Agronomic biofortification with selenium: Effects on whole blood selenium and humoral immunity in beef cattle

The purpose of this study was to evaluate Se supplementation strategies in mature beef cattle by measuring changes in whole-blood Se (WB-Se) status and humoral immune response to vaccination. Mature beef cows (n = 45) were balanced by age and randomly assigned to 1 of 3 supplementation groups that received different chemical forms of Se or Se dosages compared to a standard (control) Se treatment. Supplementation treatment groups were provided limited access (6 weeks) to either sodium selenite (200 mg/kg Se; LSe) or Se-fertilized forage (FSe) and subsequently had no additional Se in their mineral supplement for the study duration. The LSe group cows grazed non-Se-fertilized forage. The control group grazed non-Se-fertilized forage and received continuous Se supplementation (CSe) from a free-choice mineral supplement (120 mg/kg Se from sodium selenite). Cows were bled pre and post grazing and then every 4 weeks thereafter for approximately 5 months to assess WB-Se concentration. All cows were immunized with J-5 Escherichia coli bacterin at the end of the 6-week supplementation period, and serum was collected for antibody titers 2 and 4 weeks after the third immunization. Covariate adjusted WB-Se concentrations were influenced (P<0.0001) by group, time and their interaction. Cows in the FSe group had higher (P<0.0001) WB-Se concentration (186 ± 5 ng/mL) immediately post-grazing (42 days) compared to LSe (117 ± 5 ng/mL) and CSe cows (130 ± 5 ng/mL). WB-Se concentration in FSe cows remained higher (P=0.02 to P<0.0001) over the next 4 (CSe) and 5 (LSe) months. Higher (P<0.05) WB-Se concentrations were observed in CSe compared to LSe cows over the last 4 months of the study. Treatment group (P=0.036) and time post vaccination (P<0.0001) influenced J-5 E. coli antibody titers, with FSe cows having higher titers than LSe cows (P=0.01), although FSe and CSe cows were not different. Short-term exposure of cattle to Se-fertilized forage elevates WB-Se concentrations within several weeks and this exposure is sufficient to maintain adequate concentrations throughout grazing periods when there is limited access to Se supplements. Short term exposure to higher levels of inorganic Se supplementation is not equivalent to ongoing inorganic Se supplementation at lower rates.