Efficacy of Dectomax Pour-On (5% doramectin) for Horn Fly Control L. D. Foil, Dept. of Entomology; and S. M. DeRouen, Hill Farm Research Station

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1 Efficacy of Dectomax Pour-On (5% doramectin) for Horn Fly Control L. D. Foil, Dept. of Entomology; and S. M. DeRouen, Hill Farm Research Station Findings Acceptable levels of horn fly control were observed for from 4 to 8 weeks. Differences in the length of control among the three sites were most likely affected by immigration of adult flies from untreated groups. Abstract Three groups of cattle at three locations in Lousiana were treated with Dectomax (0.5% doramectin) pour-on, and horn fly populations were monitored. Acceptable levels of horn fly control were observed for from 4 to 8 weeks. Differences in the length of control among the three sites were most likely affected by immigration of adult flies from untreated groups. Introduction The horn fly, Haematobia irritans (L.), is a major pest of cattle. Accepted estimates of the economic threshold of horn flies on cattle range from 50 to 200 flies per animal, depending on geographic location. Kunz et al. (1991) reported estimated annual losses from horn flies for cattle production in the United States to be $876 million. These authors reported that weaning weights of calves of cows protected from horn flies were 6.3 kg/head heavier than those for calves from cows infested with horn flies. Extensive data are available to document the efficacy and duration of control for topical application of ivermectin (IVOMEC Pour-On); 4-8 weeks of control have been reported following a single treatment (Lancaster et al, 1991; Lysyk and Colwell, 1996; Foil et al, 1998). However, comparable studies have not been conducted with doramectin. The purpose of this study was to test the efficacy of Dectomax Pour- On (5% doramectin) for horn fly control under field conditions in Louisiana. Methods and Materials The trials were conducted during 1998 in Louisiana; the sites were three Louisiana State University Agricultural Center Research Stations (Ben Hur, Baton Rouge; Hill Farm, Homer; and St. Gabriel, St. Gabriel). Cattle at St. Gabriel and the Hill Farm were commercial crossbreds and those at Ben Hur were purebred Herefords. At each test site, groups of animals were randomly allocated to the treatments. All cattle were maintained on native pasture, and treated cattle did not share a common fence line with untreated cattle. Dectomax pour-on was administered topically at a rate of 5mg/10kg to animals over 2.5 months of age. At St. Gabriel, 18 cows were treated on 22 May 1998, and a control group of 11 received no treatment. At Ben Hur, 17 cows and their calves were treated on 23 May The control group at St. Gabriel also was used for Ben Hur: the two sites are 12 km apart. At the Hill Farm, 28 cows and their calves were treated on 19 May 1998; a control group of 70 cows received no treatment, and was isolated in a pasture in one corner of the station isolated from other station cattle. Fecal samples were collected from ten animals from each of the treatment groups before and after treatments at the Hill Farm 36

