EFFECT OF SAFE-GUARD FREE-CHOICE PROTEIN BLOCKS ON TRICHOSTRONGYLE NEMATODES IN PASTURED CATTLE FROM EASTERN SOUTH DAKOTA

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1 Proceedings of the South Dakota Academy of Science, Vol. 91 (2012) 131 EFFECT OF SAFE-GUARD FREE-CHOICE PROTEIN BLOCKS ON TRICHOSTRONGYLE NEMATODES IN PASTURED CATTLE FROM EASTERN SOUTH DAKOTA S. J. Smith 1, A. A. Eljaki 2, J. Acharya 2, R. F. Daly 1 and M. B. Hildreth 1,2 * Departments of Veterinary Sciences 1 and Biology & Microbiology 2 South Dakota State University Brookings, SD *Corresponding author michael.hildreth@sdstate.edu ABSTRACT Parasitic nematodes have significant detrimental effects on the profitability of beef production in South Dakota. Administering anthelmintics can be inconvenient and expensive when treating cattle on pasture. Free-choice anthelmintics were developed to improve the ability of beef producers to effectively deworm cattle without passing them through a cattle chute. The convenience of free-choice anthelmintics outweighs those of traditional deworming practices; however, the efficacy of the free-choice anthelmintics has not been tested under pasture conditions in the United States Northern Great Plains. The purpose of this study was to determine the efficacy of Safe-Guard (fenbendazole) Freechoice Protein Blocks in pastures from eastern South Dakota. Two adjacent cattle herds were used for this study. A group of 42 heifers (treatment) were given one free-choice SafeGuard Protein block for four days. A similar group of 22 steers (controls) were given similar, but non-medicated protein blocks. Both groups were parasitized with trichostrongyle nematodes; PCR results indicated the presence of Haemonchus spp., Cooperia spp., Ostertagia spp. and Trichostrongylus spp. in the heifer herd. Prior to treatment, the arithmetic mean trichostrongyle egg output was numerically higher in the untreated steers (35.82 eggs/gram) than the treated heifers (22.63 eggs/gram), but not statistically different. After treatment with the Safe-Guard blocks, egg output dropped significantly to 1.50 eggs/gram (greater than 93% reduction) in the treatment group, but increased significantly in the untreated group to an average of eggs/gram (greater than 92% increase). Based upon the calculated consumption rate of the SafeGuard Protein Block by the treatment group, the average dosage consumed was lower than that recommended by the manufacturer. In spite of the lower intake, access to the medicated blocks significantly decreased trichostrongyle nematode loads in the treated cattle during the study period. Keywords bovine, trichostrongyle, anthelmintic, fenbendazole, free-choice

