One central strength of many cover crops is their allelopathy – their tendency to exude chemicals from the root system that inhibit the germination and growth of other plant species. In doing this, the crop creates conditions to favor itself and eliminate competition – an excellent survival strategy. In other words, the crop behaves like a mild herbicide.
Most of the small grains are allelopathic, with cereal rye topping the list. Lesser known allelopathic small grains are oats. Even this fall, we’ve seen our research plots with oats planted only a month ago appear much cleaner than neighboring plots containing other cover crop mixes. Oat allelopathy can continue on even after the oats winter-kill, leaving a relatively clean field to plant into at the beginning of spring.
These simple small grain cover crops are turning out to be effective weapons against the dreaded “superweeds”. Early results from a 2 year Penn State Study of various cover crop species suggests that rye comes in first for weed suppression, while clovers provided at least a 75 percent reduction in herbicide-resistant marestail populations.
Even when weed size can be reduced by an aggressive cover crop, it is much easier to control it, and we tend to run into fewer issues with herbicide resistance.
A SARE cover crop survey report showed that 82% of cover crop users reported cereal rye had reduced weed problems. 26 % said that rye had helped improve control of herbicide-resistant weeds, and rye was particularly effective in controlling Palmer amaranth. Grass-legume combinations were effective in producing the most biomass, and reduced Palmer amaranth incidence by half.
Cover crops used in conjunction with pre-emergence herbicides proved to be a good strategy to combat some herbicide resistant weeds.