Relay intercropping is a form of multiple cropping where a second crop (e.g. soybean) is planted before a first crop (e.g. wheat) is harvested and can be used to increase the length of the growing season available to the second crop. A field study was conducted in 1988, 1989, and 1990 near Boone, Iowa to evaluate relay intercropping soybean into winter wheat. The objective of the study was to determine the effect of soybean variety and row spacing (in a relay intercropping system with winter wheat) on wheat and soybean yields, soil moisture usage, and canopy light interception.;Three soybean genotypes (A75-D29, 'Hoyt', and 'Pella 86') were interseeded into 'Siouxland' hard red winter wheat. Wheat rows were spaced 25 cm apart and were interseeded with soybeans at three different row spacings (25, 51, and 76 cm). Land equivalent ratios (LER's) were calculated to measure the agronomic productivity of the system. Soil moisture samples were analyzed from three depths in the profile and at two distances from the row. Light interception measurements were made in 20 cm increments from the soil surface to the canopy top.;Dry weather resulted in premature death of intercropped soybeans in 1988 and 1989. In 1990, above average rainfall resulted in adequate soil moisture throughout the growing season and all treatments reached maturity. Averaged over the 3 years, sole wheat yielded slightly more than intercropped wheat; however, neither soybean variety nor row spacing had a significant effect on wheat yields. In 1990, soybean performance was greatly diminished by intercropping as yield, seed size, plant height, and plant stand were reduced by 78, 14, 27, and 31%, respectively. Soybeans grown in narrow row spacings and those with indeterminate growth habits produced the greatest yields in both sole and intercrop systems. In 1990, the LER was 1.18 with the wheat component comprising over 80% of the total.;Sole soybeans utilized more soil water and intercepted more light than intercropped soybeans after wheat was harvested. Among the intercropped treatments, soybeans grown in narrow row spacings and those with an indeterminate growth habit were also advantageous in light interception.
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