2 132 Proceedings of the South Dakota Academy of Science, Vol. 91 (2012) INTRODUCTION Trichostrongyle nematodes have a detrimental impact on the profitability of cattle production even in herds from the United States Northern Plains (Mertz et al. 2005). Administration of anthelmintics for controlling these internal nematodes in cattle has changed over the last several decades from the use of oral liquids and pastes to the more convenient injectables, pour-ons, sustainedreleased-boluses and free-choice ingestibles. Free-choice anthelmintics have been developed to minimize the costs and problems associated with handling individual animals. At the present time in the United States, morantel tartrate and fenbendazole (FBZ) are the only anthelmintic active ingredients that are available in any free-choice formulation. Fenbendazole (brand name Safe-Guard ) is available in several free-choice (i.e. non-handling) formulations including: flaked meal and minipellets (Safe-Guard Mini-pellets, and Safe-Guard Flakes), 0.5% alfalfa-based pellets/crumbles (Safe-Guard Pellets ), mineral/salt (Safe-Guard 20% or 35% Free-choice Mineral), a molasses-based block (Safe-Guard En- Pro-AL Molasses Block) and a 20% protein-based block (Safe-Guard Protein Block). Safe-Guard free-choice formulations were first marketed in the late 1980s, and yet, surprisingly few studies have been conducted on the efficacy of these products in North America. Most of the available studies have been performed in southern (Blagburn et al. 1986; 1987; Bransby et al. 1992; Miller et al. 1992; Williams et al. 1995) and western states (Kvasnicka et al. 1996; Smith et al. 2000; Taylor et al. 2000), or in central Canada (Garossino et al. 2001; Garossino et al. 2005), but no published studies have come from the United States Northern Great Plains. The vast majority of fenbendazole free-choice studies utilized the mineral-based formulation (Williams et al. 1995; Kvasnicka et al. 1996; Smith et al. 2000; Taylor et al. 2000; Garossino et al. 2001; Garossino et al. 2005). Two studies utilized a free-grade premix and/or pellet formulation (Blagburn et al. 1986; Keith 1992), and two other studies utilized a block formulation (Blagburn et al. 1987; Bransby et al. 1992; Miller et al. 1992). Fenbedazole is in the benzimidazole class of anthelmintic drugs which binds to helminth β-tubulin, further inhibiting the polymerization and formation of microtubules necessary for cell division, nutrient absorption and intracellular transport (Lubega and Prichard 1990, 1991; Prichard et al. 2003; Robinson et al. 2004). It is a broad spectrum anthelmintic that has shown a high capability of removing both immature and adult internal parasites in cattle (Keith 1992). First released as a 10% oral suspension, administration at 5 mg/kg FBZ controls immature nematodes located in the gastrointestinal tract of cattle, while administration at 10 mg/kg will also control adult tapeworms (Crowley et al. 1977; Blagburn et al. 1987; Keith 1992). The benefits of free-choice anthelmintics include: 1) a reduced amount of time handling cattle for oral deworming; 2) reduced handling which results in less stress on livestock; and 3) ability to administer the anthelmintic at strategic times to disrupt the parasitic life cycle and minimize pasture contamination (Garossino et al. 2001). In addition to demonstrating a decrease in the average number of nematodes present in a herd, studies have found an association between cattle

3 Proceedings of the South Dakota Academy of Science, Vol. 91 (2012) 133 strategically dewormed with FBZ and increased calf weaning weights and cow pregnancy rates (Myers 1988; Keith 1992). Deworming with FBZ free-choice mineral in steers on pasture or in feedlots correlates with increased daily gain, improved feed/gain ratio, increased carcass quality resulting in decreased feed costs and increased profitability (Smith et al. 2000; Taylor et al. 2000). A major disadvantage to free-choice anthelmintics is the inability to control consumption (dosage) rates, particularly when given to a large herd on pasture (Garossino et al. 2001). This inability to guarantee adequate drug consumption is the major disadvantage of using free-choice anthelmintics. Another downside to free-choice anthelmintics, in particular the mineral salt block by Safe-Guard, is the higher treatment cost per head, although some of these increased costs are offset by lower labor costs when using free-choice anthelmintics because cattle do not have to be processed through a handling system. An additional cost with the free-choice system is that cattle must be acclimated to a mineral block before the medicated protein block is administered (Blagburn et al. 1987). Thus, the cost per animal is increased while the assurance of an adequate anthelmintic dosage is diminished with this approach to treating cattle. Providing a subtherapeutic dosage through the use of free-choice anthelmintics can accelerate the development of anthelmintic resistance in the cattle parasites (Lanusse 2010). The present trial was implemented to study this Safe-Guard product in a typical stocker beef production setting on a pasture in eastern South Dakota. The outcomes from this study should provide cattle producers in this region needed information about the efficacy of this free-choice option under normal production conditions. This study was designed to compare the fecal egg counts (FECs) in a treated herd before and after treatment in a manner similar to a standard fecal egg count reduction test (FECRT). METHODS Pastures and Stocking Density This anthelmintic trial took place from September 7th through October 13th 2011, and the treatment group was comprised of 42 crossbred yearling heifers (approx. 800 lb, 363 kg) located on a 51.6 acre pasture comprised primarily of cool season grasses, generally Kentucky blue grass and green needle grass. Stocking rate for this treated group was acres/hd, and the heifers were supplemented with cornstalks and grass hay bales. Due to the extended length of the study, an adjacent untreated herd was used to serve as a temporal control to ensure that any observed decreases in egg production in the post-treated herd were not the result of natural FEC decreases. Twenty-two crossbred yearling steers (approx. 850 lb, 386 kg) were used for this temporal control group; this group was grazed on a 30.3 acre separate but similar non-adjoining pasture. Stocking rate for this control group was acres/hd, but they were not supplemented with cornstalks or hay. These two pastures are located in eastcentral South Dakota, roughly latitude longitude Treatment All cattle were on salt and mineral blocks free choice prior to starting the trial. The treatment group (heifers) was given a nonmedicated 20% protein block for seven days prior to treatment for adaptation to the medicated