2 (19 May 1998 and 28 May 1998) and St. Gabriel (25 May 1998 & 29 May 1998) and nematode egg counts were done. The number of horn flies per side was estimated for 10 randomly selected cows from each herd weekly. All fly counts were made before 08:30 hours with the aid of binoculars. Horn fly counts were continued on all stations until the mean number of flies on the treated groups was above 50 per side. Statistical analysis was done using PROC GLM in SAS (1996). The experimental design was a randomized block. Statistical comparisons were made between treated and control groups for each location and each week. The mean of each treated group was divided by the control mean for the same day to determine percentage control. In the discussion, a mean fly count on treated animals < 50% of the mean fly count on untreated animals is considered a satisfactory level of control (Lysyk and Colwell 1996). Results and Discussion Sixty percent of the cattle at both St. Gabriel and the Hill Farm had detectable egg counts before treatment. Pre-treatment arithmetic mean egg counts were 9.7 eggs / gram (range = 0-88) at St. Gabriel, and 2.5 eggs / gram (range = 0-18) at Hill Farm. All 20 of the post-treatment egg counts were 0, which indicates a highly effective level of internal parasite control. Fly counts on the Dectomax pour-on group were statistically significantly lower than those for controls for 11 weeks at the Hill Farm, and different from control at Ben Hur until counts were discontinued at 9 weeks (Table 1). At St. Gabriel, counts were significantly different from controls for only 4 weeks. This difference represented a rapid increase in the treatment counts as well as a decrease in the control counts. When evaluated in regards to the action threshold of 50 flies (50% of the accepted economic threshold), the Dectomax pour-on provided from four weeks of control at St. Gabriel to 8-9 weeks of control at Ben Hur and the Hill Farm, respectively. Because control counts averaged less than 100 per side at the Hill Farm, a period of control for 8 weeks of less than 50% of the control level should be accepted. This period of control compares favorably with that previously reported for Ivomec pour-on (Foil et al 1998). Because most producers do not maintain a control group, treatment efficacy relative to an action threshold may be more appropriate for practical evaluation of product performance. Based upon the 50 flies per side threshold, the effectiveness of Dectomax pour-on of between 4 and 9 weeks, appeared to be a highly variable result. Explanations for this type of variance could include factors such as application of the product, climatic conditions, and other differences among the study sites. All animals were treated from 2 containers from the same stock, and applied by the same individual. Therefore, differences in application can be discounted as a significant source of variability. Because of the importance of proper storage when using endectocides, this is often the first question of product performance under field conditions. Climatic conditions were not monitored during the study other than the fact that it did not rain on the day of treatment at any of the locations, and drought conditions persisted for the duration of the experiment. The endectocides affect horn fly populations via direct adult mortality, reduction of the fecundity of adults, and by the inhibition of larval growth in the manure. The duration of these treatment effects is variable (Marley et al, 1993) If attack or invasion is predominately comprised of emerging adults from the treated herd, then this level of attack 37

3 is influenced by the period of larval control in the feces, which has been reported to be up to 6 weeks for Ivomec pour-on (Fincher, 1996). Since the period of adulticidal activity of these products may be shorter than the larvicidal activity, the apparent efficacy of the treatment may be limited by the number of untreated animals near the treated group. Ben Hur and St. Gabriel are separated by approximately 12 km. A major difference between these 2 sites was that all cattle at Ben Hur received an endectocide for fly control, whereas there were more than 50 untreated adult cattle in addition to the control group for this experiment at St. Gabriel. Therefore, fly immigration from surrounding herds at Ben Hur should have been lower than at St. Gabriel. The number of flies on the Hill Farm control group was relatively low, which may have been a reflection of the extreme drought in Louisiana during the period of this study. Additionally, the treated cattle were segregated in one area of the Hill Farm, which was 1.9km from the control cattle. Therefore, we suggest that the treated groups at Ben Hur and the Hill Farm had limited immigration of horn flies from surrounding herds when compared to those at St. Gabriel, and that this was the most probable factor contributing to the differences in the periods of control. This would indicate that the possibility of untreated cattle in the area should be considered when endectocide products do not provide fly control for the maximum time that has been reported. References Fincher, G.T Ivermectin pour-on for cattle: effects on some dung-inhabiting insects. Southwest. Entomol. 21: Foil, L.D., Strother, G.R., Hawkins, J.A., Gross, S.J., Coombs, D.F., Derouen, S.M., Wyatt, W.E., Kuykendall, L.K., and Spears, B.G. Jr The use of Ivomec (ivermectin) pour-on and permethrin ear tags for horn fly control. Southwest Entomol. 23: Kunz, S.E., Murrell, K.D., Lambert, G., James, L.F., and Terrill, C.E Estimated losses of livestock to pests. p In D. Pimental [ed.], CRC Handbook of Pest Management in Agriculture. Vol. 1. CRC Press, Boca Raton, FL. 765 p. Lancaster, J.L., Jr., Kilgore, R.L., Simco, J.S., Parham, R.W., Hubbell, D., and Cox, J.L Efficacy of a topical ivermectin formulation against naturally occurring adult horn flies on cattle. Southwest. Entomol. 16: Lysyk, T.J., and Colwell, D.D Duration of efficacy of diazinon ear tags and ivermectin pour-on for control of horn fly (Diptera: Muscidae). J. Econ. Entomol. 89: Marley, S.E., Hall, R.D., and Corwin, R.M Ivermectin cattle pour-on: duration of a single late spring treatment against horn flies, Haematobia irritans (L.) (Diptera: Muscidae) in Missouri, USA. Vet. Parasitol. 51: SAS Release SAS Institute Inc. Cary, NC USA. Sparks, T. C., Quisenberry S.S., Lockwood J. A., Byford R.L., and Roush R.T Insecticide resistance in the horn fly, Haematobia irritans. J. Agric. Entomol. 2: Williams, R.E., and Towell, C.A Comparison of insecticidal ear tags and ivermectin in a topical formulation for controlling horn flies and face flies (Diptera: Muscidae) on pastured cattle. J. Agric. Entomol. 9:

