Executive Summary
What the report is about
Weeds are widely reported in the research literature as a key constraint in organic production and anecdotal accounts confirm this.

Who is the report targeted at?
Organic growers seeking to find the most cost effective method of treatment for weeds. Weed control costs as a percentage of gross returns have been found to be an important factor affecting profitability.

Thus the examination of alternative methods of weed control is important for organic crop growers.

Background
In the past decade, there has been a surge of interest in organic agriculture and organic produce. As new growers enter the industry, the demand for information about organic production methods is increasing. The costs of weed management in organics vary depending on the farming system, with estimates ranging from 30-50% of expenses in intensive horticulture, to 10-30% in broadacre production. Organic farmers must rely on an integrated, multi-strand approach to controlling weeds based on a wide range of options.

Aims/Objectives
The organic weed management (OWM) survey was conducted to provide information about the weed control methods used by growers, attitudes towards weeds, basic characteristics of the farms and farmers surveyed, and the relationships between parameters. The organic weed management survey was prompted by the numerous anecdotal and research reports that weeds were a key constraint in organic crop production and the general lack of information about OWM practices. It provides a quantitative analysis of current weed management practices used by organic herb and vegetable growers in Australia and also gives an insight into some of the factors that influence those practices.

Methods used
It was considered that the existing knowledge base amongst Australian organic herb and vegetable growers could be accessed through a mail survey. This survey was initially conducted to examine current attitudes and approaches to weed management. Field experiments were then conducted to assess the relative performance of several commonly used weed control treatments in terms of weed growth, crop growth and treatment cost for two crops with differing growth patterns – the short season salad vegetable, lettuce, and the longer growing medicinal herb, echinacea.

Results/Key findings
A moderate response rate was achieved (43%), with the majority of respondents coming from within the sampling frame, i.e. organically certified producers of herbs and vegetables. The reported farm sizes were small (median = 2.4 hectares) and the respondents had relatively little experience with organic farming (median = 6 years). Producers grew a wide range of herb and vegetable crops (median = 3) and reported numerous types of weeds on their farms. The most common weeds were predominantly those with persistent underground parts that resist common forms of organic weed control such as cultivation (by hand or plough) and mulch. Heavily seeding annuals were also frequently reported. Respondents expressed strong concern about weed management, particularly regarding the difficult and time consuming nature of OWM and the impact of weed competition in reducing crop yields.

The respondents reported using a diverse range of weed management techniques, with over 40 specific methods or strategies being mentioned. The most common method was manual weeding, which was sometimes used as a central management technique or as a final “clean up” after relying on other methods such as tillage or mulch for the bulk of the weed control. Other commonly reported methods were organic mulches; tillage (especially rotary hoes); cultural methods such as rotations, bed preparation, timing of operations, cover crops and inter-cropping; slashing and/or mowing; and grazing. Less frequently reported OWM methods included thermal methods, synthetic mulches, organic sprays and farm hygiene practices.

The results for perceived success of weeding methods used were loosely correlated with the regularity of use. Therefore, manual weeding, organic mulches and tillage were reported to be the most successful weeding methods. No relationship was found between the perceived expense of a weeding method and the regularity of use of that method. This suggests that growers were not primarily motivated by the cost of a weeding method; instead, they were more concerned that it was successful in controlling weeds. Synthetic mulches and thermal weeding were considered to be the most expensive weed control methods.

Results from the OWM survey and the US-based Organic Farming Research Foundation (OFRF) survey (Walz 1999) highlight a number of possible research issues of concern to organic growers.

Improved control of particularly problematic weeds, such as couch, sorrel and nutgrass, was a common area of concern. Studies on the biology and ecology of such weeds may identify lifecycle vulnerabilities that can be exploited by modified management practices. There also appeared to be some interest from respondents in novel techniques such as flame weeding, steam weeding, improved tillage implements and organic sprays, although these methods require further development too.

Cultural strategies such as techniques for weed seed bank reduction and the development of weed suppressing cover crops that don't become weeds themselves were also of interest to respondents.

For lettuce, a crop with a short growing season, cheaper weed control methods such as tillage with limited follow-up hand weeding may be sufficient to ensure a reasonable crop yield. The acceptable economic return of the control treatment suggested that good weed control in the cropping area prior to planting may even be adequate. Hay mulch provided good yields but was less cost-effective due to the high labour requirement for mulch laying and follow-up hand weeding. However, this treatment greatly reduced bolting in the lettuces and may give more flexibility in terms of harvesting than tillage or hand weeding. Paper mulch stunted the growth of lettuce, presumably due to nitrogen immobilisation, although it also provided excellent weed control. It could not be recommended without reformulation with a nitrogenous fertiliser or some other method of improving available soil nutrient levels.

The results from the echinacea trials suggest that more expensive weeding methods are cost-effective for longer-season and higher-value crops. Hand weeding and hay mulch both provided cost-effective weed management. The differing effects of those two treatments on soil structure, carbon and moisture conservation were not evaluated, although these factors may be considered important by growers. The paper mulch again reduced crop yields and, combined with high purchase cost, appeared to be unreliable. The moderate adjusted crop value (ACV) (derived by subtracting the cost of each treatment at each site from the gross crop values) for paper mulch at Yarrowitch suggested that the treatment may have some potential as a weed control method in certain circumstances.

The economic analysis of the treatments is specific to the time and location of the trials, and outcomes may vary depending on the availability of labour, machinery and implements, and suitable mulch materials. However, similar trends have been reported in other organic and conventional trials.

For both crops the ranking of the weed control treatments for their weed biomass accumulation at crop harvest was hand weeding = paper mulch followed by hay mulch, tillage and then the control treatment. Weed control levels ranged from 98% through to approximately 60% for the tillage treatments.

 Implications for relevant stakeholders

It was found that manual weeding, organic mulches and tillage were reported to be the most successful weeding methods. This finding should prove useful for organic farmers. The lack of relationship between the perceived expenses of the weeding method and the regularity of its use is significant for producers of products designed for weed control. Growers were far more concerned with the effectiveness of the weeding method.

Recommendations
Given the failure of the questions about time and cost of weed management to elicit reliable data in this survey, and the general importance of economic factors in broader decision-making, another potential research area may be an economic analysis of OWM practices. A more focused study of the economic impact of weeds and the cost and efficiency of various key weed management methods may generate empirical data about the relative cost of weed management in organic production.

Such information is not currently available in Australia. The dominant weed control methods reported in the survey - manual weeding, organic mulches, cover crops and tillage - provided the focus for the experimental work reported in this project.

A range of problem weeds are still poorly controlled in such production systems; these include couch, sorrel and nutgrass. An examination of the biology and ecology of these weeds should identify lifecycle vulnerability's which can be exploited for their control. There also appeared to be some interest from the survey respondents in novel weeding techniques such as flame weeding, steam weeding, improved tillage implements and organic sprays; these systems were not examined in the current study but they are worthy of further study. Cultural strategies such as techniques for weed seed bank reduction and the development of weed suppressing cover crops (which do not become weeds) should also be assessed.

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