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Effect of a trace mineral injection on pregnancy rate of purebred and commercial Black Angus heifers synchronized using a 14 day CIDR-PG protocol
Purebred Black Angus heifers (PBA, n=207, BW = 347 ± 3.0 kg) and commercial Black Angus heifers (CBA, n=529, BW = 335 ± 2.3 kg) were used to examine the effect of a trace mineral injection (TMI), Multimin 90, on reproductive performance. Heifers were fed a hay and barley silage-based ration that contained selenium (0.7 mg/kg), zinc (151 mg/kg), copper (36 mg/kg), and manganese (108 mg/kg). Thirty three d prior to the start of breeding, a controlled internal drug releasing device (CIDR) was inserted into each heifer, and alternate heifers were injected with 4 mL TMI (0.53/45 kg BW) or no injection (CON). Fourteen d later the CIDR was removed and 16 d after CIDR removal heat-patches were placed on heifers’ tail-heads and PG was injected. Heifers with fully activated heat-patches 72 h post-PG were inseminated and injected with GnRH. Non-responding PBA were injected with GnRH 72 h post-PG and inseminated 3 h later. Non-responding CBA with activated heat-patches at 76 h post-PG were inseminated at 82 h post-PG and given GnRH. All remaining CBA were injected with GnRH at 76 h and inseminated at 93 h post-PG. Ten d after insemination, all CBA were given an injection of GnRH and inserted with a CIDR. The CIDR was removed 7 d later, estrus was monitored for 5 d, and heifers in estrus were inseminated. Twenty-three d after the start of breeding CBA were transported 160 km to native-range and co-mingled with 14 fertile bulls. Conception was determined at 33 d for PBA and 76 d for CBA after start of breeding using ultrasonography. For PBA, there was no difference (P = 0.67) in overall AI conception between CON (52%) and TMI (52%). Conception of CBA to first service AI did not differ (P = 0.52) between the CON (55%) and TMI (50%) and there was no difference in the proportion of CBA (21 vs. 22% for CON and TMI, respectively) that received second service AI (SSA). However, conception to SSA was greater (P = 0.05) for TMI (68%; 40/59) than CON (50%; 27/54). Overall pregnancy rate of CBA after AI and exposure to bulls for 27 days did not differ (P= 0.53) between the CON (87%) and TMI (85%). These data suggest that TMI has limited impact on reproductive performance of heifers being fed adequate concentrations of trace mineral.
Keywords: beef heifers, conception, trace mineral injection