Effect of treatment with an ivermectin sustained-release bolus on productivity of stocker beef calves

Endoparasite Control

The gastrointestinal nematodes are considered to be the most important group of internal parasites. Gastrointestinal parasitism is a major constraint to animal health, productivity, and profitability in grazing livestock production systems. The clinical manifestations of infection may range from no apparent effect to death. Despite years of research, assessment of production losses in many regions of the United States and of the potential impact of gastrointestinal nematode control remains to be determined.

Effect of horn fly and internal parasite control on growth of beef heifers

The effects of horn fly and gastrointestinal nematode control on body weight gain of yearling Angus-Brangus cross heifers were evaluated in three separate studies during the years 1999, 2000 and 2002. In each year, the studies started in late May and lasted for 150, 148 and 123 days, respectively. In all three studies, the tag treatment (10% lambdacyhalothrin+13% piperonyl butoxide impregnated ear tags) provided excellent horn fly control. In the three studies, the average weekly horn fly counts for tagged heifers were 1, 3, and 0 flies per side while the average on untreated heifers was 52, 163 and 90 flies per side. In studies 1 and 2, there was no difference (P>0.1) in weight gain between tagged and untreated heifers, but in study 3, tagged heifers gained 50% more weight (P<0.001) than the untreated heifers. For gastrointestinal nematode control, ivermectin (IVM) was administered on Day 0 in studies 1 and 3 using a sustained release bolus and on Day 0 and subsequent 56-day intervals in study 2 using either the injectable or pour-on formulation. Heifers treated with IVM in studies 1 and 3 had significantly lower (P<0.05) GI nematode fecal egg counts compared to control heifers. In study 2, IVM treated heifers also had significantly lower GI nematode fecal egg counts compared to control heifers, except for Day 90 when no differences in fecal egg counts were observed between IVM pour-on treated and control heifers. Weight gain of heifers that received IVM was significantly greater (P<0.005) than untreated heifers in each of the three studies. IVM treated heifers gained 45, 61 and 184% more weight than the untreated heifers during the three studies. There was no interaction (P>0.1) between the main treatment effects of fly control and gastrointestinal nematode control.

Evaluation of the impact of parasite control with the IVOMEC SR Bolus given at breeding age on first lactation yield in Holstein heifers

This field trial was conducted to test the effect of treatment with a sustained-release ivermectin bolus on first lactation milk production (projected to 305 days) and milk composition (percent fat and protein) in breeding-aged Holstein heifers. From 12 herds, 96 heifers were randomly allocated at turnout to treatment with a commercially available ivermectin bolus (n=49) or untreated control (n=47) groups. Animals were weighed at turnout, mid-season, and at the end of the grazing season. Fecal samples were taken and nematode eggs counted. Projected 305 days milk production, fat and protein content were retrieved from PATLQ reports (Quebec DHI records) for heifers that calved during the subsequent 10 months. The heifers had a mean weight of 409 and 404 kg at turnout, respectively, for control and treated groups. Nematode egg excretion was relatively low (mean of 10 egg/g (EPG) for controls at housing) throughout the pasture season. The average daily gain (ADG) and total weight gain (TWG) over the entire pasture season was found to be higher in treated heifers compared to controls (ADG: difference=0.08 kg per day, P=0.010; TWG: difference=12.82 kg, P=0.013). Taking into account a possible farm effect, the 305 days milk production for IVOMEC SR bolus-treated heifers was significantly higher than for untreated animals (mean+/-S.D.; 348+/-476 kg; t-test=2.31, P<0.05). Overall, 8 of the 10 remaining farms in the study demonstrated a positive milk production difference in treated (n=34) versus control heifers (n=33). The results of this study suggest that preventive treatment of breeding-aged, grazing dairy heifers with a sustained-release ivermectin bolus provides a significant weight-gain which may translate to a significant milk production advantage, even in situations with commercial utilization of moderately contaminated pastures.

A single pasture limited treatment approach to estimate production loss from gastrointestinal nematodes in grazing stocker cattle

The objective of this study was to evaluate the use of a single pasture, limited treatment methodology to assess the impact of gastrointestinal nematodes on weight gain in grazing cattle. From a group of 450 British crossbred, yearling spayed heifers, 60 animals were randomly selected (prospective randomization) prior to placement on summer pasture. Each of these 60 animals was weighed, a fecal sample obtained for nematode egg count and a uniquely numbered ear tag applied. A randomly assigned group of 30 received treatment with one ivermectin sustained release bolus, while the remaining 30 served as non-treated controls. The treatment and control groups rejoined the remaining non-treated 390 animals, and were grazed as a single group for 165 days. At grazing conclusion, treatment and control cattle were individually weighed, and fecal samples obtained for nematode egg counts. Treatment was associated with a 0.064 kg per grazing day gain increase, or a 10 kg increase over the grazing season (P = 0.02). Nematode egg counts at grazing initiation were not different between treatment and control (P = 0.30), though egg counts in treated cattle at study end were lower than control (P < 0.0001). Results of this study support the use of a single pasture limited treatment approach for measuring the effect that internal nematodes have on weight gain in grazing cattle under commercial range conditions.

