Soybean Growth and Development & Management Information for Replant Decisions

Even if enough healthy plants (with good distribution in the row) remain after a hail storm to warrant leaving the stand, the weed status of the field is an important consideration in determining whether or not to replant. Soybeans will grow slowly for a period of time after damage occurs, depending on the amount of leaf loss and the weather. The rate at which soybeans recover will influence their competitive ability with weeds and its sensitivity to various weed control practices.

In relatively weed-free fields, soybeans that are not too severely injured will probably recover and grow fast enough that combinations of cultivation and application of postemergence herbicides will be effective. Effective cultivation requires weeds to be small (1 - 3 inches) and the crop tall to prevent it from being buried by soil. It is not necessary to cultivate deeper than 1 to 2 inches for most weeds. A shallow cultivation will effectively control annual weeds with little additional crop stress or soil moisture loss.

Early in the growing season, if a field to be replanted is very weedy, a soil-applied herbicide may be needed prior to the second planting. To reduce the chance of crop injury to the replanted crop, select an herbicide that is suited to the soil and weed situation (see the current edition of Cultural and Chemical Weed Control in Field Crops, Minnesota Extension Service, BU-3157), but which is not in the same chemical family as the first herbicide (Table 7). For example, if the first crop of soybeans was treated with trifluralin (Treflan), replanted soybeans should not be treated with Treflan or chemicals similar to Treflan, such as ethalfluralin (Sonalan) or pendimethalin (Prowl). Use Cultural and Chemical Weed Control in Field Cropsto choose a herbicide from another chemical family that will control the weed species in the field. Care must also be taken to choose a herbicide that, when applied this late in the growing season, will not present a carryover to next year's crop. See the individual herbicide labels for crop rotation restrictions. If you do retreat the crop with the herbicide that was previously used, remember to check the label for the maximum registered amount of herbicide that can belegallyapplied and act accordingly.

In fields that require replanting but still have good weed control or in fields planted after mid-June, another application of a soil-applied herbicide is probably not necessary. The more persistent soil-applied herbicides used on the first planting should remain in sufficient quantity to give some weed control in the replanted crop because most weed seeds near the soil surface should have already germinated. In weed-free fields the crop should be planted without any soil tillage or tilled no more than 1.5 to 2 inches deep to avoid bringing weed seeds and untreated soil to the surface. Soil and air temperatures should be favorable for fast germination, emergence, and growth of the crop, giving the crop a competitive advantage over weeds. Germinating weeds can be controlled by rotary hoeing, cultivation, postemergence herbicides, or combinations of these practices. Rotary hoeing is effective on newly germinated weeds before they emerge from the soil (weeds in the white stage). After weeds emerge, the effectiveness of the rotary hoe rapidly diminishes. Soybeans up to the second trifoliolate (V2) leaf stage can be rotary hoed. Do not rotary hoe soybeans when the hook-shaped hypocotyl has just emerged from the soil (the "crook stage"). Also, be aware that rotary hoeing may compound injury to hail-stressed soybeans.

Postemergence herbicide applications can control many annual weeds in soybeans, provided the weeds are small (Table 8). Postemergence herbicides should not be applied until the soybeans have recovered from the hail damage and resumed growth.This is particularly true of the postemergence broadleaf herbicides. Most postemergence broadleaf herbicide labels require a delay in application until the stress period passes, generally when the crop and weeds resume growth. Postemergence grass herbicides for soybeans are not as restrictive and can be applied to hail-stressed beans; however, the stress may reduce herbicide effectiveness on both the broadleaf and grass weeds.

Once soybean plants have resumed active growth, many weeds may be too big to control with herbicides. Therefore, if soybean fields are severely hail-stressed and weedy and the grass and broadleaf weeds exceed the weed sizes listed in Table 8, a difficult decision needs to be made. If the weed population is large and it isn't too late to replant, then replanting is advisable even though the crop plants would probably recover. If it is too late to replant a crop, then cultivation is the only option.

The use of postemergence herbicides to control later weed flushes poses several unique problems. A grower should be aware of applicable forage, feed, and grazing restrictions imposed by the Environmental Protection Agency (EPA)(Table 9). These restrictions are included on the herbicide label and are imposed by the EPA to prevent unacceptable levels of herbicides from entering grain and livestock food sources. A large number of soybean herbicides have grazing and forage restrictions on their labels (Table 9). This does not necessarily mean that herbicide residues exist at unacceptable levels in soybean plants. Often, there is a restriction because soybeans are not usually used for grazing or forage. It is more cost-effective for a herbicide manufacturer to restrict grazing and forage rather than conduct expensive research to document herbicide residue levels in the crop residue. It is important to note however, that postemergence soybean herbicides do have minimum time intervals between time of herbicide application and soybean harvest to prevent herbicide residues from entering the soybean seed. Table 9lists these minimum time intervals as well as forage, feed, and grazing restrictions for soybean herbicides. It is very important that the applicator stay within label restrictions.

Because of the reduced competitiveness of hail-damaged soybeans, weeds that are normally shaded out are able to grow and become a major problem. Eastern black nightshade is an excellent example of this situation. Eastern black nightshade is not a very competitive plant in a normal soybean stand because its growth and development is reduced by the soybean canopy. Watch for the development of this weed in hail-damaged fields. Timely applications of Blazer, Cobra, Pursuit, Reflex, or a cultivation will reduce this problem in soybeans(Table 8).

In some situations, it may be too late to replant soybeans, necessitating a change to another crop (see Figure 8). The herbicide used on soybeans may limit the choice of crops that can be replanted. Most preplant incorporated or preemergence herbicides will persist in the soil at least 4 to 12 weeks and could injure a newly planted crop. In addition, labelled crop rotation restrictions may also limit crops that can be planted. Any crop for which the chemical is labeled may be planted. Label information regarding replanting should be followed carefully because some labels suggest tilling the soil before replanting, others do not. See the individual herbicide label for specific details. Alternative crops which can be planted when various herbicides were used on the first crop are given in Table 10.

Note: Herbicide names, and application and use restrictions were based on information available for use in 1994. Always refer to current herbicide labels for the latest information.This publication is for your information. The University of Minnesota or its officers or employees make no claims or representations that the chemicals discussed will or will not result in residues on agricultural commodities and assume no responsibility for results from using herbicides.

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