Poisonous weed species and their significance for cattle production in Serbia region Konstantinovic Branko, Meseldzija Maja University of Novi Sad, Faculty of Agriculture Novi Sad, Trg Dositeja Obradovica 8, 21000 Novi Sad, Serbia telfax: +381-21-450-616, E-mail: brankok@polj.ns.ac.yu Abstract Weed species that grow on agricultural land together with crop plants cause crop yield reduction and increased costs of agricultural production due to the cost of their control. Beside these direct damages, some weed species cause indirect damages by their adverse effects to human and domestic animals health. In our agroecological conditions the most problematic poisonous weed species are: Solanum nigrum L., Pteridium aquilinum (L.) Kuhn, Ranunculus repens L., Asclepias syriaca L., Equisetum arvense L., Datura stramonium L., Sinapis arvensis L., Conium maculatum L., Aristolochia clematitis L., and Hypericum perforatum L. Damages in cattle production caused by the presence of poisonous weed species in animal feed are great. In 17 west countries of USA losses were 230 million USD. Fodder improvement through enhancement and safe keeping of pastures, increase of lawn use technique, prevention and pasture control in regard to diseases and collection and use of hay for animal feed during winter period. Key words: poisonous weed species, fodder plants, cattle production. Introduction Fodder plants yield is reduced by weeds due to their competition for water, nutrition and light, simultaneously lowering seed quality. If until first mowing of lucerne, clover or bird's-foot trefoil prevail weed species, they significantly influence to the yield, cause problems during harvest, and indirectly cause frequency of pests and diseases occurence. With the aim of yield improvement, intensive production is applied that requires introduction of contemporary crop cultivation that also includes weed control. Weeds in fodder represents one of the cruical factors in yield reduction, and their removal is oneof more important conditions for profitable production. For achievemt of this goal, it is necessary to have excellent knowledge on weed biology and herbicides and their action mechanisms, persistence, selectivity and other featurs (Konstantinovic, 1999). Due to the specific cultivation in convenient agroecological conditions, there exist also convenient conditions for the occurence, development and formation of weed comunity. In its structure and seasonal dynamic weed comunity of fodder differs significantly from those that occure in annual crops. Several years lasting exploatation of fodder crops, stable weed comunity is formed in which dominant are perrenial weeds belonging to the group of geophytes whose control has yet not been solved in completely satisfactory manner (Strbac et al., 1996). However, fodder plants are frequently grown in crop rotation with other crops, because they, as perrenial cultures leave arrable areas relatively free of weeds. Certain weed species can be: poisonous- that are avoided by domestic animals during grazing, but they can be eaten if found in hay or silage – hosts to insects and parasitic causers of lucerne plant diseases – of low nutritional value and have unpleasant smell that can reduce quality of animal products, or they can have pricly or thorny leaves and damage skin of the mouth and alimentary tract of domestic