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* Department of Animal and Food Sciences, Texas Tech University, Lubbock 79409-2141 and
and
Clayton Livestock Research Center, Clayton, NM 88415
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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0.01) between WG classifications. Within SDOF, ADG for CON and LA45-51L did not differ (P = 0.70), but both were less (P 0.08) than for LA45-51H. Overall, these data indicate that live cultures of LA plus PF did not greatly affect feedlot performance and carcass characteristics. Some of the DFM used decreased fecal EC shedding, which might be related to the results for ileal lamina propria thickness.
Key Words: Escherichia coli O157 • Fattening Performance • Feed Intake • Intestinal Mucosa • Lactobacillus acidophilus • Propionibacterium freudenreichii
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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defined probiotics as "a live microbial feed supplement, which beneficially affects the host animal by improving its intestinal microbial balance." This definition did not include the possibility of improving microbial conditions in the rumen. Kmet et al. (1993) defined ruminal probiotics as "live cultures of microorganisms that are deliberately introduced into the rumen with the aim of improving animal health or nutrition." Thus, for ruminants, the possibility exists that probiotics improve microbial conditions in both the rumen and the lower digestive tract.
The term probiotic is generic and all-encompassing, to include microbial cultures, extracts, and enzyme preparations. As such, the Office of Regulatory Affairs of the Food and Drug Administration (FDA, 2003), as well as the Association of American Feed Control Officials (AAFCO, 1999), have recommended the term direct-fed microbials (DFM) be used to describe feed products that contain a source of live, naturally occurring microorganisms.
Krehbiel et al. (2003) reviewed the literature and suggested that feeding DFM generally results in a 2.5 to 5% increase in ADG and an approximate 2% improvement in feed efficiency in feedlot cattle. To further investigate the efficacy of using DFM for feedlot cattle, we conducted two experiments to evaluate the effects of DFM based on different strains of Lactobacillus acidophilus plus one strain of Popionibacterium freudenreichii on performance, carcass, and intestinal characteristics, and Escherichia coli strain O157 shedding in finishing beef steers.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Experiment 2.
Seven hundred twelve steers predominantly of British and continental breeding were received at the Clayton Livestock Research Center (CLRC; Clayton, NM) between August 23 and September 10, 2002. On arrival or within 3 d of arrival, steers were processed as follows: 1) uniquely numbered ear tag in the right ear, 2) vaccination with Reliant-plus (Merial, Duluth, GA), 3) vaccination with Ultrabac 7 (Pfizer Animal Health), and 4) treatment down the back line with Ivomec pour on (Merial). After processing, steers were offered 5.44 kg/steer (as-fed basis) of a common 65% steam-flaked corn-based concentrate starter diet. In addition, 2.27 kg/steer (as-fed basis) of wheat hay was offered for 3 d after arrival. Over the next 3 wk, steers were stepped up to a 91% concentrate diet (intermediate diets of 65, 74, and 83% concentrate changed weekly).
Treatment and Pen Assignment
Experiment 1.
Approximately 1 wk before the trial began, each steer was implanted in the right ear with Ralgro (Schering Plough, Union, NJ), and sorted into a 2.9-m x 5.6-m partially slotted concrete-floor pen. To accomplish this sorting and assignment to pens, groups of four contiguous pens were designated as a block until 12 blocks were arranged. Steers were then arranged in ascending order based on their individual sorting BW, and 240 were blocked into groups of 20 each. Within a block, steers were assigned randomly to treatment (five steers per pen), with treatments assigned randomly to pens. On d 0 (trial initiation), each steer was individually weighed by the methods described below to obtain an initial BW measurement.
Experiment 2.
