The prophylactic effect of ivermectin treatment of calves, three weeks after turnout, on gastro-intestinal helminthiasis

In two grazing experiments carried out in 1982 and 1983 the prophylactic effect on gastro-intestinal helminthiasis of a single ivermectin treatment of calves 3 weeks after turnout was studied in animals which were turned out early on contaminated pasture and in calves which were turned out late on mown pasture. A single ivermectin treatment 3 weeks after early turnout did not prevent heavy helminth infections. However, such single treatment of calves 3 weeks after a late turnout on mown pasture appears to be a promising way of preventing heavy infections. Late turnout on mown pasture without anthelmintic treatment was not enough to prevent heavy infections. Ivermectin treatment was more effective against Ostertagia spp. than against Cooperia spp. There was no effect on the faecal egg output of Nematodirus helvetianus. Predominantly due to the dry summer, infections were lighter in 1983 than in 1982. These lower infections, and possibly also a delay in the onset of inhibition and differences in helminth strains, may explain the lower proportion of inhibited development in Ostertagia spp. in 1983 than in the comparable groups in 1982.     

The combined effect of fenbendazole treatment and a move to aftermath 7 or 9 weeks after turnout on Dictyocaulus viviparus infections in calves

A grazing study was performed with the main objective of examining the effect of fenbendazole (FBZ) in a ‘dose and move’ system on nematode infections in calves with special emphasis on Dictyocaulus viviparus.

Three groups of six calves were grazed from May to October 1993. One group (DM7) was treated with FBZ and moved to aftermath (pasture which had only been mown) 7 weeks after turnout. The second group (DM9) was similarly treated and moved 9 weeks after turnout and the third group served as untreated pasture control group (PC) and was moved to aftermath 9 weeks after turnout.

FBZ treatment removed adult lungworms from DM7 and DM9. Tracer calves grazed during the first 7 or the first 9 weeks after turnout acquired mean burdens of 18 and 125 lungworms, respectively. In PC faecal larval counts increased until the end of August. Most of the animals in this group were then suffering from lungworm disease and emergency treatment with ivermectin was given. In both FBZ-treated groups, larvae reappeared in the faeces of some of the calves 4–5 weeks after treatment. Subsequent reinfection resulted in higher mean faecal larval counts in both groups 2 months after treatment, although variation in faecal larval counts was high. In DM7 values tended to be higher than in DM9. These higher larval counts were associated with mild signs of parasitic bronchitis in some calves of DM7, whereas no signs were seen in DM9.

At the end of the experiment, all calves, and also a group of six permanently housed non-infected control calves (HC), were infected experimentally with 5000 D. viviparus larvae to evaluate development of immunity. The worm counts at necropsy showed that all calves on pasture had developed immunity.     

Grazing study in Ireland using the morantel sustained release bolus for controlling nematodiasis in calves

The morantel sustained release bolus was administered at turnout to first-season grazing calves in order to assess its efficacy in the seasonal control of infection by nematode parasites in Ireland. The pastures grazed by control calves showed a marked increase in gastrointestinal trichostrongylid infective larvae by September, while numbers of infective larvae on pasture grazed by bolus-treated calves remained at a low level throughout the grazing season. In consequence, the controls showed significantly higher worm egg counts in late season and significantly higher worm burdens (mainly Ostertagia spp) at necropsy carried out in November on representative number of principal animals selected from each group. These reduced worm burdens were attributed to the suppression of egg output during the early part of the season as a result of treatment with the morantel sustained release bolus at turnout in the spring. Pasture contamination with Dictyocaulus viviparus larvae was present on all treatment pastures. The bolus-treated calves however were subjected to an increase in D. viviparus infection which occurred on their pasture in late season after the active life of the bolus had expired. It was concluded that bolus treatment delayed (rather than prevented) the buildup of D. viviparus infection on the pasture by 60-90 days.     

