Growth implants have not
been widely used in heifer calves because of concern by herd managers about
detrimental effects on subsequent reproductive performance of heifers kept
as herd replacements. Currently three implants Synovex-C®, Component
E-C® (estradiol and progesterone), and Ralgro® (zeranol) have been
given FDA approval for use on potential replacement heifer calves. Past
reviews of this subject have been quite thorough and generally concluded
that one implant given at or after the heifer is 2 months of age has very
little impact on future reproductive performance (Hargrove, 1994 and Deutscher,
1994). Also these reviews have both concluded that implanted heifers have
significantly greater pelvic area when measured at about one year of age,
but these differences are indeed very small at the time the heifer is delivering
her first calf at or about two years of age. Consequently, the data on
dystocia rate indicates that implanted heifers have no less calving difficulty
than do non-implanted counterparts.
The possible effect of implanting
on breeding season pregnancy rates is still the major concern of ranchers
deciding to implant heifer calves or leave them unimplanted. The following
are tables reporting trials that examined the difference in pregnancy percentages
of heifer calves implanted once at birth, once at calf-working time (approximately
2 months of age), once at weaning time, or multiple implants. Both the
36 mg zeranol implants and the 10 mg estradiol - 100 mg progesterone type
implants are examined. The available data is clear that implanting at birth
is detrimental to breeding season pregnancy rates.
Table 1. Summary of trials where heifer calves were implanted
with Zeranol at birth
Number of trials
Average difference from non-implanted controls in pregnancy rates
3
-39% (Range -37% to -50%)
Therefore, producers must be encouraged to follow label instructions
closely when implanting heifer calves after 30 days or 45 days of age depending
on implant type. The average loss in percentage pregnant due to one implant
(at calf-working time) is quite small (tables 2 and 3).
Table 2. Summary of trials where heifer calves were implanted
once with Zeranol at 1 to 3 months of age.
Number of trials
Average difference from non-implanted controls in pregnancy rates
13
-0.8% (Range -11% to +19%)
Table 3. Summary of pregnancy rate of heifers implanted once at 1
to 3 months with Estradiol and Progesterone.
Number of trials
Average difference from non-implanted controls in pregnancy rates
10
-3.8% (Range -10% to +6%)
The tremendous variation in the
trials is partly due to the relatively small numbers of heifers represented
in some treatment groups. This summary of trials should not lead to any
conclusions that one implant type is safer than another when given properly
at 2 months of age. When heifers are implanted once at weaning time, the
risk of reduced pregnancy rates is slightly greater. Most producers
can identify potential replacements at this time. Therefore, the decision
to implant stocker heifers being kept for gain and not implanting those
kept for replacements seems obvious for most operations. The summary of
trials in which heifers were implanted more than once indicate that the
risk of reproductive loss increases as the number of multiple implants
increase.
Summary
The information available
for suckling steer calves and heifer calves not intended for replacements
is clear that growth promoting implants are consistent in improving average
daily gain from implanting to weaning. Other reviewers have stated that
the decision to implant is much more important than the decision of which
implant to use (Corah and Blanding, 1992). Average daily gain responses
of approximately .1 pound per day can be expected with the zeranol and
estradiol-progesterone implants in steer calves. Slightly greater responses
may occur in heifer calves (.12 to .14 pound per day advantages). This
agrees with other findings in which heifer calves tended to produce greater
responses than steers (Mader, et al. 1994).
Potential replacement heifers
that can be identified early in life (for example heifers in seedstock
herds) should not be implanted. No advantage in puberty age or dystocia
rate exist. Heifers that cannot be identified early in the suckling phase
as a potential replacement can be implanted once at approximately 2 months
of age with very little risk of reproductive impairment. Re-implanting
of replacement heifers increases the risk of reduced pregnancy rates. Economic
analyses of a simulated commercial cow herd indicates that little economic
risk exists if all heifers are implanted once at calf working time. The
risk increased if a very high replacement heifer rate was used and ranch
history of greater than 5% reduction in pregnancy rates due to implanting
had been shown.
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