ratio of 1:5) and He as the carrier gas at a flow rate of 1.5 mL/ min. After injection (1 µL), the column temperature was held at 75°C for 2 min and then was increased to 180°C at 5°C/min. The temperature was maintained at 180°C for 33 min, followed by an increase of 4°C/min to 225°C, with the temperature finally held at 225°C for 44 min. The detector and injection temperatures were set at 250°C. Identification of peaks was accomplished by comparison to retention times of commercially prepared standards (Nu Chek Prep, Elysian, MN). Peaks were quantified by peak area comparisons with a known quantity of an internal standard (0.5 mg/mL of heptadecanoic acid; Nu Chek Prep).
The bacterial fraction of the whole fermenter samples was separated using differential centrifugation, as described by Merchen and Satter (1983). This fraction was then dried in a freeze drier (Duradry, FTS Systems; Stone Ridge, NY) and subsequently was analyzed for DM (method 930.15) and ash (method 942.05; AOAC, 1995). In addition, the samples were analyzed for N using the Leco analyzer described previously. The purine content of the lyophilized bacterial fraction and dried effluent samples was determined using the method of Zinn and Owens (1986). Diet samples were analyzed for DM, ash, N, and fatty acids by the methods described above for the fermenter and effluent samples.
Statistical Analyses. The pH and ammonia data collected from each flask on d 3 through 6 at 0900, 0930, 1000, 1100, and 2100 h were analyzed as a split-plot design using the MIXED procedure of SAS. Flask was the experimental unit. Fixed effects were treatment, sampling day, sampling time, and their interactions, with random effects of flask within treatment and flask within day x treatment. The VFA data collected for d 3 through 6 at 2 h after feeding were analyzed as repeated measures using the MIXED procedure of SAS, with fixed effects of treatment and sampling day, and flask within treatment as the subject of the repeated measures. An autoregressive covariance structure was used. All other data were analyzed as a completely randomized design. Daily effluent flow (milliliters) was evaluated as a covariate for all models. Daily flow was retained in the model as a covariate when significant (P < 0.05), but to conserve degrees of freedom, it was dropped from the model when not significant.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Final BW decreased linearly (P = 0.069) as the percentage of HMTBA increased in the diet. Similarly, adjusted final BW (calculated from HCW and the average dressing percent) decreased linearly (P = 0.063) as HMTBA concentration increased. For the overall feeding period, the difference in ADG between the 0 and 0.204% levels of HMTBA was 0.10 kg/d (linear effect; P = 0.099) based on live weight gain data and 0.13 kg/d (linear effect, P = 0.082) based on carcass-adjusted data. The difference in overall ADG between the 0 and 0.069% HMTBA diets was fairly small (0.04 kg/d). The negative effect of HMTBA on ADG was primarily attributable to decreased ADG early (e.g., d 0 to 28) in the feeding period. The linear decreases in ADG noted with increasing doses of HMTBA were a function of lower DMI (Table 3). For all cumulative periods of the experiment, DMI decreased linearly (P 0.006) as the percentage of HMTBA in the diet increased. Gain:feed ratio (Table 3) was not affected by level of HMTBA in the diet, except during the first 28 d of the feeding period, when the negative effect of HMTBA on DMI was most evident. After d 28, no differences were noted among treatments for G:F (P 0.22), suggesting that the negative effects of HMTBA on ADG were entirely a result of decreased DMI. Likewise, when mean performance data for each treatment were used to calculate NE values, the NEm and NEg concentrations of diets containing HMTBA were slightly greater than those for the control (NEm = 2.27, 2.33, 2.34, and 2.36 Mcal/ kg of DM; NEg = 1.58, 1.63, 1.64, and 1.66 Mcal/kg of DM for the 0, 0.069, 0.137, and 0.204% HMTBA diets, respectively). The slightly greater NE values for diets containing HMTBA reflect the finding that effects on DMI were consistent across the experiment, whereas effects on ADG occurred primarily in the first 28 d of the experiment. The performance-based NE values were greater for all 4 diets than values calculated from NRC (1996) tabular values for the feedstuffs used in the diets (2.15 and 1.47 Mcal/kg of DM for NEm and NEg, respectively, assuming that HMTBA liquid has the same energy value as cane molasses).
