3 Scouting for Weeds 

Soybeans can compete effectively with weeds late in the growing season if the weed population is below the economically damaging level within the first six weeks after planting.
The keys to weed management are:

To scout for weeds, follow these steps: Table 1 gives weed rating values (low, medium, and high) for common types of weeds. Do not continue counting beyond the "high" level for any weed. If you find more weeds of that species, record the "high" level number with a " + " beside it and go to the next sample site. Be sure to notice the size of the weeds, as this information is important in making control decisions. If the count reaches the "high" level in more than five samples, consider the field as needing control action and stop sampling. (NOTE: Keep broadleaf and grass weed counts separate. Do not discontinue sampling broadleaf weeds if the number of grass weeds reaches the "high" infestation level, or vice versa).

If the weeds appear in specific areas of the field, draw a map of the field and mark their locations. After all stops have been made, total the count for each weed and divide by the number of stops.

Using Economic Thresholds for Weeds

The development of highly effective and selective posternergence herbicides for soybeans has made it possible for growers to make an important change in the way they manage weed problems. Traditional weed control systems have included the use of preplant or preemergence herbicides. Although these herbicides have usually been very effective, they are preventive in nature because they must be applied before weeds germinate and emerge. The use of posternergence weed control tactics allows the grower to treat only those areas or fields where the weed populations exceed economically damaging levels, thus affording an opportunity to save money.

One of the keys to successful use of this remedial approach to weed control is being able to determine when weed populations exceed economically damaging levels.

In the vast majority of soybean fields, weed populations far exceeding the economic thresholds will develop if no control measures are used. A high percentage of fields will also have a residual weed population left after the primary weed control measures have been applied, either because of incomplete control of the susceptible species or because tolerant species are present. The process of deciding whether to treat this residual population is where information on economic thresholds is of greatest value. In addition, the residual population in most fields is made up of more than one weed species. Therefore, to make the most intelligent decision, a grower needs to know the economic threshold when many different kinds of weeds are present in a field.

One approach to developing multiple weed thresholds has been to compute a competitive index (CI), which compares the competitive ability of the various soybean weeds to that of common cocklebur, the most competitive weed in North Carolina fields. The weeds are rated using a scale from 0 to 10, with 10 being the most competitive (see Table 2).

The competitive index is used to calculate the competitive load (CL) that each weed exerts on the crop. Combined with information on soybean prices and control costs, the competitive load can be used to determine the profitability of a particular treatment. An example is given in the next section.

Deciding When to Treat for Weeds

Using the procedure described previously, scout each field 14 to 17 days after planting. If you find obvious differences in the weed population in different parts of the field, subdivide the field and treat the different parts as separate fields.
 

Table 2. Competitive Index (Cl) for Major Soybean Weeds

Weed                                           Cl        Weed                                                 Cl
Arrowleaf sida    .....................    1.2       Morningglory, pitted      ..................   3.6
Balloonvine*      ......................    2.5       Morningglory, purple      .................   4.0
Barnyardgrass     .....................    0.3       Morningglory, red      .....................   3.2
Bermudagrass      ....................    0.4       Morningglory, smallflower  .............   2.4
Black nightshade, eastern*.......    3.0       Morningglory, tall   .........................   3.2
Bristly starbur   .......................    3.8       Nutsedge, purple     ........................   0.2
Broadleaf signalgrass     ...........  0.3        Nutsedge, yellow     .........................  0.3
Carpetweed     .......................    0.1       Palmer amaranth      ........................   4.0
Cocklebur, common ...............   10.0      Pigweed, redroot     ........................   4.0
Coffee senna      .....................    2.4       Pigweed, smooth      ........................  4.0
Crabgrass      .........................    0.2       Poinsettia, wild    .............................   3.0
Crotalaria, showy*     ..............   2.4       Pokeweed, common  ......................   4.5
Crowfootgrass     ...................    0.2       Poorjoe    .......................................   0.2
Dayflower      .........................    1.2       Prickly sida        .............................   1.2
Eclipta   .................................    1.8       Proso millet        .............................    0.6
Fall panicum      .....................    0.4       Pursiane, common   .......................     1.2
Field sandbur     .....................    0.2       Ragweed, common   .......................   3.8
Florida beggarweed     ............   2.8       Ragweed, giant   .............................   9.5
Florida pusley    .....................    1.5       Red rice     ......................................  0.3
Groundcherry, cutleaf   ............   3.5       Redweed             ............................  2.0
Giant foxtail  ...........................   0.3       Shattercane         .............................   1.0
Goosegrass     ........................   0.2       Sicklepod           ...............................  2.5
Green foxtail     .......................   0.3       Smartweed, Pennsylvania .................  4.7
Hemp sesbania        ................   3.8       Southwestern cupgrass ....................   0.4
Itchgrass      ............................   0.8       Sprangletop         .............................  0.4
Jimsonweed     ........................   5.2       Spurge, spotted     ..........................   1.2
Johnsongrass (seedling)    ........   0.4       Spurred anoda       ...........................  3.2
Johnsongrass (rhizome)    ........   1.2       Texas panicum       ..........................   0.6
Lambsquarters, common  ........   5.2       Tropic croton       ............................   1.2
Mexicanweed  ........................   1.4       Velvetleaf          ...............................   3.0
Morningglory, cypressvine .......   3.4       Venice mallow       ...........................  0.8
Morningglory, entireleaf      ......   3.2       Volunteer corn      ...........................   1.0
Morningglory, ivyleaf      ..........   3.2       Witchgrass          ..............................  0.4
Morningglory, palmleaf    .........   3.0       Yellow foxtail      ..............................  0.2
*Causes problems in addition to its competitive effects.
 

