Now is the time to start thinking about the establishment of a cover crop. If herbicides such as glyphosate, paraquat, 2,4-D, or dicamba are being used prior to cover crop establishment remember to consider the replanting interval. For glyphosate and paraquat, most cover crops can be planted after application. However, depending on the rate of 2,4-D applied, at least a week to four-weeks waiting period is necessary before establishing cover crops. If small grain covers are being established, some research suggests that a minimum delay of 7-10 days after application at rates of 1 pint/acre of 2,4-D ester. Also, other studies have shown that certain clovers (e.g., red and crimson) and alfalfa may be established within the same constraints with 2,4-D ester. To be certain, a longer waiting period and rainfall help to reduce the potential for injury. Dicamba usually requires at least a couple of weeks or more before certain cover crops can be planted and this also is directly related to the amount applied. Some have asked about the use of products such as Sharpen, Liberty, and Elevore as a burndown prior to cover crop planting. Currently, we do not have adequate data on this utility for these products (see more details below from our field study). However, from reading the labels, some suggestions can be made. Sharpen (1 fl oz) has no restrictions for small grains but has a 1 month wait period for other cover crops (species not specified); Liberty has a 70 day wait period for small grains, but canola can be planted anytime, and for Elevore, small grains, ryegrass, and canola can be planted after 14 days but there is a 9 month wait for clover and mustard species. Keep in mind, once some of these products (e.g., Sharpen) are applied, they cannot be harvested or grazed for livestock feed. Refer to appropriate herbicide labels for more details.
We conducted a field study in 2020 at Rock Springs to examine the effects of commonly used burndown herbicides prior to fall cover crop establishment. Herbicides were applied on September 8 and included: 2,4-D ester (1 and 2 pints/A), dicamba (8 fl oz), Elevore (1 fl oz), Sharpen (1 fl oz), 2,4-D + Sharpen (1 pint + 1 fl oz) and Liberty (32 and 43 fl oz). Seven cover crop species – cereal rye, annual ryegrass, forage radish, crimson clover, red clover, hairy vetch, and alfalfa were planted at 0, 7, and 14 days after herbicide application (DAA). Cover crop injury was visually evaluated about 2 months after application.
Preliminary results (Figures 1-7) indicate that crop injury varied between species, herbicide, and planting date. In general, cereal rye and ryegrass were impacted the least. Only 2,4-D (2 pt) and dicamba caused 25% injury at 0d planting, while the other herbicides and planting dates resulted in less injury. Forage radish was injured most (20 to 73%) at 0d and 7d plantings by 2,4-D (both rates), dicamba, and 2,4-D + Sharpen; however only 2,4-D (2 pt) and 2,4-D + Sharpen treatments at the 14d planting resulted in 28 to 50% injury. Crimson and red clovers were injured most (18 to 90%) by the Group 4 herbicides on 0d and 7d plantings, except Elevore which caused no more than 12% crimson clover injury. The 2,4-D (2 pt), dicamba, and Elevore treatments still caused 22 to73% injury to red clover at 14d planting. Only dicamba and 2,4-D + Sharpen treatments injured crimson clover the most (18 to 23%) at 14d planting. Hairy vetch was significantly injured (22 to 87%) by 2,4-D (both rates), dicamba, and 2,4-D + Sharpen at 0d and 7d plantings, but only 2,4-D (2 pt) and 2,4-D + Sharpen treatments at the 14d planting resulted in 22 to 57% injury. Alfalfa was injured most (22 to 90%) by all the Group 4 herbicide-containing treatments across all planting dates, except 2,4-D (1 pt) at 14d which only caused 8% injury. Sharpen and Liberty did not cause significant injury to any of the cover crops at any of the planting dates.
In summary, the use of Group 4 herbicides as a burndown treatment prior to fall cover crop establishment tend to be risky. However, depending on what species is selected (e.g., cereal rye or ryegrass) and when it will be planted (>7 DAA) some Group 4 herbicide options could still be useful. Furthermore, from this preliminary data, other herbicides such as Sharpen and Liberty might have a fit since they are generally safe to many cover crop species and can control problematic weeds such as glyphosate-resistant marestail/horseweed and Palmer amaranth. These could be tank-mixed with glyphosate for broad spectrum burndown prior to cover crop establishment. Further research will need to be conducted at this and other locations to verify the results.
Figures 1-7. Effect of burndown herbicides on seven cover crop species (% visual injury)
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Source : psu.edu