Trace element status of soil and organically grown herbage in relation to animal requirements

To obtain a general picture of the herbage zinc (Zn), iron (Fe), manganese (Mn), copper (Cu), molybdenum (Mo), cobalt (Co) and selenium (Se) concentrations on organic livestock farms, we analysed soil (2001) and herbage (2001 and 2002) samples from 28 farms from four regions in Norway. We analysed animal blood plasma Cu, B12 (Co), α- and γ-tocopherol (vitamin E) and whole blood Se to investigate if the farms feeding practice met the dietary need of Cu, Co, Se and vitamin E in animals. The first cut herbage median (10th-90th percentile) Zn, Fe, Mn, Cu, Mo, Co and Se concentrations were 19 (14-34), 50 (36-88), 34 (22-86), 5.3 (3.9-6.8), 1.5 (0.6-4.8), <0.05 (<0.05-0.08) and <0.01 (<0.01-0.03) mg kg-1 DM, respectively. The herbage trace element concentration was generally higher in the second cut. The second cut herbage median (10th-90th percentile) Zn, Fe, Mn, Cu, Mo, Co and Se concentrations were 21 (16-37), 84 (52-171), 66 (36-205), 7.0 (5.7-9.3), 3.3 (1.6-10.1), 0.06 (<0.05-0.15) and 0.02 (<0.01-0.06) mg kg-1 DM, respectively. The plasma Cu and B12 (except one sheep herd) concentration were within the suggested normal range set by the Norwegian Veterinary Institute. Whole blood Se concentrations were 0.10 (0.04-0.15) µg g-1 in dairy cattle and 0.14 (0.03-0.26) µg g-1 in sheep. Vitamin E concentrations were 4.2 (2.7-8.4) mg L-1 in dairy cattle and 1.3 (0.9-2.4) mg L-1 in sheep. The results of mixed model analyses of herbage Zn, Fe, Mn, Cu and Mo indicated that soil pH, soil texture, botanical composition and phenological stage at harvest mostly influenced the herbage trace element concentrations within regions. There was a poor relationship between soil and herbage trace element concentrations, except for Zn. None of the soil and plant variables explained the variation in the herbage Se or Co concentration, but the number of samples was too low to draw clear conclusions on these two elements. There were some differences in soil and herbage trace element concentrations between regions. It was generally concluded that Zn, Fe, Mn, Cu and Mo did not limit plant growth. The herbage concentrations of Fe, Mn, Cu and Mo were sufficient to meet the dietary needs of ruminants. The herbage Zn concentration was insufficient to meet the dietary needs of dairy cattle. The herbage Co and Se concentrations and the Cu/Mo ratio were not alone balanced to meet the dietary needs of ruminants. The on-farm feeding practises fulfilled the dietary needs of Cu and Co. Selenium contents were generally insufficient on dairy farms under prevailing feeding regimes, whereas the vitamin E was insufficient on sheep farms. It is therefore highly recommended to use trace element mixtures and/or concentrates fortified with Cu, Co, Se and vitamin E on Norwegian organic livestock farms. Most open vessel digestion procedures of biological material utilize a mixture of acids that include perchloric acid. There have been many accidents associated with the use of perchloric acid where serious injury has resulted. Therefore, a microwave digestion procedure of biological material, avoiding the use of perchloric acid while maintaining accurate selenium recoveries, was developed. Biological material was digested in two steps using nitric acid followed by hydrogen peroxide. Following the addition of phosphoric acid, remaining nitric acid and hydrogen peroxide were removed by evaporation, and Se-oxides were reduced to selenite using hydrochloric acid. Samples were adjusted to a buffered pH of 1.75 and reacted with 2,3-diaminonaphthalene. The resulting piazselenol complex was extracted into cyclohexane. A normal phase HPLC method, using an amino phase column and a cyclohexane/ethyl acetate mobile phase, was used to separate the piazselenol complex from any remaining impurities before fluorescence detection on a HPLC-FLD. The relationship between peak height and selenium concentration was linear between 0 and 2 mg L-1. The mass detection limit of the complete procedure was 0.29 ng of selenium. Recoveries of Se were within the certified range for the material analysed. A pot experiment was used to investigate the relationship between ammonium-nitrate and selenate in the wheat uptake and leaching water loss of Se. Ammonium-nitrate was applied by two methods, (i) entire dose at sowing (ii) in split application as 75 % at sowing and 25 % at stem elongation. Selenate was applied at sowing, tillering, stem elongation, head emergence and at milking growth stage. Split N application increased the protein content and Se concentration in grain, but decreased the Se concentration in leaf and straw. The highest Se concentration in the plant was achieved when the soil N potentially was highest. The Se leaching losses increased with response uptake by plants, being highest at highest Se uptake by plants, but decreasing with split N application. Conclusions of the work: • Supplement of Cu, Co, Se and vitamin E are recommended to both dairy cattle and sheep and Zn to dairy cattle in organic husbandry in Norway. • It is possible to determine Se in biological material without use of perchloric acid. • Applying selenate and ammonium-nitrate together after tillering increases the wheat grain Se concentration and total Se uptake, split N application having the lowest leaching losses of Se.