Progress on Rottboellia cochinchinensis management - Bernal E. Valverde

INTRODUCTION

Itchgrass (Rottboellia cochinchinensis [Lour.] Clayton) is an erect, strongly tufted, C⁴, annual grass, characterized as a vigorous competitor and for being able to reach a height of up to 4 m (Holm et al. 1977, Figure 1). Its common names in English and other languages relate to the silicaceous, fragile, irritating hairs covering the leaf sheaths that break off on contact with the skin. It is native to the Old World (Afro-Asian) and probably was introduced to the New World at the beginning of the twentieth century. Here, in its exotic range, infestations are considered to be the most severe, perhaps because of several contributing factors, including improved climatic compatibility, human intervention in disseminating the grass, favourable agronomic practices, and the absence of co-evolved natural enemies (Ellison and Evans, 1992). In addition to Neotropical areas, where it is an important weed in several crops including maize, sugar cane, upland and rain-fed rice, beans, sorghum and some perennials, such as citrus and oil palm at early growth stages, it has been reported as a weed of many crops in several countries (Holm et al. 1977). Itchgrass is reported as a weed mostly from latitude 23°N to 23°S, within the 20°C isotherm (Reeder, 2000, personal communication) but is also has the ability to grow, flower and set seed under some of the temperate regimes found in the United States where it can reach 75- 100 percent of its growth potential (Patterson et al. 1979). Just in Central America and the Caribbean it is estimated that itchgrass affects more than 3.5 million hectares (FAO, 1992). It is also considered an important weed is West Africa (Chikoye et al. 2000). This chapter covers important concepts and tactics for the integrated management of itchgrass with emphasis on annual crops in the Neotropics.

ECONOMIC IMPORTANCE

A previous review (Labrada, 1994) stated the economic importance of itchgrass. Itchgrass infestations can result in up to 80 percent crop loss, or even abandonment of agricultural lands (Holm et al. 1977; Table 1). Poor resource farmers in tropical areas devote substantial amounts of time and inputs to control itchgrass in subsistence crops. Farmers in the seasonally dry areas of the Pacific region in Costa Rica, for example, use an estimated 34 percent of total inputs solely on itchgrass control, which is mainly done by a combination of manual (slashing) and chemical methods, particularly the application of paraquat (Calvo et al. 1996). Control costs in maize might represent up to 26 percent of the income obtained from selling the grain (Valverde et al. 1999b). Where present, farmers usually regard itchgrass as a troublesome weed. In Costa Rica farmers acknowledge its rapid growth and yield-reducing effects as the most detrimental characteristics of itchgrass and recognise the large amount of seed it produces (Calvo et al. 1996; Valverde et al. 1999b).

In slash-and-burn agriculture in South America, itchgrass invades fields cleared from forest and, especially, from fallow. Under such conditions farmers consider it one of the most undesirable weeds (Fujisaka et al. 2000). In more input-intensive crops, such as sugarcane, itchgrass is a major weed and has been widely reported in several countries including Brazil (Arevalo and Bertoncini, 1994), Costa Rica (Vargas-Acosta, 1993), Cuba (Maldonado, 2000), Guatemala (Jiménez et al. 1990), Malaysia (Anwar 2001), Mexico (Valverde et al. 2001), Trinidad (Bridgemohan and Brathwaite, 1989) and Venezuela (Valle et al. 2000).

Table 1. Effect of uncontrolled itchgrass populations on maize and sugarcane yields.

BIOECOLOGICAL CHARACTERIZATION

Itchgrass reproduces solely by seeds that are disseminated by water, farm machinery, and birds. Over long distances the main form of dissemination has been as a crop seed contaminant. Itchgrass seed has been found in rice seed lots received at the International Rice Research Institute in the Philippines (Huelma et al. 1996). It is speculated that itchgrass was introduced into the US in the early 1900s, where recent movement into new areas has been documented along railroad tracks (Hall and Patterson 1992). Similarly, there are indications of such type of dissemination in rice seed movements from Colombia to Brazil in 1961 (Millhollon and Burner, 1993). In Campeche and other areas in Mexico farmers identified contaminated rice seed as being responsible for the introduction of itchgrass to their fields (Valverde et al. 1999, 2001). Movement of livestock and farm machinery is blamed for the introduction of itchgrass from Thailand to Malaysia (Anwar, 2001).

