Machinery would be used less, and that would mean an additional savings of an estimated $2500 in machinery wear. Conservation tillage is an umbrella term that encompasses many types of tillage and residue management systems . There are several definitions for CT. For example, Allmaras and Dowdy define it as “a combination of cultural practices that result in the protection of soil resources while crops are grown.” The Conservation Technology Information Center defines CT as any tillage and planting system that leaves at least 30 percent of the soil surface covered by residue after planting. California’s CT Workgroup characterizes it as a crop production system that deliberately reduces or eliminates primary intercrop tillage operations such as plowing, disking, ripping, or chiseling, and that manages surface residues so as to permit efficient planting, pest management, and harvesting. Several U.S. states have developed innovative tillage systems that conserve soil and residue and maintain crop productivity. However, findings in these states do not transfer directly to California because of differences in climatic and soil factors, dependence on irrigation and specific types of irrigation, and the overwhelming diversity of cropping systems in California. It is interesting to note, however, that with the advent of chemical herbicides, cannabis curing the concept of eliminating both tillage and cultivation from crop production had its first evaluation in a California orchard, in 1944, using a practice called “chemical fallow” .
As herbicide-tolerant crops —mainly cotton and corn —have increased, so has interest in CT systems among California growers. Along with the availability of HTCs, several other factors including increased fuel prices, access to better CT, global positioning system technology, and environmental air quality issues have had the combined effect of increasing interest in CT systems in California. Conservation management plans , now required by the San Joaquin Valley Air Pollution Control District , can include HTCs such as Roundup Ready crop varieties and the reduction or elimination of tillage as acceptable practices for dust reduction. The SJVAPCD suggests that the reduction in the number of passes and tillage that accompanies these practices can reduce soil and water losses and mitigate dust problems. Similarly, there is increased interest in testing CT systems in other non-HTC varieties such as tomatoes, wheat, oats, and dry beans in California. Reduced tillage, however, often brings with it changes in weed species and populations, and therefore in weed-management needs, and this is a major concern for the growers who may want to adopt CT systems . Phillips and Young stated that the vital factor for success of no-till row crop production is weed control, and that this depends largely on the proper use of suitable herbicides. For this reason, our focus in this publication is on the weed management issues in CT and we will suggest some techniques for the successful implementation of CT systems in California.Tillage has been a major agricultural weed control technique for several decades, so the development of CT systems that advocate no-tillage or reduced tillage has significant implications for growers.
Tillage affects weeds by uprooting, dismembering, and burying them deep enough to prevent emergence, by changing the soil environment and so promoting or inhibiting the weeds’ germination and establishment, and by moving their seeds both vertically and horizontally . Tillage is also used to incorporate herbicides into the soil and to remove surface residues that might otherwise impede the herbicides’ effectiveness. Any reduction in tillage intensity or frequency, therefore, poses serious concerns with regard to weed management. Weed species shifts and losses in crop yields as a result of increased weed densities have been cited as major reasons why CT systems have not enjoyed widespread adoption. Some other common concerns about weed management under CT include emergence from recently produced weed seeds that remain near the soil surface, interception of herbicides by thick surface residues, lack of disruption of perennial weeds’ roots, and changes in the timing of weed emergence . Reports of weed species shifts, however, have been inconsistent. For example, Cussans reported an increase of some dicot weeds accompanying increased levels of cultivation. Conversely, Wrucke and Arnold reported similar distribution patterns for broad-leaved weeds in both CT and conventional tillage systems. Pollard et al. reported that most weeds showed no consistent response to tillage. Swanton et al. found that tillage was an important factor affecting weed composition: common lambsquarters and redroot pigweed were associated with a moldboard plow system, whereas large crabgrass was associated with no-till.
Derksen et al. suggested that changes in weed communities were influenced more by environmental factors than by tillage system. Childs et al. stated that, over time, small-seeded annual broadleaf weeds and perennial weeds become more prevalent in no-tillage fields. Culpepper , in a survey of six states , reported a shift towards Amaranthus species, annual grasses, winter annuals, and morning glories in glyphosate-resistant cotton CT systems. Shrestha et al. concluded that long-term changes in weed flora are driven by an interaction of several factors: tillage, environment, crop rotation, crop type, and the timing and type of weed management practice. Very little data exist on such weed community dynamics in CT under California conditions. Studies in California have shown that most black nightshade emerged from the top 1 inch of the soil and that effective control of this species could be achieved with deep tillage . Maximum emergence of annual morning glory seeds occurred from the top 3 inches of the soil and a significant reduction in its population was generally observed following cultivation . Wright and Vargas observed increased populations of annual morning glory in cotton under reduced tillage. Further, glyphosate does not provide consistent control of pitted morning glory and other annual morning glory species . These findings suggest that the San Joaquin Valley cotton production systems using Roundup Ready CT technology may still have to rely on some level of cultivation for control of annual morning glory to avoid costly hand weeding. Some other problems associated with reduced tillage include the difficulty in managing perennial weeds such as nuts edge , since control of these species requires integration of cultural, mechanical, and chemical methods . It is clear that CT systems remain problematic in Roundup Ready cotton production systems, but CT systems have been successfully tested in Roundup Ready forage corn in some areas of the San Joaquin Valley .Successful implementation of a CT system depends to a large extent on a good understanding of the dynamics of weed seeds in the soil seedbank. A soil’s weed seedbank is the reserve of viable weed seeds present on the surface and in the soil. The seedbank consists of new seeds recently shed by weed plants as well as older seeds, some of which have persisted in the soil for several years . Different tillage systems disturb the vertical distribution of weed seeds in the soil—in different ways . Studies have found that moldboard plowing buries most weed seeds in the tillage layer, curing weed whereas chisel plowing leaves most of the weed seeds closer to the soil surface . Similarly, in reduced- or no-till systems 60 to 90 percent of the weed seeds are located in the top 2 inches of the soil . The Figure 1 graph shows that most weed seeds remain in the top 0 to 2 inches of the soil in notill systems. These seeds are at a relatively shallow emergence depth, and with suitable moisture and temperature they would seem likely to germinate and emerge more readily than those buried deeper by other tillage systems. In fact, though, weed seeds that are on the soil surface may be more readily eaten by vertebrates and invertebrates , killed by weathering, and more harmed by pathogens than those buried deeper .
