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Summary Report of
by Paul Kristiansen, Brian Sindel, and Robin Jessop
July 2007
RIRDC Publication No 07/112 RIRDC Project No. UNE-65A
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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