Reports of the number of seeds produced per plant vary among locations. In Costa Rica, we estimated itchgrass seed production at about 10 000 seeds m^-2 with a single itchgrass plant growing in isolation producing between 570 and 730 seeds (Smith et al. 2001). Similarly, Tucuch, (1991), estimated seed production in 6 500 m^-2 under maize-field conditions in Campeche, Mexico. Hall and Patterson (1992) reported that a single itchgrass plant may produce between 2 200 and 16 500 seeds. The contribution of seed to the soil seed bank is much lower than it could be anticipated from the amount produced per plant. A large proportion of seed is lost before the next cropping season. In a heavily infested field in Costa Rica, seeds lying on the soil surface were counted at the beginning of the cropping season, giving an estimate of 324 seeds m^-2 (Smith et al. 2001). Seed dormancy and germination habits vary substantially across the world (Holm et al. 1977). Bridgemohan et al. (1991) determined that in cultivated maize soils 40-60 percent of the itchgrass seed persisted in the soil after one year of burial, with innate and enforced dormancy contributing 8.5 percent and 35 percent, respectively, to the mode of persistence. In Costa Rica, Rojas et al. (1994) showed that little viable seed remained after 18 months in the soil, underlining the importance of prevention of seed set in the weed’s management. Seed on the surface and buried at 5 and 10 cm substantially lost its persistence; at 20-cm deep less than 10 percent of seeds remained viable. Bridgemohan et al. (1991) also showed that tillage increases the depletion rate of the weed seed reserve, in their case by 32 percent per year.

Seedlings emerge intermittently in the field, especially after soil disturbance. Based on our own observations, density-dependent mortality is of practical importance when itchgrass density is equal or more than 40 seedlings m^-2. Maximum aboveground biomass was estimated at about 600 g m^-2. Under such saturation conditions, biomass per plant reached about 15 g. In Costa Rica, Rojas et al. (1993b) determined that the critical period of interference of itchgrass on maize was between 45-60 days from planting at itchgrass densities of between 66-74 plants m^-2. When itchgrass was allowed to compete unrestricted with the crop it reduced maize yields between 46-54 percent. Likewise, in Trinidad, Bridgemohan et al. (1992) determined that the critical period of interference was from 0-63 days after emergence at 55 itchgrass plants m^-2 with yield reductions of about 50 percent in unweeded plots. In more temperate areas itchgrass is also very competitive. In Louisiana (United States) itchgrass reduced maize grain yield an average of 125 kg ha^-1 for each week of interference with the crop to a total of 38 percent when it was allowed to interfere with maize for the entire season (Strahan et al. 2000b). Similarly, Rahman and Price, (2001) determined that itchgrass, even at very low densities is highly competitive with sugarcane and that crop yield loss was very closely related to itchgrass density. Under their experimental conditions in Sudan, estimated yield losses caused by itchgrass were 64 and 43 percent in plant and ratoon cane respectively. Itchgrass is very competitive for light (Bridgemohan and McDavid, 1993) and, additionally, it has been suggested to be allelopathic to crops, including to maize (Bridgemohan et al. 1992, Bridgemohan and McDavid, 1993) and rice, (Casini et al. 1998).

Itchgrass evolves distinct biotypes and several have been described within individual countries. At least five biotypes were reported in the Philippines (Pamplona and Mercado 1981) and several in Brazil (Alves et al. 2001). Millhollon and Burner (1993) divided biotypes gathered from 34 countries into five broad groups based primarily on the effect of day length on flowering, but also on general morphology and pattern of growth. Biotypes can also be distinguished by isozyme analyses, particularly esterases (Fisher et al. 1987). In Costa Rica, biotype differentiation also has been documented (Rojas et al. 1992, 1993c) according to plant morphology (height, tillering, pubescence) and length of the vegetative cycle under comparable conditions. A recent characterization of 38 itchgrass biotypes from 20 countries or territories by amplified fragment length polymorphism (AFLP) indicated an extremely narrow genetic base, there being more than 80 percent similarity among all of the biotypes, possibly as a result of the predominantly inbreeding nature of the weed and to the relatively recent expansion of its geographic range. However, AFLP allowed identifying five major biotype groups that were closely related to their geographical distribution. Two of these major groups were comprised of biotypes predominantly collected in Latin America, which suggests that the majority of Latin American biotypes might have arisen from a limited number of introductions, probably from Africa and some from Asia (R.H. Reeder, 2000, personal communication).