Further, CT systems do not bring weed seeds from deeper in the soil profile up to the soil surface. Although CT systems may have more weed seeds at shallow depths in the soil, the weed seedbank can be effectively managed by minimizing processes that replenish the weed seeds and maximizing processes that deplete the seedbank.Shifts in weed populations from annuals to perennials have been observed in CT systems . Perennial weeds are known to thrive in reduced- or no-tillage systems . Most perennial weeds have the ability to reproduce from several structural organs other than seeds. For example, nutsedge and johnson grass , two common weed species in California, generally reproduce from underground plant storage structures: tubers and rhizomes, respectively. Conservation tillage may encourage these perennial reproductive structures by not burying them to depths that are unfavorable to emergence or by failing to uproot and kill them, in contrast to conventional tillage. Most perennial weeds occur in patches, though, and mapping these perennial weed patches and attacking them regularly with herbicide applications or mechanical control could be an effective management strategy in CT systems. Wright and Vargas found that the most effective purple and yellow nutsedge control in cotton was achieved by a combination of glyphosate in a Roundup Ready system that involves mulching seed beds and cultivating two or three times using sweep-type cultivators. Similarly, Shrestha et al. found that cultivation was necessary for successful control of field bindweed in CT blackeye beans . All of this means that some level of cultivation may be necessary for the management of “difficult-to-control” perennial weeds in certain cropping systems in California.Several studies have shown the composition of weed species and their relative time of emergence to differ between CT and soil-inverting tillage systems. Their germination and emergence may be enhanced more by the types of equipment used in soil-inverting tillage systems than by CT equipment. For example, studies in Denair, California have shown a markedly lower emergence rate for wild radish under CT than under soil-inverting tillage . Studies have shown that tillage stimulates the seedling emergence of wild radish . The timing of weed emergence also seems to be species dependent. For example, Bullied et al. found that species such as common lambs quarters, field pennycress , green foxtail Beauv., wild buckwheat , and wild oat emerged earlier in CT than in conventional tillage system. However, redroot pigweed and wild mustard emerged earlier in the conventional system than in the CT. Furthermore, in CT systems the presence of residue on the soil surface may influence soil temperature and moisture regimes that affect weed seed germination and emergence patterns over the growing season ; this may mean that CT practitioners have to change the timing of weed control measures in order to ensure their effectiveness. Soil surface residues can interfere with the application of herbicides, so there is a greater likelihood of weed escapes if residue is not managed properly or if herbicide application timings or rates are not adjusted.Weeds that are present when crops are planted in a CT system will likely need to be controlled with a non-selective burndown herbicide such as glyphosate, paraquat, or glufosinate. Selective herbicides are not typically used for burndown in CT systems, since the objective prior to crop emergence is total vegetation control, and selective herbicides may not control all of the weeds present. For example, common chickweed, shepherdspurse , London rocket , filaree , mustards , and fiddlenecks are common annual weeds that are present on the fallow beds and early cotton stands in CT systems, and these need to be controlled with non-selective post emergence herbicides . The non-selective burndown herbicide can be applied before or after crop planting but prior to crop emergence . Since these herbicides lack residual activity, applications should be scheduled as close to crop planting or emergence as the label will permit in order to minimize further weed emergence prior to crop emergence. Occasionally a burndown herbicide is tank mixed with a residual herbicide; the burndown herbicide is intended to control the emerged weeds and the residual herbicide to prevent weed emergence or growth. These burndown herbicides are usually tank mixed with carfentrazone or oxyfluorfen to control broad leaf weeds. Growers using CT may see this burndown herbicide application as an increase in production costs, considering that tillage would have controlled these emerged weeds in a conventional system. However, they may be overlooking cost savings for fuel, labor, and energy that are realized when a grower practices CT.In conventional tillage systems, crop residues generally are not present at the time of preemergence herbicide application.