Effects of S-Methoprene, Altosid® IGR, on fly numbers and weight
gain in crossbred heifers in northern California1
N. L. Humphrey*
*Department of Animal Sciences, College of
Agriculture, California State University, Chico
ABSTRACT:
This study was conducted to determine the efficacy of S-methoprene
for horn fly control and determine if control of horn flies
improved weight gain in beef heifers. The study took place in
Orland, California using six to nine month old replacement
heifers. Heifers were randomly assigned to one of two
treatments. Treatment heifers received Right Now®
Emerald mineral containing 0.01% (S)-methoprene and control
heifers received Right Now® Emerald mineral without
fly control, both of which were offered free-choice. During
period one heifers achieved optimum intake of mineral, whereas
optimum intake was not achieved during period two of the study.
Results showed that (S)-methoprene did not offer significant
control of horn flies during period one (p=0.2885) or period two
(p=0.741). Since horn fly control was not achieved, improved
weight gains were not observed in methoprene treated heifers
during period one (p=0.0918). During period two, control
heifers gained significantly more weight than methoprene treated
heifers (p=0.0232). Economic threshold fly populations were
never reached in either treatment indicating the cattle used for
the study did not require fly control to reach maximum genetic
potential.
There are many pests that inhibit cattle from reaching their
maximum genetic potential for growth and health. The horn fly,
Haematobia irritans (L.), has been estimated to cost the
beef industry $876 million a year (Kunz et al., 1991). The
economic threshold for horn fly populations in a given
environment is estimated to be 50 to 200 flies per animal with
accepted levels varying by study (Andress et al., 2000 and
Swanson et al., 2003). When fly populations reach threshold
cattle begin to drop off in weight gain and expend energy on
production of new blood that is being depleted by the flies (Drumond
et al., 1981). Producers can receive premiums for their cattle
when they can achieve maximum weight gain and market calves that
are healthy and thriving.
Studies have shown that with the use of endectocides, fly counts
can be kept below the threshold level (Andress et al., 2000;
Miller et al., 2003; and Swanson et al., 2003). External
__________
1This
work supported by funds from CSU, Chico Research and Creativity
Grant, Cargill Animal Nutrition, Wellmark Int., and Superior Ag
parasite control can lead to enhanced weight gains in yearling
cattle (DeRouen et al., 2003; Sanson et al., 2003; and DeRouen
et al., 1995). Extensive use of pyrethroid and organophosphate
fly control methods have allowed flies to develop a resistance
to their lethal actions (Barros et al., 2001 and DeRouen et al.,
1995). Within the past ten years a new fly control product,
Altosid Insect Growth Regulator (IGR) the active ingredient S-methoprene,was approved for use in California. Methoprene mimics
juvenile hormone naturally produced by insects and prevents
pupae from molting into adults (Glare and O’Callaghan, 1999;
Parthasarathy et al., 2006; and Stark 2005) but has not been
well studied in northern California. Furthermore, methoprene
has a relatively short half-life in the environment and is
converted to natural biochemicals in mammals (Glare and
O’Callaghan, 1999 and Quistad et al., 1975). Thus, the purpose
of this study was to determine the efficacy of S-methoprene for
horn fly control and determine if control of horn flies improves
weight gain in heifers under field conditions in northern
California.
MATERIALS AND METHODS
The experiment was conducted at Quiet Hills Ranch in Orland,
California. Animals were allowed a 22 day acclimation period
prior to onset of the study. Period one of the study consisted
of 19 days, and period two consisted of 57 days.
Cattle
One hundred and twenty six commercial Hereford-Angus crossbred
heifers approximately six to nine months of age served as the
experimental units for period one of the study. Heifer weights
ranged from 148 to 327 kg. Heifers were identified by
individual ear tags and were given standard vaccinations for
infectious diseases. All heifers received a dose of injectable
Dectomax for the control of internal parasites (Pfizer Animal
Health, New York, NY).
Period two of the study was conducted using 20 treated heifers
and 14 control heifers. Heifer weights ranged from 250 to 330
kg.
