Rice is an important staple food and cash crop of Pakistan and it is the second staple food after wheat. It contributes 0.6% to the gross domestic product and adds 3.1% to the value of the national agricultural product. It is grown over an area of 2748 thousand hectares with an annual production of 6811 thousand tons. The average paddy yield in Pakistan is 2.48 tons per hectare which are below the yield potential of 6.0 tons per hectare. Some of the major causes of low rice yield in Pakistan include the problems associated with delayed transplanting, less plant population, establishment, inadequate or improper use of fertilizer, a shortage of water, extreme weather events or climate shifts, and poor insect pest and weed management. Weeds are a recurrent and ever-present threat to agricultural productivity and cause massive rice crop failures over vast areas. The losses due to weed infestation can be greater than the combined losses inflicted by insect-pests and diseases. The most problematic weeds in rice are Umbrella plant, Purple nutsedge, Flatsedge, Saltmarsh, Jungle rice, Barnyard grass, Goosegrass, Paper wort and Alligator weed. Normally the losses in rice paddy yield due to weeds range from 15 to 20% and these depend upon the weed species present and their density, their time of germination and the duration of the competition, the competitive capacity of the crop and crop management practices. In severe cases, paddy yield losses may be more than 50%.
Weeds out-compete the crop for different environmental resources including light, temperature, space, moisture, nutrients and also reduce its germination and growth by releasing allelochemicals. The inhibitory properties of weeds on the growth and development of crops are associated with the release of allelochemicals from different plant parts. Allelochemicals are present in virtually all plant parts and tissues including stems, leaves, flowers, buds, bark, pollen grains, seeds, fruits, roots, and rhizomes. However, their concentration is different from one part to another. Root exudation, leaching by dew or rain drops, and volatilization from aerial plant parts or decaying plant tissue results in the release of compounds into the environment. Alligator weed (Alternanthera philoxeroides) is an emerging invasive weed in Pakistan and many countries around the world. It rarely sets seed, and the seeds it does produce are usually not viable therefore this weed reproduces and spreads primarily by adventitious rooting from stem nodes and roots. It has the ability to grow successfully in tropical, subtropical and temperate regions of the world under both aquatic and terrestrial habitats. It is a serious and problematic weed in 30 countries around the globe including Australia, USA, and Asian countries. Alligator weed is considered an economic and potentially also an environmental weed. It has many well documented negative impacts on human activities, including agricultural and horticultural industries, recreational water use, and flood control. Alligator weed also has potential ecological impacts as it impedes water flow, increases sedimentation, and exacerbates flooding. Flood damage to structures such as fences may also be increased by the weight of dense mats of weed washing up against them. Alligator weed is widely recognized as an aggressive competitor, based largely on research comparing it to agricultural or other invasive plant species. Alligator weed has been recognized as an invasive and troublesome weed in rice, corn, cotton, soybean, vegetables and fruit trees and it reduces wheat production by 36%, sweet potato by 63%, lettuce by 47%, and corn by 19%.
Seed emergence is one of the most critical phases of plant development at which the weed can compete for an ecological niche and is mediated by various environmental factors. The allelopathic effect of Alligator weed aqueous extract and soil decomposed residues on rice germination and seedling growth was investigated at Department of Agronomy, University of Agriculture, Faisalabad, Pakistan. Rice emergence (%) was reduced with increasing soil residue concentration (1- 4% w/w) of Alligator weed whereas Mean emergence time increased. Root, shoot length of rice and their dry weights decreased in zero day decomposition when compared with control and the stimulatory effect was observed in 15 day decomposition beyond which again inhibitory effect was observed. Decomposed residues of Alligator weed contained water soluble phenolics namely Quercitin, Caffeic acid, Syringic acid, Sinapic acid, Quercitin, Chlorogenic acid, P- Coumeric acid, Trans-4-hydroxy3-methoxy cinnamic acid.
The threshold concept of weed density helps to decide whether or not to apply a control measure by considering the cost and benefit of weed control in the crop under study. Weed species have different modes of growth, reproduction, and levels of threshold density on specific crops. Effect of varying density levels (2, 4, 6, 8, 16 and 32 per m-2) of Alligator weed on rice was studied in a field experiment. Weed dry weight of Alligator weed showed an increasing trend as its weed density moved from 2 to 32 plants m-2. Paddy yield determinants like no. of panicle bearing tillers per plant, no. of kernels per panicle and 1000-kernels weight significantly reduced with increasing density levels from 2 to 32 plants m-2 of Alligator weed. NPK and micronutrients (Cu, Fe, Zn, and Mn) contents in Alligator weed showed decreasing trend and their uptake showed increasing trend with increasing weed density. Paddy yield of 3886 kg ha-1 and 3615 kg ha-1 has recorded in weed-free treatments in 2011 and 2012, respectively. Rice yield was decreased by 21.38% and 23.78% with Alligator weed density of 32 plants m-2 in the first and second year, respectively. The critical threshold level of Alligator weed in rice was 2 plants m-2.
The critical period of weed-crop competition is an important component of effective weed management program. The critical period for weed control is the length of time that the crop must be kept weed-free to maintain yield losses at or below a certain level. Effect of different competition periods i.e. 0 weeks after transplanting (WAT), 4 WAT, 6 WAT, 8 WAT, 10 WAT, and full season competition of Alligator weed on rice was studied under field conditions. Weed dry weight continued to increase with increasing period of competition with rice. No. of kernels per panicle, no. of panicle bearing tillers per plant and 1000-kernel weight showed decreasing trend with increasing weed crop competition period. Full season competition of Alligator weed with rice decreased paddy yield up to 21.69% and 22.28% compared to no competition in a first and second year, respectively.
The most common method currently used to manage weeds in rice crop is the use of herbicides. In spite of some crop quality and environmental perspectives, chemical weed control with herbicides is considered to be the most effective, time saving and economical way of controlling weeds as compared to other methods of weed control. Different doses of two herbicides with and without adjuvants were tested against Alligator weed in rice. The lowest dry weight of Alligator weed (76.11 g m-2 and 79.38 g m-2) was recorded in plots sprayed with Clover-20 WP @ 225g ha-1 (Bispyribac sodium @ 45 g a.i. ha-1 + 200 mL ha-1 adjuvant) in 2011 and 2012, respectively. Less amount of macro and micronutrients was uptake by Alligator weed where it was treated with herbicides. Maximum paddy yield of 3911 kg ha-1 and 3571 kg ha-1 was recorded in plots sprayed with Clover-20 WP @ 225g ha-1 (Bispyribac sodium @ 45 g a.i ha-1+ 200 mL ha-1 adjuvant) (22.52-24.82% more yield compared to weedy check) in two years.
So it can be an inference from the studies that an active allelopathic mechanism exists in alligator weed, which can release toxins during saturation with irrigation or rain in nature. Therefore, Alligator weed should be removed from the field by weeding to save crop plants from harmful effects of this weed. Alligator weed should be controlled at a weed density of 2 plants m-2 within 4 weeks after transplantation of rice with the application of Clover-20WP@225g ha-1 (Bispyribac sodium @ 45 g a.i ha-1 + 200 mL ha-1 adjuvant).
Author of the article- Dr. Azhar Mehmood Department of Agronomy, University College of Agriculture and Environmental Sciences, Prof. Dr. Asif Tanveer The Islamia University of Bahawalpur.