LH secretion around induced ovulation during early and late diestrus and its effect on the appearance of short estrous cycles in cyclic dairy heifers

The aim of this study was to investigate the effect of the stage of the estrus cycle (early vs. late diestrus) on the LH secretion after induced ovulation and on the incidence of short estrous cycles in dairy heifers. Cyclic heifers were presynchronized with dexcloprostenol and divided into Groups D7 (n = 6) and D14 (n = 6). On Day 7 (D7) or Day 14 (D14) after ovulation (Day 0), all animals were treated with dexcloprostenol followed by 0.1 mg of gonadorelin (GnRH) 24 hours later. Blood samples were taken daily for progesterone (P4) analysis and for LH analysis every 10 minutes for 3 hours on Days 1, 3, and 5 and every 30 minutes for 6 hours starting just before GnRH administration. In a control group, Group C (n = 7), estrus was synchronized with an intravaginal progesterone-releasing device (CIDR) inserted for 9 days. Blood for P4 analysis was taken daily for 16 days and for LH analysis every 30 minutes for 31 hours starting 30 hours after CIDR removal (unmanipulated LH peak) and every 10 minutes for 3 hours on Days 1, 3, and 5 after ovulation. In all groups, ovarian structures and ovulation were detected via daily transrectal ultrasound examination around first and second ovulation. In D7, all cycles (6/6) were shorter than normal (range 7-9 days). In D14, 2 of the 6 animals had a short cycle (both 7 days) whereas 4 had a cycle of normal length. All cycles were of normal length in C. Animals in D7 and D14 were divided according to their cycle length into short (SC) and normal (NC) cycle groups. The mean size of the ovulatory follicle during the 3 days before ovulation was significantly different between D7 and D14 and on 3 days and 1 day before ovulation between SC and NC. Secretion of LH during the 6 hours after GnRH administration did not differ between D7 and D14, or SC and NC. The mean basal LH secretion in D7 and D14 on Days 1, 3, and 5 was significantly different. No difference on those days was noted between SC and NC. In conclusion, when ovulation is induced with GnRH 24 hours after giving dexcloprostenol, short estrous cycles can occur during both early and late diestrus. The preovulatory LH surge did not differ between D7 and D14, or SC and NC. Also, basal LH secretion on Days 1, 3, and 5 was similar in SC and NC, and lower basal LH concentration coincided with higher P4 concentration.