author(s)
Vijyakanth N, Anil Kumar R, Ezakial Napolean R and Thavasiappan V

abstract
The effect of substituting estrogen for the 2^nd dose GnRH on the synchronization of ovulation was studied. Healthy pluriparous crossbred cows were selected and randomly divided into two groups (A and B) of six cows each. Cows from group A were treated with injection of 10 µg of GnRH on day 0, 500 µg of PGF_2α on day 7 and another 10 µg of GnRH 48 h after PGF2α (day 9) and timed artificial insemination (TAI) was done on day 9 and 10. In group B, the second dose of GnRH was replaced with 1mg of estradiol benzoate injection 24 h after PGF2α (day 8). Intensity of estrum was significantly (P<0.01) higher in group B Cows. Though ovulation rate was higher in group A, no significant difference in overall conception rate was observed. The concentration of progesterone was higher among cows which ovulated and in cows conceived at 1^st service. The estradiol-17β concentration was highest at estrum (63.17±3.62 pg/ml) and it was high in cows which ovulated and in cows which become pregnant. Concentration of IGF-I was significantly lower in group A at all stages of experiment. The concentration was lower during estrus in cows which ovulated (149.21±16.72 vs. 175.99±25.39). Increase in estradiol and decrease in IGF-1 and progesterone at estrus has positive results on the establishment of pregnancy. Though the results of replacing the 2^nd dose of GnRH with estrogen injection were similar to that of group A, less cost of estrogen, improved estrous expression and ease of AI favours this treatment.