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Effect of an injectable trace mineral on reproductive performance of beef cows grazing irrigated pasture
Injectable trace minerals can be used to increase trace mineral status of cows and may result in improved reproductive performance. Beef cows (n = 174) were blocked by age and expected calving date and randomly assigned to treatment in a 2×2 factorial of either an injectable trace mineral containing copper, manganese, selenium, and zinc (TMI) or no injection (CON) and timed AI at either 72 or 80 h post controlled-internal-drug-release device (CIDR) removal. Thirty-eight days prior to the start of the calving season and again 23 days prior to AI, cows in the TMI group were injected with Multimin®90 (0.68 mL /68 kg of BW). Liver biopsies were taken prior to pre-calving injection, prior to pre-breeding injection and again 15 days after pre-breeding injection to access mineral status. All cows were estrus synchronized using a 5 day Co-Synch plus CIDR protocol in which cows were given an injection of gonadotropin-releasing-hormone (GnRH) and inserted with a CIDR 8 days prior to AI. The CIDR was removed 5 days later and a shot of prostaglandin was given followed by a second prostaglandin injection 5.6 h later. Cows were given an injection of GnRH and inseminated either 72 or 80 h after CIDR removal with sexed semen. Cows were exposed to fertile bulls for natural service breeding 17 d after AI and remained with the bulls for 45 days. Pregnancy was determined by rectal palpation at 105 d post AI. For the responses measured there was no interaction (P≥0.19) between TMI and AI timing; therefore only the effect of injection will be reported. At 15 d post pre-breeding injection, TMI had no effect on liver concentration of manganese (P=0.26), zinc (P=0.22), or copper (P=0.11), however TMI cows did have elevated concentrations of selenium (P=0.01) when compared to CON. Liver concentrations of the trace minerals measured in both treatments were considered adequate at all-time points. There was no difference between CON and TMI in the number of days post calving (P = 0.21) or BCS (P=0.95) at AI. Trace mineral injection did not affect reproductive performance of cows as both conception to AI (P=0.41) and overall pregnancy rate did not differ (P = 0.19) due to TMI treatment. These data suggest that the use of an injectable trace mineral may not improve reproductive performance of cows that have adequate trace mineral status prior to injection.
Keywords: Cows, Injectable trace mineral, Reproduction