Drought situations affect weed control. Herbicides breakdown more slowly than normal under dry soil conditions. Therefore, an additional margin of safety should be exercised while following both rotational crop restrictions and wait periods after prior herbicide applications. The spectrum of weeds may also be different due to changes in soil moisture levels resulting in more drought-tolerant weeds in the fields. This document addresses some of the concerns about herbicide residues in soil following a drought situation, and their possible solutions. Bear in mind, however, that there is perhaps no real solution. Instead we will have to work towards minimizing losses. Tips on weed management strategies for fall are also addressed briefly.

The Problem

Some of the effects caused by the severe drought are easily recognized, but there are other less obvious problems--extended persistence of herbicides is one of them. Some of the residual herbicides that were applied in spring or summer -- or even last year -- may persist in levels that could cause injury to crops planted in the fall. Based on feedback obtained from county extension faculty, this document discusses the rotational restrictions of fall-seeded crops and includes a quick method to determine crop-damaging levels of herbicide residues in the field.

Wait Periods for Rotational Crops

When seeding potential crops in the fall, refer to Table 1 to determine the suitability of fields for the prospective crop. Be particularly careful when atrazine or other triazine types of herbicides have been used recently. Following a period of drought, an additional buffer or safety margin may be required above and beyond the normal rotational restrictions.

Is There a Solution?

How does drought affect rotational crop restrictions? Conditions may vary from county to county depending on soil type, intensity of drought, etc. If the rotational restrictions are met in accordance with the label, additional caution could be exercised by determining the approximate biologically active levels of residual herbicides. This could be determined by conducting bioassays.

Bioassays

Bioassays are simple and inexpensive tools to determine biologically active herbicide residues in soil. Materials required include: 8 plastic or galvanized trays (approximately 14" by 18"; 4" deep), a shovel, and seed of prospective crop. The following steps outline the method of conducting a bioassay.

1. Clean the trays and shovel thoroughly, preferably with a little bleach, followed by plenty of water.
2. Collect soils from the top 0 to 4-inch depth from randomly selected spots in the field and transfer them into the trays. Break large clods and mix uniformly.
3. Fill four trays as described above and label them as "Treated."
4. Clean the shovel again, and collect similar soils (0 to 4-inch depth) from areas adjacent to the field where the herbicide was not applied. Field borders may serve this purpose.
5. Fill four trays as described in step 4 and label them as "Control."
6. Place the crop seed in rows at an appropriate depth.
7. Water gently as necessary.
8. Observe germination and growth of seedlings in the trays.
9. Compare seedlings in treated and control trays for injury symptoms.
10. For 2-3 weeks, observe for injury symptoms and measure seedling heights at the end of this period.

Ideally, there should not be any differences in the appearance or growth of seedlings (for example, seedling heights) between the "treated" and "control" trays. If there are differences, these may imply that herbicide residues are still present at levels injurious for that particular crop.

Fall Weed Management

Fall is a good time to apply systemic herbicides to manage perennial weeds. These weeds have perennating organs like rhizomes and stolons that help them persist year after year. Development of these organs usually occurs during the early fall. During a drought, it is possible that drought tolerant perennial weeds are more prevalent in fields than annual weeds. Therefore, fall may be a good time to apply a systemic herbicide (an herbicide that is absorbed and moved within the plant tissues). A non-selective systemic herbicide like glyphosate could be applied as a spot application. Grass killers (graminicides) like sethoxydim, clethodim or fluazifop may also be applied during fall to manage perennial grasses. Please consult the label for proper rates and application methods. The environmental conditions during and after application of an herbicide determine their efficacy. Conditions that favor plant growth, including soil moisture and fertility, allow better performance of systemic herbicides. You may have to wait until the soil is fairly moist before applying these systemic herbicides to manage weeds. When plants are free of stress, herbicides are absorbed faster and move better within the plant system, providing more efficient kill. Application of systemic herbicides early in the morning may be more effective. Spray additives or adjuvants may enhance herbicidal effectiveness based on label recommendations. Please follow the label directions for use!

Table 1. Rotational Restrictions (wait-periods) for herbicides applied prior to fall seeding of potential crops in West Virginia. Check product label for accordance with this list and in instances of tank-mixing two or more products.