An example will illustrate how to determine whether control measures are necessary. Suppose that a soybean field has been scouted and the following weeds have been found:
 

Weed                                 Total No. Counted
Cocklebur  ..................................     8
Smartweed   ................................    32
Jimsonweed    .............................     20

Step 1. Calculate the average number of weeds per stop. Since 10 stops were made, the average is found by dividing the number of weeds per stop by 10.

Weed                                          Average
Cocklebur .............................     0.8 ( 8/10)
Smartweed  ............................    3.2 (32/10)
Jimsonweed  ...........................    2.0(20/10)

Step 2. Look up the competitive index (CI) value for each of the weeds in Table 2.

Weed                           Competitive Index (CI)
Cocklebur  ...............................      10.0
Smartweed     ...........................      4.7
Jimsonweed    ...........................      5.2

Step 3. Multiply the average number of weeds per stop (from step 1) by the Cl value for that weed (from step 2) to get the competitive load (CL) for each weed in the field.

                        Average No.     Competitive     Competitive
Species              of Plants             Index               Load
                          per Stop              (CI)                (CL)
Cocklebur    ....... 0.8  ................ 10.0 .............    8.0
Smartweed    ...... 3.2  ...............   4.7 ...............  15.0
Jimsonweed    ..... 2.0  ...............  5.2 ................  10.4
    Total competitive load (TCL)    ......................   33.4

Step 4. Add up all the competitive load values to get the total competitive load (TCL) for the field.

Step 5. Divide the total competitive load by 2 to find the percentage by which the competing weeds will reduce the yield.

Projected yield loss =   33.4  = 16.7 percent
                                      2

Step 6. Multiply this projected yield loss in percent by the yield that could be expected from the field if it were totally free of weeds during the entire growing season. Divide the result by 100 to find the projected loss due to the weeds present. For this example, assume that the field normally produces 30 bushels per acre:
 

Yield loss =    16.7 percent X 30 bushels per acre  =  5 bushels per acre
                                            100

Step 7. Multiply the number of bushels lost per acre by the price per bushel expected when the crop is sold to find the amount of money lost per acre. Assume that the expected price is $6 per bushel:

Loss = 5 bushels per acre X $6 per bushel = $30 per acre

    Thus we know that if no herbicide is applied the grower can expect a loss of about $30 per acre because of the weeds competing with the crop. Since it will cost money to apply herbicides and control the weeds, the next step is to compare the cost of control with the expected loss.

Step 8. Subtract the cost of treating the field (the cost of the herbicide) from the amount lost if the field is left untreated. If the result is positive, it indicates that it will be profitable to treat for weeds at this time.
In this example, if the cost of applying herbicide is $8 per acre and the cost of leaving the field untreated is $30 per acre (from step 7), the estimated gain from treating the field will be:

    $30 - $8 = $22 per acre

Weed Threshold Worksheet

Back to Table of Contents