Extension Weed Specialist

A new concept has developed in small grain production areas in North Dakota during the 1970s. In the decade of the 90s this trend will continue to expand and increase among efficient crop managers. These systems can be described as minimum, reduced, zero, or no-til1. The primary purpose of these systems is to minimize the mechanical working of the soil and still maintain economic yields. Increased costs of energy supplies, machinery and labor will press for these changes. Also, increased emphasis on soil erosion control and non-point source pollution will add to these changes.

New cropping systems will mean different weed control programs. Under no-till, the weed control methods will not remain static. New problems and new weed control methods will develop as experience is gained with no-till agriculture.

Weed control is required several times per year in a no-till cropping system. They are as follows:

1. In fall, after harvest
2. In spring, before crop emergence
3. Selectively in the growing crop
4. Non-selective patch treatment in growing crop

The number of weed spray applications under no-till is often a concern. Under a conventional system at least one tillage operation will normally be carried out in the fall. If perennial thistles are a problem, a fall treatment will be necessary as part of a thistle control program. Before seeding, tillage will be used to control weeds before planting a crop. In the crop, under both no-till and conventional tillage, one and generally two herbicide applications are required to control the type of weeds found in North Dakota fields. Although the choice of herbicides may be different under no-till than under conventional, the number of herbicides will likely be the same. The major change under zero tillage may be the shift from a combination of one preplant incorporated and one postemergent herbicide to two postemergent herbicide applications. In some cases tank mixing may reduce the number of herbicide applications. This comparison indicates that the number of weed control operations is similar under no-till and conventional tillage systems.

No-till is highly dependent on the use of herbicides and consequently cultural weed control is often overlooked. However, good cultural weed control practices, which are suited to no-till when combined with the proper use of herbicides, can make the difference between acceptable weed control and a total failure.

Cultural control of annual weeds is based on the following cycle of weed seed movement in the soil.

WEED SEED CYCLE

Fig. 1

No-till can break this cycle of weed seed movement in the soil. The population of annual weeds can be reduced by no-tillage. However, biennial and perennial weed species are well adapted to undisturbed soil conditions and will invade a zero tillage field if proper control measures are not carried out.

The use of clean seed is essential. Any weed seed added to the soil with crop seed will be in an ideal location to germinate. The weed seed will be close to the soil surface, will be in the disturbed drill slot, and will be close to the applied fertilizer. A second consideration in using clean seed is the prevention of the addition of new weed species to the field. Weed seed content of certified seed can be checked by referring to the seed analysis tag. Seed from other sources should be checked carefully and weed seeds identified.

Soil disturbance must be minimized to reduce weed problems under no-till. Any disturbance caused by the seeding operation will have the same effect as cultivation on stimulating weed seed germination. Weeds frequently emerge in the drill run or marker under no-till, resulting in a growth of weeds in the crop row, and a lack of weeds between the rows. If severe disturbance occurs, or if tillage is carried out, weeds will emerge both in the row and between the row.

Crop competition should be used to assist in weed control. Even crop emergence and rapid growth of the crop under no-till will suppress many late emerging weeds. Care must be taken to ensure an even crop stand. Trash management and proper drill adjustment will prevent skips in seeding. Avoidance of seeding misses will prevent crop voids, which serve as ideal spots for weed growth. Proper fertility levels will maintain crop competition, which will improve control from postemergence herbicides and suppress growth of late emerging weeds.

Crop Rotation and Crop Sequencing: Is also a part of no-till weed control. The major effect of crop rotation is to allow herbicide selection to match the weed problem. Since herbicide selection to control all weed species is difficult, crop rotation will allow for use of different herbicides so that a weed species will not go uncontrolled for two years in a row. Caution must be exercised to ensure that a major weed species is controlled every year, even if this means avoiding a crop in the rotation if a suitable herbicide is not available for the problem. Only crops with adequate postemergance herbicides should be considered for no-till.

Sanitation is another method of weed control useful in no-till. Weeds which are common in fence lines, roadways, pastures or waste areas may spread into no-till fields. These areas should be cleaned up by herbicide application and then kept clean by spraying as required. Once cleaned up, grass competition will keep most weeds from becoming established. Perennial grasses may creep into the field from headlands.

Perennial weeds such as Canada thistle and quackgrass are well adapted to grow under no-till farming. Research and farmer experiences report switches from annual to perennial weed problems within a few years of starting a no-till cropping system.

No-till should lead to reduced infestation levels of annual weeds like wild oats, green and yellow foxtail (pigeongrass) and wild buckwheat. However, no-till may encourage perennial weeds. It is highly critical that control measures be used to prevent establishment of serious thistle or quackgrass problems.

Since no-till enhances perennial weed growth, it is highly advisable to establish no-till on fields relatively free of perennial weeds and fairly clear of other weed problems. Control practices for serious perennial noxious weed problems are more flexible under conventional tillage operations.

Chemical Weed Control is a must under no-till and must be combined with other cultural methods listed above. Selection and timing of herbicides relative to the weed species present is a must to obtain effective weed control.

Fall spraying must be considered may for control of perennial and winter annual weeds.

Control procedures against perennials should occur in the fall after harvest. Limited growth in the spring before planting aid the lack of suitable herbicides for use in the crop restricts control of weeds such as quackgrass to fall operations. Canada thistle control also takes place in the fall. Early planting of an early maturing crop such as barley or wheat will allow additional time for weed regrowth and control measures. Specific control recommendations are available in the current Chemical Weed Control Guide (Circular W-243).