Effects of treatments with endectocide on the weight gain of grazing cattle in a warm temperate climate

Five groups of 20 weaned beef calves were injected subcutaneously with either an ivermectin, a doramectin, an abamectin long-acting formulation, an ivermectin long-acting formulation or a saline control, at turnout and 60 and 120 days later. The animals grazed the same pasture and were sampled and weighed at turnout and 30, 60, 90, 120, 150 and 180 days later. At turnout the mean bodyweights of all the groups were similar and faecal culture showed that they had a mixed strongyle infection of Cooperia, Haemonchus, Ostertagia, Oesophagostomum and Trichostrongylus species. After 180 days, the mean bodyweight gains of each group were respectively 62.1 kg, 102.2 kg, 106.4 kg, 107.3 kg and 110.1 kg for the control, ivermectin, doramectin, ivermectin long-acting and abamectin long-acting groups. All the products significantly improved the weight gains of the cattle, and significantly reduced their faecal egg counts.     

Control of parasitic bronchitis and gastroenteritis in grazing cattle by strategic prophylaxis with ivermectin

In May 1985 four groups of 10 calves, aged between four and five months, were turned out on to separate, permanent pastures of equal area which had been seeded during the previous few days with larvae of Dictyocaulus viviparus. One group acted as a control, the second was vaccinated with lungworm vaccine before turnout and treated with thiabendazole three, eight and 13 weeks after turnout, while the third and fourth groups were given ivermectin three times (three, eight and 13 weeks after turnout) and twice (three and eight weeks after turnout), respectively. A severe outbreak of parasitic bronchitis resulted in the death of three control calves within five weeks of turnout and parasitic bronchitis and gastroenteritis affected the second group of calves after approximately four months at pasture. The calves given ivermectin excreted no lungworm larvae and remained free of clinical parasitism throughout the trial.     

Comparison of early season suppressive anthelmintic prophylactic methods for parasitic gastroenteritis and bronchitis in calves

A comparison was made of the material costs and effectiveness of three methods of early season suppression by anthelmintic medication of Ostertagia species and two of Dictyocaulus viviparus in calves, each method suppressing faecal egg output for different lengths of time from the start of spring grazing. The anthelmintics used were: Morantel bolus administered five days before going to grazing; oxfendazole given three times at three, six and nine weeks after the start of grazing and ivermectin injected three, eight and 13 weeks after going to pasture. The effectiveness of each was estimated by comparison with worm numbers in untreated control calves. Oxfendazole, which was active for the shortest time (about 65 days) from the start of grazing (May 1), produced a 78.1 per cent reduction in Ostertagia species and an 84.4 per cent reduction in D viviparus. The morantel bolus was estimated to be active for 90 days and resulted in a 94.3 per cent reduction of Ostertagia species. The ivermectin treatment, which, because of the prolonged excretion of the chemical and different sensitivity of worm species, was estimated to suppress Ostertagia species for 105 days and D viviparus for 119 days, caused reductions of 98.7 per cent of the former and 97.4 per cent of the latter species. Material costs per calf were estimated at pounds 1.25 for oxfendazole, pounds 2.00 for ivermectin and pounds 10 for the morantel bolus.     

Persistent activity of a single late-season treatment with ivermectin against gastrointestinal trichostrongyles and lungworm in young calves

The present study was designed to assess the persistent efficacy of ivermectin against gastrointestinal trichostrongyles and lungworm (Dictyocaulus viviparus) when given late in the season to young calves naturally exposed to infection on permanent pasture. The results suggest that ivermectin prevents the re-establishment of Ostertagia spp. for 2 to 3 weeks, but Cooperia spp. for only 1 to 2 weeks. Re-establishment of lungworm is prevented for a period of at least 3 weeks. The results are discussed in the light of recent studies on the ivermectin effects on experimental or early-season natural infections.     

Epidemiological study of parasite infection in a cow-calf beef herd in Quebec.

The patterns of gastrointestinal and pulmonary nematode infections in a previously untreated Aberdeen Angus cow-calf herd were observed between May 1988 and December 1990. The cow-calf herd and replacement heifers were on separate pastures. The relatively high mean faecal egg counts of cows and heifers at the time of turnout were mainly owing to the maturation of hypobiotic worms. The strongyle egg counts of calves began to rise soon after turnout onto pasture and reached peak levels at the end of the grazing season. The number of infective larvae on pasture was highest during September/October. Ostertagia, Cooperia and Nematodirus were the most prevalent genera found at necropsy and on pasture. Larvae of these nematodes were able to overwinter on pasture and Ostertagia larvae, additionally, were able to overwinter in the host as arrested early fourth stage larvae. The high egg output of cows at the time of turnout may serve as a source of infection for their calves and be responsible for the late-season rise in pasture larval counts.     

