B.L. Frick, Southwestern Ontario Agricultural Research Corporation (SWOARC), Harrow, Ontario

Executive Summary

Evaluation Summary (Tech. Transfer Report Summaries)

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Key Words:

corn, soybeans, winter wheat, no-till, survey, weed species, weed density, conventional tillage, conservation tillage, herbicide, inter-row cultivation

Executive Summary

Conservation tillage systems have been advocated as a means of reducing soil erosion and phosphorous run-off from cropland, and of maintaining long-term soil productivity. Conservation tillage has been defined as any method of tillage which leaves a minimum of 30% residue on the soil surface. Such systems range from strict no-till to chisel plowing, but all involve a reduced frequency of tillage and/or a different type of tillage compared to conventional systems. One of the main obstacles to widespread adoption of conservation tillage is the perception by farmers that new and exacerbated weed problems will occur when tillage is no longer available as a method of weed control. To address this concern a study was undertaken to determine what weed problems resulted from a reduction in tillage, and to examine possible management strategies to handle these problems.

A survey was conducted to examine weed species composition and weed density in 593 farm fields across southwestern Ontario in 1988 and 1989. The crops selected for the survey were corn, soybeans and winter wheat. Tillage systems were categorized as conventional (including some form of soil inversion as by a moldboard plow), conservation (including some soil disturbance other than inversion), and no-till (no tillage between the current and previous crop). Fields were also grouped according to the length of time that a given tillage system had been practiced.

In general, the same weed species were found in all tillage systems, but their frequency of occurrence and density varied slightly. All three tillage types were dominated by five weed species that occurred in more than 25% of fields: green foxtail, lamb's-quarters, redroot pigweed, common ragweed, and quack grass. Dandelions also occurred in more than 25% of conservation and no-till fields, as opposed to 21% of conventional fields. No-till and conservation tillage fields had higher overall weed densities than conventional fields. Weed populations varied greatly within tillage systems.

Only a few trends with length of time in a tillage system were observed. First year conservation tillage fields generally had more green foxtail and dandelions than conventional fields, but these weeds decreased once conservation tillage was established. Quack grass decreased in frequency and density with time in no-till or conservation tillage. Established no-till and conservation fields had markedly less quack grass than conventional fields.

Reduced tillage systems present an altered environment for weed growth and so may affect the rate of development of weed species. Most weed control practices are targeted at particular growth stages. The growth and development of the more common weed species were monitored in a small subset of the surveyed fields in 1989. Lamb's-quarters, redroot pigweed, and velvetleaf showed no difference in the timing of various growth stages among tillage systems. Green foxtail and common ragweed emerged later in no-till than in conventional tillage, and so may have been more likely to escape control. Perennial species emerged at about the same time in all tillages, but grew more slowly in conventional tillage than in no-till. Dandelion was the only weed to show a difference in flowering in relation to tillage. Dandelions flowered most often in no-till fields and least often in conventional fields.

Herbicide-use patterns did not vary substantially among tillage systems, other than the increased use of burndown treatments, and decreased use of soil-incorporated chemicals in no-till. The literature on herbicide efficacy in reduced tillage systems is not conclusive. The primary concerns of farmers in the study were (1) lack of information on dandelion control, and (2) the limited number of herbicides registered for burndown applications.

Greenhouse experiments were conducted to determine the effects of simulated "tillage" treatments on the survival of seeds, vegetative propagules, and actively growing plants of the more common weed species. Burial of weed seeds in the autumn, as through plowing, favoured annuals over perennials. Overwinter survival of roots and rhizomes of perennial species was decreased if they were brought to the surface. Mechanical damage to the roots of actively growing annual weeds was an effective control method at all growth stages. Perennial species were susceptible to mechanical damage only over relatively short periods of time.

The results presented here show that problem weed species were not substantially different in different tillage systems. Weed communities were influenced by geographic region, soil type, field history and level of management, in addition to tillage. It is probably not appropriate to design a single strategy for weed management in reduced tillage systems.

The perception that weed management is more difficult in reduced tillage needs to be challenged. Changes in weed communities are small. Some changes offer a greater challenge for weed control; other changes offer opportunities for improved weed control. The transition to conservation tillage or no-till however may initially require more intensive weed management.

Evaluation Summary

The objectives of the study were:

• to determine the weed species likely to be the greatest problem under various conservation tillage systems, and to see if their stages of growth differ between tillage systems.
• to examine the susceptibility of problem weeds to control by herbicide and tillage methods available in conservation tillage systems.
• to recommend weed management strategies for field testing.

A total of 593 fields of corn, soybeans and winter wheat in southwestern Ontario were surveyed in 1988 and 1989. The three tillage systems surveyed were conventional, conservation (minimum) and no-till.

The same weed species were found in all tillage systems, but there was a slight variation in their frequency of occurrence and density. The weed species that occurred most frequently in all tillage systems were green foxtail, lamb's-quarters, redroot pigweed, common ragweed, quackgrass and dandelions. No-till and conservation tillage fields had higher overall weed densities than conventional fields. Quackgrass and dandelions decreased in frequency and density with time in no-till or conservation tillage.

Lamb's-quarters, redroot pigweed and velvetleaf were at similar growth stages in each tillage system. Green foxtail and common ragweed emerged later in no-till than conventional. Perennials emerged at about the same time in all systems, but grew more slowly in conventional tillage than in no-till. Dandelions flowered most often in no-till fields and least often in conventional fields.

The increased use of burndown treatments and decreased use of soil incorporated herbicides were the only differences in herbicide use between tillage systems. Autumn burial generally decreased the survival of seeds of perennial species but not of seeds of annual species. Roots of perennial weeds had a lower survival rate when brought to the surface in the fall versus fall burial.

Laboratory work was conducted to simulate depth of seed germination and mechanical methods of weed control. Increased seed depth decreased annual seed germination, however increased the survival of some perennial weeds. All mechanical damage treatments reduced survival of annuals compared to undisturbed plants. Reduction of survival rates was more pronounced with shallow cutting or inversion than with deep cutting. For perennials shallow cutting or inversion treatments were most effective 3 to 4 weeks after planting. This seemed to be related to depletion of root reserves used for top growth that weakened the plants. Simulated inter-row cultivation was an effective control method for annual weeds but had only a short term effect on perennials. In general weed species were not substantially different in different tillage systems and, therefore weed control should not be any more difficult in reduced tillage systems.

Comments:

The survey showed only slight differences between tillage systems. In other field studies shifts toward more perennials and fewer annual weeds have been observed in no-till. Many experienced farmers report that weed control is not any more difficult and in fact may be easier than conventional systems. The study did agree with that observation. The lab studies could use some field data to back up the results.

Associated SWEEP/LSP Research:

• SWEEP Report #19A - Weeds of Corn, Soybean and Winter Wheat Fields Under Conventional, Conservation and No-Till Management Systems in 1988 and 1989
• SWEEP Report #32 - Optimal Herbicide Use in Conservation Tillage Systems
• SWEEP Report #48 - The Feasibility of Band Spray Application in Conjunction with Inter-Row Cultivation in No-Till Corn

Future Research: ( ) indicates reviewers suggestion for priority, A - high, C - low.

(C) Conduct field studies of the lab work.

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Created: 05-28-1996
Last Revised: Thursday, May 19, 2011 02:01:00 PM