Objective—To measure the reduction in fecal nematode
egg counts and productivity impact of treatment
of yearling steers with doramectin at entry into the
feedlot, compared with control steers treated only
Animals—6,096 crossbred yearling steers with a
mean (± SD) body weight of 377.0 (± 37) kg.
Procedure—Steers were implanted with zeranol and
alternately separated to fill each of 24 pens. Groups of
steers within 12 matched pairs of pens were randomly
allocated to treatment with doramectin or no treatment
with doramectin for internal nematodes. Fecal
samples were collected from approximately every
twentieth steer from each pen at day 0 and at reimplant
(approx day 60). Each steer was weighed on day
0 and at reimplant and then mean body weights of
steers per pen were determined at 120 to140 days
after trial initiation.
Results—Treatment steers had a significantly lower
fecal egg count at reimplant than control steers.
Treatment steers had a significantly greater mean
daily gain during the study, significantly greater feed
consumption, significantly lower feed-to-gain ratio,
and significantly better quality carcass grades at
Conclusion and Clinical Relevance—Under the
conditions of our trial, there was a significant fecal
egg count reduction response to doramectin treatment,
which resulted in significantly improved productivity.
Results of economic analysis of return on
investment indicated that even with low egg counts
in heavy body weight cattle, nematode egg count
reduction with doramectin significantly improved
returns. (Am J Vet Res 2001;62:622–624)