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Response of Arrowleaf Clover to Postemergence Herbicides¹

Presently there are no herbicides cleared for weed control in arrowleaf clover (Trifolium vesiculosum Savi.) a proven legume for pasture mixtures in the sunbelt. The objective of the study was to determine the selectiveness of 2,4-DB [4-(2,4dichlorophenoxy)butyric acid], 2,4-D [ (24dichlorophenoxy)acetic acid], bromoxynil [3,5dibromo-4-hydroxbenzonitrile], MCPA [((4-chloro-o-to1yl)oxy) acetic acid], and dicamba [3,6dichloro-oanistic acid] for the control of various broadleaf weeds in arrowleaf clover. Field studies were conducted at three locations. Soil classification for these location are: Aquic Paleudolls for location I, Ultic Haplustalfs for location 11, and Mollis Albaqualfs for location 111. At all three locations the herbicides were applied postemergence to actively growing arrowleaf plants (5 to 15 cm in height) in late March or early April. Visual damage to arrowleaf clover and weeds was estimated after several weeks. Forage production from the various treatments was determined from June samplings and seed production of arrowleaf was determined from July samplings. Of the herbicides evaluated, 2,4DB at 0.56 kg/ha was consistently selective to arrowleaf clover and adequately controlled the broadleaf weeds including seed stalk formation of curly dock (Rumex crispus L.). MCPA at 0.56 kg/ha also controlled the broadleaf weeds but was not consistently selective on arrowleaf clover. The 0.56 kg/ha rate of 2,4-D also controlled the broadleaf weeds and significantly reduced the forage production of arrowleaf clover. Seed production with the 2,4-D treatment (0.56 kg/ha rate) was not suppressed as much as forage production and the decrease was only significant at one location. A combination of 2,4D and banvel (0.42 and 0.14 kg/ha respectively) severely damaged the arrowleaf and could be used as a treatment to control arrowleaf clover.

More than 1,000-fold differences were found among products in viable rhizobia content. Plate counts adequate discriminated products when 10⁷ rhizobia per gram was considered to be the lower limit for acceptability. The close correlation of plate counts and MPN (soybean nodulation) indicated reliability of plate counts despite the difficulty of enumerating rhizobia among numerous contaminants. Nodulation of soybeans in greenhouse and field plantings in soil free of R. japonicum was directly related to the number of rhizobia applied per seed, with no nodulation produced by products supplying fewer than 108 rhizobia per seed, and generally abundant nodulation by products providing 10⁵ to l0⁶ rhizobia per seed. Some products performed poorly despite apparent adequate viability; for example, where a fungicide was added at planting. Various strains of R. japonicum and formulation processes, as well as number of rhizobia applied, may account for differences in nodulating effectiveness. In no case, however, was a product of low plate count found to be an effective inoculant. Plate count provides a relatively rapid, inexpensive evaluation that is supported by other techniques involving soybean nodulation response.

Key Words: Broadleaf weed control • Trifolium vesiculosum • 2,4-DB • Rumex crispus • Curly dock

² Former graduate research assistant and associate professor, Dep. of Agronomy, Oklahoma State Univ., Stillwater 74078.

Received for publication August 20, 1979.