4 Table 1. Average number of horn flies per side and % fly reduction on cattle treated with dectomax pour-on endectocide and on untreated controls. Week Posttreatment Ben Hur Cows % Contro l BH St Gabriel Cows SG Control % Control SG Hill Farm Cows HF Control % Control HF * * * * * * * * * * * * * * * * * * * * * * * * *Control and Pour-On groups statistically significantly different at p =

5 Long-term Effects of Stomach Worms and Liver Flukes on Mature Beef Cattle in Central Louisiana Alvin F. Loyacano, Dean Lee Research Station; J. C. Williams, Dept. of Vet. Science; Jeff Gurie, Dean Lee Research Station; and Andrew A. DeRosa, LSU School of Vet. Medicine Findings Untreated internal parasites can reduce overall beef cow performance. Both nematode and liver fluke infections can reduce mature cow weights. Nematode infections can reduce calf weaning weights. Introduction Internal parasites are of continual concern to Louisiana beef cattle producers. Many studies have documented the detrimental effect of stomach worm (gastrointestinal nematodes) infections on the gains and over all condition of weaned calves and stocker animals. Bovine liver fluke (Fasciola hepatica) infections have also been reported to affect calf and stocker gains. A four year study of replacement heifers with sub clinical nematode (primarily Ostertagia) and fluke infections was conducted at the Dean Lee Research Station in Central Louisiana to determine the effect of these parasites on the growth and reproductive performance of yearling replacement heifers. The treatment groups were: Group1, untreated controls; Group 2, treated for nematodes only; Group 3, treated for bovine liver flukes only; and Group 4, treated for nematodes and flukes. Nematodes were controlled with subcutaneous injections of either Ivomec7 (ivermectin, Merial) or Dectomax7 (doramectin, Pfizer, Inc.), each at the recommended dose of 200 ug/kg-1 body weight. Clorsulon (Curatrem7 Merial) drench was administered at the recommended rate of 7 mg/kg-1 body weight to control flukes. Treatments were initiated at weaning and were repeated at approximately 60 day intervals until the beginning of the heifers initial breeding seasons. This trial demonstrated that effective treatment for nematodes increased heifer gains and condition scores. Treatment for liver flukes did not significantly affect heifer gains or condition. Treatment for either parasite alone did not increase pregnancy rate. However, simultaneous treatment for both parasites resulted in increased pregnancy rates. Treatment for nematodes also increased calf weaning weight. These data indicated that both types of parasites affected some phase of heifer performance but affected them via different mechanisms. Gastrointestinal nematodes are known to reduce nutrient absorption in the digestive tract and to decrease feed consumption. This results in reduced gains and body condition scores and lower calf weaning weights. Liver flukes have been shown to reduce the metabolism of reproductive hormones in the liver by damaging tissue and causing blockage in the bile ducts in the liver. This combination of effects appears to delay the onset of puberty in the heifers and thus reduce pregnancy rates. It has been documented that beef cattle develop a resistance to most nematode parasites but it is believed that liver fluke damage in the liver is cumulative. The cows generated from the heifer study were retained and monitored over eight calf crops to determine the long term effect of nematodes and liver fluke on mature beef cows. Experimental Approach At the end of each of the four years of the heifer study, the two year old cows that were pregnant when their first calf was weaned were allotted by treatment to one of 40

