Weeds occurrence

Last years the concern of the scientists and contributors to chitin, chitosan and chitincontaining connections has increased. It is connected to their widespread occurrence in the nature, paiticulai properties, and also feasibility in many areas of a national economy. The raw sources for obtaining chitincontaining of products are the testas of crabs, lobsters, shrimps, and also cabbage-weeds, funguses. 

A weed, which is a form of a natural resource, is simply a living organism whose presence conflicts with the interests of people. Undesirable as they may be, weed species are natural occurrences in the agricultural ecosystems created by man s food and fiber production. Over 300,000 species of plants inhabit the earth, but only 30,000 of these are weeds. About 1,800 weed species cause serious economic losses in crop production, and about 300 weed species are serious in cultivated crops throughout the world. Most cultivated crops are plagued by 10 to 30 weed species that must be controlled to avoid yield reductions (1). 

Additional benefits derived from cover cropping include weed and disease suppression. Approaches for managing weeds are discussed below however, the main mechanism of weed suppression by cover crops appears to be resource competition, rather than factors such as allelopathy (Bond and Grundy 2001). The different occurrence of the fungal pathogen... 

There are publications within the organic movement that relate the occurrence of specific weeds to specific soil conditions and that refer to the possibility of weeds being indicators of soil status (e.g. Pfeiffer 1970, Walters 1996). The presence of a given weed therefore suggests that the land has too much or too little of a certain soil feature (e.g. drainage, pH, copper levels) that may be modified by the farm manager. Although many of the recommendations are based on careful observation and commonsense, some of the relationships have not been conclusively established. 

The genus Eleusine belonging to the family Poaceae and subfamily Chloridoideae comprises nine species. E. indica is found now in many places of the world and is treated as a weed and has been found in various archeological sites in the United States. Only E. tristachya is found in South America, the rest being endogenous to Africa. The domestication of E. coracana is discussed in Hilu and De Wet (1976). They reported that the occurrence of . africana and E. indica overlaps in Africa. The two "species" interbreed and the hybrids are weed-like. 

The high efficacy of triazine herbicides and their repetitive use in crops and noncrop situations has resulted in the selection of weeds that are resistant to these herbicides or are not well controlled at the lower rates now being used. In most instances, triazine resistance is due to an alteration in the herbicide-binding site in PS II. Despite the widespread occurrence of triazine resistance, these herbicides are still widely used, even in fields in which triazine-resistant biotypes are known to occur. The rate of increase in the selection for triazine-resistant weed species depends in part on the integration of alternative weed control strategies, in addition to the use of triazine herbicides, for control of these weed species. Due to their resistance mechanism, many triazine-resistant weeds are less competitive than their susceptible counterparts. 

Anderson, D.D., F.W. Roeth, and A.R. Martin (1996a). Occurrence and control of triazine-resistant common waterhemp (Amaranthus rudis) in field com (Zea mays). Weed Technol., 10 570-575. 

Heap, I.M. (1997). The occurrence of herbicide-resistant weeds worldwide. Pestic. Sci., 51 235-243. 

Rola, H. and J. Rola (1996). Occurrence of Amaranthus retroflexus, Chenopodium album and Echinochloa crus-galli in southwest Poland and their control by herbicides. In R. DePrado, J. Jorrin, L. Garcia Torres, and G. Marshall, eds., Proceedings of the international Symposium on Weed and Crop Resistance to Herbicides. Cordoba, Spain Graficas TYPO, S.L, pp. 42-44. 

Table 11.2 Occurrence of resistant weed biotypes to different herbicide groups up to 2006...

Both slaframine and swainsonine were isolated from the fungus Rhizoctonia leguminicula. The fungal infection of red clover hay or pasture is associated with the occurrence of the slobbers syndrome in cattle and horses which consists of excessive salivation (slobbering), lacrimation, feed refusal, bloating, stiff joints, abortion, diarrhoea and/or violent behaviour. Some of these clinical symptoms are similar to loco-weed poisoning but the first two are not observed upon administration of swainsonine to animals [114]. 

Fluroxypyr 0.5-0.6 l.ha-1 postemergently in occurrence of catch weed Galium aparine), possible divided dose 2 x 0.3 1 (from 14 BBCH)... 

Allen, B. L. and B. F. Hajek. 1989. Mineral occurrence in soil environments. In J. B. Dixon and S. B. Weed (eds.), Minerals in Soil Environments. Madison Wis. Soil Science Society of America, pp. 199-278. 

BLACK NIGHT SHADE Solanum nigrum, L., Solanaceae, is a common weed in gardens and waste places. The whole plant is toxic due to the occurrence of the alkaloid solanine, giving the same symptoms as Bittersweet. 

FOOLS PARSLEY Aethusa cynapium, L., Family Apiaceae, is a European weed. The whole plant is poisonous due to the occurrence of very low content of the alkaloid coniine, a cicutoxin-like substance and essential oil. There have been lethal poisonings among humans and cattle. Symptoms of poisoning are stomach-ache, increased production of saliva, vomiting, diarrhoea, headache and blurred vision. 

Intraspecific triazine resistance was first discovered in common groundsel (Senecio vulgaris L.) in western Washington in the late 1960s. The subsequent widespread and frequent occurrence of other triazine resistant weeds over the past 20 years, has made triazine resistance the best known and most studied example of herbicide resistance (1). Triazine resistance has also been of great interest because of the importance and extensive use of this group of herbicides. 

TABLE V. Occurrence and Distribution of Weed Biotypes Resistant to Various Nontriazine Herbicides... 

Based on our knowledge of herbicide modes of action and other characteristics, we are able to predict, with some confidence, which herbicides will have a high risk for resistance. There are wide margins for error due to uncertainty about genetic variability of weeds (i.e., the relative frequency of occurrence of different resistance mutations), the lack of knowledge on the primary sites or modes of action for certain herbicides, and the lack of understanding of cross-resistance between classes of herbicides. 

Table VIII shows some interesting trends in the first occurrences of herbicide resistant biotypes. Not only are herbicide resistant weeds appearing after fewer repeat annual applications of some of the newer herbicides (e.g., 4 to 5 years of treatment with sulfonylures herbicides), but there seem to be more species that have potential for resistance, as shown by the 26 new cases in 1988. In addition, the resistant biotypes are more fit and competitive than most biotypes resistant to triazine herbicides.

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