Pasture
During period one heifers grazed 350 acre lots of native
rangeland pasture with ad libitum water via water troughs and
reservoirs throughout the study. Treatment groups were
separated by one mile of pasture.
During period two heifers grazed 50 acre lots of irrigated
pasture with ad libitum water via water troughs
throughout the study. Treatment groups were separated by a
fence line.
Group allocation and treatment
During period one heifers were randomly assigned by lottery to
one of two treatments, heifers remained in their respective
treatments throughout both periods of the study. One group
received horn fly control in the form of an insect growth
regulator, methoprene in a standard mineral granule, and one
group received no horn fly control. The heifers in the control
group received free-choice Right Now® Emerald
mineral. The heifers in the treatment group received Right Now®
Emerald mineral containing 0.01% (S)-methoprene offered
free-choice. All heifers were offered free-choice Right Now®
Emerald mineral for 22 days prior to the start of the study to
acclimate the heifers to mineral consumption.
Mineral Consumption
Mineral consumption was monitored on a weekly basis. During
period one mineral weight was only obtained as mineral was
placed in tubs. During period two, mineral remaining in tubs
was weighed as was new mineral placed in tubs.
Determination of inhibition of adult emergence of the horn
fly
On-animal estimates of adult horn fly numbers were made once per
week beginning on day 0. Horn fly numbers were estimated by
randomly selecting 10 heifers from each treatment group and
estimating the number of horn flies on one side of each animal.
All fly counts were made before 09:00 h with the aid of
binoculars (DeRouen et al. 1995). Final fly counts for period
one were taken on day 19 and for period two on day 57. Fly
populations were considered to be under control when fly counts
averaged less than 100 flies per side per animal.
Evaluation of weight gain of heifers
During period one individual weights of heifers in each
treatment group were taken on days 0 and 19. During period two
individual weights of heifers were taken on days 0 and 57.
Weights were measured using a Powell digital scale. Handling of
the heifers was minimized to avoid shrink.
Calculations and Analysis of Data
Fly Counts and Body Weights. The following
equation was used to calculate percent horn fly control for the
treated heifers.
(White, 2006)
Replicated fly counts were analyzed using a two sample T-test
(Statistix8, 2003). Additionally, average daily gain data for
each trial period were analyzed using ANOVA (Baldy, 2001) as a
completely randomized design. Pairwise comparison of mean
weight gains between methoprene-treated versus non-treated
heifers was made using Tukey’s multiple comparison test. The
accepted level of statistical difference was p≤0.05.
Results
Mineral consumption
Label recommendations for intake of Right Now® Emerald
mineral is 0.04 ounces per 100 pounds of body weight.During
period one, mineral consumption by heifers in the treatment and the
control exceeded the label recommendation by 75.9% and 69.1%
respectively.
During period two, treated heifers consumed 52.3% less mineral than
was recommended, while the control heifers consumed the mineral at a
rate of 53.6% below label recommendations. Because heifers in both
the treatment and the control were pastured with surplus cows,
intake by designated animals was estimated from the entire 70 cow
herd for the treatment heifers and the entire 64 cow herd for the
control heifers.
Inhibition of adult emergence of the horn fly period one
Fly counts were not significantly different between treated and
control groups during period one of the study (p=0.2885) (Figure
1). Throughout period one fly counts in both the treated and
control groups never reached the accepted economic threshold of 100
flies per side.
Figure 1: Mean
number of horn flies per side of control and methoprene treatment
during period one. Treatment differences were not significant with
p>0.05. (two sample T-test)
Inhibition of adult emergence of the horn fly period two
As with period one, fly counts among treatments in period two were
not significantly different (p=0.0741) (Figure 4). Again, fly
counts in both the treated and control groups never reached the
accepted economic threshold of 100 flies per side.
Figure 2: Mean
number of horn flies per side of control and methoprene treatment
during period two. Treatment differences were not significant with
p>0.05. (two sample T-test)
Weight gain of heifers
There was not a statistically significant enhanced weight gain in
heifers treated with methoprene (p=0.0918) during period one of the
study (Figure 3). During period two heifers in the control gained
significantly (p=0.0232) more weight than heifers treated with
methoprene (Figure 4).
