In CT systems, however, residues may be present at the time of herbicide application and may decrease the herbicide’s effectiveness as the residues intercept the herbicides, reducing the amount of herbicide that can reach and kill germinating weed seeds . Since most preemergence herbicides can be surface applied and then incorporated into the soil by rain water or sprinkler irrigation, incorporation should not be an issue in CT systems. It may be that the increased organic matter on the soil surface binds up some of the herbicide, so a grower may need to increase application rates in order to achieve adequate control. Cover crops left on the surface present a different situation for preemergence herbicides. Cover crop mulches are seldom uniform; it is common to see thick mulch and bare ground in the same field. Researchers have observed that in areas with a thick mulch, the mulch may block an herbicide from reaching underlying weeds but may be sufficient by itself to control weeds, whereas in areas of the same field where the mulch is thin or nonexistent, the herbicide can reach the weeds and provide effective control . A planter implement also moves mulch and crop residue away from the seed line, creating a relatively clean zone for good herbicide action where it is needed most.Post emergence herbicides work equally well in CT and conventional tillage systems, cannabis protective tray though it should be noted that residues on the soil surface in a CT system may interfere with effective herbicide contact with emerging seedlings.
Hartzler and Owen suggest that growers wait until weeds become established and then control them with postemergence herbicides since the timing of weed emergence is less uniform in CT than in conventional systems. A grower should not wait too long to apply treatment, however; weeds that emerge together with the crop may cause greater yield losses than those that emerge later in the growing season. Similarly, crop emergence and development may be less uniform in CT systems than in conventional tillage systems, particularly for plantings made during cool periods of the year and in fields that have a lot of surface residue. In spring and summer plantings, growers can expect this difference in weed emergence timing to be much less. Adoption of CT has increased as a result of the development of HTCs that allow post emergence herbicides to be applied during the growing season with a relatively low risk of crop injury. However, when post emergence herbicides are to be aerially applied, growers should not wait so long as to allow the crop canopy to close, since the crops might then intercept the aerially applied herbicide, reducing the contact between the herbicide and the weeds under the crop canopy. Correct identification of the best time frame for post emergence herbicide application is critical in CT systems. Again, the complexity of cropping systems in California makes it difficult to provide a blanket recommendation on the optimal time frame for application.Increased persistence of residual herbicides may be a concern in CT systems. For example, Vargas and Wright observed crop injury that was attributable to the persistence of prythiobac sodium in a CT study.
Staple was applied to cotton but the following tomato and corn crops suffered considerable stand loss due to herbicide carryover . Among the tillage systems compared, injury was most severe in the completely no-till system. Similar carryover of another herbicide, sulfonylurea , applied in corn caused crop injury in the subsequent wheat crop. In the absence of the soil mixing that usually comes with tillage, residual herbicides may not be diluted sufficiently in the soil profile, and this may lead to injury of the subsequent crop. When selecting herbicides for a CT system, then, it is important to make choices that minimize losses to subsequent crops in the rotation. Some herbicides, such as Smetolachlor , are less likely to persist into the following crop.Herbicide-tolerant crops have made it easier for growers to begin to transition to CT systems in California and other states. The advantage of HTCs, mainly Roundup Ready crops, is the ease with which a grower can apply glyphosate over the top of the crop with excellent crop safety and weed control. As a result, production costs have also decreased as growers reduce the number of trips across the field, herbicide applications, cultivations, and hand weeding operations. By reducing cultivation and eliminating hand weeding, growers have reduced their costs by $25 to $150 per acre depending upon weed species and density. UCCE cotton cost studies indicate an average savings of $60 per acre with Roundup Ready cotton compared to the costs of growing conventional cotton . The potential for weed resistance to specific herbicides is always a concern with herbicide programs, and that concern increases with HTCs in a CT system. Roundup Ready promotes the continuous use of glyphosate, and we probably can expect that to induce shifts in weed species or the development of glyphosate-resistant weeds in HTC fields. Glyphosate-resistant horseweed or marestail, has been reported in no-till Roundup Ready corn-soybean rotations in the Midwest and mid-Atlantic regions of the United States , in the cotton producing areas of the mid-South , and recently in California on canal banks with a history of repeated glyphosate use . The elimination of tillage takes away an important tool for managing herbicide-resistant weeds. Studies by Wright and Vargas in the San Joaquin Valley have already shown weed shifts in Roundup Ready fields, with increases in annual morning glory over conventional tillage plots . Although CT systems are most often practiced in conjunction with HTCs, conventional varieties, herbicides with different modes of action, or tillage may also be needed to manage herbicide resistant weeds. Another concern as acreages of HTCs in CT systems increase is growers’ greater reliance on post emergence herbicides applied prior to planting and during the cropping season. Herbicides such as glyphosate and carfentrazone are sometimes applied by airplane or helicopter. With more than 200 different crops grown in the San Joaquin Valley, the potential for herbicide drift and associated damage to non-target crops is very high. Even ground applications of herbicides carry with them some risk of movement to sensitive non-target crops. Herbicide drift management is an important issue when using HTCs in a CT system.Any material that blocks light will suppress or prevent the growth of weeds. Layers of organic mulches such as municipal yard waste, straw, hay, or wood chips, for example, can be used for control of annual weeds . Thicker layers provide better results. Organic mulches break down over time, and the original thickness can typically reduce by 60 percent after one year. Coarse green waste works better than dry organic residue as a mulch. Organic residue mulches are rarely used in vegetable production in California because they are so costly to obtain, cannabis storage as well as to haul and to spread. Organic mulches are used particularly in areas close to cities that have implemented programs to collect organic wastes and either to compost the material or to sell it as green waste for agricultural use. Cover crops can be grown and then undercut and left on the same beds to form organic mulch . Plants that are used to produce this type of organic mulch include various cereals, clovers, vetches, and fava beans . Two advantages of growing the mulch in place are that it is rooted and so will not blow away in windy locations and that it does not have to be transported and spread. Organic mulches provide some weed control, depending on their thickness and ability to block light, besides offering other benefits to row crops.