Effect of strategic treatments with invermectin on parasitism of set-stocked calves exposed to natural trichostrongyle infection in Lithuania

The effect of strategic treatments with ivermectin in first-season calves exposed to trichostrongyle nematodes on naturally contaminated pasture was studied. Twenty first season heifer calves were divided into 2 groups, according to live weight, and on 22nd May each group was turned out onto a 1 hectare pasture. Group A (Plot A) was treated with ivermectin at weeks 3, 8 and 13 after turn out, while group B (Plot B) served as an untreated control group. The study showed that control calves exhibited increase in trichostrongyle egg counts in August, while treated calves were excreting low numbers of trichostrongyle eggs. Pasture larval counts on Plot B (control animals) were low during the first part of the grazing season, followed by a steep rise towards the end of July. In contrast, the numbers of infective larvae recovered from Plot A remained low throughout the season. Both groups showed comparable weight gains from May up to the middle of July. However, from then on, Group B (controls) had lower weight gains than ivermectin treated Group A. From the end of July onwards, most untreated calves (Group B) showed clinical signs of parasitic gastroenteritis. It can be concluded that the strategical ivermectin treatments were successful, and faecal egg counts, pepsinogen levels and herbage larval counts clearly demonstrated that this was accomplished through suppression of pasture contamination with nematode eggs and subsequent reduction of pasture infectivity.     

Immunity of ivermectin treated cattle to challenge from helminth parasites in the following season

Two groups of yearling cattle which had been treated with ivermectin either three and eight, or three, eight and 13 weeks after turn out to trichostrongyle contaminated pasture in their first grazing season were exposed in the following season to natural challenge with helminth parasites. To assess their immunity to this challenge each group shared a pasture with parasite naive first season calves. No anthelmintic treatments were administered at any time during the year. Throughout the grazing period the yearlings showed normal respiratory rates, negative faecal lungworm larval counts, and, relative to the calves, low faecal trichostrongyle egg counts. All the first season calves developed patent lungworm infections and on one occasion the mean respiratory rates of each group of calves were significantly greater than those of the yearling cattle. At the end of the grazing period, from early May until late September or October 1986, the cattle were removed from pasture and together with parasite naive controls challenged with either 10 or 22 third stage larvae of Dictyocaulus viviparus/kg bodyweight and necropsied between 18 and 23 days later. Although the experimental challenge resulted in relatively heavy lungworm infection of the naive controls, none of the yearlings and only three of the 11 calves which had been at pasture were found to be infected. However, large numbers of arrested fourth stage larvae of Ostertagia ostertagi were present in all the naturally infected yearlings and calves.     

A 3-year field evaluation of pasture rotation and supplementary feeding to control parasite infection in first-season grazing cattle: dynamics of pasture infectivity

A 3-year grazing trial (2002-2004) was conducted on a commercial beef cattle farm in south-central Sweden to assess different methods of parasite control. This paper focuses on the dynamics of the free-living larval stages, whereas data on performance and within-host parasitological variables are presented in a complementary paper. Each year in May, 4 groups of 10 first-season grazing (FSG) steers were turned out on to separate 2ha paddocks and subjected to the following strategies: (1) spring turn-out on to pasture which had been grazed the previous year by second-season grazing (SSG) steers (paddock RT), followed by a move to aftermath (paddock AM) after 10 weeks (mid-July), (2) supplementary feeding with concentrate and hay for 4 weeks following turn-out (paddock FD), set stocked, (3) untreated control (paddock UT), set stocked and (4) anthelmintic treated control (paddock DO), set stocked. All paddocks were assigned a new set of FSG cattle each year whereas the treatments remained the same. Pasture infectivity were monitored partly by two tracer calves that grazed each paddock along with the FSG calves for 3 weeks after turn-out and prior to housing, partly by analysis of herbage samples for infective larvae (L3) that were collected from each paddock at monthly intervals between April and October. The predominant genera found were Cooperia and Ostertagia. Tracers grazing paddock RT overall harboured less worms, and in particular less Ostertagia spp., and tracers grazing paddock AM in mid-July harboured insignificant numbers of nematodes compared to tracers on the FD and UT paddocks. Although total worm counts varied between groups, smaller numbers were generally observed early in the grazing-season (May), compared to close to housing (September) when inhibited early L4 larvae were almost exclusively found. Results observed from herbage samples showed high numbers of L3 in spring before the time of turn-out, compared to around housing. In conclusion, the rotation control strategy showed promising results and provided a turn-out pasture that was 'nematode safe' to FSG cattle the following spring, whereas the feeding strategy failed as applied in this experiment.     