The steers used in this trial were from two distinct weight groups (WG). The first group, classified as a short days on feed group (SDOF), because of heavy initial BW (average BW at arrival = 352 kg), had grazed before arrival at the CLRC. The second group, classified as a long days on feed group (LDOF; average BW at arrival = 314 kg), had been in a grower yard before arrival at the CLRC. The steers that had been grazing were obtained from northwest Oklahoma, and the specific forage type that the steers had been grazing was not known. The steers from the grower yard had received a corn-silage based 50% concentrate diet. Steers were stratified by BW within a WG classification by the methods described in Exp. 1. In order to have equal replication within a pen, 59 of the heaviest steers from LDOF were grouped with SDOF. Once blocks were arranged, steers were assigned randomly to pens (approximately 9 m x 30 m; 22 steers per pen) within blocks. On d 0 (start of the trial), steers from SDOF were weighed individually with a single-animal squeeze chute suspended from two load cells (Silencer; Moly Manufacturing, Lorraine, KS), implanted with Revalor S (Intervet), and returned to their home pen. Likewise, steers from LDOF were weighed individually, implanted with Component E-S (Vet Life, West Des Moines, IA), and returned to their home pen.
Experimental Diets and Treatments
Experiment 1.
All cattle received the same basal diet (Table 1) for the entire experiment. The four direct-fed microbial (DFM) treatments consisted of 1) control, lactose carrier only (CON), 2) 1 x 109 cfu of Lactobacillus acidophilus (LA) strain NP51 plus 1 x 106 cfu of LA strain NP45 plus 1 x 109 cfu of Propionibacterium freudenreichii (PF) NP24 per animal daily (LA45-51H), 3) 1 x 109 cfu of LA strain NP51 plus 1 x 109 cfu of PF NP24 per animal daily (LA51), and 4) 1 x 106 cfu of LA strain NP51 plus 1 x 106 cfu LA strain NP45 plus 1 x 109 cfu of PF NP24 per animal daily (LA45-51L). Treatments were stored in a freezer in individually marked packets. Each day one packet per treatment was reconstituted with 2.5 L of distilled water in an individual sprinkler can labeled with markings that corresponded to the specific treatment. When a quantity of diet sufficient to feed all the pens in a treatment had been delivered to a self-propelled mixer/delivery unit (Rotomix, Dodge City, KS), the contents of the appropriate sprinkler can was poured on the feed and allowed to mix for approximately 3 min before the feed was delivered to the pens of cattle assigned to that treatment. Clean-out batches were sent to the mixer/delivery unit between treatment diets to prevent any cross-contamination.
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) for the entire experiment. The three DFM treatments consisted of: 1) control, lactose carrier only (CON), 2) 5 x 106 cfu of LA strain NP51 plus 5 x 106 cfu of LA strain NP45 plus 1 x 109 cfu of PF NP24 per animal daily (LA45-51L), and 3) 1 x 109 cfu of LA strain NP51 plus 5 x 106 cfu LA strain NP45 plus 1 x 109 cfu of PF NP24 per animal daily (LA45-51H). Treatments were packaged and stored as described in Exp. 1. Immediately before diets were mixed, each packet was reconstituted with 1 L of water in a treatment-coded bottle and poured into a treatment-coded 18.93-L container that was tared on a platform scale. Water was then added to the container until the scale display read 18.14 kg. Another container with the same coding was then tared individually on the scale, and 9.07 of the 18.14 kg were poured into the container. The contents of one container were added to the batch of feed designated for a corresponding treatment group of pens within a weight group. Respective containers were added to the diet and allowed to mix for 3 min. To prevent cross-contamination, the addition of DFM treatments to the diet was ordered by increasing concentration of DFM, and clean-out batches of feed were mixed each day before the CON diet was mixed.
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; Table 3).
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Feed bunks were cleaned, and orts were weighed (Ohaus electronic scale, ± 0.045 kg; Ohaus, Pine Brook, NJ) at intervals corresponding to weigh dates throughout the trial. Dry matter content of these samples was determined in a forced-air oven by drying overnight (typically 20 h) at 100°C. All weights for DM determinations were obtained on an Ohaus electronic balance (± 0.1 g). To calculate the average DMI by a pen, dried feed refusals obtained at the end of each period were subtracted from the total DM delivered to the pen for the period. The corrected total of DM delivered was then divided by the number of animal days in the period.