Thick mulches have created some difficulties in direct-seeded crop fields , but less so with transplant fields. Cover crop mulches are currently the subject of a great deal of research on crops in California’s interior valleys, but at present their use is not widespread. Small grain cover crops are being tried by some growers in CT processing tomato fields in western Fresno County. The cover crops are planted after tomato harvest in the fall, allowed to grow over the winter, and killed in late winter with a post emergence herbicide such as glyphosate. Considerable residue is present on the beds in this system , but this can also have the negative effect of interfering with post emergence herbicides, resulting in weed escapes. Such weed escapes have been observed in a tomato CT system with small grain cover crops in Fresno County .Herbicide resistance is the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide that would normally be lethal to the wild type. In a plant, resistance may occur naturally due to selection or it may be induced through such techniques as genetic engineering. Resistance may occur in plants as the result of random and infrequent mutations; there has been no evidence to date that demonstrates herbicide-induced mutation. Through selection, where the herbicide is the selection pressure, susceptible plants are killed while herbicide resistant plants survive to reproduce without competition from susceptible plants. If the herbicide is continually used, resistant plants successfully reproduce and become dominant in the population. The appearance of herbicide resistance in a population is an example of rapid weed evolution . Research on early cases of herbicide resistance showed that resistant plants were found infrequently in weed populations before use of the herbicide. In some cases this was because the resistant plant was not as fit as other plants in the population and therefore would not persist in large numbers. Recent research, however, has shown that in some cases resistance doesnot come at a cost , so resistant plants may be just as capable of surviving to reproduce as are susceptible plants. These results suggest that the frequency of resistant plants in a population might be high even before the herbicide selection pressure is applied. Herbicides are active at one or more target sites within a plant. Target sites are enzymes, proteins, or other places in the plant where herbicides bind and thereby disrupt normal plant functions. One example is an enzyme—acetolactate synthase —that is involved in making branched-chain amino acids. Some classes of herbicide bind to the enzyme, causing dysfunction of the enzyme and reducing the synthesis of certain amino acids that are necessary for protein synthesis. These ALS herbicides differ in chemical structure but are active at the same target site. Plants resistant to ALS herbicides have altered acetolactate synthase that does not bind the herbicide. Often, a resistant weed that has been selected by pressure from one herbicide will be resistant to all herbicides that act on that herbicide’s target site. When a plant expressing resistance to a herbicide also demonstrates resistance to other herbicides that target the same plant process even though the plant has not been exposed to the other herbicides, the resistance is termed cross-resistance. For example, a population of yellow starthistle in Washington State evolved resistance to Picloram, a picolinic acid herbicide. When that population was subsequently exposed to Clopyralid, another picolinic acid herbicide, it also expressed resistance. Most cases of herbicide resistance in weeds involve a single mutation or modification in some function so that the weed is resistant or cross-resistant. Rarely does a single plant express resistance to several herbicides that affect different target sites. When a weed that has been exposed to herbicides that attack different target sites expresses resistance to more than one of these herbicides, that is termed multiple resistance. Plants of rigid ryegrass in Australia have multiple resistance to a number of herbicides in the cyclohexanedione, sulfonylurea, dinitroaniline, triazine, substituted urea, and triazole classes to which the weed has not been exposed. These classes include all of the herbicides currently registered in areas where this weed is a problem. The mechanisms of multiple resistance in rigid ryegrass include changes to the herbicides’ sites of action and the detoxification of herbicides by plant enzymes called cytochrome P450 mixed-function oxidases . This family of enzymes is similar to those found in many insects resistant to insecticides.Herbicide resistance was first reported in 1970. Common groundsel in a Washington tree nursery was shown to be resistant to herbicides in the triazine chemical class. Since that time, plants of 61 species have evolved resistance to the triazine herbicides. Resistance did not evolve in plants as early as it did in insects or fungi due to fundamental differences in the life cycles and genetics of plants, insects, and fungi.