Observations on the resumption of development of inhibited Ostertagia, Cooperia and Nematodirus infections in calves stabled overwinter.

Two groups of three month old, parasite-free calves grazed a permanently infected pasture for 14 days, Group A during the first two weeks of September and Group B during early November. Half of each group was killed 14 days after removal from the pasture and the remainder stabled overwinter before slaughter and parasitological examination. Marked inhibition of development occurred for Cooperia oncophora with a variable lower level of inhibition for Ostertagia ostertagi and practically none for Nematodirus helvetianus in those calves grazing late in the fall. Under the conditions of this study, inhibited Cooperia larvae resumed development in several calves soon after they were stabled while small numbers of Ostertagia resumed development regularly during the winter and spring with a considerable number of Ostertagia still present when the calves were slaughtered at the end of the stabling period. On the other hand, Nematodirus and practically all Cooperia worms were lost during the stabling period. In three of seven calves grazing late fall pastures, large Cooperia infections were either not established or failed to become patent.     

Efficacy of toltrazuril in the prevention of coccidiosis in naturally infected lambs on pasture

The efficacy of toltrazuril (Bay Vi 9142) in preventing ovine coccidiosis due to an infection acquired immediately after turnout on pasture was evaluated by comparing the faecal consistency, weight gain, and oocyst output of treated and untreated lambs in 3 trials. The lambs were either given a single treatment with toltrazuril at 15 or 20 mg/kg, or they were given a dose of 10 mg/kg on 2 separate days. A single treatment with toltrazuril at 20 mg/kg on day 10 after turnout on pasture almost completely prevented coccidiosis in 2 trials. In a third trial the acute phase of a severe Nematodirus battus infection coincided with the outbreak of coccidiosis, and thus partly masked the clinical effect of the anticoccidial treatment. In lambs treated with toltrazuril at 15 mg/kg on day 10 after turnout, the coccidial infection caused a softening of the faeces, but the lambs were not severely affected by the coccidia. In lambs given a dose of 10 mg/kg of toltrazuril twice, either on days 10 and 11 after turnout, or on days 10 and 20, the coccidial infection caused a softening af the faeces, including some cases of diarrhoea. Oocyst production due to the initial coccidial infection on pasture was markedly reduced by all treatments with toltrazuril. The reduction was most pronounced after a dose of 20 mg/kg. Lambs treated with single doses of 15 or 20 mg/kg of toltrazuril had a better weight gain than the untreated controls in 2 of the trials. Lambs treated with toltrazuril on day 10 after turnout were partially resistant to the coccidial reinfection acquired immediately after treatment, and they had a similar level of immunity as the untreated controls to the subsequent reinfection on pasture.     

Efficacy of abamectin against natural infections of gastrointestinal nematodes and lungworm of cattle with special emphasis on inhibited, early fourth stage larvae of Ostertagia ostertagi.

The anthelmintic efficacy of abamectin (avermectin B1) was evaluated against gastrointestinal nematodes, including Ostertagia ostertagi inhibited larvae and lungworm, in yearling crossbred beef heifers during late spring. The calves were grazed on contaminated pasture for 10 weeks and then held under conditions free of nematode infection for 3 weeks prior to allotment and treatment on 5 June. Thirteen calves were randomly assigned to two groups of six by restricted randomization on body weights; the extra lightest calf was assigned to the non-treated control group. Group 1 calves were treated with abamectin at 200 micrograms kg-1 body weight by s.c. injection and Group 2 calves were not treated; all were killed at 14 days after treatment. Ostertagia ostertagi was present in all controls; arithmetic mean numbers of adults, developing fourth stage larvae (L4) and inhibited EL4 were 7683, 605 and 36,102, respectively. Other nematode genera present in controls in sufficient numbers for the experiment were Haemonchus placei adults, Trichostrongylus axei adults, Cooperia spp. adults, Oesophagostomum radiatum adults, Bunostomum phlebotomum adults, Dictyocaulus viviparus adults and E5 (immature adults). Abamectin was highly effective (consistently greater than 99% efficacy and P less than 0.05) in removing all nematodes present in treated calves as represented in non-treated controls, including the primary target of Ostertagia ostertagi inhibited EL4. The lowest efficacy was 93.8%, against D. viviparus E5.     