The effect of first season chemoprophylaxis in calves on second season pasture contamination and acquired resistance and resilience to gastrointestinal nematodes

Two groups of second grazing season cattle, which had been treated with an ivermectin-sustained-release bolus (ISRB) in their first grazing season, were monitored during consecutive years (1995 and 1996) on the same second grazing season pasture. In the preceding year (1994), this pasture had been grazed by yearlings that had not received chemoprophylaxis in their first grazing season. The objectives of the study were, firstly, to investigate whether the chemoprophylactic-treated yearlings were less resistant to gastrointestinal nematodes upon subsequent exposure, and hence excreted more strongyle eggs compared to the control yearlings; secondly, whether an increased susceptibility of the previously treated animals resulted in a yearly increase of the pasture infestation on the second grazing season pasture; and finally, whether this affected the second year weight gain of the animals. In 1996, the yearlings that had been chemoprophylactic-treated in 1995 excreted higher numbers of nematode eggs, compared to the previously 'untreated' yearlings. In addition, the proportion of Cooperia larvae was markedly higher in the faecal cultures from the chemoprophylactic treated-animals, suggesting a negative effect of preventive treatment with an ISRB on the acquired resistance of the animals. However, there was no evidence that the slightly higher egg output in the previously treated yearlings had an effect on the larval contamination of the second grazing season pasture. A significant yearly decrease in the second season average daily weight gains was observed, but it could not be inferred from the results of the parasitological parameters that the differences in second year growth were caused by different levels of resilience between chemoprophylactic-treated and -untreated animals. As the study covered three consecutive second grazing seasons, an effect of differences between years (e.g. in weather conditions or grass growth) on the results cannot be excluded.

Effect of chemoprophylaxis with an ivermectin sustained-release bolus on acquired resistance to gastrointestinal parasites in cattle

The influence of chemoprophylaxis with an ivermectin sustained-release bolus in the first grazing season on the resistance of cattle to gastrointestinal nematodes during the following grazing season was investigated. In 1993 and 1994 dairy replacement calves were either given one bolus at the start of their first grazing season or left untreated. The two groups were grazed separately on a pasture that was divided into two similar sized paddocks. Faecal egg counts, serum pepsinogen and antibody levels were measured to evaluate host-parasite contact. Pasture infection levels were estimated by pasture larval counts and worm counts in tracer calves. After winter housing the animals were monitored during their second grazing season on a pasture that was also divided into two similar sized paddocks. Acquired resistance to gastrointestinal nematodes was evaluated by faecal egg counts and weight gains. Again, pasture infection levels were determined and pepsinogen and antibody levels were measured. During the first grazing seasons gastrointestinal nematode infections were controlled very effectively by the bolus, as shown by the greater weight gains, the negligible faecal egg counts and the low serum pepsinogen and antibody levels in the treated calves. In contrast, all parameters showed extensive parasite-host contact in the untreated animals. The efficient prophylaxis in the treated groups resulted in low levels of larval contamination on the paddocks grazed by the treated animals, compared to moderate infection levels at the end of both first grazing seasons on the paddocks grazed by the untreated animals. During the second grazing seasons (1994 and 1995) the faecal egg output was low in all groups. Although in the previously treated animals faecal egg counts were consistently higher, the differences were minimal, resulting in comparable levels of larval contamination on both paddocks. Serum pepsinogen and antibody levels were not significantly different between the groups and indicated a similar level of larval uptake on both paddocks. No negative effect of the previous chemoprophylaxis on the clinical condition and the weight gain of the second season grazing animals was observed.

Efficacy of a spring strategic fenbendazole treatment program to reduce numbers of Ostertagia ostertagi inhibited larvae in beef stocker cattle

Efficacy of a spring strategic treatment program with fenbendazole (FBZ) to reduce the accumulation of Ostertagia ostertagi inhibited early fourth-stage larvae (EL4) was investigated in two groups of crossbred beef heifers which were 7-9 months of age and ranged in weight from 155 to 223 kg. The cattle were allocated to groups and treated on 27 April (Day 0). Group 1 calves served as nontreated controls. Group 2 calves were treated with FBZ 10% drench suspension at 5 mg kg-1 on Day 0 and with 6-day courses of FBZ free-choice mineral (to provide 5 mg kg-1 per animal) on Days 28 and 56. Each group grazed on a separate 4.9 ha pasture for 105 days to 10 August. On 10 August the cattle were taken off pasture and each original group was re-allocated to subgroups of three cattle which were treated orally with FBZ (5 mg kg-1), oxfenbendazole (OXF, 4.5 mg kg-1) or left nontreated (CONT). The cattle were necropsied on Days 120 and 121. Mean actual and cumulative fecal egg counts indicated near total suppression of egg output in the strategically treated group (high of 5.2 eggs g-1 feces (EPG) on Day 28). Egg counts of the nontreated group remained above 100 or 200 EPG to Day 63 and then decreased to less than 100. Ostertagia was the predominant genus, followed by Cooperia on most sampling dates. The largest O. ostertagi worm burdens were recovered from the CONT-CONT subgroup; numbers of EL4 ranged from 18,922 to 51,137. Reduction in numbers of EL4 in original controls, treated with FBZ or OXF in August, were low, being 60.2% and 74.3%, respectively. The numbers of O. ostertagi recovered from subgroups originally treated strategically with FBZ were generally lower than in original controls. The largest reduction in O. ostertagi numbers was in the FBZ-CONT subgroup, which was not treated in August. Percent reduction values for O. ostertagi adults, developing L4 (DL4) and EL4 were 84.6%, 96.7%, and 99.0%, respectively. Percent reduction values for adults, DL4 and EL4 in the FBZ-FBZ and FBZ-OXF subgroups were 90.7%, 61.3%, 85.6% and 92.4%, 45.5%, and 73.0%, respectively. Variability in numbers of EL4 acquired during grazing by original controls and strategically treated cattle and variability in efficacy of August treatments was evident as observed in high outlier numbers of worms in all subgroups except the FBZ-CONT subgroup. Liveweights and gains were nearly identical in the two groups on 10 August.