Agronomy Journal • Volume 108, Issue 3 • 2016 1 S oybean is the world’s most widely grown legume, as it is an important source of protein and oil (Ainsworth et al., 2012). Approximately 30% of the world’s soybean is produced in the United States and Iowa is the leading soybean producing state with an annual production of approximately 14 million tons of soybean in 2014 (USDA-National Agriculture Statistics Service, 2014). In Iowa, soybean is usually planted in rotation with corn in either C-S or C-C-S rotations. Farmers are interested in how diferent crop rotation and tillage systems afect soybean yields. Tere is a general consensus that crop rotation increases soybean yield and economic return (Pedersen and Lauer, 2004; Kelley et al., 2003). Numerous studies have shown that soybean yield decreases when grown continuously as a monocrop rather than in rotation with a cereal crop (Crookston et al., 1991; Meese et al., 1991; West et al., 1996). However, the length and the type of the rotation can afect the “rotation beneft” for soybeans. In Wisconsin, soybean yield in an annual rotation with corn aver- aged 7% lower than soybean yield afer 5 yr consecutively of corn (Pedersen and Lauer, 2002, 2003). Diferent tillage systems also afect soybean yields through changes in soil water content and temperature, among many other factors (Licht and Al-Kaisi, 2005). Across years with no-tillage, soybean yield was 9% higher than yield with conventional tillage, which was attributed to higher early-season soil water content (Pedersen and Lauer, 2004). Soybean response to diferent tillage and crop rotation systems is highly complex and infuenced by location-specifc soil condi- tions, including soil drainage, soil texture, soil organic matter content, soil water-holding capacity, and weather conditions, including temperature, amount and distribution of precipitation, and frost-free days (Grassini et al., 2014). To understand soybean yield changes, we have to explicitly consider and incorporate temporal and spatial variability efects on soybean growth and yield, and their response to various cropping systems including crop rotations and tillage systems (Guastaferro et al., 2010; Basso et al., 2007; Batchelor et al., 2002). In addition to spatial-temporal variability, environmental concerns (Kumar et al., 2012), grain price, and the desire of farmers to reduce production costs further infuence the choice of management strategies for soybean production (Guastaferro et al., 2010). Tillage and crop rotation infuence soil quality and the sustainability of cropping systems’ productivity (Munkholm et Soybean Spatiotemporal Yield and Economic Variability as Affected by Tillage and Crop Rotation Mahdi M. Al-Kaisi,* Sotirios Archontoulis, and David Kwaw-Mensah Published in Agron. J. 108:1–14 (2016) doi:10.2134/agronj2015.0363 Received 30 July 2015 Accepted 12 Dec. 2015 Copyright © 2016 by the American Society of Agronomy 5585 Guilford Road, Madison, WI 53711 USA All rights reserved AbstrAct Tillage and crop rotations are practices that infuence yields and economic returns. Tis study was conducted at seven locations in Iowa from 2003 to 2013 with a split-plot design. Tillage systems [no-tillage (NT), strip-tillage (ST), chisel plow (CP), deep rip (DR), and moldboard plow (MP)] were the main treatment and rotations [corn (Zea mays L.)–soybean ( Glycine max [L.] Merr.) (C-S), corn–corn–soybean (C-C-S), and continuous corn (C-C)] were the subplot. Te objectives were to investigate: (i) annual soybean yield variability, (ii) appropriate tillage systems and crop rotations within location, and (iii) rotation efect on soybean yield and economic returns for the C-S cropping system. Soybean yield varied from 1.5 to 5.0 Mg ha -1 across Iowa with 21% variability. Te yield response to tillage systems at diferent locations was not signifcant and the economic return with NT ($1258 ha -1 ) exceeded that with conventional tillage ($1241 ha -1 ). Input costs excluding land rental and crop insurance were lower with NT ($463 ha -1 ) than with conventional tillage ($512 ha -1 ). Te C-C-S rotation resulted in greater soybean yields (9%) and economic returns (11%) than the C-S rotation in fve out of seven locations. Te efect of C-S, C-C-S, and C-C was consistently higher in the southern locations (well-drained soils, warmer temperatures) than northern locations (poorly drained soils, cooler temperatures). Site-specifc efects of rotation on soybean yield were greater than tillage system efects. Stable economic returns over time for corn and soybean were more associated with the C-S rotation than with C-C-S and C-C. Dep. of Agronomy, Iowa State Univ., Ames, IA 50011. *Corresponding author (malkaisi@iastate.edu). Abbreviations: C-C, continuous corn; C-C-S, corn–corn–soybean; C-S, corn–soybean; CP, chisel plow; DR, deep rip; MP, moldboard plow; NT, no-tillage; ST, strip-tillage. soil tillAge, conservAtion & MAnAgeMent core ideas • C-C-S rotation resulted in 9% greater soybean yield than that of C-S. • C-C-S rotation resulted in 11% greater economic return than that of C-S. • Tillage systems did not result in soybean yield diferences with both rotations. • NT input cost was $49 ha –1 less than that for CP, DR, and MP. • C-S cropping system yields and economic returns were superior to C-C. • Spatial and temporal soybean yield variability was 16–26% and 20–22%, respectively. Published March 11, 2016