4 134 Proceedings of the South Dakota Academy of Science, Vol. 91 (2012) block (salt and mineral blocks were removed during the study). All mineral, protein and medicated blocks were placed on the ground in the same location, which was toward the center of the pasture. The control group (steers) had continuous access to salt and mineral blocks throughout the entire trial. One Safe-Guard Protein Block (FBZ kg 20% protein block) was given to the treatment group on September 19, 2011 (day 0) and was completely consumed by day 4. This would provide an average dosage of 446 mg FBZ/hd when completely consumed over the four days. This is 25% of the dosage recommended by the manufacturer. Three additional FBZ blocks were placed in the treatment group pasture on day 9 (September 28) to ensure that adequate consumption of FBZ eventually occurred in the study. These blocks were removed on day 14 (October 3) and weighed. The total consumption for these last 3 blocks was 26.4 kg, resulting in an average consumed dose of 1,039 mg/hd; this is 57% of the recommended dosage. One heifer from treatment group became injured during the trial but was not removed. It is possible that she did not access the medicated blocks. Temperatures during the trial averaged 14.8 C with a high of 30.6 C and low of -2.2 C. Sample Collection and Analysis For each collection day, attempts were made to collect 20 samples from each herd because this sample size was found to be representative of herd parasite loads (Gasbarre et al. 1996), but because it was not possible to collect paired samples before and after treatment, samples from multiple days were collected before and after treatment in order to improve the statistical power of the study. Fecal samples were collected on days -12, -7, -6, -5, -3, 0, 7, 16, 18, 22, and 24 from both groups. The number of fecal samples collected each day is reported in Figure 2, and attempts were made to collect samples from the freshest fecal pats found in each pasture during the sampling time. Three-gram samples were analyzed using the Modified Wisconsin Sugar Flotation Technique to establish trichostrongyle egg counts (Cox and Todd 1962). Results are reported as eggs/g (EPG) of feces, and summarized as arithmetic means and geometric means. Statistical comparisons were made using the Kruskal-Wallis (Nonparametric ANOVA) Test in Graphpad InStat (version 3.05 for Windows 95/NT, GraphPad Software, San Diego California USA, www. graphpad.com). FECs were analyzed as raw data. Comparisons with P-values greater than 0.05 were not considered statistically different. A polymerase chain reaction (PCR) assay was performed on DNA isolated from trichostrongyle eggs from 16 heifer fecal samples (containing the highest number of trichostrongyle eggs) prior to treatment to determine which trichostrongyle species were present. The method for isolating DNA was as described by Harmon et al. (2006, 2009). The PCR assay was a simplex assay based upon a multiplex assay first described by Zarlenga et al. (2001), and refined by Harmon et. al. (2009). This gel-based assay was able to identify the following trichostrongyle genera: Haemonchus, Ostertagia, Trichostrongylus, and Cooperia. Two separate primer pairs were used for detecting Haemonchus eggs. The ITS2 (2nd internal transcribed spacer) primers were as listed by Harmon et al. (2006); the ETS (external transcribed spacer) primers were as listed by Zarlenga et al. (2001). The intensity of each PCR product band was visually scored from 1 to 3.

