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The effects of exogenous estrogen on growth, mammary development, and the age at which the first estrous symptoms are detected were investigated for the first time in Korean native heifers at 6 months of age. At 90 days after treatment, weight gains of heifers implanted with 10 mg of estradiol were not different from those of the control heifers. However, weight gams of heifers implanted with 20 mg of estradiol were significantly increased at 90 days by 18%, relative to control values. At 180 days, cumulative weight gains of heifers implanted with either 10 mg or 20 mg of estradiol averaged 6.7% or 17.8%, respectively, greater than controls. But these values were not statistically significant. Regardless of dosage, teat diameter gains in the treatment groups were significantly increased at 90 days about 159%, relative to controls. Cumulative teat diameter gains in the heifers implanted with either 10 mg or 20 mg of estradiol were also significantly increased at 180 days by 100% or 128%, respectively, compared to controls. Teat length gains in the heifers implanted with either 10 mg or 20 mg of estradiol were significantly increased at 90 days by 200% and 295%, respectively, compared to controls. Cumulative teat length gains in the heifers implanted with either 10 mg or 20 mg of estradiol were significantly 265% and 325%, respectively, higher than controls. The heifers implanted with either 10 mg or 20 mg of estradiol showed a significant increase in teat volume gains at 90 days of 282% or 246%, respectively, over those of the control heifers. Cumulative teat volume gains in the heifers implanted with either 10 mg or 20 mg of estradiol were significantly increased at 180 days by 251% and 244%, respectively, compared to control values. However, there were no significant differences m gams of teat diameter, length and volume between heifers implanted with 10 mg and 20 mg of estradiol regardless of 90 days or 180 days. Heifers implanted with 10 mg or 20 mg of estradiol showed the first estrus 20 or 124 days, respectively, faster than controls. Overall, These data strongly suggest that estradiol implantation into heifers stimulates growth and mammary development and hasten the age at which the first estrus in Korean native heifers is observed.
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