Mechanisms for Overyielding in a Sunflower/Mustard Intercrop

Two intercrop patterns and sole crops of mustard (Brassica hirta Moench) and sunflower (Helianthus annuus L.) were planted in 1988 and 1989 on a Waukegan silt loam soil (fine-silty over sandy or sandyskeletal, mixed, mesic Typic Hapludoll) at Rosemount, MN, to examine mechanisms for overyielding [Land Equivalent Ratio (LER) > 1.0] in this intercrop system. We hypothesized that reduced competition between species for N and water were the mechanisms for previously observed yield benefits in both crops. Secondary objectives were to examine the importance of intercrop structure and N fertilizers on resource use and yield advantage. A strip intercrop pattern where 2.28-m strips of sunflower (76-cm rows) alternated with 2.28- m strips of mustard (15-cm rows) was compared with a more intimate row intercrop pattern of 76-cm sunflower rows inlet-planted with four 15-cm rows of mustard. Nitrogen was applied at planting at 0 or 112 kg N ha−¹ to whole plots, with planting patterns allocated to subplots in a split-plot design. Soil water content, nitrate N, and total N were measured at different locations and depths in the intercrop and sole crop patterns during the growing season. Mustard rows adjacent to sunflower in the strip intercrop yielded an average of 61% more than sole crop rows. Sunflower rows adjacent to mustard in the strip intercrop yielded an average of 40% more than sole crop rows. Yields of both sunflower and mustard were lower in the row intercrop compared with respective sole crops. Land equivalent ratios ranged from 0.96 to 1.43 in the strip intercrop and were generally below 1.0 in the row intercrop. Application of N did not consistently affect LER. Soil depletion patterns indicated that border rows of mustard obtained both soil water and N from the strips planted to sunflower at a time when demand for these resources by sunflower was low. Sunflower border rows obtained water and N from mustard strips later in the season. These data implicate complementary use of water and N as important mechanisms for the positive land equivalent ratio observed in this strip intercrop. Contribution of the Minnesota Agric. Exp. Stn. Sci. J. Ser. no. 19195.