ITCHGRASS MANAGEMENT

Successful management of itchgrass depends on the depletion of its soil seed bank and preventing production of seed (Bridgemohan and Brathwaite 1989; Valverde et al. 1999b). No single control tactic is able to achieve this goal, thus a truly integrated strategy is required to decrease itchgrass populations steadily. Available and promising tactics include mechanical, cultural, chemical and biological options. Eradication as a strategy does not seem entirely feasible even when a new, very localised infestation at the farm or country level is detected early. For example, eradication efforts were initiated at Wales Estate, one of the major sugarcane- producing units in Guyana, soon after itchgrass was first identified in 1991. Areas affected were designated as restricted. All agriculture-related movement required previous approval, including that of workers and equipment, and the use of planting material from infested sites was banned. Several tactics were implemented to eliminate the weed: roughing, herbicide application, legume cover cropping or flooding during fallow periods (under their production system, flooded fields remain under fresh water for 6-12 months after tillage but before planting). Although the eradication programme did not eliminate the weed after five years, it reduced the density of the infested sites and limited new infestations (Bishundial et al. 1997).

Mechanical control

Shallow tillage can be used to promote itchgrass germination prior to planting. Emerging seedlings could then be controlled by additional mechanical means or with herbicides. Failure to control itchgrass seedlings after soil preparation, however, may result in extremely high densities that would substantially reduce crop yields. In small-scale farming interrow slashing or cultivation are frequently used but the practice results ineffective as seedlings growing within the crop rows escape control and will then add to the seed bank and reduce crop yield (Bridgemohan and Brathwaite, 1989). In-crop cultivation may damage crop roots, bringing seed to the surface where it easily germinates and increases the risk of erosion (Maillet, 1991; Bridgemohan and Brathwaite, 1989).

Cultural control

Since itchgrass is easily dispersed with crop seed, an important tool for preventing its introduction to new fields and spread is the use of certified crop seed.

Several agronomic practices also can help in decreasing itchgrass densities and depleting the soil seed bank once the weed is established. Crop rotation could help in disrupting the close association between itchgrass and some crops (such as maize and sugar cane) by allowing the use of alternative control tactics such as other herbicides (selective graminicides) and flooding. Maize monoculture facilitates the rapid establishment of itchgrass as a dominant weed (Fisher et al. 1985).

One of the most successful and researched tactics to smother itchgrass plants is the use of cover crops. Cover crops have been developed in Mesoamerica, Africa and Asia to improve soil characteristics and control weeds (Buckles and Triomphe, 1999, Carangal et al. 1994, Tarawali and Ogunbile, 1995). They can be used either as part of a rotation scheme or as intercrops. Preference has been given to nitrogen-fixing legumes, including Cajanus cajan, Calopogonium mucunoides, Canavalia spp., Crotalaria spp., Dolichos lablab, Pueraria phaseoloides, Mucuna spp. (velvetbean), and Stylosanthes spp.

Selection of the cover crop species should take into account local conditions and cropping systems as well as farmers’ needs, including consideration of negative competitive effects on the crop, additional problems related to pest control, management costs, and the positive value of the cover crop as a food supplement and in the prevention of soil erosion (Kirchhof and Salako, 2000). Legume covers also increase soil organic carbon and phosphorus levels and improve the cation exchange capacity and Ca contents (Obi, 1999). Some of these cover crops also are allelopathic. Allelochemicals present in velvetbean are inhibitory of weed growth (Anaya, 1999, Caamal-Maldonado et al. 2001, Fujii et al. 1991).

Thirteen legume species were originally screened for their adaptation and usefulness for itchgrass suppression in the Guanacaste region in Costa Rica (De la Cruz et al. 1994). The best cover crops were Mucuna deeringiana, P. phaseoloides, Canavalia ensiformis, Vigna unguiculata and D. lablab. Of these, velvetbean was the most suppressive of itchgrass. Velvetbean (Figure 2), C. ensiformis and V. unguiculata were further evaluated as cover crops. Itchgrass density was reduced about 60 percent in the presence of either velvetbean or C. ensiformis and by 55 percent with V. unguiculata compared with the unweeded control, 90 days after planting (DAP). Itchgrass substantially reduced maize yields, which were almost ten times higher in the presence of the suppressive legumes. Itchgrass suppression and ground cover was better when the cover crops were planted simultaneously with maize or a week later, compared with two weeks after maize planting. Velvetbean seemed more suitable for growers’ adoption since it is an annual species, easier to manage and with a better growth habit (Valverde et al. 1999b).

