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| The Framework for Weed Control Programs in California Rice Presented at the UC Cooperative Extension Rice Meetings, Feb. 1999 James E. Hill and Albert J. Fischer University of California, Davis | |
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| Introduction | |
| Weed resistance to the herbicides used in California rice is a relatively new event. However, weed populations have always been dynamic and the continuous use of almost any management practice has resulted in the loss of weed control. In other words, about the only certainty in California rice weed management is change. In fact, within a few years after the introduction of rice in 1914, weeds were running rampant in the dry-seeded culture established at the time. Dr. Jenkins Jones wrote in 1924, that "practically all, if not all of the lands–and these represent the major portion of the rice acreage–are quite foul with water grass," and that on these lands it was "practically impossible to grow profitable rice crops." Jones’ research led to water-seeding, but large seeded biotypes of water grass better able to emerge through the continuous flood became the dominant weed problem along with a new set of aquatic species. These included the sedge species, the aquatic broadleaf species and the late watergrass biotypes or so-called "mimics" which evolved in Asia from selection pressure of hand weeding. As weeds that looked different from rice were hand pulled the ever evolving survivors looked more and more like rice; hence, the name "mimic." Since 1992, some weed species that commonly infest California rice fields are evolving with resistance to herbicides, thus increasing the difficulty of weed management and control. Adding to the complexity of rice weed management are regulatory aspects related to herbicide drift, buffer zones and water holding periods which limit weed control choices and shape decisions. The following discussion and tables provide a framework for decision-making in the increasingly complex business of rice weed control. | |
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The Weeds: Species, Recordkeeping and Resistance | |
| When the broad-spectrum combination of Londax and Ordram was the primary treatment, rice weed control was very simple, but this is no longer true. For example, many of the new herbicides control one or only a few species, so incorrect weed identification can lead to poor control. It is not enough to group weeds broadly into sedges, "lilies" and grasses. Rather, we need to know with certainty that the weed is ricefield bulrush instead of smallflower umbrella sedge; or to know with certainty that the weed is California arrowhead rather than ducksalad or some other broadleaf species. Moreover, knowledge of the species and its competitive ability are critical to target the most important and potentially damaging weeds. For example, even though California arrowhead may be the dominant species in a field, will it be the most damaging? For information on weed species in California rice fields, look at Integrated Pest Management for Rice, University of California Publication number 3280, or on the internet at website: http://agronomy.ucdavis.edu/uccerice/weed/weed.htm | |
| Field history is a valuable tool for understanding the changes in weed populations. Although it is common to keep field records of varieties, yields and quality, it is relatively uncommon to see good records and maps of the weed species present in a field. Records of weeds (complete with field maps) coupled with good documentation of management and herbicide practices provide very useful information about the buildup of certain weed species, weed resistance and other aspects related to weed control (such as whether or not the weed infestations are related to field operations–field equipment, etc.). Furthermore, the ability to use certain herbicides may depend on the ability to document resistant weed populations in the field. An example is Shark under the expected 1999 Section 18 registration which will require certification of resistant weeds before you can use it outside the propanil buffer zones. Most importantly, however, good field records will likely improve the ability to select management practices and herbicides to minimize weed problems.Lastly, it is imperative to know what weed species are resistant to which herbicides. With limited exception related to the research of UC Davis and the companies whose products are involved, there are no diagnostic services available to determine whether or not the weeds are resistant. Thus, such diagnosis depends on the records of field history. The best indicator is whether or not properly applied herbicides are able to control the weeds. If not, the chances are good that the species may be resistant. However, other possibilities should be eliminated before concluding that the weed is resistant. One telltale sign, assuming that all conditions such as weed growth stage, weather and management practices were ideal, is the survival of a single, normally susceptible species while all others are controlled. The survival of a single species year after year when it was previously controlled is also a reasonable indicator of resistance. However, allowing weeds to reproduce over time eliminates the option of prevention to keep resistant weed seed banks at low levels in the soil. Certainly, the early identification of weed resistance and even draconian efforts to reduce seed production are essential to combat resistant weeds–especially on a farm scale where resistant populations could be restricted to single fields rather than be allowed to spread. | |
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| The Herbicides | |
| When Londax and Ordram dominated the California market for weed control in water-seeded rice in the early 1990s, there was relatively little interest in new products. With the onset of widespread weed resistance, many old and new products have entered, or are about to enter the market, including new technologies such as rice varieties genetically engineered for tolerance to herbicides such as Roundup and Liberty (Table 1).While all the new products hold promise for improving weed management in rice, they add to the puzzle of information needed to use them safely and efficiently. For example, if a foliar applied herbicide is translocated in the plant, it may not be necessary to completely drain the field; but, in combination with a foliar herbicide that does not translocate (contact), weed control could be greatly compromised by not having the field completely drained. Thus, it will be important to know the behavior of each herbicide in the plant and the environment. In the case of transgenic varieties, will it be safe to use them in fields adjacent to nontransgenic varieties–or next to transgenic varieties engineered for a different herbicide? Most of the newly introduced herbicides are