Abstract
Less than 10% of beef cow-calf producers in the United States currently implement reproductive technologies such as estrous synchronization and artificial insemination (AI) as a component of their breeding management program. Synchronization protocols aid in the synchrony of estrus and ovulation to improve cattle fertility, however, many protocols often require multiple hormone injections and animal handlings. Development of estrous synchronization protocols that limit number of injections, cost, and labor inputs, without compromising fertility, may improve the adoption of fixed-time artificial insemination (FTAI) within the beef industry. The main objective of the first study was to compare the effect of a single injection of high-concentration (HICON) prostaglandin F2⍺ (PGF) or two conventional PGF injections (2PGF) administered 8 h apart at controlled internal drug release insert (CIDR) removal, on reducing serum progesterone (P4) before FTAI and on subsequent pregnancy per artificial insemination (P/AI) in postpartum beef cows (n=404) synchronized with a 5-d CO-Synch + CIDR breeding program. Serum P4 concentration at the time of insemination is a determining factor of cattle fertility in FTAI protocols, as lower concentrations have been associated with increased P/AI. A secondary objective of the first study was to assess the relationship between P4 concentration at the time of insemination and probability of pregnancy to AI. Progesterone concentrations at AI were lesser in 2PGF than HICON, however, P/AI was not different between treatments. Probability of pregnancy decreased as P4 concentration at AI increased and the optimal P4 cutoff concentration to optimize fertility was determined at 0.43 ng/mL. Intramuscular (i.m.) route of injection is commonly used to administer products used in estrous synchronization in beef cattle; however, this route causes greater incidence of injection-site lesions and tissue damage associated with reduced beef carcass quality and tenderness than subcutaneous (s.c.) routes. Therefore, use of s.c. administration is favorable to reduce carcass damage. The objective of the second study was to compare the ability of a single s.c. injection of high-concentration PGF (HICON-SC) with two i.m. injections of conventional PGF 6 h apart (2PGF-IM) to reduce P4 concentration and induce luteolysis by the time of AI in a 5-d CO-Synch + CIDR synchronization program in postpartum beef cows. A similar decline in P4 concentration occurred in both treatments following PGF, indicating s.c. administration of HICON could be an effective alternative to i.m. administration of the conventional double PGF dose to induce luteolysis before AI in the 5-d CO-Synch + CIDR protocol.