Velvetbean intercropped with maize at either 50 000 or 80 000 plants ha^-1 reduced itchgrass biomass at maize harvest between 75-95 percent (Valverde et al. 1995). Conversely, itchgrass density did not affect velvetbean biomass nor were differences found between the two velvetbean densities. But both velvetbean (planted one week after maize) and itchgrass reduced grain yield up to 40 percent. These results prompted additional research to better define planting dates and densities for the cover crop in order to minimize negative effects on crop yield.

The interaction between velvetbean, maize and itchgrass was further studied during two cropping seasons. Two velvetbean varieties (variegate and grey seeded) similarly suppressed the natural itchgrass infestation at or after 60 DAP. By the end of the critical period of competition (45 DAP), velvetbean suppressed itchgrass biomass from 60-80 percent. In the first year the improved maize variety (Diamantes) yielded more than the local (Criollo) one and velvetbean did not decrease maize yield. But in the second year velvetbean slightly reduced maize yield and the criollo variety was more competitive with itchgrass and yielded about 70 percent more grain than Diamantes. The local variety has a shorter cycle and probably escaped the negative impact of severe water stress present late in the second cropping season (Valverde et al. 1999b).

Repeated experiments studied the impact of velvetbean density (25 000 or 50 000 plants ha^-1) and planting time (0, 5, 10 or 15 DAP) on itchgrass and maize (cv Diamantes). Velvetbean was more effective in reducing itchgrass density at the higher density. Better soil cover by velvetbean was obtained when it was sown simultaneously with maize than when planted later in relation to the crop, probably because of the competition imposed by maize on the cover crop. At 45 DAP, velvetbean (planted at 50 000 plants ha^-1 simultaneously with the crop) reduced itchgrass density to 23 and 46 percent of that recorded in the unweeded controls. Concomitantly, itchgrass biomass decreased between 10-15 percent when velvetbean density increased from 25 000-50 000 plants ha^-1. Lower maize grain yields were obtained when maize was grown in association with velvetbean at its highest density and, especially, when the cover crop was planted simultaneously with maize. Itchgrass itself decreased maize grain yield by about 46 percent (Valverde et al. 1999b).

The taxonomy of velvetbean is still confusing. Some previously designated species are now considered as varieties of M. pruriens, a notion recently supported by genetic characterization using molecular markers (Capo-chichi et al. 2001). Another type of velvetbean (M. cochinchinensis, also designated as M. pruriens var. cochinchinensis) has been shown to be suitable for control of speargrass (Imperata cylindrica) in maize and cassava in West Africa, although similar concerns regarding its effect on crop yields have been risen (Chikoye et al. 2002). Based on their growth characteristics, Akanvou et al. (2001) indicated that the woody erect-growing legumes, Crotalaria juncea and C. cajan and the creeping, M. pruriens are more competitive than C. mucunoides (also creeping) and the semi-erect shrubby-type species, Stylosanthes hamata and Aeschynomene histrix. M. pruriens and C. mucunoides accumulated 77 and 63 kg of N ha^-1, respectively and were similar in total dry biomass production (3.5-4.0 T ha^-1). For nitrogen to become available for crop growth, the cover crops first must undergo decomposition.

Velvetbean fallows also have been widely adopted in parts of West Africa, mostly because of their ability to suppress speargrass (Tarawali et al. 1999). Under fallow conditions in northern Nigeria, N content of standing vegetation (litter not included) at 16 weeks after planting was 228, 143, and 241 kg ha^-1 for M. cochinchinensis, Lablab purpureus and Crotolaria ochroleuca, respectively (Carsky et al. 2001). Grain yield increases as a result of improved nitrogen uptake can be observed when maize is planted following legume cover crops (Tian et al. 2000). Rice yields in plots preceded by a legume fallow were about 30 percent higher that those obtained in plots preceded by a natural weedy fallow (Becker and Johnson, 1998).

A cropping system based on velvetbean as a cover crop is well established in some areas in Mesoamerica, particularly in Honduras, where farmers have widely adopted this species to improve soil conditions, water conservation, weed suppression and maize yields for over 25 years. Interestingly, in Honduras, the velvetbean system has been threatened by the introduction of itchgrass in the mid- to late-80s, increasing weed control costs and reducing maize yields (Buckles and Triomphe, 1999).

