Land equivalent ratios, light interception, and water use in annual intercrops in the presence or absence of in-crop herbicides

Increased crop production (overyielding) often observed in intercrops compared to sole crops has been attributed to enhanced resource use. The objective of this study was to investigate intercropping complementarity of wheat (Triticum aestivum L.), canola (Brassica napus L.), and field pea (Pisum arvense L.) for light and water use. Sole crop and intercrop combinations were evaluated for effects on land equivalent ratios (LERs), canopy light interception, soil moisture, water use (WU), and water use efficiency (WUE), with or without in-crop herbicides at two field sites in Manitoba, Canada. The mean LER was 1.1, but LERs varied greatly between site-years and herbicide treatments, and were significantly greater than one in 22% of the site-year-treatment combinations. The wheat-canola-pea and canola-pea intercrops showed the greatest frequency of overyielding for dry matter (50%) and grain yield (38%), respectively. Peak light interception tended to occur earlier with canola than with field pea, thus increasing the potential for light use complementarity between these crops. There was a positive correlation between LER and light interception in half of the site-years with applied herbicides and a negative correlation between LER and weed biomass at most site-years without herbicides. Although crop treatments used water differently within the soil profile, there were no differences in WU, but some differences in WUE, between crop treatments; however, WUE generally was not greater in intercrops compared to sole crops. In this study, overyielding in intercrops was inconsistent, and seemed to be related more to light interception than to water utilization.