Weed management implications of a broccoli-winter rye intercropping system

Interseeded cover crops have the potential to improve agricultural productivity and reduce reliance on agrichemicals through suppression of crop pests and improvement of soil quality. We hypothesized that interseeded winter rye ( Secale cereale L.) could suppress weeds in transplanted broccoli, without reducing crop yield. In field studies, broccoli was cultivated at 0, 10 or 10 and 20 days after transplanting (DAT) with or without rye at the final cultivation. When seeded early (0 DAT), rye suppressed weeds and improved broccoli yields relative to unweeded controls, but resulted in yield losses relative to weed-free controls. When seeded late (10 or 20 DAT), rye neither reduced crop yields, nor suppressed weeds. A single cultivation was sufficient to avoid broccoli yield losses, but resulted in seed production among Powell amaranth (Amaranthus powellii S. Wats.) plants ranging from 1,500 to 30,000 seeds m-2.; A mechanistic plant growth model was developed in order to understand observed variability in Powell amaranth fecundity. Parameters required for the model were estimated based on field experiments in which plants were grown under a gradient of competitive conditions. Lower irradiance and red-far red ratios associated with more competitive environments were correlated with increased specific stem length and specific leaf area. For Powell amaranth, more competitive environments had no effect on flowering phenology, but reduced fecundity, and increased the primary dormancy of seed. Results from model simulations using historic climate data provided estimates of the probability of different levels of Powell amaranth seed production under different cropping systems.; Weed seed production estimates were incorporated into a population dynamic model of Powell amaranth to estimate the cost of a single year's seed rain on future crops. Results were sensitive to assumptions regarding future crop types, tillage practices, and weed seed characteristics. For producers of broccoli planning to grow other high-value crops in the near future, the costs associated with weed seed production levels observed in interseeding systems are likely to be very high. Future research should examine methods of improving the weed suppressive ability of rye, or combining interseeding with complementary weed management practices to minimize these costs.