For growers who may have planted cover crops last fall, it will be very important to apply an effective burndown to these species in the coming weeks.

Cover crops that aren’t effectively controlled prior to planting this spring can become a weed that takes moisture and nutrients away from the developing corn or soybean plants. Tables 1-3 show some of our recent research results related to the effects of some common herbicide burndown programs on the control of different cover crop species.

Tables 1 and 2 show the response of wheat, cereal rye, annual ryegrass, crimson clover, hairy vetch, and winter pea to a variety of herbicide burndown treatments. Table 1 shows the control provided by these treatments when applied earlier in the spring on April 5th, while Table 2 shows the control provided by these same treatments applied later in the spring, on May 1st. 

As you can see from the results presented in both tables, it is very important to make a timely and effective burndown herbicide application, regardless of which cover crop species you have present. Also, it seems clear from our results that the effectiveness of these different herbicide treatments will vary by cover crop species, but overall some of the species that have proven the most difficult to control in our research are annual ryegrass, wheat, and crimson clover. 

On the other hand, some of the cover crop species that these burndown treatments have controlled fairly well include cereal rye, hairy vetch, and winter pea. It is also important to note that tillage radish and oats winter-killed in our experiments, although they were a part of the study initially. In our experiences so far in central Missouri, neither of these species will over-winter. 

Due to the troublesome and persistent nature of annual ryegrass, we also conducted a separate experiment to evaluate more treatments and timings for the control of this species. The results from this experiment are shown in Table 3. Once again, one of the most important take-home messages from this experiment is that the timing of the burndown herbicide application is critical to the level of annual ryegrass control achieved. 

For example, in this experiment the average level of annual ryegrass biomass reduction with the glyphosate-containing treatments evaluated was 91% when applications were made to 5¾-inch annual ryegrass, but declined to 77% and 58% when these same herbicide treatments were applied to 14- and 36-inch annual ryegrass, respectively. 

There are two other things to keep in mind when it comes to the results of this study. First, our base rate of Roundup PowerMax that we used in all of these treatments was 36 ounces per acre. Based on the results from last year’s study it seems clear that this rate may need to be even higher for ryegrass. Second, we weren’t able to include tank-mixes mixes of glyphosate plus certain grass herbicides like clethodim (SelectMax, Arrow, etc.) in the trial last year but there is some data coming out of the southern U.S. that shows these mixes are effective. For example, several weed science colleagues in the southern states where ryegrass problems are more severe have seen good results with glyphosate plus Select Max at 10 to 16 ounces per acre.

Because ryegrass can be especially difficult to control in the spring, consider the following tips to achieve a more effective burndown of this species: 

1) Make sure to adjust spray settings (higher GPA, nozzle selection, etc.) to optimize spray coverage

2) Spray during daylight hours when annual ryegrass is actively growing, preferably at temperatures around 60 F 

3) Spray at least 4 hours prior to sunset to allow for maximum herbicide translocation

4) Try to avoid spraying when day or night time temperatures are forecasted in the 30s for an extended period of time.