5 Proceedings of the South Dakota Academy of Science, Vol. 91 (2012) 135 RESULTS PCR results confirmed the presence of Haemonchus, Ostertagia, Cooperia and Trichostrongylus in the pre-treatment heifer samples (Table 1). Haemonchus was the most commonly identified genus (both by ITS2 and ETS primers), being found in 75% or more of the samples. Trichostrongylus was found in only two samples. More than half of the samples contained Ostertagia and Cooperia species. Throughout the study, at least 90% of the daily fecal samples from the untreated steers contained at least one trichostrongyle egg in a 3-gram sample (Figure 1). The mean daily trichostrongyle fecal egg counts (MDFECs) from the untreated steers ranged from 15.6 to 81.6 EPG over the length of the trial (Figure 2). The individual FECs during each of the sampling days in the untreated steers varied considerably, as indicated by the large standard errors for each time period. This high variance exists because most of the cattle had low numbers of worms (shedding few eggs), while most of the worms were aggregated into a few animals (shedding many eggs). For example, in the collection days prior to treatment of the heifers (days -12 through day 0), 59.41% of the 101 samples collected from the steers contained fewer than 26 EPG, but four steer samples contained more than 140 EPG (Figure 3). Eight samples contained fewer than 1 EPG. Because samples were randomly collected, some days were under-represented by samples with high egg numbers (Figure 2, untreated day -6 to day 0 and day +16). As shown in Table 2, the arithmetic mean FEC from the 90 untreated steer samples Figure 1. Prevalence of trichostrongyles infecting treated (heifers) and untreated (steers) cattle before and after treatment of the heifers.

6 136 Proceedings of the South Dakota Academy of Science, Vol. 91 (2012) collected after treatment of the heifers (days +7 to +24) was 92.71% higher than before heifer treatment (days -12 to 0); the geometric mean was 87.62% higher (P < 0.05). Prior to treatment, the prevalence of trichostrongyle eggs (in 3-gram samples) was 100% except for day 0 when it dropped to 90% (Figure 1). Before treatment, the lowest MDFEC in the heifers (15.4) was similar to that of the steers, but the highest heifer MDFEC (28.6) was much lower than the highest value in the steers (Figure 2). Only two heifer fecal samples contained more than 100 EPG ( and EPG), and 14 samples contained less than one EPG. More than half (55.37%) of the samples contained 2-40 EPG, demonstrating less aggregation in the pre-treatment heifer samples (Figure 3). The arithmetic and geometric means of the 121 heifer samples prior to treatment were numerically higher than those of the steers during this time, but these values were not statistically different (Table 2). Trichostrongyle prevalence in the heifer samples dropped to between 15% and 35% after treatment with the Safe-Guard Block. The MDFECs in the heifer (treatment) group decreased by 90.25% from the day of treatment (day 0) to the seventh day after treatment (day +7). In the 6 collection periods prior to treatment (day -12 to 0), the overall arithmetic mean FEC of the heifers was EPG; this decreased to 1.50 EPG (93.39% decrease) during the 5 collection days after treatment (P < 0.001). After treatment, only 26 of the 100 three-gram samples contained eggs, accounting for a total of 450 eggs (in the 300 g of sample) Figure 2. Mean fecal egg counts (in eggs/gram of feces) of treated (heifers) and untreated (steers) cattle before and after treatment of the heifers. Error bars represent the mean plus and minus the standard error. The number in parentheses next to each data point is the sample size for that data point.

7 Proceedings of the South Dakota Academy of Science, Vol. 91 (2012) 137 Figure 3. Aggregation of all the trichostrongyle egg counts in fecal samples from treated (heifers) and untreated (steers) cattle before and after treatment of the heifers. collected during the five sampling days (Table 3). More than 80% of those eggs were in eight samples, suggesting that these heifers did not receive a sufficient dose of FBZ. Five of the 26 infected samples came from collections prior to the second treatment; heifers excreting these samples would have received (on average) only 25% of the dosage recommended by the manufacturer. For each day, there were three to seven samples that contained eggs. It is possible that these infected samples were from the same few heifers that consumed significantly less FBZ than the other heifers. DISCUSSION The herd of yearling heifers used in this study contained trichostrongyle worm loads (i.e EPG) that were representative of other South Dakota beef herds recently studied. Untreated yearling stockers used to evaluate the economic impact of cattle nematodes on nine herds in South Dakota were excreting an average of 14.4 EPG during the fall grazing season when samples were collected (Mertz et al. 2005). Two of these herds were excreting more than 20 EPG, which was very similar to the pre-treatment FEC average in the heifers used in the present study. Cows tend to shed fewer eggs than yearlings, and FECs from 951 cows in 98 different Northern Plains (i.e. South Dakota, North Dakota and Minnesota) herds contained an average of 5.4 trichostrongyle EPG in this age class of cattle (Hildreth et al. 2007). Spring-born calves tend to shed more trichostrongyle eggs than yearling cattle by the end of the grazing season, as illustrated

