Classical biological control agents such as herbaceous insects or plant pathogens are only effective against a single species or at best a few closely related species and there has been research on biocontrol agents for single species of common corn and soybean weeds. But the weed complex found in row crops such as corn and soybeans consists of many annual weeds and if a single species is controlled other species will replace it. We may be able to increase the spectrum of weeds by combining biocontrol agents or tank mixing biocontrol agents with herbicides. But much more research will be required. Production systems with allelopathic crops or rotational cover crops may be a way to manage a range of weed species using a biological control approach.
Many plants release chemicals which suppress a wide spectrum of weeds.
There has been limited research on allelopathic row crops and no attempts to release corn or soybean cultivars with high levels of allelopathy. Even if allelopathic row crops are developed, allelochemicals will not control all weed species. Allelochemicals are also unlikely to control perennial weeds. Another hurdle is that allelopathic crops must control weeds through the critical weed-free period which can be as much as 12 weeks and can not reduce yields or delay maturity of the desirable crop. Allelopathic cropping systems are probably best considered a replacement for preemergent herbicides.