ABSTRACT
Weeds are not benign in sugarcane cropping systems and become important biotic constraints on production. They reduce sugarcane yield by competing for water, nutrients and light, thereby potentially causing considerable economic loss. Although sugarcane is neither a short stature nor a short-season row crop, its wide row spacing,1,2 and slow emergence and growth rate early in the season allow for an upsurge in weed pressure,3 making the crop vulnerable to weed competition before canopy closure shades and suppresses weed growth.4-8 Competitiveness of weeds with sugarcane is attributed to their ability to grow and develop rapidly, shade emerging and developing shoots and reduce the formation of tillers.6 Early in the season in resource-rich sugarcane production systems with ideal temperatures, high precipitation, full or supplementary irrigation3,9,10 and high inorganic fertilizer inputs,11-16 weed infestation and competition is accentuated by rapid weed growth as compared to sugarcane which utilizes much of its carbohydrate reserves for the production of shoot-roots which are used for the absorption of nutrients and water.3 In these resource-rich environments, weed competition is also likely for light because more individuals develop leaves that reduce light available for the crop.17,18
Competition from numerous weed species with sugarcane can result in significant yield losses. Total crop failure can occur when no adequate and timely control measures are taken. Perennial grass species are the most competitive and difficult to selectively control in sugarcane because of similarities between the species. Sugarcane yield losses of up to 91% caused by competition from individual weed species or mixed weed populations under field conditions have been reported in different sugarcane production regions.4,19-32 The degree of sugarcane yield loss from weed competition depends on the weed species, weed density, weed distribution, duration of weed-sugarcane competition, sugarcane cultivar, climatic condition, agronomic practices and the life history stage of the weedsugarcane system at which the interaction occurs.6,19,25,33-39
Weeds infesting sugarcane also serve as alternative hosts for disease (Table 1) and insect pests (Table 2), increase production cost and lower land value. Combination of weeds and other pests can also favour occurrence of other pests. Showler et al.40 showed that weed and disease stress factors favoured higher sugarcane nematode populations. Weeds can also create physical hindrance of sugarcane growth and interfere with harvesting operations.27,41,42 For example, vines of Ipomea species climb and wrap sugarcane plants and increase the need for frequent removal from mechanical harvesters, thus interfering with harvesting.26 Weedy fields can also harbour rats that feed on sugarcane stalks by providing them with seeds that become an important source of protein required for their reproduction.43-45 The diet of the canefield rat (Rattus sordidus) during breeding in Australia predominantly consists of summer grasses in sugarcane fields followed by a switch to a diet of predominantly sugarcane at the end of the breeding season.46 Furthermore, weeds that mature and produce seed become sources of seed bank replenishment and reinfestation in subsequent years.
