This Is AuburnElectronic Theses and Dissertations

Improving Reproductive Efficiency and Calving Distribution of Heifers Through Estrus Synchronization, Natural Service, and Fixed-Time Artificial Insemination




Cofield, Lawrence

Type of Degree

Master's Thesis


General Veterinary Medicine


Artificial insemination has is considered to be one of the most important innovations in cattle management. Fixed-time artificial insemination (FTAI) protocols have offered new opportunities to implement AI, however, these technologies have not been widely implemented in beef heifers. The FTAI pregnancy rates in beef heifers have been inconsistent, leading to frustration and often discontinuation of a program that offers significant genetic and economic potential. The FTAI conception rates in heifers have been especially variably as some heifers come into estrus and ovulate too early and some ovulate too late for FTAI to be effective. Despite these inconsistencies, a major benefit of a FTAI program in heifers is that more heifers calve earlier in the calving season creating “calving momentum.” The calves conceived via AI have improved genetics resulting in improved carcass characteristics This improvement does come at some cost in terms of time, materials and labor. While many protocols and recommendations regarding the timing of hormone administration have been given, little information in available on breeding protocols involving estrus synchronization protocols combining natural service and AI. The goal of this study was to identify a breeding protocol that will result in more heifers calving within the first 21 days of their first calving season, as well as identifying the calving distribution (AI vs. natural sired). In this study, a 7-day CO-Synch + CIDR® protocol was combined with immediate natural service to critically evaluate the effectiveness of a FTAI protocol in beef heifers in order to provide scientifically based recommendations regarding a FTAI combined with natural service in terms of conception, calving rate and sire distribution that will result in 85% of pregnant heifers calving in the first 30 days of the calving season. Seventy-five pubertal heifers were randomly placed into 2 groups based on reproductive tract scores, ensuring that a relatively even allotment of peripubertal and pubertal heifers were in each group. They had free access to native forages, hay and water throughout gestation. The control Group (n=25 heifers; control) underwent a commonly used 7-day CO-Synch + CIDR® FTAI protocol, followed by introduction of a bull 10 days following AI. A 60-day breeding season was implemented. The experimental Group (n=50 heifers) underwent a similar 7-day CO-Synch + CIDR® protocol. Following CIDR removal at 7 days and FTAI on day 10, bulls were immediately placed with the heifers. Again, a 60-day breeding season was implemented. Three mature Angus bulls were used in the study, which were determined to be “satisfactory” on their breeding soundness examination. The experimental group was split into two groups (n=25) with one bull per group for logistic and management purposes. Initial pregnancy examinations were performed via trans-rectal ultrasound utilizing a 10 MHz probe at 120 days after the initiation of FTAI protocol. A follow-up pregnancy examination at five to six months of gestation was performed via trans-rectal palpation. Additionally, parentage testing of calves was performed on all calves to determine AI-sired calving distribution. The goal was to identify a breeding protocol that resulted in 85% of the heifers calving within the first 30 days of their first calving season. The calving distribution (AI vs. natural sired) following the induction of the breeding protocol involving estrus synchronization, natural service, and fixed-time artificial insemination was evaluated. . The preliminary results indicated that total conception rates may be improved combining a FTAI protocol with immediate natural service in heifers. While the percentage of heifers calving in the first 30 days was similar between experimental and control groups, subsequent 30-day calving intervals may be improved by a combination of natural service and AI, utilizing synchronization protocols to more efficiently tighten the calving season. Calving distribution in the experimental group drastically favored the natural service sire, which was expected. Thus, utilizing a FTAI protocol in beef heifers using the 7-day CO-Synch + CIDR® combined with immediate natural service may be a viable option for improving the pregnancy rates and increasing calving season momentum. However, if desiring a greater number of calves from AI sires, bulls should be turned out two weeks following the synchronization of estrus, as the percentage of AI to NS sired calves on the experimental compared to the control were 18% and 44% respectively.