Pathophysiologic effects of Ostertagia ostertagi in calves and their prevention by strategic anthelmintic treatments.

Pathophysiologic effects of Ostertagia ostertagi infection and their prevention by strategic anthelmintic treatments were studied in 3 groups each of 6 steer calves. Group-1 calves were noninfected controls. Group-2 calves were inoculated with 100,000 third-stage larvae on the 1st and 28th days of the experiment and grazed on pasture initially free of contamination. Group-3 calves were on a similar regimen as those in group 2, but were also treated with ivermectin 9 days after each larval inoculation. Group-2 calves had increased plasma pepsinogen and gastrin values and decreased weight gains, and total serum protein and albumin concentrations from the 2nd week of infection onward. They were anemic at 10 to 12 weeks and had lower carcass and meat quality at slaughter. Strategic anthelmintic treatments were effective in preventing these effects and calves in groups 1 and 3 had similar performances. On the basis of our findings, high pepsinogen values were related to worm burdens, whereas high gastrin concentrations were related to gastric lesions.     

Effect of ivermectin on performance of beef cattle on Georgia pastures

A total of 469 cows and calves from 2 herds, each on 6 pastures, was used to evaluate the anthelmintic efficacy and animal-performance benefits of ivermectin given subcutaneously at a dosage of 200 micrograms/kg to nursing beef calves and their dams during a grazing season. Pastures were paired across the 2 herds. Three pasture groups from 1 herd were randomly assigned to either a nonmedicated control or to a medicated group. Treatment assignments were reversed in the other herd. The control groups comprised 110 cows and 108 calves, whereas 127 cows and 124 calves were treated with ivermectin (200 micrograms/kg). The cows were treated once, in late spring, and the calves were treated twice, once in late spring and again in midsummer. Cattle from one herd were weighed on days - 12, 21, 49, 77 (day of 2nd treatment for calves), and 105, and the other herd was weighed on days - 6, 23, 57, 86 (day of 2nd treatment for calves), and 113. Rectal fecal samples for nematode egg counts were obtained from approximately 25% of the cattle in each pasture on weighing days; usually, the same cattle were sampled each time. Calves treated with ivermectin gained (P less than 0.05) more weight than control calves up to the 2nd treatment date and up to the termination of the study. There was no significant difference between treated and control cows, with regard to weight gain over either interval. Treated calves had fewer positive fecal egg counts (P less than 0.01) and passed fewer eggs (P less than 0.05) after both treatments than did control calves. There were no differences in either number of eggs or number of negative cows between treatment groups. Adverse reactions attributable to treatment were not seen.     

Evaluation of the parasitological and production responses of a cow/calf operation to an anthelmintic program with ivermectin

The efficacy of a treatment schedule using ivermectin, given subcutaneously at the rate of 200 micrograms kg-1, to control gastrointestinal parasitism and its effect on liveweight gains was assessed. Two herds with a total of 466 Hereford X Brangus cow/calf pairs were used. Each herd was on six pasture plots of comparable size, stocking rate, and quality and quantity of forage. Pasture groups were paired across the two herds. Pasture groups from one herd were randomly assigned to nonmedicated control and the other three to ivermectin treatment. Treatment assignments per pasture group were reversed in the other herd. The control group contained 118 cows and 112 calves and the medicated group 121 cows and 115 calves. Cows were treated in early July and the calves in late July or early August. Cattle from one herd were weighed on Days -28, 0 (day of cow treatment), 30 (day of calf treatment), 58, and 86, while the other herd was weighed on Days -29, 0 (day of cow treatment), 27 (day of calf treatment), 61, and 89. Day 0 was not the same calendar day for the two herds. Fecal samples for parasite egg counts were obtained from the rectum on weight days from five cows and six calves from each pasture; in most cases the same cattle were sampled each time. Calves treated with ivermectin gained (P less than 0.05) more weight than control calves between day of treatment for cows and the end of the trial.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of ivermectin delivered from a sustained-release bolus on the productivity of beef cattle in Oregon

