Estrus characteristics, ovarian response to synchronization hormones, and fertility of crossbred dairy heifers managed under a semi-intensive system

Abstract

This study aimed to characterize estrus sign/s which best predict the time of ovulation and determine ovarian responses to different  synchronization protocols and conception rates to fixed-time artificial insemination in Boran*Holstein crossbred heifers. In the first part of the study, twenty-seven heifers were observed for estrus signs (from induced and natural) and the relationship of various estrus signs with ovulation time was evaluated by using ultrasonography. In the second part, 91 Boran*Holstein crossbred heifers were randomly grouped into three groups. Group one heifers (n=28) received 100μg gonadotropin-releasing hormone (GnRH) on day zero (D0), 500μg  prostaglandin F2α (PGF2α) on day 7 and 100μg of GnRH on day 9. Group 2 heifers (n=32) were treated as group one but additionally  received progesterone as controlled internal drug release (CIDR). Group three heifers (n=31) were treated as those in group 2 but without injection with GnRH on day 0. In all the 3 groups’ insemination was made at 19h of the second GnRH. The results showed that irrespective of estrus source type (induced or natural), score for standing to be mounted, mounting other heifers, and non-receptive mount by other heifers showed a strong positive correlation with ovulation time (r=0.67, P<0.05). Standing estrus duration, and time elapsed from standing estrus to ovulation were shorter (P<0.05) in induced estrus. The conception rate was 39.3% in synchronized ovulation in the absence of CIDR. The conception rate in timed insemination was 56.3% when ovulation was synchronized by combining CIDR, PGF2α, and GnRH. In conclusion injection of GnRH on day zero together with treatment with CIDR improved ovulation rate, the number of new corpus luteum, and conception rate. Heifers that ovulated within the first 24h after timed insemination had a higher (P<0.05) conception rate than heifers ovulated after 24h of timed insemination. Cycling heifers at day zero had a greater (P<0.05) conception rate than non-cycling.