Source: Beef Farmers of Ontario
Sixty-four cow calf pairs were assigned to one of three anthelmintic treatments (ivermectin, fenbendazole, or control) at the beginning of the grazing season in 2014. Animals that had calving difficulty, twins, or were ill around the time of calving were excluded from the study. Treatment groups were balanced for cow age. Prior to turn-out to pasture, animals were weighed, a fecal sample was collected, and the assigned treatment was given. Fecal samples were analyzed for gastrointestinal nematode (GIN) eggs using a modified Wisconsin technique. Sixteen groups of 4 cow-calf pairs were placed in identical sized pastures, with all pairs assigned to one pasture given the same treatment. Fecal samples were collected and animals were weighed every 4 weeks throughout the grazing season, from May to October. This protocol was repeated during the 2015 grazing season. Animals enrolled in the previous year were treated again with the same product (n=43) and animals not previously enrolled and with no history of anthelmintic treatment (n=21) were used to replace animals from the previous year (culled due to being open or calving too late in the calving season). No animals were lost to follow-up or had to be excluded from the study during either year. No adverse effects of treatment were seen in the study.
Following animals monthly over 2 grazing season showcased the variation in fecal egg counts over time in cows and calves. It also showed the variation in individual groups and animals. Overall, the peak fecal egg counts were seen in mid to late July. Control animals tended to have higher fecal egg counts than either treatment group, especially as the grazing season progressed. However, even without treatment fecal egg counts tended to decline after mid-July, likely as the weather became less hospitable to GIN larvae (hotter and drier). Calves had GIN eggs present at the beginning of the grazing season. As the GIN require pasture to develop to an infective stage, this is not surprising. There was no significant difference in GIN fecal egg counts between treatment groups over the trial period.
In 2015, fecal samples were collected from all enrolled animals 2 weeks after treatment to determine the risk of anthelmintic resistance with a fecal egg count reduction test (FECRT). Samples were collected from 56 of the 64 cows at both time points. Eight animals (2 control, 2 ivermectin, and 2 fenbendazole) were missing one sample. Initial fecal samples from calves had no GIN eggs present, so they could not be used in the FECRT. Animals that were not treated averaged an 82.9% increase in GIN egg counts over the 2 weeks (range 100% decrease to 2400% increase). Animals treated with ivermectin averaged an 81.4% decrease (range 100% increase to 100% decrease) and fenbendazole treated animals averaged an 69.7% decrease (range 33% increase to 100% decrease) in GIN eggs counts over 2 weeks. Anthelmintic resistance is suspected if fecal egg counts in samples taken 2 weeks after treatment are not reduced at least 95% compared to initial samples.
Overall, there was no significant effect of anthelmintic treatment on adjusted calf weaning weight However, the effect on weaning weight varied by year. In the first year of the study, there was no significant difference in adjusted calf weaning weight. In the second year of the study there was a tendency for ivermectin treated calves to weigh more at weaning then control calves, but there was no difference between control and fenbendazole treated calves.
Anthelmentic treatment did not have an effect of maternal body weight, body condition score, or pregnancy rate. There was no difference in health between the groups and no animals had to be treated due to clinical parasitism.
Anthelmintic treatment had no significant effect on any of the economic measures collected in this trial. However, there was a tendency for ivermectin treated animals to outperform control and fenbendazole treated animals. There is evidence suggestive of anthelmintic resistance in this herd, which is particularly interesting as they do not routinely use anthelmintics.
This study showcased nicely the epidemiologic curve of GIN in a southern Ontario pasture grazed herd. It demonstrated the change in parasitism over time and the large variation seen between individuals. Though there were no significant differences in the economic measurements used in this trial, the tendency of ivermectin treated calves to weigh more at weaning deserves further investigation. More research is also needed to determine the risk of anthelmintic resistance in this herd and in Ontario beef cattle as a whole. The evidence of resistance in this herd suggests that resistance may be a problem elsewhere. However, FECRT would be more accurate if performed at peak parasitism. So future FECRT should be performed in mid to late July. It is also interesting that fenbendazole treated animals routinely performed more poorly than ivermectin or control animals and the level of resistance was higher. Fenbendazole is being promoted to producers as a good alternative to ivermectin in herds that suspect resistance, but evidence from this trial suggests that may not be the case. As the sample size in this trial was quite small, it would be important to continue this research in more herds and more cows across the province to be better able to advise beef producers on proper anthelmintic use.