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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, 1991). 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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