8 138 Proceedings of the South Dakota Academy of Science, Vol. 91 (2012) in the results from 483 calves from the same 98 Northern Plains herds; these calves were shedding an average of EPG by September through November (Hildreth et al. 2007). The genera of trichostrongyles identified in the heifers from this present study were similar to those identified in calves from 13 cow-calf herds in eastern South Dakota (Harmon et al. 2009). In the present study, Haemonchus was the genus most commonly identified (75% of all samples) in the heifer herd; whereas, only 26.8% of the calves from the 13 herds were infected with Haemonchus. The steer herd (in a pasture very close to the heifer pasture) was simply used as temporal control to ensure that any observed decreases in egg production in the post-treated herd were not the result of natural FEC decreases. Stromberg and Corwin (1993) have shown that FECs in cattle will eventually begin to decrease during the fall season, especially during October and November. The MDFECs within the control steer herd actually increased during the experimental time period, suggesting that no natural decreases would be expected within the adjacent heifer herd. At the initial treatment period (day 0), the treatment (heifer) group was inadvertently under-dosed and received only one-fourth the recommended dosage of Safe-Guard block material, and yet, the MDFECs in this group showed a 90.25% decrease 7 days after treatment. Many of the eggs still being shed after the first treatment were likely in heifers that did not receive an adequate dose of FBZ. After 3 additional Safeguard blocks were given to the treatment herd at day +9, only 3 out of the 20 heifer samples were shedding eggs during the next collection period (day +16) even though the combined FBZ consumption level was only slightly over half the recommended dosage. By the end of the study, the number of fecal samples with eggs had increased slightly to 7 out of the 20 samples, but after the second treatment, the MDFEC stayed below 2 EPG. Incorporating all of the pre- and post-treatment data from the multiple collection days into the FECRT evaluation clearly showed that the addition of Safeguard blocks to the heifer pasture very significantly decreased both the prevalence (by more than 65%) and the trichostrongyle FECs intensity (by more than 93%) in the treated herd. Utilization of the recommended dosage may have slightly improved the outcome of this study, but only by a small amount. The low number of infected fecal samples suggests that the vast majority of heifers consumed a sufficient quantity of FBZ. Results from this South Dakota study are consistent with findings involving Safe-Guard Blocks in post-weanling calves from Alabama and Louisiana (Blagburn et al. 1987; Miller et al. 1992). The 50 calves from the Alabama study were first grazed on a contaminated pasture and then housed individually in dirt pens during the evaluation period. Block consumption rates were monitored and adjusted to enable each calf in the treatment groups to consume a total of 5 mg/kg FBZ. Under these conditions adult and immature trichostrongyles numbers decreased by more than 99%, and the FEC dropped from a mean of 1,620 EPG in the untreated controls down to 0 in the treated groups (Blagburn et al. 1987). Calves were kept on pasture throughout the Louisiana study, and the effectiveness of the block treatment in this study was significantly less than in the Alabama study (Miller et al. 1992). The 93% FEC reduction measured in the present South Dakota study was slightly less effective

