Start the Growing Season on a Positive with a Successful Burn-down
Travis Legleiter, Extension Weed Scientist
Herbicide burndown applications have begun in Kentucky and are likely to continue in earnest over the next two weeks. This spring has been tough so far for herbicide applications, with wide temperature swings every couple of days, but the forecast looks promising for good burndown weather. As the sprayers head to the field, I want to give a few quick reminders and tips to help start the growing season with successful herbicide burndowns.
Pay Attention to the Weather
The temperatures this winter and early spring have been warmer than average, allowing for the advanced growth of winter annual weeds. The majority of species are one to two weeks in growth ahead of a typical year. Unfortunately, as mentioned previously, the unusually warm weather has been mixed with overnight below-freezing temperatures. Additionally, March brought above-normal precipitation in Western Kentucky. This all adds up to conditions for advanced weed growth but poor conditions for burndown spray applications. The short-term forecast for April is daytime temperatures in the 60 to 80F range, with overnight temperatures staying above freezing. These temperatures, especially the overnight lows, are ideal for effective spring herbicide burndowns. Unfortunately, those nice spring temperatures are typically accompanied by the wind, which is not favorable for spring burndowns. Each year I receive numerous calls from specialty crop growers, homeowners, and fellow grain crop farmers with complaints of drift from spring burndown applications. Typically, we are including either growth regulators (2,4-D or dicamba) and/or contact herbicides such as saflufenacil in our burndowns which can cause significant off-target injury at very low rates. As the warm temperatures and calendar give us all spring fever and the urge “to do something in the field,” be aware of wind conditions and avoid the costly mistake of drifting onto a neighboring crop.
Another weather condition we must be aware of is rain and forecasted rainfall. Spring can bring unpredictable rain patterns, and a pop-up storm can wreak havoc on-field activities, especially herbicide applications. While you cannot control the rain, be aware of the weather forecast and any potential chances of rainfall. Additionally, and more importantly, be aware of the rain-fastness and any rainfall restrictions of the herbicide products you are applying. Refer to Table 1 for the rain-fastness (amount of time needed between application and rainfall to avoid wash-off of the herbicide product) of some common burndown herbicides. Also, be aware that the labels of many newer formulations, including Xtendimax and the Enlist products, have a language to prohibit applications if excessive rainfall is forecasted in the next 24 to 48 hours (refer to specific labels for exact language).
Ryegrass Needs Special Attention
Ryegrass is an ever-increasing issue on Kentucky corn and soybean acres, with numerous failed burndowns occurring over the past two years across the state.
Annual ryegrass is one of the first weeds to green up in late winter and has unfortunately gotten a head start this year with the warm winter temperatures. One essential key to a successful annual ryegrass burndown is making applications within the window of the three conditions outlined in Figure 1.
This window capturing both the correct growth stage, air temperatures, and soil conditions has pretty much been impossible to find this year with the rollercoaster temperatures that occurred in March. While we have not been able to find the window with the correct overnight temperatures, the ryegrass has continued to grow, and most have already surpassed the 6-inch height in our fields. Now we must capitalize on warming overnight temperatures and dry field conditions to achieve control of this problematic weed. To capitalize, we must use the correct burndown tank mixtures; the following keys based on our research are essential (See Figure 2 for further data):
Use at least 1.5lb ae/a glyphosate (40 fl oz Roundup PowerMax 3)
The addition of 1 fl oz Sharpen (or 15 fl oz Verdict) to 1.5 lb ae glyphosate results in the consistently greatest ryegrass control in our research.
Increasing rates of saflufenacil (Sharpen or Verdict) does not help or hurt glyphosate efficacy on ryegrass.
Avoid tank mixing glyphosate and atrazine or metribuzin as these products will antagonize glyphosate activity on ryegrass.
The best non-glyphosate mixture is Gramoxone plus atrazine or metribuzin plus 2,4-D or dicamba. These tank mixtures work best on small ryegrass and under warm sunny conditions.
Adjuvants
Make sure you understand what adjuvants are needed to ensure your herbicide applications are effective. Adjuvants are often needed to ensure the product can effectively find its way into the weed and to its target site of action. The exclusion of an adjuvant such as MSO from a Sharpen application can be the difference in a successful and a failed burndown. You can either refer to the herbicide label or AGR-6 (http://www2.ca.uky.edu/agcomm/pubs/agr/agr6/agr6.pdf) for recommended or required adjuvants for the products you plan to apply.
Carriers
We have received a few questions about the use of liquid nitrogen as a carrier for spring burndown applications. While the inclusion of a small amount of nitrogen (such as ammonium sulfate) can be beneficial in getting herbicides into plants, larger amounts, such as liquid N as a carrier, may have the opposite effect. Liquid nitrogen can cause rapid plant tissue necrosis and antagonize the movement of a systemic herbicide to its target site of action, allowing weeds to survive the herbicide application.
We would recommend using water as your burndown carrier for the most effective herbicide applications. Although all water is not created equally, and we must be aware of the properties of the water we use for herbicide applications. As we start a new growing season, it may be wise to go ahead and check your water sources’ pH and hardness. Adjustment of water hardness and pH can be critical for successful herbicide applications throughout the season. In the challenging conditions of spring burndown, having a quality water carrier can go a long way.
Table 1. Rain-fastness and additional rainfall restrictions for some commonly used burndown herbicides.
Figure 1. The optimal window for Italian (annual) ryegrass burndown occurs when all three of these parameters occur at the same time.
Figure 2. Visual control of winter annual grasses twenty days after burndown application prior to corn.
