The most important grass species on the ovine white-liver disease (OWLD) pastures (S) were Poa spp., Agropyron repens and Lolium perenne, while the control pastures (H), where lambs grew well, consisted of Poa spp., Festuca rubra and Agrostis tenuis. The soil was more acidic on the H pastures as compared with the S pastures. OWLD grass (S grass) contained marginal to deficient amounts of cobalt during the first 2 months of grazing. During 2 years out of 3, the average Co content was slightly lower in the S grass as compared with the content in the H grass. The lowest average grass Co was, however, found during one year in the H grass, in spite of the fact that the H lambs also this year grew well, and were 13 kg heavier than the S lambs after 3 1/2 months on pasture. Results thus indicate that the H lambs some years were subclinically Co deficient, without developing clinical symptoms or OWLD, and that factors other than marginal/deficient grass Co are of importance as to whether OWLD will develop or not. S grass differed from H grass by having significantly lower copper, molybdenum, manganese and zinc content, lower protein N/amid N ratios and higher aluminium and iron contents. According to the results, marginal to deficient grass Co is essential for development of OWLD, but cofactors play a part.
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