9 Proceedings of the South Dakota Academy of Science, Vol. 91 (2012) 139 than in the Alabama study. This may be due to the fact that this study was conducted under pasture conditions, but it is more likely that this lower deworming efficacy was due to the suboptimal dosing with FBZ in the South Dakota study. Cost is a very important factor for cattle producers when considering anthelmintic approaches for use in their herd. A simple comparison of web-based prices for various anthelmintic products demonstrates that free-choice anthelmintics are generally not the least expensive approach; however, these products don t require the labor costs needed for working cattle in the chutes during the treatment process. Working pastured cattle through a chute also adds stress to these animals which can also risk injuries and negatively impact production factors. Therefore, with free-choice anthelmintics it is possible to treat cattle multiple times when necessary. Free-choice anthelmintics could be particularly useful in bison production. In summary, this study has found that there are both benefits and challenges to using free-choice anthelmintics. The Safe-Guard Mineral Block showed significant reduction in fecal egg count numbers; however egg reduction was not seen in the entire herd, indicating that there may be consumption issues in providing a proper dosage for each animal. When inadequate dosages of anthelmintics are used, there is a risk for increased anthelmintic resistance. Use of free-choice anthelmintics not only decreases labor costs, but also reduces the amount of stress put on animals at times of processing. Future studies should monitor the development of anthelmintic resistance in a herd treated with free-choice anthelmintics during extended time periods. ACKNOWLEDGEMENTS This work was supported in part by the South Dakota State University Agricultural Experiment Station. Special thanks to Alex Hegerfeld for technical support. This project also utilized the South Dakota State University Functional Genomics Core Facility supported in part by the National Science Foundation/ EPSCoR Grant No and by the State of South Dakota. LITERATURE CITED Blagburn, B. L., D. S. Lindsay, C. M. Hendrix, and L. A. Hanrahan Evaluation of three formulations of fenbendazole (10 per cent suspension, 0.5 per cent pellets, and 20 per cent premix) against nematode infections in cattle. American Journal of Veterinary Research 47: Blagburn, B. L., D. S. Lindsay, C. M. Hendrix, and L. A. Hanrahan Efficacy of fenbendazole-medicated feed blocks against gastrointestinal nematode infections in calves. American Journal of Veterinary Research 48: Bransby, D. I., D. E. Snyder, and W. B. Webster Medicated supplement blocks effective for deworming beef cattle. Highlights of Agricultural Research at the Alabama Agriculture Experiment Station 39: 6.

10 140 Proceedings of the South Dakota Academy of Science, Vol. 91 (2012) Cox, D. D., and A. C. Todd Survey of gastrointestinal parasitism in Wisconsin dairy cattle. Journal of the American Veterinary Medical Association 141: Crowley, J. W., Jr., A. C. Todd, D. H. Bliss, and W. J. Foreyt Further controlled evaluations of fenbendazole as a bovine anthelmintic. American Journal of Veterinary Research 38: Garossino, K. C., G. Royan, M. E. Olson, D. N. Milligan, B. J. Ralston, and T. A. McAllister Individual intake and antiparasitic efficacy of free choice mineral and fenbendazole in range calves. Veterinary Parasitology 94: Garossino, K. C., B. J. Ralston, M. E. Olson, T. A. McAllister, D. N. Milligan, and B. M. A. Genswein Individual intake and antiparasitic efficacy of free choice mineral containing fenbendazole for grazing steers. Veterinary Parasitology 129: Gasbarre, L. C., E. A. Leighton, and D. Bryant Reliability of a single fecal egg per gram determination as a measure of individual and herd values for trichostrongyle nematodes of cattle. American Journal of Veterinary Research 57: Harmon, A. F., D. S. Zarlenga, and M. B. Hildreth Improved methods for isolating DNA from Ostertagia ostertagi eggs in cattle feces. Veterinary Parasitology 135: Harmon, A. F., B. C. Lucas, and M. B. Hildreth PCR comparison of trichostrongyle genera present in South Dakota cattle with and without springtime deworming Proceedings of the South Dakota Academy of Science 88: Hildreth, M. B., W. B. Epperson, and K. J. Mertz Effect of longitude and latitude on fecal egg and oocyst counts in cow-calf beef herds from the United States Northern Great Plains. Veterinary Parasitology 149: Keith, E. A Utilizing feed-grade formulations of fenbendazole for cattle. Agri-Practice 13: Kvasnicka, W. G., L. J. Krysl, R. C. Torell, and D. H. Bliss Fenbendazole in a strategic deworming program. Compendium on Continuing Education for the Practicing Veterinarian April: S113-S117. Lanusse, C Anthelmintic Therapy in Ruminant Species: Understanding of the Host-Drug-Parasite Interaction. Anti-Infective Agents in Medicinal Chemistry 9: Lubega, G. W., and R. K. Prichard Specific interaction of benzimidazole anthelmintics with tubulin: high affinity binding and benzimidazole resistance in Haemonchus contortus. Molecular and Biochemical Parasitology 38: Lubega, G. W., and R. K. Prichard Interaction of benzimidazole anthelmintics with Haemonchus contortus tubulin: binding affinity and anthelmintic efficacy. Experimental Parasitology 73: Mertz, K. J., M. B. Hildreth, and W. B. Epperson Assessment of the effect of gastrointestinal nematode infestation on weight gain in grazing beef cattle, pp , Journal of the American Veterinary Medical Association.

