Border Effects on Yields in a Strip-Intercropped Soybean, Corn, and Wheat Production System

Strip-intercropping of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] normally results in corn producing a negative border effect on soybean production. This study was conducted to determine whether including a small grain strip between corn and soybean strips could reduce the negative border effects of corn and enhance soybean yields. Corn, soybean, and wheat (Triticum aestivum L.) were grown as strip intercrops (15-ft wide) in a ridge-till system at two locations in southern Minnesota from 1991 through 1994 and were compared with a two-crop corn-soybean system. Rows were oriented east-west at one location and north-south at the other. Soybean yield in the three-crop system was reduced by 17% for the north row adjacent to corn and 8% for the south row next to wheat compared with nonborder east-west rows. In north-south rows, soybean yields were reduced by 21% in the east row next to corn compared with nonborder rows with no yield reduction in the west row next to wheat. In the two-crop corn-soybean strip system, soybean yields were reduced by 34 and 11% in the south and north border rows, respectively, compared with the nonborder rows in east-west rows. In north-south rows, the outside east row yielded 19% less and the west row yielded 21% less than the nonborder rows. Corn yield of the outside north row next to wheat in east-west rows was 6% greater while the south row next to soybean yielded 18% greater than the nonborder rows. In north-south rows, the east outside row next to wheat yielded 23% greater and the west row next to soybean yielded 27% greater than the nonborder rows. In the two-crop system, yield of the outside corn rows was enhanced similarly compared with the nonborder rows in both row orientations. Wheat yield in the 5-ft section next to soybean was 4% greater than the center 5-ft section and 6% greater than the 5-ft section next to corn in east-west rows and 9 and 17% greater in northsouth rows. Results from this 4-yr study indicate that wheat planted between corn and soybean strips improved soybean production over the two-crop system without adversely affecting wheat yields. Corn production was enhanced by 9 to 12% in north-south rows and 2 to 7% in east-west rows in both the two and three-crop strip systems. Research QuestionIn traditional two-crop strip intercropping, where corn and soybean are grown in alternating strips, the increased corn yields due to border effects are generally offset by decreased soybean yields in the outside rows. Some farmers and researchers in the Midwest are trying to incorporate a small grain strip between the corn and soybean strips in an effort to offset the negative border effects of corn on soybean. Since small grain matures earlier than either corn or soybean, it competes less with those crops for light. The objectives of this study were to: (i) determine the border effects of wheat introduced between corn and soybean on grain yield of soybean, corn, and wheat and on corn grain moisture at harvest, and (ii) compare grain yield of soybean and corn in the three-crop system to that in the two-crop system. Literature SummaryResearch with three-crop strip intercropping indicates that although small grain reduces negative border effects of corn on soybean, it competes with corn for moisture early in the growing season. Corn yields were reduced significantly in the corn row adjacent to small grain in dry years, but the effect of small grain on adjacent soybean rows was variable. Yields of corn and soybean in the three-crop system have to be compared with those of a conventional corn-soybean system in order to determine the direct impact of adding a small grain strip to a corn-soybean system. Study DescriptionThe study was conducted with corn, soybean, and wheat as strip intercrops on ridges on a poorly drained Webster clay loam soil at two locations in southern Minnesota from 1991 to 1994. Each crop was planted in a 15-ft wide by 120-ft long strip allowing for six rows of corn and soybean. Rows were oriented north-south at one location and east-west at the other. To maximize light interception and minimize shading, soybean strips were located on the south side and wheat on north side of corn in east-west rows. In the north-south rows, wheat was located on the east side and soybean on the west side of the corn strips. Corn was planted at 30 200 plantdacre in the center four rows and 36 000 plants/acre in the outside rows. Soybean was planted at 9 to 10 seeds/ft of row in all rows. Wheat was planted at 94 lb/acre in rows spaced 8-in. apart. Similar management practices were used for the two-crop (corn-soybean) strip-intercropping system. Applied QuestionsWhat effect did a wheat strip have on corn and soybean yields in adjacent rows? Corn grain yield of the outside north row next to wheat in east-west rows was 6% greater than the nonborder rows when averaged across the 4 yr (Fig. 1) while in north-south rows, the east outside row next to wheat yielded 23% greater. The soybean row next to wheat in east-west rows yielded 9% less than the nonborder rows during the 4 yr (Fig. 2) while in north-south rows the soybean row next to wheat yielded similar to the nonborder rows. Was wheat grain yield affected by corn and soybean border crops? Four-year average yields for the north 5-ft section next to soybean in east-west rows yielded 4 and 6% greater than the center and south 5-ft sections, respectively. In north-south rows, the east 5-ft section next to soybean had 9 and 17% greater yield than the center and west 5-ft sections, respectively. Does row orientation have an influence on crop yields in the border rows? Although the two sites were 30 mi apart, the similarity of soils, climatic conditions, and experimental procedures for the two sites and 4 yr of investigation across varying weather conditions allows the following observations and conclusions. Averaged across the 4 yr, corn yields in the outside two rows were 12% greater in east-west rows and 25% greater in north-south rows compared with the nonborder rows, while soybean yields were 13% less in east-west rows and 12% less in north-south rows compared with the nonborder rows. This indicates that corn yields in the border rows are enhanced to a greater extent in north-south rows than in east-west rows, while the border row soybean yields were affected in both row orientations to the same extent. On the other hand, average wheat yields in the outside 5-ft sections were similar to the center 5-ft section in both row orientations. Are corn and soybean yields in the strips of the three-crop system improved over those in the two-crop system? Relative grain yield calculations indicated that, in a three-crop system, whole-strip corn yields were enhanced 2 and 9% in east-west and north-south rows, respectively, compared with a whole-field. In a two-crop system, corn yields were enhanced 7 and 12% in east-west and north-south strips, respectively, compared with a whole-field. Soybean yields were reduced by 11% in strips in a two-crop system and by only 5% in a three-crop system, compared with a whole-field. This suggests that while corn yields in the strips benefit in both two- and three-crop systems, soybean yields are reduced less in the three-crop strip system. Fig. 1Four-year (1991–1994) average corn grain yield as influenced by row position in east-west and north-south rows in a corn-soybean-wheat strip-intercropping system. Fig. 2Four-year (1991–1994) average soybean yield as influenced by row position in east-west and north-south rows in a corn-soybean-wheat strip-intercropping system.