Planting date and seeding rate are agronomic decisions that producers can use to maximize soybean [Glycine max (L) Merr.] seed yield and economic return. Current information on the response to planting date and seeding rate may underestimate the yield penalty for delayed planting in northern climates and overestimate the seeding rate required to optimize yield. Th e objective was to determine the eff ect of planting date and seeding rate on soybean seed yield at environments with diff erent yield potentials in Iowa. Field experiments were conducted in six locations between 2003 and 2006 for a total of 13 environments. Four seeding rates were planted at four dates between late April and the middle of June. No yield diff erence existed between the late April and early May planting dates and the yield decline rate between each date was consistent among the six locations. Harvest plant populations were not infl uenced by planting date indicating that plant establishment does not pose a limitation to late April planting. Harvest plant populations required to reach 95% of the maximum yield were as great as 290,800 or as low as 194,000 plants ha -1 , for locations seeded in 38-cm row spacing and 157,300 to 211,800 plants ha -1 for locations seeded in 76-cm row spacing. Planting in late April or early May increased economic return ha -1 but there was no diff erence between seeding rates of 185,300 and 556,000 seeds ha -1 , accounting for additional seed costs. In the cool spring climate of the upper Midwest soybean producers can increase yield by planting soybean 1 to 2 wk earlier than current planting dates. Th ese data also suggest that the optimal seeding rate for most locations is less than current seeding rates.

Soybean [Glycine max (L.) Merr.] yield response to narrow row spacing has been consistently positive in the upper Midwest and new split-row planters have made narrow row soybean production feasible, yet adoption has been slow in Iowa. Wide (76-cm) and narrow (38-cm) row spacing and four seeding rates (185,000; 309,000; 432 000; and 556,000 seeds ha -1 ) were evaluated at three locations during 2004, 2005, and 2006 to determine seed yield in wide and narrow row spacing and four seeding rates and evaluate economic advantages associated with changes in row spacing. Soybean planted in 38-cm row spacing yielded 248 kg ha -1 greater than soybean planted in 76-cm rows aft er adjustment for diff erences in fi nal plant populations. Maximum yield at all locations was attained at a fi nal harvest population of 462,200 plants ha -1 but >95% of the maximum yield was achieved with fi nal populations as low as 258 600 plants ha -1 . Increased production costs associated with greater seeding rates removed the yield benefi t from greater harvest plant populations. Farm size of 144 ha with at least 50% of the land base dedicated to soybean production would benefi t from conversion from wide to narrow rows. To break even on the investment in a split-row planter a yield increase of 124 kg ha -1 was necessary for farms with 30% of 288 ha dedicated to soybean production. Th ese data indicate that yield and economic benefi ts are suffi cient to support the production of soybean in narrow rows and at seeding rates below current seeding rate recommendations.

Soybean [Glycine max (L.) Merr.] yield has increased at a rate of 25 to 30 kg ha -1 yr -1 due in part to improved genetic gain, and has been further advanced by the addition of resistance to soybean cyst nematode (Heterodera glycines Ichinohe; SCN) in new cultivars. Th e objective was to determine specifi c growth changes that explain the yield improvement from old to new cultivars and the further yield improvement gained from the addition of SCN resistance. Studies were conducted at three Iowa locations during 2005 and 2006. Two old and two new SCN-susceptible, and two new SCN-resistant cultivars were evaluated for total dry matter (TDM) accumulation and leaf area index (LAI) through the season along with yield and yield components at harvest. New cultivars produced yields superior to older cultivars due to increased crop growth rate (CGR) culminating in greater TDM 105 days aft er emergence (DAE). Yield was strongly associated with the number of seeds produced m -2 and this yield component accounted for almost all of the yield diff erences among cultivars. Seeds m -2 was positively related to CGR between 42 and 105 (growth stage R1-R5.5) DAE and to LAI 105 DAE. New SCN-resistant cultivars produced yields 17 to 19% greater than new susceptible cultivars across three locations. Increased TDM and CGR explained the yield response at the low-yield location, but not at the high-yield locations. Apparent harvest index (HI) was similar among all cultivars at each location. Selection for increased yield has indirectly selected for increased TDM and CGR with a similar amount partitioned to seed dry weight. Future yield gains will be made by (i) increasing the amount and the rate of dry matter (DM) and (ii) through the increased production and duration of leaf area.

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lock, 1994), the ability to scavenge excess soil nitrate N and reduce nitrate leaching following corn (Staver and There is a need for improved soil and water conservation in the corn Brinsfield, 1998; Strock et al., 2004), weed suppression (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation common to for up to 5 wk from rye mulch (Liebl et al., 1992; Wilthe upper Midwest, and an appropriate cover crop may fulfill this need. A corn-soybean rotation that included a rye (Secale cereale L.) liams et al., 1998), and the production of allelopathic cover crop was studied at two Minnesota locations in 2002 and 2003compounds that increase weed suppression (Barnes and to evaluate rye management method and timing for no-till soybean Putnam, 1987). production. Fall-planted rye following corn harvest at Waseca andDespite the potential benefits of rye, its adoption as Rosemount was managed the next spring by: (i) mowing once, (ii) a cover crop in the corn-soybean rotation has been mowing twice, (iii) applying glyphosate herbicide once, (iv) applying minimal. The limited use of rye can be attributed to cost herbicide twice, and (v) mowing once followed by applying herbicide, of establishment and termination as well as possible interwith four mow dates beginning 1 May separated by approximately ference with the subsequent crop growth. When rye was 1 wk. Rye regrowth after mowing but before stem elongation in early used as a cover before corn, yield was reduced in part to mid-May was similar to that of uncut rye but decreased dramatically due to N immobilization (Tollenaar et al., 1993; Vaughan when mowed at anthesis in early June. At Rosemount, low weed populations and the presence of the rye cover crop, when properly man-and Evanylo, 1998; Wagger, 1989). Soybean grown folaged, had only a minimal affect on soybean yield, resulting in the onelowing rye has not shown the same yield reductions as pass mowing system being equally profitable as the no-rye two-pass corn. In Ontario, Wagner-Riddle et al. (1994) found that herbicide system. At Waseca, where weed pressure was high, the rye while soybean growth was reduced early in the season, cover crop treatments without subsequent herbicide application asthere was no yield difference at harvest. Bauer (1989) well as the early one-pass herbicide applications did not provide adereported that soybean yield was not reduced when rye quate control, making these systems less profitable. Our research was managed with a herbicide but was reduced when indicated soybean yields following a rye cover crop were often comparye was mowed without subsequent application of a rable to yields where no rye cover crop was grown, but economic herbicide due to rye regrowth. Studies by Bauer (1989) returns were usually reduced. and Eckert (1988) indicated that soybean stand establishment was reduced when planted into rye residue. Bauer (1989) also reported delayed physiological devel-1991 Buford Circle, St Paul, MN 55108.