6 two replicate pastures of each treatment. They were annually administered their assigned parasite control treatments in January prior to the calving season and in July when their calves were weighed for their 205 day weight. Random fecal samples were collected from each group at each treatment date to monitor parasite infections. The cows remained on the study as long as they weaned a calf each year. Heifer calves produced by the cows in this trial received the same treatments as their dams and were added to the trial herds when they were pregnant with their second calf. Cows produced their first calf on this study at three years of age and at the end of the study cow age ranged from four to ten years. A total of 476 cows were utilized during the eight years that the study was conducted. These cows were Angus sired, with a mixture of Hereford, Brahman, Simmental and Gelbveigh breeding in their back grounds. Due to the inherent fertility of the soils at this location, it was possible to meet the nutrient requirements of the mature cows with a total forage diet and minimal free choice mineral supplementation (primarily salt and Di-Calcium phosphate). The warm season forage available in the pastures was primarily a mix of Bermuda grass and Dallisgrass. The pastures were overseeded with ryegrass each fall to provide late winter and spring grazing. Free choice bermudagrass hay was provided when sufficient growing forage was not available. The overall average annual body condition score of the cows on this trial was 5, indicating that their minimal nutrient requirements were being met. Results Analysis of the mature cow data (TABLE 1.) revealed that cows treated for nematodes and or flukes (Groups 2, 3 & 4) had heavier average weights than control cows (Group 1). Cows treated for nematodes (Groups 2&4) had higher condition scores than control cows (Group 1). The average condition scores for cows treated for both parasites were not significantly different from those of cows treated only for flukes (Group 3). Pregnancy rates for cows treated for nematodes (Groups 2&4) were higher than those of cows treated only for flukes,(group 3) but were not significantly higher than those of control cows that received no treatments for internal parasites (Group 1). Calf birth weights were not affected by treatment for either parasite, but calves from cows treated for nematode infections (Groups 2&4) were heavier at weaning than calves from cows with untreated nematode infections (Groups1&3). Treatment for liver fluke infections did not significantly affect weaning weights. Calving intervals were unaffected by treatment for either parasite. Average parasite burdens, as measured by standard fecal egg counts (parasite eggs per gram of feces in a standard sized sample) were relatively low for both nematodes and liver flukes. Treatments for the parasites did not reduce the egg counts. In fact, cows that never were treated for nematodes had lower average egg counts than those cows which received multiple treatments as yearling and two year olds and bi-annual treatments for nematode parasites as mature cows. Fluke egg counts were not different between any of the treatment groups. Year and cow age effects on cow and calf data were analyzed to determine if the effects of the parasites varied between years and were cumulative and increased with the age of the cows. Year and cow age significantly affected all animal performance data, but did not affect nematode or fluke egg counts. Cow age data are presented in Table 2. Analysis of the data demonstrated that there was no significant cow age by treatment interactions; therefore no interaction data are 41

