Exhaustion of storage reserves by fallow cultivation

All types of perennial weeds can be reduced in abundance by repeated tillage or cultivation. Complete eradication of severe infestations of some perennial weeds may require many operations over two or more growing seasons (Bakke et al., 1944; Timmons & Bruns, 1951; Hodgson, 1970). Such intensive tillage should be avoided for reasons of soil conservation, but these studies provide insight into the effects of tillage on perennials. Bakke et al. (1944) found that during the first year of fallow cultivation, most of the weight lost from roots of Convolvulus arvensis was due to depletion of carbohydrates; large numbers of roots did not die until the second year of fallowing.

The optimal period between successive cultivations depends on the depth of cultivation (Timmons & Bruns, 1951; Hákansson, 1969&) and season of the year. However, many perennials decline most rapidly when cultivation is repeated at two to four week intervals (Table 4.3). If the interval is longer than optimal, new growth has an opportunity to replenish perennating organs, and eradication will be delayed. Ifthe interval between cultivations is shorter than optimal, the maximum amount of stored carbohydrates will not yet have been converted to shoot growth prior to each operation, and more operations will be required for eradication. The interval between cultivations can be prolonged once the weeds begin to weaken.

Often full eradication is not required, and in many situations significant reduction in weed pressure can be accomplished with one or two extra operations during the normal fallow season. In a New Zealand study, two rotary cultivations in early spring were as effective as glyphosate for controlling Achillia millefolium in spring barley (Bourdot & Butler, 1985). In Florida, two diskings reduced Imperata cylindrica rhizome density more than any herbicide tested, although only the combination of herbicides and disking gave good control (Willard et al., 1996). In southern England, tine cultivation following barley harvest was more effective than dalapon plus aminotriazole in controlling a mixture of Agropyron (Elytrigia) repens and Agrostis gigantea (Hughes & Roebuck, 1970). Other studies have similarly shown the effectiveness of fallow season tillage for management of perennial weeds (Fail, 1956; Lym & Messersmith, 1993).

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