Winter annual weeds such as pennycress, tansymustard and shepherdspurse will be resistant to most spring herbicide applications but can be readily controlled in the fall. Farmer experience has shown that nearly all fields should be sprayed in the fall to control winter annual weeds. These weeds are difficult to locate in the stubble. A field may appear to be clean when in fact there are numerous weeds present. If the field was sprayed for perennial thistles, winter annual weeds may have been controlled by the same treatment. Check these fields carefully and respray for winter annual weeds if necessary.

Initial weed control in the spring should take place before or after seeding but before the crop emerges. This treatment controls most seedling weeds but will not control perennial weeds. Canada thistle may be suppressed and top growth of quackgrass may occur but regrowth can be expected.

Spraying should take place just before seeding or 2 or 3 days after seeding. Maximum weed control will be obtained if spraying is carried out just before crop emergence when a maximum number of weeds have emerged. However, the risk of delayed spraying by rain or wind must be considered. Spraying cannot be carried out if the crop has emerged. Spraying should not be done when dust covers the leaf surface of weeds. This can inactivate several of the chemicals used. This prevents spraying at the time of seeding and for 1 or 2 days after seeding. Spraying several days in advance of seeding will allow emergence of new weeds ahead of crop emergence.

Delayed seeding will allow a heavy weed growth to occur and control will be difficult. Seeding should usually be carried out before heavy weed growth develops. In the event that seeding must be delayed, two herbicide applications may be required, both at an early stage of weed growth.

Selective weed control in the crop involves the use of postemergence herbicides. Preplant incorporated herbicides cannot be used. The weed species spectrum should be carefully identified so that the proper herbicides can be selected. In the event that a suitable tank mix cannot be used, two sprayings will likely be required.

Annual weeds such as wild oats, green and yellow foxtail, kochia, pigweed, lambsquarters and wild buckwheat will usually be reduced under no-till. Since it is important to prevent the return of weed seed to the soil, even light infestations of weeds should be controlled. This requires a careful check for seedling weeds at an early stage of growth. Most weeds will emerge in the row and may be hidden by the crop. Wild oats in wheat or barley may be especially difficult to see.

Perennial thistles require a sequence of treatments in the crop and after harvest. Suitable herbicides should be selected for postemergence spraying.

Patch treatment for control of perennial weeds can be used effectively with no-till. Patches are dense and easy to locate when they are not spread by cultivation. Non-selective herbicides can be used to control the weeds while sacrificing the crop in the patch. Milkweed can be handled by patch treatment, as can small infestations of quackgrass, perennial sowthistle and Canada thistle.

No-till systems may increase volunteer crop problems in numerous ways. Under no-till it is generally advised to continuous drop rather than a crop-fallow rotation. Dry after-harvest conditions, combined with shallow tillage as in conventional tillage may stimulate fall germination of volunteer crop seeds. Under no-till, when grain fails to germinate in the fall, it likely will germinate the following year, possibly after the new crop has emerged.

Control of volunteer crops can occur by one or more of the following:

1. Fall germination - Winterkill will usually control germinated plants. If growth is heavy fall spraying may be considered. However, this increases total cost and should be avoided where possible.
2. Early spring germination - volunteer growth can be controlled by herbicides applied at the time of seeding.
3. Selective control in the crop --some herbicides will control or suppress volunteer crops. Crop rotations should be planned to allow use of available herbicides.

An environmental and energy concern trade-off centers on the increased herbicide use under no-till. This concern must take into account the following points:

1. A reduced fuel consumption accompanies no-till and should compensate for a partial increase in herbicide use
2. No-till reduces soil erosion. Eroded soil, while being a pollutant in itself, can carry soil bound herbicides and result in off target movement of the herbicide.
3. Actual herbicide use under no-till will likely be less or similar to that under conventional tillage. See table 1.

The following examples assume a common weed population consisting of wild oats, green foxtail, wild buckwheat, Canada thistle and pennycress. Two zero tillage system is compared to two alternative conventional systems.

Table 1. Comparisons of no-till and Convention tillage herbicide uses.

Dr. Steve Miller, NDSU agronomist, initiated a field experiment in the fall of 1977. Research to study the effects of tillage on wheat yields was conducted in 1978 and 1979 on a silty clay soil, pH 7.2 and 6.4% organic matter. The tillage treatments included no-till (seeding directly into stubble with Haybuster drill), minimum till (fall discing or chisel plowing with spring cultivating and harrowing) or conventional-till (fall moldboard plowing, with spring cultivation and harrowing).

Weed populations were influenced by tillage treatment (Table 2). Weed populations were almost two times greater in minimum or no-till compared to conventional-till treatments. Grassy weeds (green and yellow foxtail) were dominant both years regardless of the tillage system. Wheat yields were greater in the conventional-till than minimum or no-till treatments without good weed control (Table 2); however, when both grassy weeds and broadleaf weeds (kochia) were controlled, wheat yields were similar in the conventional and no-till system.

Weed control under no-till involves a combination of cultural and chemical operations combined into an organized year-round process. Timing of operations is critical. Effective control at all steps in the program is critical in achieving a high level of weed control in the crop.

New weed problems will arise as no-till is continued. Careful observation will allow for these new problems to be brought under control before they become serious. Weed problems are continuously adapting to adjust to control measures. Weed adaptations under no-till is just another example of this change.

Table 2. Influence of tillage and herbicides on weed populations and wheat yield at Fargo, North Dakota in 1978 and 1979. Miller, NDSU.