Weed control in tropical ley farming systems: a review

Abstract

The first sustained effort to develop a ley farming system (a pasture legume rotated with a grain crop) for the Australian semi-arid tropics began in the late 1970s at Katherine, Northern Territory, where various strategies were identified and implemented. It was soon discovered that a main constraint to success was the invasion of the legume ley by grass weeds. This occurred despite the replacement of Townsville stylo (Stylosanthes humilis), which provided the base for the early work, with more competitive legumes such as Verano (Stylosanthes hamata) and Centurion centro (Centrosema pascuorum). Early weed control work focused on the use of chemicals, but later competition and population dynamics were studied in S. humilis pastures. The ley farming system comprised a number of essential elements, each of which offered opportunities for weed control First, a legume ley was rotated with grain crops. Past work concentrated on legume leys, but nitrogen (N)-fertilised grass leys may be successful if the N was economically supplied. Grasses can effectively suppress weeds. Rotation of herbicides is possible. Sicklepod (Senna obtusifolia) can be controlled with atrazine in the sorghum phase, and emergent Sida spp. can be controlled in the pasture phase using a herbicide roller. Second, cattle graze the legume ley and crop residues in the dry season. Cattle can be used to suppress grass weeds in the wet season. The major use of the legume ley, however, was to provide high quality food in the dry season when there was little other available food. Future research should investigate the economics of balancing the need for grass weed control with food provision in the dry season. Third, crops sown directly into a ley killed with a knockdown herbicide. Although the need for improved soil surface management was recognised early, development of reduced tillage was delayed until the availability of glyphosate. Initial work concentrated on improving plant establishment under mulch. Recent work has shown that mulches can effectively control weeds in crops without the use of herbicides; best weed control occurred when glyphosate was applied as a single application at sowing. Fourth, ley regenerated as an intercrop in the grain crop. There is no doubt that the presence of an intercrop reduces the yield of the grain crop. Future research should investigate the economics and risks associated with intercrop-induced changes in yield and herbicide use patterns. The extreme climate and sandy soils pose problems for herbicide use. There is marked seasonal variation in effectiveness of pre-emergence herbicides, ranging from zero to marked crop phytotoxicity. The effectiveness of knockdown herbicides is reduced by stressed target plants and rain soon after spraying. Mulch dynamics, grazing, competition and interference, and herbicide interactions with target plants and the environment, were identified as key features requiring attention in a ley farming system to achieve practical weed management.

Dormancy and germination characteristics of herbaceous species in the seasonally dry tropics of northern Australia

“…Apart from Themeda triandra , this mixed group of annual and perennial grasses, legumes and dicots are all good colonizers of bare areas and a number are major weeds in Australia and other tropical areas – Echinochloa colona , Eleusine indica and Sida acuta ( Holm et al 1977 ; Kleinschmidt & Johnson 1977; Mott 1980; Akobundu & Agyakwa 1987; Martin 1996). Dactyloctenium radulans is an annual grass which increases under heavy stocking ( Anderson 1993; Roberts & Silcock 1993) and can be a weed of cultivated areas and roadsides ( Tothill & Hacker 1983).…”

“…These two annual grasses are common colonizers of waste land and can be weeds in cropping areas ( van Rijn 1968; Kleinschmidt & Johnson 1977; Mott 1980; Martin 1996). This dormancy pattern results in the retention of the seed bank until the start of the wet season when most of the seed becomes germinable thus providing for mass germination events due to the high seed production of these species ( McIvor et al 1996 ).…”

Orchardgrass Ley for Improved Manure Management in Wisconsin: I. Forage Yield, Environmental Impact, and Production Costs

“…In a GL system, the land is planted to grass for several years and then followed by annual crops (Webster et al, 1999; Martin, 1996). Research has shown that grasses are very efficient in capturing soil N (Simmelsgaard, 1998), in increasing P and K uptake (Heathwaite et al, 1998; Gustafson, 1987; Unwin et al, 1986), as well as, improving soil structure and organic matter levels (Min et al, 2003; Nelson et al, 2009; van Eekeren et al, 2008; Riley et al, 2008).…”

Integrated Weed Management Practices for Adoption in the Tropics

“…It was found that intercrops may inhibit weeds by limiting resource capture by weeds or through allelopathic interactions [31], and that weed biomass was reduced in 90 % of the cases when a main crop was intercropped with a "smother" crop. It has also been reported that self-regenerating intercrops reduce establishment costs and can provide weed suppression over years [37].…”