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Integration of Mechanical and Cultural Control Treatments to Manage Invasive Shrub Chromolaena Odorata and Other Weeds Under Drought Conditions in Pasture Area

A field experiment was conducted on Chromolaena odorata dominated pasture with the objectivesof evaluating efficacy of integrated of mechanical and cultural control on weed suppression anddetermining botanical composition of plant species after treatment started. The treatments were, T1slashing of Chromolaena every month, T2 digging up of Chromolanea, and exposed to dry (mulching),T3 digging up of Chromolaena followed by burning, T4 digging up of Chromolaena followed byburning and sowing with Centrosema pubescens and T5 digging up of Chromolaena followed byburning and planting with Brachiaria decumbens. Results of experiment showed that under droughtconditions, digging up Chromolaena was very effective in suppressing regrowth of Chromolaena but itwas not effective to other weeds. Among treatments, digging out of Chromolaena followed by burningand planting with Brachiaria decumbens was the most effective and slashing of Chromolaena everymonth was the least effective in suppressing weeds. Botanical composition was shifted with treatments.Stachytarpheta jamaicensis and Chromolaena were the dominant species in monthly slashedChromolaena plots, Stachytarpheta and Calopogonium muconoides were the dominant species inmulched plots while Mimosa pudica, Brachiaria and Centrosema were the dominant species in burntplots.


INTRODUCTION
Chromolaena odorata (L.) King and H. Robinson (hereafter is called Chromolaena), known in South Sulawesi as jonga-jonga, meaning deer weed, is a major invasive weed of pasture and plantation crops in many countries in Africa, South and Southeast Asia and the Pacific. Chromolaena has become one of the worst terrestrial invasive plants in humid tropics and subtropics overly the past century (Gautier, 1992).
Chromolaena can grow rapidly and form infestation and can affect agriculture, pasture and biodiversity, as Chromolaena interferes with the functions of natural ecosystems. It can be very invasive, forming impenetrable thickets in open areas such as pastures. It can suppress pasture plants by competing nutrients and water, overshading and its allelopathic effect lowering productivity of desirable forage species with a concomitant loss of livestock production. Chromolaena leaves, especially the young ones, are toxic to animals due to high levels of nitrate (5 -6 times above toxic level) (Sajise, 1974).
Control of Chromolaena is difficult due to its ability to thrive in a wide variety of soils, rapid attainment of reproductive maturity, large production of easily dispersed seed, a significant proportion of seed persisting in the soil more than one year and strong ability to resprout after burning (Witkowsky and Wilson, 2001). Mechanical control is one of the common methods used for control of Chromolaena in many countries. Mechanical controls include labor intensive hand weeding, slashing, digging and burning. In conventional areas, the initial slashing operation is followed by digging and sometimes burning. Slashing and burning reduces the standing biomass but regrowth will occur from rootstock , usually more profusely. Cultural controls include the use of mulch and various fast growing forages and other plants, still have been rarely used in controlling of Chromolaena.
In Maiwa pasture, more than 50 % of area has been covered by Chromolaena that severely reduced grazing area. The owner has spent much money on slashing, digging and burning Chromolaena without much of success. This is because mechanical control requires repeated follow operation to achieve complete eradication. Notwithstanding these restrains, this method is still widely used, because a plentiful unskilled cheap labor is available.
In recent years, investigation is in the progress in examining the use of integrated mechanical, chemical and cultural control methods for controlling Chromolaena. The present experiment was conducted in Chromolaena dominated pasture with the purpose of evaluating the efficacy of mechanical control and integrated of mechanical and cultural control methods in suppressing the regrowth of Chromolaena and other weeds and determining botanical composition of plant species during dry season.