Body weight measurements taken on the basis of a pen or individual animal were obtained without withholding feed or water. Cattle were weighed in the morning before feeding on scheduled weigh days. After 28, 84, 112, and 140 d on feed, cattle were weighed on a pen basis using a platform scale (± 2.27 kg). On d 56, BW measurements were obtained on an individual animal basis using the single-animal squeeze chute described previously, and each steer was reimplanted with Revalor S (Intervet). On the day before each scheduled weigh day, the scale to be used was calibrated with 453.6 kg of certified weights (Texas Department of Agriculture). Based on performance, as well as visual appraisal, blocks of cattle were sent to the Excel Corp. slaughter facility in Plainview, TX, when the group was expected to have sufficient finish to grade USDA Choice. On the morning of shipment, final BW measurements were obtained on an individual animal basis by the methods described previously.
Experiment 2.
Feed was mixed each day in a Butler-Oswalt mixer (Model 1830, Garden City, KS) and feed allocations for each pen were weighed (0.45-kg scale breaks) into a feed truck with six compartments. Each compartment was equipped with a horizontal auger to deliver the preweighed feed allocations to individual pens. Feed bunks were managed to contain approximately 0.227 kg/steer at 0700, before feeding at approximately 0800. When accumulated feed became compromised by moisture or excessive fines, it was removed, weighed, analyzed for DM, and subtracted (DM basis) from the respective pen’s feed delivery log at the end of the experiment. To determine the DM content of treatment diets, samples of feed were taken weekly from half the pens within each treatment and dried in a forced-air drying oven at 100°C for 24 h. These bunk sample DM values were used to compute average DMI by the cattle in each pen. The dried samples were composited by treatment every 28 d, ground to pass a 1-mm screen in a Wiley mill, and analyzed for CP, ADF, ash, Ca, and P (AOAC, 1990). These values for each 28-d period were averaged at the end of the experiment (Table 3
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Steers were not weighed on regular intervals as in Exp. 1; however, on d 49, steers were weighed on an individual-animal basis as described previously. At this time, steers in LDOF were reimplanted with Revalor S (Intervet). Steers in SDOF were not reimplanted. Using the same criteria as in Exp. 1, steers were shipped to the IBP, Inc., slaughter facility in Amarillo, TX, when a block of cattle was expected to have sufficient finish to grade USDA Choice. The weighted average days on feed for steers in SDOF was 104 d, whereas the weighted average days on feed for steers in LDOF was 131 d.
Carcass Evaluation
For both experiments, personnel of the West Texas A&M University Beef Carcass Research Center obtained all carcass measurements. Measurements included hot carcass weight (HCW); liver abscess score; after a 36- to 48-h chill, longissimus muscle area and marbling score of the split lean surface at the 12th/13th rib interface; percentage of kidney, pelvic, and heart (KPH) fat; and fat thickness at the 3/4-measure opposite the split lean surface between the 12th and 13th rib. The data collected were used to calculate USDA yield grade and USDA quality grade. Liver abscess scores were recorded on a scale of 0 to 6, with 0 = no abscesses, 1 = A-, 2 = A, 3 = A+, 4 = telangiectasis, 5 = fluke damage, and 6 = fecal contamination that occurred at slaughter.
Intestinal Samples
Experiment 1.
At slaughter, intestinal tissue samples were obtained from one randomly selected steer from each pen. Intestinal tissue samples from selected steers were identified at the time of evisceration. Identified tracts were collected, and sections of the ileum were obtained by the following criteria: 1) the ileocecal junction was located; 2) moving cranially through the ileum and immediately following its emergence from the mesenteric fat, the first 10- to 13-cm section was obtained; 3) again moving cranially, and following the natural curvature of the bowel, the second 10- to 13-cm section was obtained from the ileum transversely adjacent to the site of the first section. Each section was gently washed in 10% (vol/vol) formalin-buffered saline to remove digesta, and then submerged in 10% (vol/vol) formalin-buffered saline in an individually marked container. Samples were fixed in the formalin for at least 48 h before further processing. Formalin-fixed samples were trimmed and placed in cassettes for processing and embedding in paraffin. Tissues were processed with a Path Center tissue processor (Thermo Shandon, Pittsburgh, PA). Paraffin-embedded samples were thin-sectioned, secured on microscope slides with a standard mounting medium, stained with hematoxylin and eosin Y stain, and covered with a standard No. 1 cover slip. Microscope slides were evaluated under 400x magnification, with a micrometer-calibrated eyepiece. Lamina propria thickness was measured at the level of crypts and intermediate glandular areas, ignoring the contribution of the villi, at a minimum of five different locations per ileal section. Specifically, measurements were initiated at the termination of the muscularis mucosae and included the mucosal lamina propria and the epithelial lining.