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); however, a nonsignificant (P = 0.156) decrease in HCW was noted with increasing HMTBA dose, which supports the decrease noted previously in final BW. Somewhat surprisingly, 12th-rib fat did not differ among treatments, despite the somewhat lower HCW noted at the higher doses of HMTBA. In addition, marbling score tended to increase linearly (P = 0.107) with increasing HMTBA concentration in the diet, as did the percentage of steers grading Choice or greater (P = 0.078).
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Final BW (P = 0.225) and carcass-adjusted final BW (P = 0.149) did not differ between treatments (Table 5). The difference in final BW between the 0 and 0.204% levels in Exp. 1 was approximately 15.4 kg, or nearly twice that noted in Exp. 2. Thus, the approach of gradually increasing the dietary concentration of HMTBA in Exp. 2 seemed to have lessened potential negative effects of HMTBA on ADG and BW. Dry matter intake was not affected by treatment for d 0 to 28, d 0 to 56, or for the overall feeding period (P 0.132; Table 5). Nonetheless, DMI was consistently less by steers in the HMTBA treatment group than by control steers. The slight decrease in DMI of 1.6% for the HMTBA treatment for d 0 to 28 suggests that the approach of gradually increasing the dietary concentration of HMTBA was successful in minimizing the negative effects on DMI and ADG that was observed in Exp. 1. Gain:feed ratio did not differ between the 2 treatments, except for the d 0 to 56 period, when it was less (P = 0.046) with HMTBA added to the diet than for the control diet. No differences (P 0.149) were detected in carcass measurements or USDA quality grade between the 2 treatments (Table 6).
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Digestibility, microbial N synthesis estimates, and selected fatty acid flow data are presented in Table 7. There were no treatment differences for apparent or true OM digestibility (P > 0.31). The fatty acids identi- fied from the standard used to analyze effluent samples were 18:0, trans11–18:1, cis9, trans11–18:2, cis11, trans13–18:2, and trans10 cis12–18:2. Concentrations of cis9, trans11–18:2, cis11, trans13–18:2, and trans10 cis12–18:2 in the samples used for analysis were below detection limits. There were no (P > 0.55) differences between the control and HMTBA treatments for effluent concentrations of 18:0 or 18:1 trans 11 (18:0 = 0.72 vs. 0.68 mg/g; SE = 0.095, and 18:1 trans 11 = 0.40 vs. 0.38 mg/g; SE = 0.033) or in percentage of total fatty acids that these fatty acids represented in the effluent (data not shown).
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0.063) and ammonia concentrations (P 0.149; data not shown). Neither overall fermenter pH (6.02 vs. 5.98; SE = 0.122) nor overall fermenter ammonia concentrations (2.14 vs. 1.91 mg/100 mL; SE = 0.108) differed (P > 0.10) between the control and HMTBA treatments, respectively. Similarly, for molar proportions of VFA measured at 2 h after feeding, no treatment x day interactions (P 0.10) and no differences (P 0.10) were detected between treatments (data not shown). For total concentration of VFA, a treatment x day interaction was detected (P = 0.038); however, the nature of the interaction did not preclude evaluation of treatment main effects. Thus, averaged over days, control fermenters tended (P = 0.095) to have a greater total VFA concentration than HMTBA fermenters (113.0 vs. 104.9 mM). |
DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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supplemented 3 g/(steer·d) of Hydan (E. I. Du-Pont de Nemours and Co., Wilmington, DE) a dry Ca salt of HMTBA to a high-grain (sorghum-based) diet for finishing beef cattle and reported lower ADG in the first 21 d of the trial. In contrast to our findings, results with forage-based diets seem to indicate positive effect of HMTBA on ADG. For example, Rodriguez et al. (2002) reported improved ADG compared with controls when growing cattle were fed bermudagrass hay-based diets and supplemented with HMTBA at 5, 10, or 15 g/(animal·d) in a molasses-based supplement. In beef heifers fed hay ad libitum and a soybean hull-based supplement, Venable et al. (2005) reported a trend for linear effect of HMTBA on ADG for the first 30 d of an 85-d trial in heifers fed 0, 7.5, or 15 g/d of HMTBA. In addition, heifers fed 15 g/d of HMTBA had greater reproductive tract scores than those in other treatment groups.
