Introduction

Molybdenum (Mo) is an essential plant nutrient. It acts as a metallic cofactor in plant and animal enzymes. At high concentrations in forages, it can be toxic to ruminants by interfering with assimilation of copper (Cu). The range between toxicity and deficiency in animals is narrow, and therefore careful control of Mo in animal diets is essential.

Bear (1956) reviewed the early literature dealing with Mo in soils and plants and in animal nutrition in a special issue of Soil Science. The agricultural importance of Mo has been discussed (Mortvedt, Giordano, and Lindsay, 1972), and various aspects of the presence of Mo in the environment have been examined (Chappell and Petersen, 1977). Underwood (1977) and Beeson and Matrone (1976) have reviewed the biochemical importance of Mo in animal and human nutrition.

Molybdenum deficiency in tropical and subtropical soils is more widespread than elsewhere because soils in the tropics are highly weathered. In fact, many soils in the tropics are so highly weathered that little remains except sesquioxides and some 1:1 layer silicates (Pasricha and Fox, 1993). The acidic nature of many of these soils with a dominant content of sesquioxides is the major cause of Mo deficiency.