BROOKINGS, S.D. - Long residual pre-emergent or early post-emergent herbicides may cause stand reduction or complete failure of cover crops.
"Growers need to consider the rotational restrictions and intervals of herbicides before application," explained Gared Shaffer, SDSU Extension Weeds Field Specialist. "This can become a larger issue if the cover crop will be grazed."
Rotational restrictions can be found on most herbicide labels under the title of similar wording to "rotational crop restrictions" or "rotational crop guidelines." Specific guidelines, usually found under "forage restrictions," must be followed for cover crops that are grown for feeding livestock whether for grazing or forage.
Shaffer added that depending on efficacy of the herbicide, residual can both affect in-season and/or post-harvest cover crop establishment.
Crop rotational interval
A few chemical companies add common cover crops and rotation intervals to their labels. If a cover crop is not listed on the label, it then falls into the "other" category.
Most corn, soybean and small grains herbicide labels do not have rotational intervals for non-harvested or harvested cover crops.
A crop rotation interval is the required time between application time and the time of next planting.
The crop rotation interval is required for two main reasons, Shaffer explained. First, a rotation interval ensures potential herbicide residues in the soil will not affect plant establishment. And, it ensures there are no unsafe levels of herbicide in plant tissues.
"If a producer does not intend to harvest the cover crop, the rotation interval requirement is not a legal requirement, but if the producer plans on harvesting the cover crop the label restrictions must be followed," Shaffer said.
For example, if a producer grows wheat and applies an herbicide with plant back restriction of six months for cover crops. If the grower goes ahead and plants cover crops for non-forage use five months after application, the producer would be fully responsible for any damage that occurs to planted cover crop as a result of residual herbicide.