Experiment 2.
For Exp. 2, intestinal samples were collected from the cecum. The decision to collect samples from the cecum was a result of the observations obtained from the ileal samples in Exp. 1. Ileal sections that were taken closer to the ileocecal juncture showed greater treatment effects than sections taken closer to the jejunum. However, as a result of timing complications, cecal sample collections for SDOF were incomplete. Samples from two steers from each pen in Blocks 4 and 5 of SDOF, and two steers from each pen in every block of LDOF were collected. As in Exp. 1, steers were randomly selected for intestinal sampling, and intestinal tracts were identified as described previously for Exp. 1. Once again, the ileocecal junction was used as the initiation site. Sections, 8 to 10 cm in length, were removed from the cecum immediately adjacent to the ileocecal valve and moving toward the anterior end of the organ. Isolated sections were processed in the same manner as were those from Exp. 1. Cecal lamina propria measurements included the same mucosal and epithelial features as in Exp. 1.
Serum IgA Concentrations
Steers from Exp. 1 identified for intestinal sampling also were selected for measurement of serum IgA concentrations. Blood samples were collected in 50-mL conical-bottom sterile centrifuge tubes at the time of exsanguination at the slaughter plant. Blood samples were allowed to coagulate, and serum was obtained by the following procedures: 1) 50-mL conical-bottom tubes were centrifuged for 20 min at 1,000 x g; 2) the supernatant fluid was aspirated, placed in a sterile 16-mm x 100-mm blood tube, and recentrifuged at 1,000 x g for an additional 15 min; 3) the supernatant fluid obtained from the second centrifugation was collected and stored at -10°C in capped, 14-mL, round-bottom tubes. Before IgA concentrations were measured using the Bovine IgA Quantitative Enzyme Linked Immunoassay kit (Bethyl Laboratories, Inc., Montgomery, TX), serum samples were diluted to 1:250 with Tris-buffered saline that contained 1% (wt/vol) bovine serum albumin and 0.05% (vol/vol) Tween 20.
Microbiological Analyses
In Exp. 1 only, fecal grab samples for determining prevalence of E. coli O157 were collected from each steer 7 d before slaughter as well as at the time of the final BW measurement taken immediately before transport to the slaughter plant. In addition, a hide swab was taken from various sites on each steer. The method used to indicate shedding prevalence of E. coli O157 has been described by Laegreid et al. (1999). Briefly, 10 g of feces was enriched in GN-VCC broth, subjected to immunomagnetic separation by mixing 1 mL of the enriched broth with 20 µL of Dynal O157 beads (Dynal Biotech, Lake Success, NY), and then streaked for isolation on various agars. Positive samples from these isolations were then confirmed with an O157 agglutination test.
Statistical Analyses
Experiment 1.
Data for BW, DMI, ADG, gain:feed (G:F), HCW, carcass-adjusted variables (calculated using carcass-adjusted final BW, which was calculated as HCW/average dressing percent), IgA concentrations, normally distributed carcass characteristics, and ileal lamina propria thickness were analyzed as a randomized complete block design using the Mixed procedure of SAS Release 8.02 (SAS Institute Inc., Cary, NC). Nonparametric USDA quality grade data were transformed using Friedman’s test by listing the percentage of Choice and Select for each pen within a block and then analyzed as the normally distributed data from above. Pen was the experimental unit. The model statement included treatment, and the random statement included block. All animals removed from the trial because of death or chronic illnesses (four steers, total) were removed from pen-average calculations. When the overall F-value for treatment was significant (P 0.05), least squares means were separated using the PDIFF statement in SAS. In addition, preplanned contrasts were used to compare 1) CON vs. the average of the DFM treatments, 2) LA51 vs. the average of LA45-51L and LA45-51H, and 3) LA45-51L vs. LA45-51H.
