The higher richness and diversity in spring-summer observed for edges relative to field plots concurs with other studies which may be due to the absence of chemical control in edges as ocurred in an-other study .Crop fields are characterised by considerable herbicide applications which may partially explain why their weedy vegetation is different from edges not subjected to these inputs . However, in our study in winter, the opposite trend was observed as higher or similar diversity values were determined for field plots relative to edges. The effect of herbicides on weed diversity has also been erratic in other studies where herbicides applied over more than one year either reduced or maintained diversity. In both spring-summer and winter surveys and in both years between 52% and 67% of the observed species were annuals which concurs with another study in arable fields . n arable fields, the generalized adoption of glyphosate- weed community in many other studies . Glyphosate shows very effective control of a wide range of species including non-target species and changes in weed populations in response to the adoption of glyphosate-resistant soybean has been reported elsewhere.
Although there is no evidence to suggest that herbicides such as glyphosate lead to the elimination of species at the field scale , in the present study, weed cover in average in the field plot was always low. The absence of glyphosate application in edges may favour potentially rare arable, broad-leaved weeds. The crop planted at high density and the use of herbicides and fertilizers favour crop production, and increase the growth of the crop relative to the weed species which can account for the low weed cover in the field. In our study the species composition differed between years. Some of the most common species in spring- summer were A. cristata and P. oleracea which are also quoted as important weed species in other studies.In our study, cannabis grow system the most comon grassy perennial weeds were S. halepense, C. dactylon and B. catharticus. Those grassy perennials species usually form a dense canpy which exerts high competitive pressure for many other weeds. S. halepense and C. dactylon aerial biomass is killed by frosts but the dead canopy remains during win-ter. B. catharticus is a short-lived perennial densely- tufted and robust. Its aerial biomass is not killed by frosts and consequently its green canopy is present during the whole year. Some species common in crop edges are adapted to grow under the canopy of perennial species. Among these especies, P. debilis is a broad-leaved annual species which showed higher cover in edges rela-tive to the field crop.
The canopy of the grassy preenial weeds can accout for this different behaviour as edges protect P. debilis from freezing, allowing plants to achieve more biomass accumulation than in the field plots . Another annual broad-leaved species common in winter was B. incana, present in both the crop field and edges. This species is also mentioned as an important winter weed in the region in another study . The most frequent annual grassy annuals were D. sanguinalis, E. colona and E. indica. In the last years, grassy annuals are more abundant due to the adoption of no- tillage systems.In our study, the only species found with high frequency that is tolerant to glyphosate was A. cristata. No resistant weeds were detected. In edges, several species tolerant to glyphosate in high frequency were observed: C. erecta, C. dactylon and P. debilis. Our results indicate that the community in crop edges relative to field plots differs in structure and abundance and many weed species are only present either in crop edges or in field plots. Cotton the “white gold” or “money spinner” enjoys a predominant position among stall cash crops in India and cultivated since Indus valley civilization.
Cotton occupies the prime in India constituting more than 70% of the total fibre consumption in the textile sector. Weed growth offers severe weed competition in the initial stages of crop growth as cotton is a wider spaced crop with slow vegetative growth during initial stages there by causes yield reduction to an extent of 74 percent . In India, cotton is grown under varied environments and seasons. In Tamil Nadu, the zone of summer rice-fallow cotton is confined to the delta districts of Thanjavur, Thiruchirapalli, Villupuram and Vellore. Karaikal is located in the tail end of Cauvery deltazone. Of the estimated 0.5 lakh hectares under the summer cotton,one third of area comes from the rice-fallows of Cauvery delta under cotton . Cotton, in Karaikal region, is often grown only under rice-fallow condition.