ISU Extension Integrated Crop Management Newsletter
April 16, 1993

Timing planting with anhydrous ammonia applications
by Regis Voss, extension agronomist

The wet fall and spring will cause anhydrous ammonia application and corn planting date to be close. This will lead to the oft asked question, "How long do I have to wait to plant corn after ammonia application?" If there is a soil separation between the ammonia zone and the seed, planting can be done the same day the ammonia is applied. If the seed is to be placed in the ammonia zone, the longer the waiting period the less potential for root injury. There is no magic number of days to wait.

Most of the diffusion of ammonia from the point of injection takes place the first 24 hours. Typically, ammonia will diffuse 2 1/2 to 3 inches from the point of injection causing a 5- to 6-inch diameter cylinder of ammonia. If soil conditions are wet, the injection knife will seal the sides of the injection slit, which will limit the diffusion and cause a very high concentration of ammonia below the soil covering or seal.

The depth of ammonia application is extremely important. The depth should be checked in the field. Use a spade to dig down through the injection zone to check depth. Ammonia should be injected 7 to 8 inches deep to minimize potential injury. Where possible, ammonia application at a slight angle to row direction will reduce the number of plants affected.

Ammonia injury causes desiccation of roots and die-back from the root tips. Injury is first noted in the field by uneven emergence, slow growth of some plants, and wilting of plants in dry weather. Root injury is noted by browning of the roots and in severe cases roots will be completely killed and turned black back to the seed. Ammonia injury is detected more frequently in dry weather because roots are slow to develop and destruction of a portion of the root systems limits water uptake.

ISU Extension Integrated Crop Management Newsletter
April 16, 1993

Burndown likely for no-till systems
by M.D.K. Owen, extension weed management specialist

Due to the wet conditions, weeds will likely have emerged prior to herbicide application. Early preplant applications will not be effective unless a burndown herbicide is included. Preemergence applications will also need a burndown herbicide in most instances.

There are a number of options available for corn; selection should be based on weed type and size. If only broadleaf weeds are present, treatments that include a triazine herbicide, 2,4-D or Banvel will provide acceptable control. Triazines also have some activity on small grass. If grass size is larger than 1.5", control may not be satisfactory. The inclusion of a surfactant, crop oil concentrate and using liquid nitrogen will improve the activity of cyanazine and atrazine.

When grasses are larger or the predominant vegetation in the field, more consistent control will be obtained with a nonselective herbicide such as Gramoxone Extra or Roundup. These herbicides can be tank-mixed with residual herbicides to improve control. Nonselective treatments are typically used in soybeans. Due to the later planting dates, weeds are often larger than found in corn production. This increases the importance of herbicide rate for Roundup. Control can be improved with the addition of 2,4-D ester or Sencor. Sencor plus 2,4-D ester also will provide burndown activity. Gramoxone Extra is also an effective burndown option in soybeans.

The key to effective burndown weed control is to scout the field prior to application to determine the type and size of weeds. The proper treatment and rate can be determined with timely scouting.

Grazing restriction chart is available on request through the Extension Entomology office at Iowa State University.

ISU Extension Integrated Crop Management Newsletter
April 16, 1993

Herbicide mixing guidelines

by M.D.K. Owen, extension weed management specialist

Herbicide incompatibility problems are usually the result of poor mixing techniques. Typically, people neglect to "pre-wet" the herbicide products. It is important to pre-wet all products whether liquid or solid formulation. The general quidelines for mixing herbicides include:

1) Refer to the specific herbicide label for exact mixing instructions.
2) Determine compatibility of herbicides and carriers with small containers rather than large spray tanks.
3) Pre-wet dry or suspension herbicide formulations, add to the spray tank and agitate. Pre-wet herbicides with an appropriate amount of water. This is especially important when fluid fertilizer is the intended carrier.
4) Pre-wet the EC formulated herbicide and add to the spray tank during filling. Use agitation during the process.
Herbicide labels will vary with the amount of carrier in the spray tank prior to the addition of the herbicides. Follow the instructions closely. Compatibility agents may be necessary if fluid fertilizer is the primary carrier. Another important compatibility problem is water temperature. Cold water may inhibit the proper mixing of some herbicide formulations. This is particularly a concern during the early spring when cool, cloudy conditions are common. It may be necessary to allow the water to warm slightly before adding the herbicides. It is illegal to mix herbicides near a well. One last consideration is when herbicide mixtures are allowed to set long periods of time. Some products tend to settle and cake at the bottom of the spray tank. Mechanical stirring, a compatibility agent or both may be necessary to resuspend the herbicide mixture. It is better to mix only as much herbicide as will be applied at one time.

