Effects of prepartum and postpartum bolus injections of trace minerals on performance of beef cows and calves grazing native range12

Our objective was to evaluate the effects of pre- and postpartum bolus injections of a trace mineral solution on reproductive performance, BW change, and BCS change of beef cows and on growth of suckling calves. Mature beef cows (n = 460; initial BW = 497 ± 89 kg, initial BCS = 5.4 ± 0.74) managed in 2 locations were stratified by BCS, age, parity, and predicted calving date and assigned randomly to 1 of 2 treatments: 1) supplemental s.c. trace-mineral (TM) injection containing 15 mg/mL Cu, 5 mg/mL Se, 10 mg/mL Mn, and 60 mg/mL Zn or 2) s.c. injection of physiological saline (SA). Injections were administered to cows (1 mL/90 kg of BW) 105 d before the first projected calving date and again 30 d before fixed-time AI. Calves received the same treatment as their dams and were injected (1 mL/45 kg of BW) at birth and again at 71 ± 21 d of age. Cows grazed native pastures for the duration of the study; trace-mineral supplements and white salt were available to all cattle ad libitum before and during the study. Ovulation was synchronized using a 5-d CO-Synch + CIDR protocol, and cows were inseminated 72 h after CIDR removal. Cows were exposed to fertile bulls for natural-service breeding 10 d after AI for 50 d. Conception to AI and final pregnancy rate were assessed 36 d after AI via ultrasound and 120 d after AI via rectal palpation, respectively. Change in BW and BCS from initiation of the study to calving and from AI to weaning did not differ (P ≥ 0.15) between TM and SA cows. Conversely, TM cows had greater (P = 0.04) BCS increase than did SA cows between calving and AI. Calf BW at birth, ADG, and age-adjusted weaning BW did not differ (P ≥ 0.36) between treatments. Proportion of cows with estrous cycles 17 and 8 d before fixed-time AI was similar (P ≥ 0.51) between treatments. In contrast, conception to fixed-time AI was greater (P = 0.05) for cows receiving TM (60.2%) than for cows receiving SA (51.2%). Final pregnancy rate did not differ (P = 0.24) between treatments and averaged 92%; however, calving distribution by TM-treated cows was more favorable (P = 0.01; i.e., calving was generally earlier) than calving distribution of SA-treated cows. Under the conditions of this study, pre- and postpartum trace-mineral injections improved conception to fixed-time AI and subsequent calving distribution of beef cows.