SOYBEAN PLANTING DATE, ROW WIDTH, AND SEEDING RATE RESPONSE IN 3 TILLAGE SYSTEMS

Soybean [Glycine max (L.) Merr.] and corn (Zea mays L.) are commonly grown in rotation, but information regarding soybean production in conservation-tillage systems has lagged behind corn. This study was conducted to determine the influence of planting date, row width, and seeding rate on soybean grain yields, plant stands, and other agronomic characteristics, as affected by tillage intensity. Two field studies were conducted at Arlington, WI, with no-tillage (NT), reduced tillage (RT), and complete tillage (CT) systems, using 'Hodgson 78' (Group I) and 'Corsoy 79' (Group II) varieties. In one 4-yr study, soybeans were seeded in 8- and 30-in. rows in mid-May, late May, and mid-June. Yields in NT were 5.3 and in RT 2.8 bu/acre lower than in CT. Solid-seeded soybeans (8 in.) yielded 3.1 bu/acre more than wide rows (30 in.). This yield advantage for solid-seeding, however, reversed at the latest planting dates in CT and RT. Early and late plant populations, plant height, and lodging were also lower in NT and RT than in CT. In a second study, soybeans were seeded in 8-in. rows at 50 000 to 300 000 viable seeds/acre in three tillage systems for 3 yr. Plant emergence was 13 and 15% greater in CT than either RT or NT, respectively, when averaged across seeding rates. Late-season stands did not differ among tillages at the lower seeding rates (50 000 and 100 000 viable seeds/acre). At the higher seeding rates (150 000 to 300 000), however, the NT and RT systems required an additional 50 000 seeds/acre to achieve harvestable plant stands equivalent to CT culture. When averaged across all seeding rates, CT outyielded NT and RT by an average of 4.5 bu/acre. When yields at equivalent final stands were compared, however, yields in CT were less than 3 bu/acre higher than RT or NT. Soybean seeding rates in NT and RT systems must be approximately 15% to 32% higher than those normally used in CT if equivalent yields are to be expected.