Effects of eCG are more pronounced in primiparous than multiparous Bos indicus cows submitted to a timed artificial insemination protocol.

The effects of eCG on follicular growth, ovulation, and pregnancy per artificial insemination (P/AI) in multiparous and primiparous Bos indicus beef cows submitted to timed artificial insemination (TAI) were evaluated in three experiments. In experiments 1 (follicular responses; n = 64), 2 (follicular growth and ovulation rate; n = 662), and 3 (P/AI; n = 2092), cows submitted to TAI were assigned to receive one of two treatments on Day 8 of the synchronization protocol: control (no additional treatment) or eCG (300-IU of eCG intramuscularly). In experiment 1, largest follicle (LF) diameter on Day 8 (P = 0.56) and the interval from progesterone (P4)-device removal to ovulation (P = 0.79) did not differ between treatments. However, the maximum diameter of the LF (P = 0.05) and ovulation rate (P = 0.03) were greater in cows that received eCG. In experiment 2, the diameter of the LF on Day 10, follicular growth, and ovulation rate were greater in eCG-treated cows (P < 0.01). However, CL diameter was similar between treatments (P = 0.11). In experiment 3, there was a treatment-by-parity interaction (P = 0.003) on P/AI, such that treatment with eCG was more effective in primiparous cows. In conclusion, eCG treatment resulted in increased final follicular growth, ovulation rate, and fertility in B indicus cows submitted to TAI protocols, especially in primiparous cows.