The effects of HMTBA on DMI in the literature are variable. In most studies with dairy cattle, feeding liquid HMTBA at much greater doses than fed in the present study has not generally affected DMI (e.g., 20 or 50 g of HMTBA/(cow·d) for transition and lactating cows; Phillips et al., 2003). Greater total DMI by dairy than by beef cattle would affect the concentration of HMTBA in the diet, but even considering differences in DMI, previously used HMTBA doses in dairy cows would typically yield greater concentrations than used in the present study. Similarly, in limit-fed (DMI at 2.25% of BW) growing beef cattle, feeding HMTBA at 10 g/d did not affect DMI or ADG with soybean hull-based diets (Löest et al., 2001). Venable et al. (2005) reported no negative effect on supplement consumption in beef heifers fed HMTBA at 7.5 and 15 g/d with forage-based diet. Results of other studies have indicated negative effects on DMI in response to feeding HMTBA. Colby (1970) noted a decrease in DMI associated with decreased ADG in the first 21 d of a trial as a result of feeding 3 g/(steer·d) of a dry analog of HMTBA to finishing steers and suggested that the decrease was attributable to a palatability problem. Ray et al. (1983) reported a 4% decrease in grain intake (from 14.3 to 13.7 kg/d) in dairy cattle fed 34 g/(animal·d) of the dry Ca salt of HMTBA. Similarly, Higginbotham et al. (1987) reported an initial decrease in grain intake (1 to 3 d) when introducing both the dry analog and liquid HMTBA to dairy cattle, but the levels of supplement fed were much greater than in our study [30 and 90 g/(animal·d), respectively], and the intake levels returned to baseline by d 4. Ruminal escape of the HMTBA in liquid HMTBA has been estimated to average 39.5% in duodenally cannulated lactating cows (Koenig et al., 2002) and to range from 21.3 to 43.2% based on continuous culture work (Vázquez-Añón et al., 2001). Absorbed HMTBA is converted to Met, resulting in increased serum concentrations of Met within 3 to 6 h of dosing (Koenig et al., 2002). The effect that increased serum concentrations of Met might have on DMI by cattle fed a highly processed grain diet like the one fed in the present study is not known. Loor et al. (2002) reported that 30 g of supplemental HMTBA/(cow·d) increased blood plasma fatty acid concentrations in cows grazing mixed clover-grass pasture. Hence, potential negative feedback effects of increased fatty acid concentrations on DMI might partially explain the present results.
It also is possible that supplemental Met in excess may be toxic to ruminants. In rats, Met is the most toxic of the AA, and it is accepted that that Met toxicity can be exhibited through decreased intake and performance in swine and poultry (Benevenga, 1974). This toxicity is considered to result from metabolites such as methanethiol and hydrogen sulfide produced by the transamination pathway (Smolin and Benevenga, 1989). Methionine toxicity has been reported in both mature dairy cattle and growing calves. Robinson et al. (2000) reported decreased DMI when cows were infused abomasally with Met at 140% of their requirement. Infusing Lys at 140% of the dietary requirement did not affect DMI, but the decrease in DMI was again observed when the cows were infused with Met and Lys together. Abe et al. (1999) reported that Met toxicity was associated with decreased feed intake and loss of BW in 103-kg calves fed a concentrate and chopped rice straw diet ad libitum. Similarly, Abe et al. (2000) reported a linear decrease in total DMI with postruminal administration of graded levels of DL-Met. Interestingly, there was a linear decrease in the concentrate fraction of DMI but no response with the forage fraction (rice straw). Effects on DMI were associated with a linear decrease in ADG and with loss of BW at postruminal administration rates of 18 and 24 g/d of DL-Met. Thus, even if Met is the first-limiting AA in ruminant diets, an imbalance in postruminal AA flow could decrease intake and performance (Abe et al., 2000). It is not clear whether an imbalance in postruminal Met occurred in the present study; however, such a scenario could have contributed to the decreased DMI.
