Kent Harrison and Jim Beuerlein
1. Should weeds be controlled pre-emergent or post-emergent for corn and soybeans?
The answer to this question depends upon the specific weed species that need to be controlled. In general, there are economical methods available for both preemergent and post-emergent treatments as well as combination pre-emergent/post emergent treatments.
As a general rule of thumb, it is a good, cost effective policy to use a preemergent, soil applied herbicide to control routine, annual weed species; this should be followed up with post-emergent control measures as the need arises. For perennial weeds, post-emergent control is still the only option available.
2. How can we reduce costs? Can we use less herbicides and cultivate more? What system is reliable and economical?
There is no one perfect system for weed control. Factors such as soil type, cultural practices, specific weed problems, and objectives of the grower will influence the choice of a weed control program. Cultivation requires wide rows, so for soybeans there is a yield loss associated with having wider rows to accommodate cultivation. Cultivation does not remove weeds close to the row and can destroy some of the crop's root system. For specific tips on reducing costs, refer to Reducing Crop Production Costs, Bulletin #749.
3. What are common herbicide carry over problems?
Atrazine carry over is a common problem when corn is followed by small grains, alfalfa, or soybeans. Although only limited data has been collected thus far, there seems to be potential carry over problems with some of the new soybean herbicides. For example, a crop of corn, sugar beets, tomatoes, or alfalfa following use of Scepter, Preview, or Lorox-Plus may suffer herbicide injury. In addition, carry over problems have been found when small grains follow use of Command.
4. Triazine carry over problems. How can I determine if the level is too high to establish grain and/or meadow seeding?
A bioassay is the most reliable method to determine whether or not levels of triazine are too high. Take several random soil samples (0-4 inches) in the field that is in question. Plant the crop in this soil the season prior to seeding and observe the seedlings for symptoms of herbicide injury.
There are also testing labs that will do soil analysis for triazine residues. In general, for samples taken at a depth of 0-3 inches, the following guidelines are recommended:
(1) If the triazine level is 0.17 parts per million (ppm) or less, it is safe to plant oats and alfalfa.
(2) If the triazine level is less than 0.35 ppm, it is safe to plant soybeans.
(3) If the triazine level is greater than 0.35 ppm, it is only safe to plant corn.
If soil samples are taken from 0 to 6 inches, these recommended tolerances can be cut in half.
5. How much do herbicides injure crops if no weeds are present? Can you put a dollar value on this? (Example 2,4-D and Banvel on corn)
Although most herbicides do not cause sufficient crop injury to cause yield reductions, some herbicides will cause severe crop injury under certain environmental conditions. Normally, these conditions are specified on the label of the product, and the consumer is warned to avoid application under these conditions. For example, 2,4-D and Banvel can cause corn injury if applied during extremely hot, humid weather or if applied at the wrong stage of plant growth. It is impossible to put a dollar value on "typical injury" to a weed-free crop since the amount of injury that can occur is directly influenced by the environment.
Most commercial herbicide products have had research conducted on them to determine their safety in weed free crops. In general, if herbicides are applied according to label instructions and if all restrictions are carefully observed, no reduction in yield would be expected.
6. What is the health safety, both personal and environmental, of commonly used chemicals?
Some herbicides are more dangerous for the grower to handle than others. We know that the herbicides on the market today are, in general, safe as far as short term effects, but research has not yet addressed the long term effects of these products.
The U.S. Environmental Protection Agency (EPA) has been assigned the responsibility to oversee the safety of all agronomic chemicals. They require toxicology data on each pesticide, and they establish tolerance levels. Only those products that fall within the window of "reasonable" safety will be marketed, and their registration is always subject to review by the EPA.
7. How do I control perennial vegetation (sumac, milkweed, dogbane, etc.) in no-till corn?
Each weed species may require a different control method. In general, control of perennial vegetation in no-till corn usually involves repeated treatment with Banvel (or Banvel plus 2,4-D) or spot application with Roundup. Fall applications of Roundup or Banvel are generally more effective than spring or summer treatments, and must be made before the first killing frost. When perennial weeds become a serious problem in no-till corn, the best solution is to rotate to soybeans and then to control these weeds with a different group of herbicides.
8. What can be used to control broadleaf weeds in corn on very high organic matter soil?
There are very few soil applied herbicides that can be used to control broadleaved weeds in organic soils. In most situations of this type, you must depend on postemergent herbicide programs: target specific weeds with specific post-emergent chemicals.
9. Considerations and concerns in the selection and use of spray adjuvants.
In general, use only the adjuvants specified on the herbicide label. In addition to the traditional surfactants and crop oils, many products are now labeled with low rates of liquid fertilizers. Herbicide labels which claim increased performance on certain weeds using low rates of liquid fertilizer (e.g. 28% N) are generally true.
10. Explain the use of Apron-Ridomil on different crops.
Ridomil (2E and SG) is a systemic fungicide for control of certain diseases in tobacco, soybeans, alfalfa, nonbearing deciduous fruits and nuts, conifers in nurseries, and certain vegetable crops. Check the product label for the specific diseases it will control in each crop and for the recommended methods of soil application.
Ridomil MZ58 is a wettable powder labeled for controlling diseases in potatoes, tomatoes, onions, cucumbers, melons, and squash. Ridomil SG is a granular fungicide for control of Phytophthora root rot in avocado and soybeans. Specific information concerning proper application methods are found on the product labels.
Apron contains the same active ingredient as Ridomil, but is formulated for seed treatment to control seed rot and damping-off diseases of certain crops (soybeans, alfalfa, wheat, corn, field beans, ryegrass, forage grasses, etc.).