The E. coli shedding data, deemed to be binomially distributed, were analyzed as a randomized complete block design with the Genmod procedure of SAS. Block and treatment were included in the model statement, and a binomial distribution was specified. Pairwise comparisons, as well as the same contrasts noted previously, were used to test treatment effects.
Experiment 2.
Pen-based performance data, as well as normally distributed carcass characteristics and cecal lamina propria thickness data, were analyzed as a split plot with the Mixed procedure of SAS Release 8.02 (SAS Institute Inc.). Nonparametric USDA quality grade data were analyzed using the same procedures as in Exp. 1. Weight group classification was the whole-plot unit, and DFM treatment was the split-unit factor. The model statement included WG, DFM treatment, and their interaction. The random statement included block nested in WG. When the overall F for WG classification or DFM treatment was significant (P 0.05), main-effect means were separated using the PDIFF option. When the overall F for the WG classification x DFM treatment interaction was significant (P 0.05), simple-main effects were tested using the Slice option of SAS. When the overall F for the simple-main effects of a particular DFM treatment was significant (P 0.05), the two simple-effect means were considered to be different across WG classifications. When the overall F for the simple-main effects of a WG classification was significant (P 0.05), DFM treatment simple-effect means were separated within that weight group classification using the PDIFF option of SAS. In addition, preplanned contrasts that compared 1) CON vs. the average of the DFM treatments and 2) LA45-51L vs. LA45-51H were completed for the aforementioned variables.
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Results and Discussion |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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. No differences for final BW (P = 0.90) or carcass-adjusted final BW (P = 0.96) were detected among treatments in Exp. 1. Furthermore, no differences for period DMI (P 
0.30; data not shown) or overall DMI (P = 0.35) were detected among treatments, nor were the preplanned orthogonal contrasts for DMI different (P > 0.10). Period data for ADG in Exp. 1 revealed no differences (P 0.34) among treatment means; however, the contrast for CON vs. the average of the DFM treatments revealed a 7.5% increase (P = 0.07; data not shown) in d-0 to -28 ADG for the DFM-treated steers. Overall ADG and carcass-adjusted ADG did not differ (P 0.82) among treatments. Treatment means for G:F by period did not differ (P 0.37; data not shown) through d 112. In addition, G:F for the entire feeding period as well as carcass-adjusted G:F did not differ (P 0.18) among treatments; however, G:F for d 0 to 140 tended to differ (P = 0.09; data not shown) among treatments, with cattle on the LA45-51H treatment being more efficient than those on either LA51 or LA45-51L. Moreover, the preplanned orthogonal contrast for LA45-51L vs LA45-51H revealed the same pattern (P 0.09) for d-0 to -140, d-0 to end, and carcass-adjusted G:F.
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. For Exp. 2, the overall F for WG classification was significant (P 0.03) for both final BW and DMI. These results can be explained because DMI is a function of BW (NRC, 1996), and the steers from SDOF were heavier at the start and end of the trial (15.2 and 4.2% heavier, respectively) than steers from LDOF. As in Exp. 1, no differences (P 0.67) were detected for DMI or BW measurements among the DFM treatments. Furthermore, preplanned orthogonal contrasts did not reveal any differences (P 0.43).
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. The interaction for WG classification and DFM treatment was significant (P = 0.05) for ADG and approached significance (P = 0.08) for carcass-adjusted ADG. The simple-effect ADG and carcass-adjusted ADG means for all DFM treatments were greater (P 0.01) for SDOF steers than for steers in LDOF. In addition, within SDOF, DFM treatments tended to differ (P = 0.09) for ADG; the CON and LA45-51L DFM treatments did not differ (P = 0.70) from each other, but both were less (P 0.08) than for the LA45-51H treatment. Carcass-adjusted ADG did not differ (P 0.23) for any DFM treatment within a WG classification.
