Degree Type


Date of Award


Degree Name

Master of Science


Animal Science


Animal Physiology

First Advisor

Patrick Gunn

Second Advisor

Curtis R. Youngs


Removal of the initial GnRH administration within the 5-d CO-Synch + controlled internal drug release (CIDR) protocol (5dCO) on ovarian parameters and fixed timed artificial insemination (FTAI) pregnancy rates was investigated. In the first study, the objective was to evaluate the effects of GnRH removal on ovarian follicle dynamics and steroid hormone concentrations in non-lactating beef cows. Animals received either: 1) standard 5dCO hormone administration including 100 µg of GnRH at CIDR insertion and two concurrent, 25-mg doses of prostaglandin F2α (PGF2α) delivered at CIDR removal (G1-2PG), 2) no GnRH at CIDR insertion and two concurrent, 25-mg doses of PGF2α at CIDR removal (NoG1-2PG), or 3) no GnRH at CIDR insertion and a single, 25-mg dose of PGF2α at CIDR removal (NoG1-1PG). Cows in the NoG1-2PG treatment group displayed elevated peak plasma estradiol concentrations when compared to the NoG1-1PG treatment group with G1-2PG remaining intermediate. However, the remaining steroid hormone concentrations analyzed were not impacted by treatment. Furthermore, the ovarian parameters measured were not different among treatments.

In order to determine if fluctuations in estradiol concentrations observed in the first study would impact fertility, a field study was conducted. The objective of the field study was to evaluate the effect of initial GnRH removal at CIDR insertion on dominant follicle parameters and FTAI pregnancy rates in lactating beef cows. Cows were administered one of three treatments identical to the first study with the exception of FTAI occurring 72 h following CIDR removal. Although dominant follicle diameter was similar upon CIDR insertion, dominant follicle was greater in the NoG1-2PG treatment group upon CIDR removal. Estrous response prior to FTAI was not different among treatments, but NoG1-1PG did display more advanced (lower) estrous detection aid scores at FTAI. However, FTAI pregnancy rates did not differ among treatments.

In summary, the data from these two studies indicate that removal of the initial GnRH administration in the 5dCO does not negatively affect ovarian parameters or FTAI pregnancy rates. However, differences in dominant follicle diameter and changes in estradiol concentrations, as well as behavioral estrus, prior to FTAI warrant further research.


Copyright Owner

Tyler Grussing



File Format


File Size

138 pages