7 presented. Cow age significantly affected cow weight and condition scores at weaning with weights and scores tending to increase with cow age. Calf birth and weaning weights significantly increased as cow age increased. Differences in calving intervals were also significant between ages with the youngest and oldest cows tending to have the longest calving internals. Cows bred for their second calf at three years of age had significantly lower pregnancy rates than any other cow age group. Differences between other age groups were not statistically significant and ranged from 84 to 97 percent. Summary In summary, the data generated during the course of this study indicate that under the circumstances of the study, untreated nematode and or fluke infections reduced overall beef cow performance. Control cows, which received no treatments for either parasite during the course of the study, had lower average body weights than any of the groups of treated cows. Cows with untreated nematode infections had lower body condition scores than cows treated for nematodes. Fluke infections alone did not affect condition scores. Nematodes reduced the pregnancy rates of cows treated only for flukes, but did not significantly reduce the pregnancy rates of control cows. Pregnancy rates for control cows and those of cows treated only for flukes were not different. Neither parasite infection significantly affected calf birth weights. However, untreated nematode infections reduced calf weaning weights. Calving intervals were not affected by either parasite. Both nematode and liver fluke infections were relatively low and treated cows tended to have higher counts than untreated animals. Discussion As heifers, the cows in this study were treated for their respective parasites from shortly after weaning until the end of the study. They received seven treatments during their year on the heifer phase of the study. During this time period, cattle usually begin to develop some immunity to nematode parasite infection. It is expected that the frequent early treatments prevented the animals from developing this normal immunity by the reducing their exposure to immature and adult parasites in their digestive tracks. This would explain why treated cows had higher nematode egg counts than untreated cows and the lack of differences for some performance parameters. The untreated animals appeared to develop a more effective immunity to the parasites, which may have offset some of the advantages that would have been obtained by treatments. It was expected that the effect of parasites would be somewhat cumulative and that the performance of the cattle would be reduced by age. Liver fluke damage especially was expected to increase with cow age. We did not examine any livers from these animals, but our results indicate that any decrease in liver function was not significant. Cow production tended to increase with age and there were no statistically significant differences in performance due to cow age. Recommendations Replacement heifers and young cows should not be treated for internal parasites more than its necessary to maintain adequate growth and condition. This will allow the animals to develop a natural resistance to the parasites. Since treating for nematode parasites twice a year increased weaning weights by 28 pounds and cow weight by 77 pounds, it is economically beneficial to control 42

8 internal parasites in beef cows. The use of a product that controls both flukes and nematodes in the winter is recommended in areas in which the bovine liver fluke exist. A second treatment with a pour-on formulation that controls nematodes and horn flies should be applied sometime during the summer when horn flies are active. This will prevent long term nematode and fluke infections and will also provide some short term relief from horn fly irritation. 43

9 Table 1. Treatment effects on performance. TREATMENT COW WT BCS* PREG % BWT WN WT CAV INT N EPG F EPG Neither 1195 a 4.61 a 78 ab a ab.08 Nematodes 1272 b 4.80 c 84 a b b.19 Flukes 1259 b 4.67 ab 72 b a ab.16 Both 1273 b 4.75 bc 81 a b ab.09 Average *BCS = Body condition score; Preg % = Pregnancy rate; BWT = Birth weight; WN WT = Weaning weight; CAV INT = Calving intertval; N EPG = Nematode eggs per gram of feces; F EPG = Fluke eggs per gram of feces. Table 2. Effect of Cow Age on Performance. COW AGE COW WT BCS* PREG % BWT WN WT CAV INT N EPG F EPG c 4.61 cd 63 b 70 c 451 c c 4.57 d 84 a 79 b 489 d 380 a bc 4.79 bcd 91 a 80 ab 547 c 364 b bc 4.82 bc 96 a 81 ab 606 ab 360 b bc 4.94 b 94 a 82 ab 576 bc 371 ab b 5.2 a 97 a 79 ab 606 ab 366 b ab 4.9 b 88 a 80 ab 621 a 363 b a 5.0 ab 86 a 86 a 566 ab 372 ab Average *BCS = Body condition score; Preg % = Pregnancy rate; BWT = Birth weight; WN WT = Weaning weight; CAV INT = Calving intertval; N EPG = Nematode eggs per gram of feces; F EPG = Fluke eggs per gram of feces. 44