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0.12) across or within a WG classification.
For Exp. 2, the increase in ADG and improvement in G:F for steers in SDOF was likely attributable to differences in days on feed. As will be noted in the subsequent discussion, ADG and G:F tend to be more consistently improved during the early stages of an experiment for cattle receiving DFM treatments. If DFM are capable of advantageously altering gastrointestinal flora, cattle treated with DFM should show beneficial signs via improved ADG and G:F. Rivera et al. (2003) reported data suggesting that cattle shedding E. coli strain O157:H7 had lower ADG values than steers that were not shedding that strain. However, as time on feed increases, it would be reasonable to speculate that innate immunological mechanisms would provide the same protection to control animals that have had continual stimulation from an adverse microbial population. Furthermore, DFM might alter the efficiency of production and proportional concentrations of VFA in the rumen (Kmet et al., 1993). If propionate production is both energetically enhanced and proportionately increased by DFM, then it is likely that the energy available to the animal also is increased.
Kiesling et al. (1982) reported no differences in final BW, DMI, or ADG after a 28-d receiving study or in a subsequent 209-d finishing study when steers were supplemented with or without a viable Lactobacillus culture. However, calves that received some form of DFM treatment during the 28-d receiving study had increased DMI and a 7.4% greater ADG in the finishing trial compared with calves that received no DFM treatment during the receiving period. Nonetheless, the increase in ADG relative to the increase in DMI was not sufficient to improve feed efficiency. Gill et al. (1987) reported no differences in DMI between control and DFM-supplemented calves; however, in that study, DFM-treated calves had a 9.3% increase in ADG, which resulted in a 9.5% improvement in G:F relative to control steers. Orr et al. (1988) and Ware et al. (1988) reported no differences in DMI between control and LA strain BT1386-supplemented feedlot steers. In both studies, the DFM-treated steers had increased ADG, which resulted in a significantly improved G:F ratio. Swinney-Floyd et al. (1991) reported increased ADG by calves receiving a LA and Propionibacterium DFM in the early stages of a finishing trial; however, the response diminished for the entire 120-d feeding period. Overall-DMI results for the Swinney-Floyd et al. (1991) study were not provided, but results over the entire feeding period demonstrated that DFM treatment improved G:F without increasing ADG. Similarly, Krehbiel et al. (2001) reported no differences in ADG between control calves and calves that received a DFM gel at the time of their first antimicrobial treatment for morbidity. Huck et al. (2000) supplemented heifers with a LA or PF DFM in both the receiving and finishing periods or altered the DFM treatments between the two periods and reported no differences for carcass-adjusted final BW or DMI data. In addition, carcass-adjusted ADG in heifers that were sequentially fed a LA and then a PF DFM or a PF and then a LA DFM between the receiving and finishing phases tended to be greater than in control heifers, which also had a slight advantage in G:F. Rust et al. (2000) reported that steers treated with LA plus PF DFM had heavier (P < 0.05) final BW than control steers, but no differences for total DMI were reported between control and DFM-treated steers. In addition, overall ADG for the average of all DFM-treated steers was increased 6.2% over control steers, resulting in an improvement in G:F. Galyean et al. (2000) used preplanned orthogonal contrasts to express differences between control and LA plus PF DFM-treated steers. In that study, the contrast results for total DMI revealed no differences; however, the contrast for control vs. the average of all DFM treatments demonstrated that final BW and carcass-adjusted final BW were heavier for the DFM-treated steers. Furthermore, the same contrast revealed a significant increase in d-0 to -56, d-0 to end, and carcass-adjusted ADG (6.3, 4.3, and 4.2% increases, respectively) for DFM-treated steers.
