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Effects of Lime, P, and Mo Application on Mo Concentration in Subclover¹

Subterranean clover (Trifolium subterranium L.) is a winter annual legume grown in the hill pastures and hay fields of western Oregon. Soils are acid (pH 4.5 to 5.5) and yield responses to lime, P, and Mo are common. Lime, P, and Mo application can result in increased Mo concentrations in plants. Forage containing more than 10 µg Mo/g is potentially toxic to ruminants. An experiment involving subterranean clover and perennial ryegrass (Lolium perenne L.) was established in the fall of 1976 on a Typic Haploxeralf, fine, mixed, mesic soil to evaluate the effects of lime, P, and Mo applications on: (1) the Mo concentration in subterranean clover leaflets and petioles and in the clover-grass mixture and (2) the longevity of the treatment effects on Mo concentration in the clover-grass mixture. Lime was applied at 0, 4.5, or 9.0 metric tons/ha, P was applied as concentrated superphosphate at 0, 19, or 57 kg/ha, and Mo was applied as Na₂Mo0₄ 2H₂0 at 0 or 0.5 kg/ha. All treatments received adequate K, S, and B. Clover leaflet and petiole samples were analyzed to determine the Mo critical level for the clover and establish the maximum Mo concentration that would be consumed by a grazing animal. Clover grass mixture samples were analyzed to determine the average Mo and Cu concentration that would be consumed. The Mo concentration in clover leaflet and petiole samples collected 18 Mar. and 12 Apr. 1977 was increased from 1.5 to over 25 µg Mo/g by application of lime, P, and Mo. The Mo concentration decreased to less than 15 µg/g by 12 May 1977. Application of lime, P, and Mo increased the Mo concentration in the clover-grass mixture from 1 to over 5 µg Mo/g and decreased the Cu:Mo ratio to below 2, which is potentially toxic to livestock.

The highest Mo concentration was less than 10 µg Mo/g in clover leaflet and petiole samples collected the second growing season and the Mo concentration in the clover-grass mixture was less than 5 µg Mo/g for all treatments except one. The Mo concentration was below 2 µg Mo/g in all samples collected the third growing season. Although the Mo concentration was as low as 0.1 µg/g, yield responses from Mo application were not evident.

Key Words: Molybdenosis • Ruminant nutrition

² Graduate research assistant and professor dep. of Soil Science, Oregon State Univ., Corvallis, OR 97331.

Received for publication April 18, 1982.