Effect of Injectable Trace Minerals in Pasture-Based Dairy Cattle: Responses in Calves and Early-Lactation Cows

Injectable trace mineral supplementation (ITMS) has been proposed to support physiological adaptation during critical stages in dairy cattle; however, field-based evidence from pasture-based seasonal-calving systems remains limited. This study evaluated the effects of ITMS, administered at birth in calves and approximately 30 days before fixed-time artificial insemination in early-lactation cows, primarily on erythrocyte glutathione peroxidase activity and plasma trace mineral concentrations, while health, growth, production, and fertility were monitored as secondary outcomes. Thirty newborn calves receiving isotonic saline control (CNTRL; n = 15) or ITMS (n = 15) and 50 early-lactation cows (CNTRL, n = 24; ITMS, n = 26) were enrolled. In calves, erythrocyte glutathione peroxidase activity was higher in ITMS calves (322 vs. 359 U/g Hb; p < 0.01), and plasma selenium was higher on day 15 (0.57 vs. 0.62 µmol/L; p = 0.03). A treatment × week interaction was detected for body weight, but overall body weight and average daily gain did not differ. In cows, plasma selenium was transiently higher in multiparous ITMS cows on day 15 (1.04 vs. 1.09 µmol/L; p = 0.03), while antioxidant and inflammatory biomarkers, production, mastitis incidence, and fertility were unaffected. Overall, a single ITMS administration elicited modest biochemical and selenium responses without statistically detectable improvements in growth, health, production, or fertility.