The previous discussion offers little insight into intrinsic mechanisms by which DFM might improve ADG and G:F. However, in a review, Krehbiel et al. (2003) offered several mechanisms by which DFM might benefit inoculated animals. Mechanisms included competitive inhibition of pathogenic microorganisms for attachment in the gastrointestinal tract; antibacterial effects, such as hydrogen peroxide production; immunomodulation via enhanced phagocytosis and natural killer cell activity; and beneficial manipulation of ruminal fermentation. As discussed earlier, these mechanisms could benefit ruminants by increasing nutrient uptake via decreased thickening of the intestinal wall as a result of inflammation. Furthermore, DFM could directly and indirectly enhance energy production and efficiency of utilization by altering ruminal fermentation and decreasing the amount of energy used for tissue turnover in the gastrointestinal tract.
Carcass Characteristics
Results for the carcass characteristics of steers in Exp. 1 are presented in Table 7. None of the carcass measurements differed (P 0.53) among treatments, and no preplanned orthogonal contrasts differed.
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. Grade fat, taken at the 3/4 measure opposite the split lean surface between the 12th and 13th ribs differed (P = 0.03) among DFM treatments; LA45-51H steers were fatter than CON steers. The interaction between WG classification and DFM treatment was significant (P 0.04) for both marbling score and the quality grade data. The simple-main effects for the LA45-51L DFM treatment differed (P 0.01) between the WG classifications for both variables. The percentage of Choice vs. Select (42.5 vs. 57.5%, respectively) in SDOF was reversed (64.2 vs. 35.8%, respectively) in LDOF. No other carcass characteristics differed (P 0.24) among DFM treatments.
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reported a trend for smaller (3.7%) longissimus muscle area in steers treated with a Lactobacillus DFM during the finishing phase compared with control steers. In that same study, all other carcass characteristics did not differ between control and DFM-treated animals. Ware et al. (1988) reported no differences in USDA yield and quality grade or dressing percent between control and LA BT1386-treated steers. Similarly, carcass characteristics were not altered by DFM treatment in the studies conducted by Swinney-Floyd et al. (1991) and Rust et al. (2000). The preplanned orthogonal contrast for control vs. all DFM treatments used by Galyean et al. (2000) was significant for hot carcass weight (2.1% reduction in controls), but contrasts for other carcass characteristics were not significant. Huck et al. (2000) reported that the percentage of Choice + Prime carcasses was greater in heifers receiving a Propionibacterium DFM in both the receiving and finishing phases than for control heifers. Improvements in carcass characteristics as a result of DFM treatment would be difficult to explain, except for HCW. Because DFM often increase ADG, HCW would be increased if DFM supplementation increased ADG in treated animals. In addition, it might be possible that energetic manipulation of VFA production as a result of DFM inoculation could lead to differences in fat synthesis and distribution; however, the mechanisms by which DFM would cause such changes are not well defined.
Escherichia coli O157
Escherichia coli O157 shedding data from Exp. 1 are presented in Table 9. The percentage of steers shedding E. coli O157, 7 d before and on the day of shipment to slaughter, differed (P 0.10) among treatments. Results indicated that LA51 steers had lower (P 0.10) prevalence of E. coli O157 shedding than CON and LA45-51L steers. In addition, the prevalence of E. coli O157 on the hide of steers immediately before shipment for slaughter differed among treatments; the prevalence E. coli O157 on the hide of LA45-51H steers was lower (P 0.05) than all other treatments. Furthermore, the preplanned contrast for LA51 vs. the average of LA45-51L and LA45-51H for d-7 fecal shedding and the contrast for CON vs. the average of the DFM treatments for hide data were significant (P = 0.04).
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found no differences in fecal coliform counts between calves receiving no DFM and those receiving either a mixed culture of lactobacilli or a LA DFM. Conversely, Cole and Fuller (1984) demonstrated the ability of Lactobacillus salivarius to suppress the growth of E. coli in the gut of neonatal rats. Lema et al. (2001) demonstrated that several Lactobacillus and Streptococcus spp. DFM have the ability to decrease fecal shedding of E. coli O157 in experimentally infected sheep via competitive inhibition.