The effect of ivermectin delivered from a sustained-release bolus (I-SRB) on the weight gain of beef cattle through a grazing season was evaluated using 20 yearling beef steer calves randomly divided into two groups of ten animals each. Calves in the control group each received a placebo bolus, while those in the treatment group each received an I-SRB designed to release 12 mg ivermectin day-1 for approximately 90 days. All animals were weighted and samples of feces were collected from the rectum at monthly intervals, beginning on Day 0, until trial termination (Day 148). Pasture larval counts were also conducted on herbage collected on each sampling date. On Day 119, two control and two treated calves were removed from pasture, housed in isolation for 3 weeks, then necropsied for recovery of gastrointestinal nematodes. Three sets of parasite-naive tracer calves were utilized to evaluate the initial, interim and final levels of pasture contamination by nematode larvae. A fourth set was used to evaluate the level of pasture contamination the following spring. The use of the I-SRB resulted in a greater than 99% reduction in fecal egg counts of trichostrongyles and numbers of gastrointestinal nematodes in the treated principals, as well as an average daily gain advantage of 0.114 kg over the 148 day period. A 67-98% reduction in pasture larval nematode contamination occurred on pastures grazed by the treated animals, as indicated by the parasite burden in tracer calves and pasture larval counts. The treatment effect was eventually lost by the following spring since tracer calves on the treated pasture had only 33% fewer nematodes than those on the control pasture.     

Efficacy of ivermectin administered via sustained-release bolus against gastrointestinal nematodes in cattle

Twelve calves (mean weight, 175.5 kg) were used to confirm efficacy of ivermectin delivered from a prototype sustained-release bolus against naturally acquired gastrointestinal nematodes including early fourth-stage (inhibited) larvae of Ostertagia ostertagi. The calves were allocated by restricted randomization on weight to 1 of 2 groups: controls, to which a placebo bolus was given orally, and treated calves, to which a sustained-release bolus designed to deliver 8 mg of ivermectin/day at a steady rate was given orally. After treatment, the 2 groups were housed in separate pens with concrete flooring. Twenty-eight days after treatment, all calves were euthanatized and necropsied. The ivermectin-treated calves had no larval or adult Ostertagia spp and significantly (P less than 0.01) fewer adult Trichostrongylus axei and adult Cooperia (C oncophora, C punctata and C surnabada) than control calves. Efficacy of ivermectin was greater than 99% for Cooperia spp, and 100% for other parasites. Drug-related adverse reactions were not observed.     

Evaluation of gastro-intestinal nematode parasite control strategies for first-season grazing cattle in Sweden

A three-year grazing experiment (1998-2000) was conducted with first-season grazing cattle (FSGC) on improved pastures in central-eastern Sweden. Comparison was made between five groups with 10 calves in each group where four of these were set stocked and either (1) untreated, (2) ivermectin bolus treated, (3) subjected to biological control with the nematophagous fungus Duddingtonia flagrans, or (4) treated with a copper wire particle bolus. The fifth treatment was an evasive grazing strategy, whereby untreated calves were turned out onto pasture used by older cattle the previous year and then these calves were moved to silage aftermath in mid-July. To introduce low-levels of parasite infection to the experiment, each animal received a 'priming dose' of approximately 5,000 Ostertagia ostertagi and 5,000 Cooperia oncophora infective third stage larvae immediately prior to the start of the first grazing year of the trial.Results showed that efficient and sustainable parasite control of FSGC was possible to achieve without the use of anthelmintics by using turnout pastures that the previous year had been grazed by older cattle, in combination with a mid-July move to aftermath leys. Biological control also proved beneficial but the efficacy was impaired if high faecal egg counts coincided with rapid dung pat degradation due to heavy rainfall. No indication of parasite control was observed with the copper wire particle bolus. It was also demonstrated that the impact of gastrointestinal (GI) parasitism varied between years and that the level of overwintering contamination is important but likewise, is unpredictable. Although faecal egg counts in 1999 were low, due both to a delayed turnout and drought for the major part of the grazing season, deposited eggs successfully developed to infective larvae and overwintered in large numbers. The population of overwintered infective larvae at the time of turnout in early May played an important role in the course of infection in 2000 and resulted in an average 65 kg advantage of the ivermectin treated calves compared with the untreated calves.Thus, this three-year grazing experiment has emphasised the importance of subclinical gastrointestinal nematode infections in FSGC in Sweden. In addition, the study has shown that adequate parasite control may be achievable without the use of anthelmintics.