Originally written by Gared Shaffer, former SDSU Extension Weeds Field Specialist.
Weed control has focused on resistance management, with a greater reliance on residual herbicide programs, use of multiple sites of action at full rates and over lapping residual programs. As use of cover crops have increased in recent years managing weeds through a longer residual herbicide program has become more complicated.
Cover Crops have a wide range of values including soil erosion protection, increases soil organic matter, beneficial fungi/nematodes, habitat for beneficial insects, help nutrient retention, possible nitrogen source, may increase infiltration capacity, used as a feed source and can assist in weed suppression. Trends of adopting cover crops have seen the use of shorter-season cash crops to allow more time for diverse cover crops to grow. Many producers are finding new ways to get cover crops established quicker, such as following combines with seed drills or spreading cover crop seed from a combine header. Most producers are growing cover crops to help give another option for grazing or for forage production. With all this comes the question of herbicides effects on cover crop establishment and rotation restrictions.
Herbicides can be one of many possible reasons for decreased cover crop stands or decreased cover crop production. Moisture, planting time, previous crop residue, saline/sodic soils and planting uses such as seed-to-soil contact and seed depth can come into play and may be a larger issue than herbicides alone on cover crop establishment.
Wheat Herbicides of Least Concern
Herbicides of least concern, if applied in crop, on label for residual concerns on cover crops would include: carfentrazone, triallate, karmex, linex, bromoxynil, bentazon, 2,4D Amine, 2,4D Ester, dicamba, fluroxypr, MCPA, and clopyralid. For grass cover crops after wheat there may be concern after an incrop application of fenoxaprop, pinaxaden or clodinafop, but no concern for these in broadleaf covers.
Research Discussion and Results
Multiple studies have shown that many group 2 herbicides have residual effects on cover crops planted after wheat. Therefore, in a recent study conducted at SDSU, we used many group 2 herbicides such as Ally XP, Amber, Glean XP, Teamate, Harmony SG, Olympus, Outrider, Peak, Rimfire Max, Sierra and Varro. These ALS inhibitors were observed to have a wide range of effects on different cover crop species. High and low moisture environments had more significant effects on the cover crop stands when ALS inhibitor herbicides were used. The highest cover crop losses were observed in the high moisture areas. This is thought to be due to the higher pH in these fields. As when chemicals break down by hydrolysis, the soil pH present in those fields is most likely higher. Therefore, the risk of carryover increases as soil pH increases. Acid hydrolysis stops with a soil pH above 6.8.
Our research shows that Axial, Discover and Tacoma had little to no residual effects on stands of crimson clover, flax, radish, rapeseed, field pea and sunflower planted after wheat harvest. There were slight stand effects on oats from Axial XL and Discover NG. Low stand effects in Pearl Millet from Axial XL in a low moisture environment were observed. These herbicides are ACCase Inhibitors where the majority of decomposition is from soil microbes. In recent years many producers are using premixed herbicides such as Goldsky, Huskie, Huskie Complete, PerfectMatch, and Talinor. These premixed products showed more pronounced effects on cover crop establishment when planted after wheat. The components of many of these herbicides are broken down by soil microbes with ALS inhibitor components being at risk to carryover due to pH.
For more information on this research, view the SDSU Extensions publication Herbicide Residual Effects on Cover Crops after Wheat.