ISU Extension Integrated Crop Management Newsletter
April 16, 1993

Corrected Command deadlines

The deadline dates for Command applied early preplant were not correct in the previous newsletter. The correct dates for the deadlines are; April 1, south of Interstate 80, April 10, north of Interstate 80. Applications after these dates should be applied preplant incorporated unless applied to no tillage soybeans. Command can be applied prior to planting when soybeans are planted no tillage in narrow rows. The soybean drill must be equipped with an appropriate harrowing device or drag.

ISU Extension Integrated Crop Management Newsletter
April 16, 1993

Planters await weather change
by G. O. Benson, extension agronomist

The 1992 crop has been discussed and analyzed. Meetings have been held, management and buying decisions for 1993 made and now everything waits on the weather. Soils are very wet in most of Iowa. This provides a good soil moisture reserve, but may delay spring planting. Oat seeding is behind normal and by mid-April one expects to see considerable field activity and even a little corn planting.

It is far too early to talk about corn planting being delayed, but wet soils, less fall work than usual, ruts in fields and even some 1992 corn yet to be harvested are causing anxiety. However, being in such a rush that soils are worked a day or two before they are ready can have negative consequences. Those who will no-till for the first time are also reminded soil moisture conditions below the surface need to be examined when making planting decisions. Unless soil temperatures are very cold I would pay far less attention to temperature than soil moisture in making planting decisions. There continues to be considerable discussion relative to weather prospects for 1993 and how this might influence corn plant population decisions. Unless you believe there will be serious drought in 1993 I see little reason to reduce plant populations below what is considered optimum for a given location and management combination. Getting the intended stand is important, but in high population and potentially high yield fields it may be of equal importance to achieve even plant emergence and early growth. This can be a special challenge when soil conditions are not ideal.

ISU Extension Integrated Crop Management Newsletter
April 16, 1993

Pasture Weed Management and Grazing Restrictions
by M.D.K. Owen, extension weed management specialist

Proper pasture management is the key to avoiding weed problems. A healthy pasture will effectively compete with most weeds. When weeds become a problem, typically the pasture has a fertility problem or has been subjected to poor grazing strategies. If these factors are corrected, the weed problem will be significantly lessened. Herbicides may have a role in pasture weed management but can not resolve poor cultural practices.

Biennial thistles are common weeds in pastures. Best control of these weeds is obtained when the herbicide is applied while the thistle is in the rosette stage. Once the plant bolts (initiates the flower stalk) herbicidal control is more difficult. Bull and tall thistles can be controlled with 2,4-D applied at 1 to 1.5 qts/A while musk thistle will typically require 1.5 to 2 qts/A. A combination of 1 qt/A 2,4-D plus 0.5 pt/A Banvel may provide better control of thistles, particularly if the application has been delayed until bolting occurs.

                                        Interval between
Herbicide          Type of Animal     application & grazing    Comments

2,4-D amine           Dairy                 14 days            2,4-D labels vary.
                      Other              No restriction        See specific label.
2,4-D ester            All                   7 days            2,4-D labels vary.
                                                               See specific label.
Ally                   All               No restriction
Banvel            Lactating dairy        7 days if < 1pt.

ISU Extension Integrated Crop Management Newsletter
April 16, 1993

Identify White Grubs Before Treating
by Marlin E. Rice, extension entomologist (see jpg image: larva)

Two groups of white grubs are found in Iowa croplands: the true white grub, which can cause significant stand loss in corn, and the annual white grub, which does not cause stand loss.

True white grubs, larvae of May or June beetles (the big brown ones that fly around yard lights during summer evenings), feed on the roots of seedling plants, causing them to wilt and die. They have a three-year life cycle and can cause stand loss during two of these three years.

Annual white grubs do not cause stand loss to corn or soybeans. They have a one-year life cycle and the grubs are nearly finished feeding and ready to transform to adults about the time corn planting occurs.

The occurrence of true white grubs in any field is difficult to predict. Examining the soil surface during spring tillage, especially near wooded areas with willow or cottonwood trees, may provide clues as to the occurrence of white grubs. If grubs are found, they should be collected and correctly identified to determine their potential for economic damage to corn.

White grubs are C-shaped, creamy-white in color, and covered with tiny bristles. True white grubs can be separated from annual white grubs by examining the pattern of bristles on the bellyside of the last tail segment (called the raster). The raster of true white grubs has a narrow, smooth space with two rows of parallel bristles (patterned like a zipper). In addition, there are many scattered bristles on both sides of the "zipper." Annual white grubs have scattered bristles with no "zipper" on the raster. Entomologists at North Dakota State University have estimated that one or more true white grubs per cubic foot of soil will cause significant stand reductions in a seedling crop. This would be a reasonable threshold to use in Iowa.