One thing to consider this fall when planting cover crops is how these crop species will respond to herbicide carryover, according to University of Missouri weed specialist Kevin Bradley. Relatively little research has been published on this topic, and the factors that affect carryover, such as moisture and soil temperature, will likely vary from year-to-year.

With that in mind, Bradley, along with Mandy Bish a member of the MU weed science department, summarized one year of data on the effects of herbicide carryover on cover crops. This is preliminary data collected in 2013 from Columbia and the experiments are currently being repeated.

PROBLEMS WITH CARRYOVER: Cover crops can emergence can suffer due to herbicide carryover in corn and soybean fields.

Bradley and Bish offer their findings from the cover crop carryover study below:

Tables 1 and 2 show stand reduction of cover crops planted in the fall following either corn or soybean, which had one of 14 different herbicide treatments. Green boxes indicate stand loss of <15% relative to the same cover crop grown on non-treated soils. Yellow boxes specify stand loss of 15% to 30% and red boxes denote stand loss of >30%.

In the two experiments conducted last year, annual or Italian ryegrass (Lolium multiflorum) that was planted following either corn or soybean that was treated post-emergence (POST) with pyroxasulfone (Zidua) had >50% stand reduction relative to annual ryegrass grown on non-treated soils (Tables 1 and 2).

Table 1

Tillage radish stand reduction was greater than 30% when following corn that had a POST application of flumetsulam (Python) or following soybean that had a POST application of either imazethapyr (Pursuit), S-metolachlor + fomesafen (Prefix) or fomesafen (Flexstar) (Figure 1). Similar effects of fomesafen on tillage radish have also been observed by Dr. Mark Bernard’s group at Western Illinois University in which they applied different concentrations of fomesafen (from 12.5% of the labeled rate up to the labeled rate to mimic possible carryover rates) 5 days prior to planting the tillage radish.

The group observed greater than 40% damage to the tillage radish at the 1X rate of fomesafen. Bernard’s group also studied wheat and cereal rye across many of the same herbicides that we tested, including: mesotrione (Callisto), pyroxasulfone (Zidua), flumioxazin (Valor), cloransulam methyl (Firstrate), fomesafen (Flexstar), sulfentrazone (Spartan), isoxaflutole (Balance Flexx), and atrazine (Aatrex). They found less than 20% visual damage at the 1X rate across all herbicides for both crops.

While this data provides some insight into cover crop selection based on previous herbicide usage, it is only preliminary.

Table 2

For more information regarding cover crops and weed science, including herbicide carryover, see the Weed Science data in the slideshow located at http://bit.ly/1s3DHuf.

Source: MU-IPM Newsletter