There are also other types of interactions between cover crops and itchgrass. In southern Nigeria, Centrosema pubescens is one of the most popular forage legumes. However, its performance is diminished by a leaf blight caused by Rhizoctonia solani. It was determined that itchgrass, which grows within the Centrosema cover, may serve as a reservoir for the pathogen (Oben et al. 1997). The weed has also been implicated as a host of R. solani AG-1, which causes Rhizoctonia foliar blight of soybean (Black et al. 1996).

Biological control

Because itchgrass thrives in exotic ranges, a very promising and complementary management alternative is classical biological control. Of several itchgrass pathogens screened as possible biocontrol agents, a head smut, Sporisorium ophiuri (P. Henn) Vanky (Ustilaginales), has been thoroughly studied (Ellison, 1987, 1993; Reeder et al. 1996). The smut is a soil-borne, systemic pathogen, infecting itchgrass seedlings before they emerge from the soil. S. ophiuri has been recorded as a head smut of itchgrass in Africa and Asia and is restricted to the Old World (Reeder and Ellison, 1999). Infected plants grown individually in pots grew similarly to healthy plants but when gown under competitive conditions (8 plants per pot), smut-infected plants produced less tillers than healthy plants (Reeder et al. 1996). Experimentally, infected plants produced substantially less seed than healthy plants. In the endemic range of the weed, natural epiphytotics of the smut are common, often with a high percentage of plants infected within a population. Isolates of the smut were found to be itchgrass-biotype specific but one from Madagascar was found to infect a wide range of biotypes including a number from Latin America, and hence selected for a comprehensive host range screening. The smut was found to be extremely host specific; none of 49 species/varieties of graminaceous test plants other than itchgrass became infected. Screened species included pastures, weedy grasses, graminaceous crops (rice, sugar cane, maize, sorghum) and teosinte (Zea (Euchlaena) mexicana), the maize ancestor (Reeder and Ellison 1999, Valverde et al. 1999b).

Leaf rust, Puccinia rottboelliae P. and H. Sydow (Uredinales), also was observed to cause severe damage to itchgrass in the field, particularly to seedlings, and could complement the effect of the smut fungus by reducing the competitive ability of the weed within a cropping system (Reeder et al. unpublished). Unfortunately, none of the rust strains screened proved sufficiently virulent towards any of the South American biotypes that were challenged. Therefore, further host range screening was suspended.

Itchgrass is also highly susceptible to Exserohilum monoceras, a fungus being considered for Echinochloa colona biocontrol (Zhang and Watson, 1997). There is also potential to use native pathogens as a complement to other control methods for itchgrass (Zúñiga et al. 2000). Jiménez et al. (1990), described a spike rot disease on itchgrass in Guatemala that was caused by Fusarium moniliforme. Infected plants failed to produce viable seed and, in preliminary tests, the pathogen showed some specificity towards itchgrass. In Costa Rica, pathogenic strains of Curvularia sp., Drechslera sp. and Fusarium sp. have been shown infective to itchgrass, their severity being enhanced by stress factors, including the application of sub-lethal herbicide doses (Zúñiga et al. 2001).

Chemical control

Labrada (1994), compiled a list of conventional herbicides for itchgrass control. Typically, selective itchgrass chemical control has been achieved with some triazines (e.g. dimetamethrin), dinitroanilines (e.g. pendimethalin) and acid amides (e. g. diphenamid). Pendimethalin has proved very effective against itchgrass and can be easily included as a tactic for the integrated management of this weed in maize (Valverde et al. 1999b). In Trinidad, pendimethalin (1.5 kg ha^-1) and interrow cultivation at 14 and at 28 days after planting effectively controlled itchgrass in maize during the critical period of interference (Bridgemohan and Brathwaite, 1989). More recently, selective systemic graminicides that inhibit acetyl-CoA carboxylase (aryloxy-phenoxy propanoates and cyclohexanediones) have also been used to eliminate itchgrass. The weed, however, has evolved resistance, in one instance, to fluazifop-p-butyl in soybeans in Louisiana, United States (Heap, 2002). Total herbicides, especially paraquat and glyphosate are also widely used to control itchgrass

Sulfonylurea herbicides that inhibit the enzyme acetolactate synthase (ALS) are now commercially used to selectively control itchgrass. One of the most widely used compounds for this purpose is nicosulfuron. For example, it is commonly used in Campeche, Mexico, where itchgrass, along with Sorghum halepense, are particularly problematic in maize production (Valverde et al. 2001). In Louisiana, nicosulfuron (35 g a.i. ha^-1) controlled itchgrass in maize better than primisulfuron (39 g a.i. ha^-1) when applied at the six-leaf growth stage (Strahan et al. 2000a). When formulated as a dispersible granule, nicosulfuron requires the addition of a non-ionic surfactant for its activity. The new post-emergence sulfonylurea herbicide for cotton and sugar cane, trifloxysulfuron-sodium (CGA 362622), controls itchgrass in sugar cane at 15-50 g a.i. ha^-1 in mixture with ametryn (Hudetz et al. 2001).