MATERIALS AND METHODS
This experiment was conducted during dry season in a pasture owned by Faculty of Animal Husbandry Hasanuddin University located at Maiwa, Enrekang Regency South Sulawesi, from July to November 2012. The site was heavily infested by combinations of Chromolaena odorata, Stachytarpheta jamaicensis, Borreria sp and some other weeds and herbage species. There were five mechanical and integrated mechanical and cultural control treatments i.e. T1 slashing of Chromolaena every month at 10 cm above soil level, T2 digging up of Chromolaena and exposed to dry (mulching), T3 digging up of Chromolaena followed by controlled burning, T4 digging up of Chromoalena followed by burning and sowing with Centrosema pubescens, and T5 digging up of Chromolaena followed by burning and planting with Brachiaria decumbens. The plants were harvested at one, two and three months after treatment imposed. The seeds of Centrosema pubescens and tillers of Brachiaria decumbens were sown into plots immediately after burning. Super Phosphate 36 (SP36) and urea fertilizer at the rates of 40 kg P 2 O 5 and 46 kg N/ha, respectively were applied to Centrosema pubescent sown plot at the time of sowing, while urea was applied to Brachiaria decumbens planted plots two weeks after planting.
Plot sizes were 5.0 x 5.0 m and a 1.0 m space between plots was allotted to prevent treatment effects of one plot to other plots. The study area was fenced off using barbed wire and a height of 2.0 m was maintained all around the study area, to keep out of animals and unauthorized persons. The fenced area measured was 50 x 40 m. The area of 100 m wide outside of fences was ring weeded using motorized brush cutter to prevent flowering of Chromolaena and accidental burning.
The efficacy of treatment was assessed on the basis of dry matter weight of Chromolaena and other weeds regrowth sampled at one, two and three months after treatment. The less dry matter of weeds yielded, the high were their efficacy. Sample of regrowth was taken from cutting of plants at different places at 3 cm above soil surface in quadrants measuring 1 m x 1 m. To determine dry matter contents, the fresh samples obtained were dried in oven at 80 o C for 72 hours and weighted.

Statistical Analysis
This experiment was conducted using a split plot in time design with five integrated mechanical and cultural control treatments as main plot and three times of cutting as sub plot with three replications. All data were subjected to analysis of variance and probability value <0.05 is considered significant. Difference among each treatment were further analyzed using least significant difference method.

Efficacy of Treatments
The seeds of plants in the soil were continued to germinate and the slashed plants were continued to regrow after treatments imposed. The dry matter yields of weeds of each treatment are shown in Table 1. Table 1 shows that for three months after treatment, dry matter yield of Chromolaena as influenced by digging up of Chromolaena combined with other treatments was negligible. This indicated that under drought conditions, digging up is the most effective in controlling regrowth of Chromolaena, but not to other weeds. The negligible seed germination and dry matter yield of Chromolaena indicated that during dry season, there was no seed and seedling of Chromolaena that germinated and grew in the soil, meanwhile other weed seeds continued to germinate and grow. It seems that there was still large viable seed population in experimental site that had been covered by dense stand of Chromolaena for over ten years. The negligible seed germination of Chromolaena in this study may be attributed to the low available water in the soil as most of this experiment occur in heavy drought in which total rainfall during the study (August to November) was only 259 mm.
The low seed germination of Chromolaena seed during dry season was also stated by McFayden (2003) and Agarwala and Das (2012), although some seeds appeared to remain dormant for several years (Jeffery, 2010). The low density of seedling emergence of Chromolaena experiencing severe drought was reported by Fittschen (2006) that in dry areas during three years of his study, seedling density of Chromolaenawere only 0.5, 0.56 and 1.06 per 5 m 2 in the three sites, respectively.
The control of Chromolaena by digging up is very effective under drought conditions but it was an on going process as new seeds continue to blow in from distant infestation. If these seeds and seed bank in the soil get enough available moisture and light, these seeds can germinate and grow rapidly.
Across the digging out treatments, burning of Chromolaena followed by planting with Brachairia decumbens was significantly more effective in suppressing weeds than sowing with Centrosema pubescens or burnt only treatment ( Table 1). The higher efficacy of Brachiaria than that of Centrosema in suppressing weeds may be attributed to the higher of dry matter yield and covering ability of Brachiaria than those of Centrosema. This could be due to vigorous nature of Brachiaria growth and its ability to extract growth resources from the soil. Shelton (2012) stated that when established, Brachiaria decumbens will suppress weed effectively and within three months, a complete ground cover could be obtained. In this experiment, until the end of study a complete cover of Brachiaria decumbens was not attained, however, dry matter yield of this grass steadily out yielded of weeds (Table 2). This was in parallel with report of Renrun and Xaejun (2012) that in the first two years of establishment, Brachiaria decumbens in pasture had effectively prevented germination of the seeds and seedlings growth of Chromolaena plus other weeds and in the third year virtually no Chromolaena was noted in the pasture.
Although sowing with Centrosema was less effective than planting with Brachiaria, however sowing with Centrosema was more effective than burning only in suppressing weeds, although it was not significantly different (Table 1). This indicated that to gain control of weeds form burnt vegetation, the vigorous trailing and twining Centrosema can be used, as this plant can cover the ground and vegetation well and stop weeds growing beneath them by competing with nutrients, space and light.
In this experiment, burning was not effective in suppressing the regrowth of weeds. This result is not agree with Ossom et al. (2007) in Swaziland that planned and controlled use of fire was the most effective and beneficial in controlling Chromolaena and other weeds. Most Chromolaena seeds germinated at soil surface (Bhagirathet al., 2008) that may be killed by burning and presence much seed of Mimosa in the soil may be a causative factor for the low efficacy of burning in controlling weeds, as burning stimulated hard coat seed of Mimosa pudica. Application of Chromolaena mulch was more effective in suppressing weeds than slashing of Chromolaena every month (Table 1). The differences in growth factor conditions might be a causative factor. The low light transmission in mulched plots probably reduced most plant growth in study area, while the higher light intensity in slashed plots enhanced sun plant growth that probably comprises most of plants in the area.