Figure 3: Mean
weight gain of control and methoprene treatment during period one.
Treatment differences were not significant with p>0.05. (Tukey-Kramer
test)
Figure 4: Mean
weight gain of control and methoprene treatment during period two.
Treatment differences were significant with p<0.05. (Tukey-Kramer
test)
Discussion
Despite a lack of difference in fly counts during period one between
the treated and control herds, it should be noted that fly numbers
in both treatments never went above the economic threshold of 100
flies per side. Due to late rains the fly season was delayed, which
may have impacted the results obtained during period one.
Methoprene mimics juvenile hormone action in the horn fly and stops
the horn fly life cycle at the pupae stage. Methoprene is not
effective at killing adult horn flies, thus it may take up to 21
days to eliminate initial fly loads. Since period one only lasted
19 days this was not an adequate amount of time to evaluate the
efficacy of methoprene for reducing horn fly numbers. Lack of range
feed made it necessary to end period one on day 19 and relocate the
study to irrigated pasture.
Upon relocating heifers to irrigated pasture heifers were mingled
with cattle that had not previously received fly control treatment.
Furthermore, those cattle on methoprene for control of horn flies
were surrounded on all sides by cattle receiving no horn fly control
treatment (including the control group for the study). Fly
migration from untreated herds is a likely explanation for the lack
of difference in fly counts between the methoprene treated and
control heifers during period two. Another possible explanation for
the lack of difference in fly populations is the lower than label
recommended intake of methoprene by the treated cattle, which would
indicate improper levels of methoprene in the manure of these
heifers, thus, leading to decreased control effects.
Previous studies investigating horn fly control in yearling beef
cattle (DeRouen et al., 2003 and Floate et al., 2001) showed percent
control levels ranging from 68% to 100%. During period one of this
study an overall horn fly percent control of 16.7% was achieved.
Period two of this study provided an overall horn fly percent
control of 3.6%. The lower percent control percentages obtained in
this study could be explained by the fly populations being below the
economic threshold in both the control and treated groups and may
not be a good indicator of the efficacy of methoprene for fly
control.
Despite the lack of significant difference in horn fly control
throughout this study, several studies demonstrate the efficacy of
S-methoprene for control of various insects. A study conducted by
Young and colleagues (2004) found S-methoprene significantly reduced
cat flea populations in beagle dogs. Another study demonstrated
effective emergence inhibition of the parasitoid Cotesia
congregate with the use of methoprene (Beckage et al., 2002).
Methoprene in combination with diflubenzuron was shown to
effectively control S. oryzae and R. dominica in
stored sorghum (Daglish and Wallbank, 2004).
Although there is overwhelming evidence suggesting methoprene is an
effective control tactic for insects, a study in Florida showed an
isolated population of mosquitoes has developed some level of
resistance (Dame et al., 1998). Prolonged use of methoprene to
control the mosquito, Ochlerotatus nigromaculis, showed
higher lethal concentrations (LC) 50 and LC 90 were required and
lower levels of control were achieved (Cornel et al., 2002 and
Cornell et al., 2000). Furthermore, a study conducted on field
populations of the house fly showed some resistance to the IGRs
diflubenzuron and methoprene (Kristensen and Jespersen, 2003). In
order to prevent methoprene from suffering the same fate as
pyrethroid, organophosphate, and endectocide insecticides it would
be wise to consider a horn fly control protocol that either rotates
products from year to year or uses products in combination.
Further studies investigating
the efficacy of methoprene should be conducted at several ranch
locations in the northern California area during years when horn fly
infestations are heavier. While this study ended in August, horn
fly populations also exhibit a peak in the months of September and
October, so it would be wise to conduct a study that continues for
the entire fly season. To address the intake issue associated with
this study, it may be wise to consider formulating a methoprene
sustained release bolus to ensure the proper concentrations of
methoprene end up in the manure to adequately prevent pupae from
molting into adults.
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