In addition to possible direct effects of an increased Met supply on DMI, the effect of HMTBA on the dietary cation-anion balance (DCAD; [Na + K] – [Cl + S]) should be considered as a potential contributor to the decreased DMI. Given that the S content of HMTBA is 21.3% (presumably 18.74% S in HMTBA liquid), the DCAD (mEq/kg of DM) decreased from -2.99 for the 0% HMTBA diet to -11.79, -24.27, and -30.51 for the 0.069, 0.137, and 0.204% HMTBA diets. Decreasing dietary DCAD from +14.5 to -18.5 mEq/100 g of DM decreased DMI markedly in beef steers fed a 70% concentrate diet (Spears et al., 2003). To address the issue of total S, in Exp. 2, ammonium sulfate was removed from the supplement to decrease the S concentration of the HMTBA diet. Perhaps this lower S concentration along with the gradual increases in HMTBA concentration was responsible for the lesser effects on DMI noted in Exp. 2 vs. Exp. 1. Based on the concentrations fed during Exp. 2, the days on feed, and the number of blocks of cattle at each slaughter date, the overall average percentage of HMTBA fed was 0.152% of the dietary DM. In Exp. 1, the decrease in DMI with a diet containing 0.137% HMTBA in the dietary DM for the entire feeding period was 6.3% compared with the 4.4% decrease for the HMTBA-containing diet noted in Exp. 2. If postruminal Met supply, lower DCAD and total S intake, or a combination of these, are responsible for effects on DMI, it might be possible that a gradual increase in HMTBA concentration allowed the cattle to more effectively adapt to these factors.
Previous research published with addition of either Met or HMTBA to finishing diets has shown no indication of effects on carcass characteristics. In a 266-d growing-finishing trial with Holstein steers, Hussein and Berger (1995) reported no improvement in any carcass characteristics in a diet that contained ruminally protected Lys and Met. Additionally, Colby (1970) reported no change in carcass characteristics when HMTBA was fed to steers consuming a high-concentrate finishing diet. Our results confirm these previous observations and suggest that adding HMBTA to finishing diets would not likely affect 12th-rib fat, marbling score, or USDA quality grade.
In Exp. 3, no changes in digestion OM, microbial growth, pH, ammonia, VFA proportions, or concentrations of selected long-chain fatty acids in the fermenter effluent were noted between the control and HMTBA treatments. Thus, these results suggest that effects of HMTBA on DMI noted in Exp. 1 were not likely associated with changes in OM digestion, ruminal fermentation patterns, or microbial lipid metabolism.
A negative effect on DMI were observed in the present study when feedlot cattle were supplemented with HMTBA at concentrations that yielded intakes 5 g/d of HMTBA in high-concentrate, SFC-based diets. This effect seemed most dramatic during the initial 28 d of the finishing period. Thus, HMTBA might be useful as an intake limiting/control agent or in scenarios where intake control of high-grain diets is crucial. Nonetheless, because very few studies have been conducted with the inclusion of HMTBA in high-concentrate beef cattle finishing diets, further research is needed in this area.
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3 Current addresses: C. S. Abney (Quality Distiller’s Grain, Hereford, TX); J. P. McMeniman (Nutrition Service Associates, Kenmore, Queensland, Australia).
2 Corresponding author: k.wilson{at}ttu.edu
Received for publication December 11, 2007. Accepted for publication March 28, 2008.
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