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Control of Volunteer AdzukI bean and Mungbean in Pedigree Plots of Soybean

S. J. Park, R. I. Buzzell and A. S. Hamill Agriculture and Agri-Food Canada Research Centre, Harrow, Ontario NOR 1G0

Adzuki bean [Vigna angularis (Willd.) Ohwi & Ohashi] and mung bean [Vigna radiata (L.) Wilczek] are new casin crops being introduced in Ontario. Production of these pulses may be limited to the southwestern region of the province where crop heat units and soil types meet the required conditions for the crops and may become part of the rotation system with soybean. Both of these small seeded beans tend to have some combine losses due to shattering and tend to produce a high proportion of seed with impermeable seed coats. Therefore, seed of these pulses emerge persistently In the field two to three years after the bean crops are harvested. This causes serious problem in soybean production, particularly in pedigree seed production plots. Therefore, a selective herbicide metribuzin was applied to determine whether volunteer seedlings of adzuki bean and mungbean could be controlled. Results of a preliminary field trial and reaction of the beans to metribuzin in screening tests are summarized.

Materials and Methods: Adzuki bean Martyn bulk and mung bean VC1973A were seeded in rows or broadcast in plots of two soybean cultivars, Elgin and Elgin 87. Elgin is tolerant to the recommended rate of metribuzin whereas Elgin 87 is tolerant to twice the recommeded rates (Welacky. unpublished). Three rates of metribuzin, 0, 0.3 (recommended rate for soybean) and 0.6 kg a.i. ha "^ were applied as PPI, in addition to metolachlor (1.68 kg a.i. ha ') and imazethapyr (0.06 kg a.i. ha "'') PPI. Plots were 60cm X 5m in single row and arranged in a split-split block desgin in three replications: row or broadcast as main plot, different beans as sub-plot and level of metribuzin as sub-sub plot. Two soybean cultivars were randomized within the metribuzin levels to test the uniformity of the herbicide application. Twenty seeds of adzuki bean or mungbean per plot were sown in a sandy loam field at Harrow on June 23, 1994. Obsen/ations were made on seedling emergence on July 7 and injury on soybean and beans were scored on July 8, 12 and 21 and final counts of the survived plants of the beans were made at harvest. Seed of the beans were harvested individually to progeny test their response to metribuzin. Further, seedlings of the pulses and soybeans were grown in perlite/vermiculite (2:1 v/v) and transplanted to a 12L tank containing nutrient solution (Buzzell and Hamill 1988). Then, the seedlings were subjected to the treatment of metribuzin by adding 0.15 mg L' . Reaction of the seedlings was read between 11-14 days after adding the metribuzin. Several subsequent screening tests were carried out in smaller tanks by diluting the rate for mungbeans and by increasing the rate for adzuki beans.

Results: Most of volunteer bean seedlings began to show injury by July 8. Affected plants became pale and the margins of the unifoliolate leaves became brown and scortched toward the midribs and eventually died within a week, injury increased as the rate of the methbuzin increased from 0.3 to 0.6 kg a.i. ha"\ However, some plants survived and although stunted, produced a few pods containing a few seed per pod (Table 1). Soybean cv. Elgin showed slight injury at the low rate, and Elgin 87 showed no visible injury at the low rate but some had slight injury at the high rate of metribuzin.

In the hydroponic tests the response of the two pulses to metribuzin was different. Mungbeans were much more susceptible to metribuzin than soybean by showing injury at the half of the rate (0.075 ppm of metribuzin) used to screen soybeans. However. Adzuki bean was much m.ore tolerant than the tolerant soybean Elgin87: the rate of metribuzin that caused severe injury was at about 3.5 times 316

(0.525 ppm of metribuzin) stronger than soybean as the rates were increased from 2x, 2.5x, 3x and 3.5x oí the initial screening rate of 0.15 ppm of metribuzin.

The greater effect of metribuzin in reducing the average tota! number of seeds of the more susceptible mungbean vs adzuki bean is shown in Table 1. However, even v/ith soybean cullivars with above average tolerance, metribuzin would not eliminate all volunteer mungbean seedlings. Therefore, it is concluded that metribuzin could not be used effectively to control volunteer seedlings of adzuki bean and mungbean in soybean field.

Literature cited: Buzzell, R. I. and A. S. Hamil!. 1988. Improved tolerance of soybean {Glycine max) to metribuzin. Weed Tech. 2:170-171.

Acknowledgments: T. Rupert and M. Whaley for their technical assistance.

Table 1. Number of surviving plants and total number of seeds per plot produced by adzuki bean and mungbean grown in broadcast and rows treated with metribuzin.

Adzuki bean Mungb ean

Metribuzin No. of Total no. No. of Total no. Seeding (kg a.i. ha') plant of seed plant of seed

Broadcast 0.0 23.4 77.2 17.2 41.3

0.3 9.4 21.6 4.5 16.2

0.6 5.2 22.9 1.2 3.6

Row 0.0 16.5 38.0 13.0 29.9

0.3 8.0 4.8 5.7 16.0

0.6 1.7 1.7 0.5 0.6

Average 0.0 20.0 57.6 15.1 35.6

0.3 8.7 13.2 5.1 16.1

0.6 3.5 12.3 0.9 2.1

20 seeds were sown in each plot which was 60cm x 5m (3 m^)