Profit margins of soybean [Glycine max (L.) Merr.] in the United States have recently declined as a result of an increase in production and land costs. Decreased profits coupled with increasing environmental concerns such as erosion and runoff prompts more cost-efficient production practices. No-tillage production practices offer a lower cost of production in addition to decreased soil erosion and runoff. Additionally, soil quality can be improved over time in a no-tillage system, greatly increasing the yield benefits over time. Today, only 41% of Iowa's soybean production is under no-tillage production (Conservation Technology Information Center, 2010), although even more of the state has land that lends itself well to no-tillage systems. However, the Des Moines Lobe in north central Iowa is known for its poorly-drained soil, and is a result of late Wisconsinan glacial deposits (Steinwand and Fenton, 1995).

Soybean planted in fields with different soil types and drainage properties respond differently to tillage practices. No-tillage production of soybean is often less successful in poorly-drained soils (Dick and Van Doren, 1985), in part because of cooler and wetter soil conditions at planting (Meese et al., 1991). These soil conditions can lead to slower soybean germination and emergence, which makes the seedlings more vulnerable to seedling disease.

Little information exists regarding soybean growth and development in different tillage systems from Iowa and if the soybean plant compensates for slow early growth with different growth and development processes. The overall goal of this research was to evaluate soybean growth, development, and yield under conventional and no-tillage conditions in Iowa. This was accomplished based on two separate studies. Chapter two is a literature review, and the rest of this thesis is divided into two manuscripts, which constitute chapters three and four.

Chapter three examines the relationship between cultivar selection and tillage systems across Iowa. Twelve cultivars were used in conventional and no-tillage systems to determine how yield was affected by cultivar performance in different tillage systems. Four cultivars were also used in both tillage systems and at two locations to closely monitor the growth and development throughout the growing season.

Chapter four explores the relationship between plant population density and tillage system across Iowa. This study was conducted in conventional and no-tillage systems in two row spacing and with four seeding rates. A yield portion of the study was performed at six locations across Iowa to determine the widespread effect of row spacing and plant population density on yield in different tillage systems. A growth and development portion of the study was performed at two locations to examine the biomass accumulation, crop growth rate, and light interception and the effects these parameters would have on yield.

Together these manuscripts show the importance of tillage considerations when determining management practices to maximize yield. The overall goal of this work was to provide growers with more concrete management recommendations in different tillage systems across Iowa.