Botanical Composition
Shift in botanical compositions as influenced by treatments are shown in Table 2  Burning of Mimosa seeds can kill surface seeds but not buried seeds and may stimulate seeds germination due to removal of seed coat (de Menezes and Rossi, 2011) and seedling growth might be enhanced by absence of competing vegetation after burning (Paynter and Finlayson, 2003). In contrast, in this study the botanical composition of Stachytarpheta in burnt plots was lower than to that of unburnt plots. This may be attributed to killing much of Sachytarpheta seeds by burning as its seed germination and emergence was restricted to seeds planted at the soil surface that easily killed by burning (Diaz-Filho,1996). Botanical compositions of plants varied with time (Table 2). In slashed plots, botanical composition of Chromolaena decreased over time; conversely, botanical composition of Stachytarpheta increased with time ( Table 2). The same trend also found in mulched plots, botanical composition of Stachytarpheta increased with time, while botanical composition of Calopogonium decreased. The decrease in botanical composition of Chromolaena in slashed plots may be partly due to the reduced reserve carbohydrate levels in the roots and stump, as regrowth of plants depends total nonstructural carbohydrate reserve contents in the remaining plant parts after cutting (Garcia et al., 2001) while increasing botanical composition of Stachytarpheta in both plots may be due to higher its adaptability to different light intensity, wide range of soils and soil moisture conditions (Gilman, 2013).
In burnt and forage planted plots, botanical composition of Brachiaria increased with time, while botanical composition of Centrosema peaked at second cutting and then decreased. Besides, botanical composition of Centrosema always lower than that Mimosa pudica (Table 2). This indicated that Centroema was less suitable to control weeds than Brachiaria and this may be attributed to its lower growth rate and covering ability.

CONCLUSION
Under drought conditions, digging up of Chromolaena integrated with cultural control is very effective in controlling regrowth of Chromolaena, but not to other weeds. In the long term, digging up of Chromolaena followed by burning and planting with the fast growing forage like Brachiaria decumbens probably to be the most promising method in controlling Chromolaena and other weeds in pasture area. Planting of burnt area with other fast growing forage species in pasture needs to be investigated to find more effective species to suppress the regrowth of Chromolaena.