Sustainable Weed Management in Organic Herb and Vegetable Production

Summary in the original language of the document

Weed management is a major constraint in organic production. It can be expensive and time-consuming and severe crop yield losses may be incurred when weeds are not adequately controlled. Research on organic weed management (OWM) in herb and vegetable production is increasing internationally, although very little work has been done in Australia to assess current OWM knowledge among growers, and to test the efficacy and cost effectiveness of the weed management practices used by organic growers. The project described here sought to fill these knowledge gaps by reviewing the existing industry and scientific literature, conducting a national mail survey of organic growers regarding OWM attitudes and practices, and by carrying out field and glasshouse experiments investigating a range of pre- and in-crop weed control methods.
A mail survey of 219 organic herb and vegetable producers in Australia (43% return rate) indicated that respondents were very concerned about weed control, had smaller farms and less experience than other organic growers, and were mostly troubled by perennial weeds with persistent underground parts and some heavy seeding annuals. Growers used hand weeding mostly, and mulches, tillage, rotations, cover crops and slashing were also common. With more experience, growers were less concerned about weeds and there was a shift from physical to cultural weed control methods. Respondents were not primarily motivated by the cost of a weed control method, but were more concerned with effectiveness. The survey produced new data that will be useful for planning research in Australian organic agriculture.
The field and glasshouse trials evaluated the effect of various weed control techniques on weed growth (density, relative ground cover and biomass), crop growth (size, relative cover, biomass and flowering) and cost effectiveness of the treatments. The pre-crop treatments were a bare fallow (rotary hoed), a green fallow (unweeded) and three different cover crops; Indian mustard (Brassica juncea [L.] Czern. cv. Fumus F-L71), fodder radish (Raphanus sativus L. cv. Weedcheck) and Italian ryegrass (Lolium multiflorum Lam. cv. Conquest). Hand weeding, tillage, hay mulch, pelletised paper mulch and an unweeded control treatment were used for the in-crop treatments and the test crops were lettuce (Lactuca sativa L.) and echinacea (Echinacea purpurea Moench. [L.]), selected for their contrasting growth habit, growing season length and wholesale value.
In the field, the cover crops and bare fallow controlled weeds effectively during the pre-crop phase compared with the unweeded green fallow, but did not affect weed and lettuce growth in the following in-crop phase and there was no interaction between pre- and in-crop treatments. Reducing the delay between the pre- and in-crop phases from four weeks to one day did not affect weed and lettuce growth. The brassica cover crops performed poorly when they failed to establish adequately or when sowing rates were too low. Cover crops reduced pre-crop weed levels by suppressing weed growth rather than weed emergence, while the bare fallow also lowered weed emergence. Weed control was positively correlated with reduced light transmission by the cover crops, but competition for nutrients and water was not correlated with weed suppression. In glasshouse trials, brassica cover crops grown and incorporated in pots had a positive effect on subsequent lettuce seedling growth when extra fertiliser was added during the cover crop phase, but not when no fertiliser was added. These results indicate that nutrients, rather than inhibitory phytotoxins, were determining plant growth in the species tested.
In-crop weed management in lettuce (smaller plant size, shorter season, lower value than echinacea) was cost effectively achieved using cheaper weeding methods such as tillage. The unweeded control was also cost effective, indicating that good weed control prior to planting could be sufficient to achieve reasonable yields in lettuce. More expensive methods such as hand weeding and hay mulching produced low weed levels and good yields, but were less cost effective. Paper mulch provided excellent weed control, but was very expensive and severely reduced crop yields. More bolting occurred in bare, tilled plots (hand weeding and tillage) than covered, undisturbed plots (mulches and weedy control). The increased bolting was related to higher soil temperature maxima and diurnal fluctuations in the disturbed plots.
For echinacea (larger plant size, longer season, higher value), cheaper in-crop weeding methods (e.g. tillage, unweeded control) had poor weed suppression and low crop yields, while the more expensive weeding methods, hand weeding and hay mulch, controlled weeds well and were cost effective. Paper mulch controlled weeds very well but, again, had lower yields and was therefore not cost effective.
The poor crop yields under paper mulch were investigated further and found to be caused by nutrient immobilisation, particularly nitrogen. Leaf nutrient analyses indicated that nitrogen was limiting in lettuce (though not echinacea) in the field, and mulch nutrient analyses showed that the carbon:nitrogen ratios were 39:1 and 171:1 for hay and paper mulch respectively. A pot trial showed that lettuce growth was inhibited by paper mulch but not unmulched (bare soil) or hay mulch treatments, i.e. similar results to the field trials. In the pots, the inhibition by the paper mulch was not overcome by additional nitrogen fertiliser, indicating that the extra nitrogen was also immobilised, whereas the unmulched and hay mulch treatments had positive lettuce growth responses to additional nitrogen fertiliser.
Bioassays using aqueous mulch extracts showed that paper mulch extract was only weakly inhibitory to lettuce and echinacea seedlings, while hay mulch extract was extremely inhibitory, the reverse of what would have been expected had allelopathy been responsible for the results in the field. This result suggests then that paper mulch phytotoxins were not responsible for lower crop yields. More generally, these findings highlight the limits to extrapolating bioassays results to the field.
This research has provided a glimpse of the attitudes of organic herb and vegetable growers to weeds and the practices and principles used in managing weeds in Australia, and has highlighted several important advantages and disadvantages of currently used OWM methods in the field.