GEOGRAPHICAL VARIATION IN THE COPPER AND MOLYBDENUM CONTENTS OF FORAGES GROWN IN NORTHWESTERN MANITOBA

Abstract

At the rate to two to three sites per township (93.24 km2), in the cattle-producing area of northwestern (NW) Manitoba, 612 grass samples and 271 legume samples were collected and analyzed for copper (Cu) and molybdenum (Mo) content. Grass samples from north of Riding Mountain National Park (RMNP) generally had concentrations of Cu less than 9 mg/kg on an air-dry basis of approximately 90% dry matter (DM), while grass samples from south and east of RMNP had a more variable Cu content. In general, legumes had a higher Cu content than grasses; however, there were sites south and east of RMNP where either a grass or legume, grown together, had the higher (P < 0.05) Cu content. For cattle consuming forage, Cu deficiencies due solely to an inadequate dietary intake of Cu are to be expected throughout NW Manitoba; only 16% of grass and 38% of legume samples provided adequate Cu to meet the requirement for cattle. A modified boundary, 10 km east of the eastern boundary of the Ashville Geological Formation divides the study area into regions of high (> 5.0 mg/kg DM) and low (< 5.0 mg/kg DM) Mo content for grass samples; the Ashville, Favel and Vermilion River Geological Formations contain Mo-rich shales. Throughout NW Manitoba, either a grass or a legume grown at the same site may have the higher Mo content. For cattle consuming forage, the potential for a Cu deficiency, due to a Mo toxicity (> 5.0 mg Mo/kg DM), is highest for grass forage west of the aforementioned modified Ashville boundary. For legume forage, the potential for a Mo toxicity is high (> 5.0 mg Mo/kg DM) throughout the study area, and highest (> 10.0 mg Mo/kg DM) within that part of the study area delineated by soil that is in close proximity to the geological formations of a high Mo content. Key words: Forage copper, forage molybdenum, beef cattle, copper deficiency, molybdenum toxicity, northwest Manitoba