Integrating tactics for itchgrass management

Rojas et al. (1993a), conducted a four-year field study on the effects of integration of control tactics on itchgrass populations. The experiment was located in the seasonally arid zone of north-west Costa Rica, where typically there are three cropping seasons per year: maize, maize or beans, and a fallow dry season. The study addressed itchgrass management in a maize-beans-fallow rotation beginning in 1991. Tactics evaluated were fallow management: hand-weeding, paraquat application (0.5 kg ha^-1) and no weeding; tillage practices: zero tillage and conventional tillage (one pass of a disk plough to 20 cm depth plus two passes of a disk harrow) and in-crop chemical control: 1.0 kg ha^-1 pendimethalin plus 2.4 kg ha^-1 alachlor (H1), 1.25 kg ha^-1 pendimethalin (H2), 1.5 kg ha^-1 pendimethalin (H3), and no control (H4). Herbicides were applied pre-emergence following planting in both crops. Fallow management practices were initiated during the dry season of September 1991, prior to the maize planting in May 1992. Subsequently, maize was planted each year at the beginning of the rainy season in May and beans were planted immediately after the maize harvest in September. Adverse weather conditions resulted in the bean crop being lost every year. Average itchgrass population on the trial site in September 1991 (before implementation of the treatments) was 58 plants m^-2.

Itchgrass density was substantially higher in plots without control in the fallow period but use of in-crop herbicides decreased the weed populations to similar levels, regardless of fallow management. Lower itchgrass populations also were observed in plots with zero tillage compared with conventional tillage. In-crop control with herbicides had the largest effect on itchgrass populations during the crop cycle and this was greater than the effect of either tillage or fallow management. The lowest itchgrass population was observed in plots with the higher dose of pendimethalin. Maize yields were always lower in plots with no fallow and in-crop itchgrass control. When the weed was controlled chemically early in the cropping season, yields were moderately higher in plots with fallow management. However, there was no evidence of maize-yield improvement in plots with zero tillage compared with conventional tillage (Rojas et al. 1993a, Valverde et al. 1999b).

Integrated tactics to control itchgrass were also evaluated for three years in on-farm validation plots (about 1000 m² each) in small, subsistence growers’ fields at three locations in Costa Rica (Valverde et al. 1999a, 1999b). Maize is grown twice per year at two of the sites. In the third location, the cropping system is based on a maize-dry beans-fallow rotation. Validation plots integrated no-tillage, use of the selective herbicide pendimethalin in the first maize crop (to lower the initial density of itchgrass), planting of velvetbean between maize rows, and prevention of itchgrass seed set in the fallow period. In growers’ plots itchgrass control was based on a combination of slashing and direct applications of paraquat. Pendimethalin effectively controlled itchgrass and allowed the establishment of velvetbean during the first maize crop. At all sites, itchgrass densities were lower in validation plots than in growers’ fields and infestation levels decreased throughout the years with integrated management. In general, maize and dry beans yields were higher in validation plots at all locations and cropping seasons. Soil core samples also revealed substantial reductions in the itchgrass soil seed bank in validation plots (Merayo et al. unpublished results). Partial budget analyses demonstrated that integrated itchgrass management also is economically feasible for smallholders.

Biocontrol of itchgrass with the head smut could be incorporated as part of an integrated strategy. The dynamics of the itchgrass-head smut system was explored within a modelling approach (Smith et al. 1997). This work suggested that a very high annual infection rate (above 85 percent) would be required for the smut to be effective as the sole agent of control. Further refinement of the model and additional simulations suggested that the smut, in combination with a cover crop, could be highly effective. A low-density cover crop, plus 50 percent smut infection rate resulted in 6 plants m^-2 in each crop. Simulations of a high-density crop plus smut predicted a reduction of itchgrass density to 0.1 plant m^-2 (Smith et al. 2001).

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