somewhat limited in the spectrum of weeds controlled, requiring the proper selection either alone or in combination to give adequate weed control. The weed spectrum for the herbicides available for the 1999 season is shown in Table 2 and those expected by 2002 in Table 3. Potential weed control given in the tables is based on both company and UC Davis research and represent the control that could be consistently expected of a particular product, assuming that the weed species are not resistant. Different uses of the same product, application timing, field management and environmental conditions (weather) may all increase or decrease control. For example, SuperWham or Stam (propanil) works better at or above 75o F and with eight or more hours of sunlight following application. Light is required because propanil blocks photosynthesis. Shark into-the-water may control a broader range of species than indicated in Table 2 if used as a foliar applied herbicide, but higher rates are required. For best control, carefully read and follow the label which will state the rates, adjuvants, combinations and other requirements of the product.By mixing and matching the herbicides in Table 2, a complete spectrum of weed control may be possible. However, in addition to the weed spectrum, it is important to know how the herbicide is taken up by the weed, if it is translocated in the plant, the range of application timings for weed control and crop safety, if the herbicide has residual activity and whether or not the weeds are resistant. Tables 4 and 5 provide additional information on the behavior of current and future herbicides respectively. | |
| Foliar or Soil Activity: Most of the newer herbicides are active only as foliar sprays. However, Ordram, Bolero, Abolish, Londax and Shark have soil activity. Generally, when the product is formulated and used as a granule such as Ordram 15G and Bolero 10G, the activity is through the soil. Abolish, which is the same active ingredient as Bolero, is also adsorbed by the soil, but the product is designed as a spray which improves foliar contact for pinpoint flood management. Like Abolish, Londax and Shark are also adsorbed to the soil when sprayed into the water. Generally rates can be lower when used as a foliar spray than into the water, but each chemical varies so the manufacturer’s label should be followed. Products that are effective when applied into the water are weakly adsorbed and concentrated by the soil from where they are released and taken in through the plant roots. Some herbicides such as Roundup are so tightly bound by the soil that they become unavailable for root uptake. Thus, Roundup is a contact-only herbicide. Most contact-only herbicides are not adsorbed by the soil, and if they are applied into the water, concentrations are diluted to far below active levels. This is why field drainage to expose the weeds is so important for most foliar only herbicides. | |
| Contact or Translocated: Another important factor affecting the proper use of herbicides is whether or not they move in the plant. Two herbicides may be foliar active but are used quite differently with respect to field management. Translocated herbicides, such as MCPA, 2,4-D, Grandstand and Roundup move from the site of uptake to other parts of the weed to kill the growing point. Contact herbicides move very little from the point of impact, and kill only that part of the plant covered by the spray. Shark, SuperWham or Stam (propanil), Whip and all the future herbicides excepting Roundup, move very little in the plant. The translocation index given in Tables 4 and 5 are indicators of the relative movement of rice herbicides in the plant. Numbers above seven mean that the herbicide is highly mobile and below four generally means little movement. Matching water management to the translocation characteristics of the herbicide used is extremely important to the success of the application. For example, the label for Grandstand, a translocated herbicide, specifies that only 70% of the foliage need be exposed, whereas contact-only herbicides may require complete drainage. | |
| Window of Application: Herbicides vary widely in their ability to kill weeds of different sizes and in their safety to rice at different stages of growth. The application timing on the product label is given to minimize rice injury and optimize weed control and is the "application window." Abolish and Bolero have by far the smallest application window, requiring rice to be at least 1 1/2 leaf but watergrass not greater than two leaf. Many of the new herbicides have relatively broad windows of application. Some, like Whip, require rice to be in early tillering before the crop is safe. Most of the new herbicides have fairly broad ranges for application, both with respect to crop safety and weed control. Regardless of the window, it is important to remove weeds before competition reduces yield. Most research shows that the onset of weed competition is about twenty days after seeding, depending on the severity of the weed pressure and rate of growth. Competition notwithstanding, the new herbicides offer the opportunity to remove weeds where applications have been delayed by weather or to cleanup where weeds have been missed by earlier applications. | |
| Residual Activity: Residual activity is an important attribute in preventing reinfestation by subsequent germination of a new flush of weeds. Residual activity is generally determined by the amount and strength of soil adsorption and by the rate of degradation of the herbicide in the environment. Ordram, for example, has a half life of only about five days and hence a short residual activity, whereas Londax residual is 35 days. Residual activity is much more important for early applications before the rice canopy is capable of shading out weeds. Mixing a residual herbicide with early applications of foliar herbicides such as propanil can sustain control long enough for the rice canopy to cover. It is, however, a double edged sword in that selection pressure for weed resistance continues as long as the herbicide remains soil active. | |
| Mechanisms of Action: It is essential to know which herbicides have similar mechanisms of action because weeds are resistant to the mechanism that kills them, not to the herbicide per se. Once the weeds become resistant to a herbicide with a particular mechanism of action, all other herbicides with a similar mechanism of action will likely fail to control the weed. Table 6 shows the current and future rice herbicides grouped by mechanism of action. Thus, it would not be a good idea to use Abolish or Bolero (thiobencarb) where resistance to Ordram has been documented. To prevent the further buildup of resistant weed seed banks, herbicides with different mechanisms of action should be rotated or used in combination to prevent resistant species from setting seed. | |
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