The mechanisms by which DFM decrease the prevalence of pathogenic microbial species in the digestive system have been attributed to several different phenomena. Reiter and Härnulv (1984) suggested that Lactobacillus lactis has an antimicrobial capacity as a result of its ability to produce hydrogen peroxide via a lactoperoxidase-thiocyanate system. Gilliland and Speck (1977) associated the same phenomenon with LA. Walter et al. (1992) reviewed the scientific literature and compiled a host of antimicrobial substances produced by lactobacilli, including bacteriocins and other broad-spectrum antibiotic-like substances, such as acidophilin and lactocidin, that have not been chemically characterized. Reid and Burton (2002) stated that DFM prevent E. coli O157 infections by inhibiting penetration of the intestinal mucus through stimulation of more mucus production, or by interfering with binding to nutritive substrates; however, once E. coli O157 attaches and releases its toxins it would be difficult for DFM to nullify the aftermath.
Serum IgA and Lamina Propria Thickness
Serum IgA concentration and ileal lamina propria thickness data from Exp. 1 are presented in Table 10. No differences (P = 0.98) among treatments were noted for serum IgA concentrations. Likewise, lamina propria measurements taken from ileal sections closer to the jejunum did not differ (P = 0.47) among treatments. However, lamina propria measurements for the ileal sections taken near the cecum tended (P = 0.08) to differ; LA45-51H- and LA51-treated steers had a thinner lamina propria than steers in the CON treatment. Furthermore, the preplanned contrast for CON vs. the average of the DFM treatments demonstrated that steers receiving some form of DFM had a thinner (P = 0.03) lamina propria.
Cecal lamina propria thickness data from Exp. 2 are not presented in tabular form because no differences (P = 0.48) among DFM treatments were observed. The average lamina propria thickness for CON, LA45-51H, and LA45-51L steers was 0.637, 0.627, and 0.593 mm, respectively, with a standard error of 0.066 mm. In addition, the preplanned orthogonal contrasts did not reveal any differences. However, it should be noted that steers from SDOF, for which ADG was increased, were not equally represented in the cecal lamina propria thickness data because only samples from cattle in Blocks 4 and 5 in SDOF were collected, whereas all the pens in all of the blocks in LDOF were sampled.
Phillips et al. (2000) demonstrated the ability of E. coli O157 to cause attaching/effacing lesions in the bovine intestine, which lead to inflammation. Visek (1978) and Parker (1990) characterized germ-free animals as having less small intestinal mass than conventional animals. Higher intestinal water content, a thicker lamina propria, and more reticuloendothelial elements are associated with the overall greater mass of small intestinal tissue in conventional vs. germ-free animals. Furthermore, the germ-free intestine absorbs such substances as monosaccharides and amino acids more efficiently than the intestine of conventional animals (Visek, 1978). Helmut and Bruckner-Kardoss (1961), in an effort to evaluate these characteristics, used rats reared in germ-free or conventional settings. In their study, BW and small intestine length of germ-free and conventional rats did not differ; however, mucosal surface area of the small intestine and the DM per centimeter of gut length were significantly decreased 37.0 and 34.5%, respectively, in germ-free vs. control rats. Moreover, the cumulative length of intervillous spaces per unit length of serosal surface in cross-sections obtained from the jejunum to the ileocecal valve was significantly less in germ-free animals. Humphrey et al. (2002) fed chicks rice that had been genetically produced to express human lactoferrin or lysozyme (lactoferrin and lysozyme are not DFM per se but are naturally produced products found in mucosal secretions and milk). Chicks supplemented with lysozyme had a thinner (30.3%) lamina propria in the ileum than control chicks.
The feedlot performance and E. coli O157 prevalence data presented herein provide evidence to support the use of DFM in feedlot cattle. Direct-fed microbials effectively decreased the prevalence of E. coli O157 without negatively altering important feedlot performance characteristics. The lamina propria data offer insight into possible mechanism(s) by which DFM might improve ADG and G:F. If a thinner lamina propria results in more efficient nutrient absorption, then ADG and gain efficiency should be improved. Moreover, because DFM effectively decreased E. coli O157, there was likely a decrease in energy expenditure for the necessary regeneration of damaged gastrointestinal tissue as a result of pathogen-induced inflammation.
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Received for publication May 2, 2002. Accepted for publication July 21, 2003.
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