Establishment is one of the most important factors in the management of a cover crop for weed suppression. With later planting dates this year followed by a very dry June, conditions were right for herbicide carryover to be a concern for fall planted cover crops. The increase in precipitation events throughout July likely decreased that risk. It is still important to consider which herbicides were used during the growing season when selecting cover crop species. Potential interactions between cover crops and herbicide residue were covered in depth previously in this article (hyperlink to https://agcrops.osu.edu/newsletter/corn-newsletter/2020-29/herbicide-residue-considerations-fall-cover-crop-establishment).
Herbicide persistence is difficult to predict and varies by field and year. If there are specific concerns, it is best to perform a field bioassay now to determine potential impact of herbicide residues. To do this, collect soil from the fields where carryover is a concern, and soil from a field with no herbicide residue and a similar soil type. Plant cover crop species in each soil, water, and monitor emergence after 2-3 weeks. If emergence and plant health look similar between the soil with and without the herbicide, it is likely that the cover crop can be planted without risk of injury.
There has been some discussion amongst weed scientists over the benefit of a cover crop with reduced stand or biomass due to herbicide residue versus no cover crop, and whether a reduction in plant health has any effect on the ability to suppress weeds. More research is needed in this area. What we do know for certain is that high levels of biomass and ground cover provide the most effective weed suppressive benefits. Cereal rye tends to be the most effective species for weed suppression and is also the least sensitive species to herbicide carryover.
For more information on herbicide carryover to fall established cover crops, check out: https://iwilltakeaction.com/uploads/files/20210623-factsheet-cover-crop-carryover-usdadraft.pdf