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Soybean postemergence weed control begins

MISSISSIPPI STATE, Miss. — With over 70 percent of the Mississippi soybean crop planted and emerged, farmers are turning their attention to spraying postemergence materials. Summer annual weeds that emerge early grow quite differently from those that emerge after day and night temperatures begin to increase. Early-emerged weeds grow much slower, allowing a longer-than-normal delay of the first post application — an advantage of early planting.

My observations indicate the major weed flush has yet to occur (as of May 1) due to cool nights and lack of moisture in some areas. You do not have to hurry in April as much as you do in May and June. If you did not burndown early or did not achieve an optimum burndown, you may need to spray early.

If fields get wet, you could get behind quickly. Nevertheless, thus far I've observed only pockets of emerged weeds. The major first flush has yet to occur. If you spray too early where a pre- herbicide was not used, you may be setting yourself up for an additional post application.

Every day you prolong spraying early-planted acreage, you buy time or allow the crop to get closer to canopying. Weed competition data for summer annual weeds does not apply to the early plantings. Spraying as needed or spot-spraying in areas can cut the number of in-field applications and save money.

Various pre- materials put out with an early burndown have kept many fields clean and probably will eliminate one post application.

Given the efficacy and cost of glyphosate, many farmers want to spray early, but decisions should be based on the weed species present, size and population.

In areas where little rainfall was received in late April (areas of the south Delta), there is concern about moisture. Some growers may think planting shallow was wrong. Soil temperatures early in the year, potential seed quality and the uncertainty of rainfall amounts and intensity, however, make it a sound decision.

Soybeans will begin germinating when soil temperatures reach 55 degrees — not as cool as for corn, but much cooler than for cotton. It is imperative, therefore, that we take advantage of early planting.

Soybeans need to absorb 50 to 55 percent of their weight in water to begin germination. Both temperature and moisture have to be considered — you need good seed-to-soil contact.

Planting deep early will prove disastrous most of the time. The unpredictability of rain late in spring makes shallow planting a sound decision. Seed in dry soil or seed beginning germination in early April can take quite a while to emerge — due primarily to cool temperatures.

In recent years, I have seen soybeans take 30 to 35 days to achieve a stand. Last week a grower in the south Delta told me it took 27 days to achieve a stand from beans planted in early April. "They just kept coming up."

Once soil temperatures reach 65 degrees germination and emergence will occur in about seven days. As long as soil temperatures do not exceed (approximately) 94 degrees for several days (hot, dry conditions), seed will remain viable for an extended time. Planting early is often like having the seed in the bag.

With proper seed treatments on early plantings, we have avoided significant stand losses in recent years. Since most growers have begun using an Apron-type material (for pythium), replanting has been minimal. There are several broad-spectrum materials available, but some growers are still using just Thiram and Apron. This is not the broadest-spectrum option, but this mix should be sufficient, with the exception of a few unusual circumstances. If a problem occurs, however, the lack of a broad-spectrum material may be the problem.

Proper depth of planting leads to faster, uniform emergence. Soil temperatures are warmer near the surface and young germinating seedlings do not have to struggle as hard — especially after a hard, packing rain that often occurs in late March and April. As temperatures warm and fronts spread out, adjust planting depth.

We have come a long way in the last few years in precision-planting equipment, in understanding and using seed treatments, in seeding rates, in planting depth, and in varietal selection.

Alan Blaine is the Mississippi Extension soybean specialist. e-mail: [email protected]

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