Non-tillage systems are a means of managing the disposal of crop residues with minimum cultivation (Unger and McCalla, 1980). Synonyms for this approach include conservation tillage, direct drilling, eco-fallowing, limited tillage, minimum tillage, no-tillage, reduced tillage and stubble mulching. The process resembles the use of organic mulches. The aims are:
The impact of this form of husbandry on yields varies depending on the region in which non-tillage is practised. Where there are heavy soils, wet climates and colder regions, yields may be depressed due to the following:
Since the effects of tillage vary with soil type, so the degree of weed control will also alter.
Experiments with spring cabbage (B.oleracea var. capitata) by Knavel and Herron (1981) showed that with no-tillage culture, yield was less than that obtained with conventionally tilled crops when using the same nitrogen and spacing treatments. Yields obtained from non-tillage systems were increased by raising the density of plant populations and by applications of additional nitrogen, but head size remained smaller than with conventional systems. Large head size tended to correlate with the nitrogen and calcium content of the wrapper leaves.
It is only economical to use non-tillage systems for cabbage (B. oleracea var. capitata) where the soil is susceptible to erosion or drought. Non-tillage systems produce greater weed emergence compared with conventional tillage. This is probably due to increased weed seed mortality and seed burial resulting from the use of tillage implements. Tilling soil at three different depths significantly decreased weed emergence (Egley and Williams, 1990) compared with non-tillage. The effects varied, however, depending on which weed species was dominant within a particular field.
Fallowing land in the absence of crops allows the use of intensive non-selective weed control and can be used to eliminate tenacious plant species, particularly perennials, that are otherwise difficult to control. Land selection techniques can be linked with fallowing such that those parts of the holding infested with weeds which are difficult to control in Brassica crops are avoided. This technique is a form of crop rotation and can be used to exploit forms of husbandry or herbicidal control that totally eradicate weeds in advance of specific Brassica crops.
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