Allelopathic Interactive Effects Of Rice, Cymbopogon, Desmodium, Mucuna And Maize In Uganda

Abstract

Rice is a major food and cash crop under various mixed cropping systems in Uganda. Rice and some intercrops exhibit allelopathic properties and productivity is on the decline yet little is known about the causes of allelopathy, declining ecosystem productivity and the potential benefits from the ecosystems. The main objective of this study was to determine the compounds in Oryza sativa, Cymbopogon nardus, Desmodium uncinatum, Mucuna pruriens and Zea mays and their allelopathic interactive effects. Specific objectives were: (i) to identify the compounds in rice, cymbopogon, desmodium, mucuna and maize; (ii) to determine the allelopathic potential of compounds in rice, cymbopogon, desmodium, mucuna and maize; (iii) to determine the effects of allelopathy on striga, crop growth, nutrient uptake and productivity of rice based intercropping with cymbopogon, desmodium, mucuna and maize; (iv) to evaluate the allelopathic potential of rice, cymbopogon, desmodium, mucuna and maize mixed mulches on weeds and rice; (v) to assess the effects of plant powders and aqueous extracts from rice, cymbopogon, desmodium, mucuna and maize on weeds. Objective (i) was implemented between June and August 2013 at the National Crops Resources Research Institute (NaCRRI), Namulonge, Uganda. Plants were uprooted at 45 days after emergence (DAE) and samples analysed for organic compounds using the National institute of Science and Technology library. In objective (ii) pot screening, equal compartments agar, germination tests and donor to receiver plants in the same pot studies were conducted during 2013B. In objective (iii) a screen house experiment was done at NACRRI during 2013B and two field studies were conducted during 2014A. In the screen house two different crop species were planted in the same pot and as sole crops. In the field studies, maize, mucuna, desmodium and cymbopogon were tested in sole crop and intercrop systems. In objective (iv) on-station and on-farm experiments were conducted during 2015A. Cymbopogon, mucuna, rice and maize species were each planted and xvii uprooted at 55 DAE and combined in sets of three equal proportions of individual stover to give four types of mulches that were applied under 12 treatments at planting of rice. Objective (v) was conducted at NACRRI during 2013B and in the field during 2015A. Leaf, stem and root (LSR)) powders from rice, maize, cymbopogon, desmodium and mucuna were combined in equal proportions of three to make eight powdered mixtures that were applied to 72 pots in a screen house. Field studies were conducted on station and on farm with maize, mucuna, rice and cymbopogon mulches, Butanil herbicide, hand hoeing and rogueing of weeds with a weedy check. Data were recorded on the compounds in blank forest soil (control), soils potted with cymbopogon, desmodium, rice, mucuna and maize crops. Data were also collected on compounds in cymbopogon, desmodium, rice, mucuna and maize stover. Radicle and plumule length were measured, germinated seeds counted and percent germination, mean germination time (MGT) and seed germination indices (SGI) calculated for desmodium, rice, mucuna and maize seedlings. Root lengths, plant height, dry biomass, width and length of leaves, number of weeds and plants per pot, plant nutrient uptake and reserves in the soil at harvest were established in potted experiments. Data were collected on maize green leaves, striga per 100 rice plants, total and filled panicles per plant, grains per plant, filled grains per panicle and grain yield of rice under field studies. Land equivalent ratio, competitive ratio, relative growth rates (RGR), gross returns, net returns and returns on investments (ROI) were calculated for different enterprises. Blank soil, cymbopogon, desmodium, rice, mucuna and maize produced 24, 7, 5, 11, 7 and 6 major compounds respectively. Cymbopogon, desmodium, rice, mucuna and maize predominantly produced 10, 6, 6, 6 and 9 compounds respectively from their stover. Maize, rice and mucuna leachates reduced the root lengths, height and biomass for weeds. G. parviflora root and stem growth reduced on application of root exudates. Mean germination time for test plants increased while the SGI reduced at 75% leachate concentrations. Growth of rice, mucuna and desmodium reduced when potted with mucuna and maize. Maize potted with xviii desmodium and cymbopogon recorded reduced RGR, NPK crop uptake and reserves in the soil at harvest unlike rice potted with mucuna, desmodium and cymbopogon. Conversely, the maize growth increased when potted with mucuna and desmodium and with 75% concentration of rice/ desmodium and rice/maize leachates. Striga weed counts reduced in rice intercropped with mucuna, desmodium and cymbopogon but increased with maize and thus, reduced rice tillering under the latter treatment with 63% of the weeds recorded on rice plants. Sole rice and rice intercropped with cymbopogon produced higher rice grain yields and partial land equivalent ratio (LER) for rice. The highest combined LER were recorded under rice + desmodium and rice + cymbopogon cropping systems. Rice + maize and rice + mucuna intercropping systems were least productive and mucuna was the most competitive crop followed by maize and cymbopogon. Butanil, hand hoeing twice or thrice and mulching rice with maize/mucuna based mulches increased rice growth, grain yields and ROI compared to rice/cymbopogon based mulches which most effectively controlled weeds including striga. Increasing LSR powder concentrations reduced the number of weeds and mean weight per weed. Rice/cymbopogon and maize/ mucuna LSR powdered mixtures and liquid bio-extracts reduced weed densities and weight while Butanil recorded the lowest weed densities and weight. Highest weed weight/m2 and unit weed weight were under the weedy check. The highest striga count was recorded under Butanil, 3 and 2 hand hoeing (hh) and there were no striga under cymbopogon mulches and weedy check. Highest ROI were under 2hh, 3hh and maize/mucuna based mulches given 1 hand rogueing of weeds. There is high potential to utilise allelopathy as a weed management technology for increased crop productivity. Further studies are recommended on the allelopathic potential of the compounds identified in the current study and in use of C. nardus and M. pruriens for Striga hermonthica weed control