Mineral status, metabolism and performance of dairy heifers receiving a combined trace element bolus and out-wintered on perennial ryegrass, kale or fodder beet

The effects of a cobalt (Co), copper (Cu), selenium (Se), and iodine (I) trace-mineral ruminal bolus on the mineral status and performance of out-wintered, pregnant dairy heifers was investigated. Nine commercial farms grazing pasture (G), kale (K), or fodder beet (F) were used (n=3 per forage), with forty heifers on each farm randomly allocated to not receive (B-) or receive (B+) two combined mineral boluses. Mean plasma Co concentrations were 0.021 and 0.041 μmol/L in B- and B+ respectively (p < 0.001), with serum vitamin B12 also higher in heifers receiving B+ than B- (p < 0.001). Mean plasma Se concentration was 0.50 and 0.82 μmol/L in B- and B+ respectively, with heifers that received B+ also having a higher (p < 0.05) mean blood GSH-Px concentration (30 and 76 U/mL haematocrit in B- and B+ respectively). Providing a mineral bolus did not affect plasma Cu concentration in heifers receiving G or F (p <0.05), but was higher in KB+ compared to KB- (p < 0.05) at the middle and end of the out-wintering period. Heifers receiving KB- also had a lower haemoglobin and red blood cell count, but a higher mean corpuscular volume than KB+ at the end of the out-wintering period. Animals receiving B- had a higher plasma thyroxine concentration (p < 0.05). Neither the bolus nor forage type affected body weight (p > 0.05), however condition score was higher (p < 0.05) in B+ at the end of the study. It is concluded that the provision of a trace mineral bolus increased plasma concentrations of the minerals supplied, with the greatest benefits in animals grazing kale, but these increases were not translated into improved performance.