11 Proceedings of the South Dakota Academy of Science, Vol. 91 (2012) 141 Miller, J. E., T. A. Olson, M. T. Kearney, G. H. Myers, and J. C. Williams Effect of fenbendazole molasses supplement block treatment on nematode infection and subsequent weight gain of weanling beef calves. Veterinary Parasitology 44: Myers, G. H Strategies to control internal parasites in cattle and swine. Journal of Animal Science 66: Prichard, R. K., S. G. Forrester, A. Njue, Z. Feng, J. Liu, and R. N. Beech Receptor mechanisms of antiparasitics. Journal of Veterinary Pharmacology & Therapeutics 26: 3. Robinson, M. W., N. McFerran, A. Trudgett, L. Hoey, and I. Fairweather A possible model of benzimidazole binding to β-tubulin disclosed by invoking an inter-domain movement. Journal of Molecular Graphics & Modelling 23: Smith, R. A., K. C. Rogers, S. Husae, M. I. Wray, R. T. Brandt Jr, J. P. Hutcheson, W. T. Nichols, R. F. Taylor, J. R. Rains, and C. T. McCauley Pasture deworming and (or) subsequent feedlot deworming with fenbendazole. I. Effects on grazing performance, feedlot performance and carcass traits of yearling steers. The Bovine Practitioner 34: Stromberg, B. E., and R. M. Corwin Epizootiology of Ostertagia ostertagi in cow-calf production systems in the American midwest. Veterinary Parasitology 46: Taylor, R. F., D. H. Bliss, R. T. Brandt, W. T. Nichols, J. R. Rains, J. P. Hutcheson, and R. A. Smith Pasture deworming and (or) subsequent feedlot deworming with fenbendazole. II. Effects on abomasal worm counts and abomasal pathology of yearling steers. The Bovine Practitioner 34: Williams, J. C., A. F. Loyacano, S. D. Broussard, D. F. Coombs, A. DeRosa, and D. H. Bliss Efficacy of a spring strategic fenbendazole treatment program to reduce numbers of Ostertagia ostertagi inhibited larvae in beef stocker cattle. Veterinary Parasitology 59: Zarlenga, D. S., M. B. Chute, L. C. Gasbarre, and P. C. Boyd A multiplex PCR assay for differentiating economically important gastrointestinal nematodes of cattle. Veterinary Parasitology 97:

12 142 Proceedings of the South Dakota Academy of Science, Vol. 91 (2012) Table 1. Genera identified from egg DNA isolated from heifers prior to treatment; values associated with each genera represent intensity scores (1-3) for resultant PCR gel bands SAMPLE NO. EPG HAEMON- CHUS ITS2 HAEMON- CHUS ETS OSTER- TAGIA COOPERIA TRICHO- STRON- GYLUS Positive Samples Prevalence 81% 75% 63% 56% 13% Table 2. Fecal egg counts in the treated heifers and untreated steers before and after treatment of the heifers. Mean values presented in eggs/g. Values with different superscripted letters were statistically different. TREATED HEIFERS BEFORE TREATED HEIFERS AFTER UNTREATED STEERS BEFORE UNTREATED STEERS AFTER Sample Number Arithmetic Mean (Standard Deviation) Arithmetic Mean Change 93.39% decrease 92.71% increase Geometric Mean (EPG) Geometric Mean Change 95.92% decrease 87.62

13 Proceedings of the South Dakota Academy of Science, Vol. 91 (2012) 143 Table 3. Individual Trichostrongyle Egg Counts (eggs/g) from the randomly collected fecal samples from the heifer pasture days after treatment with the Safeguard blocks SAMPLE NO. DAY 7 DAY 16 DAY 18 DAY 22 DAY