In the years 1996-2002 evaluation of herbicides' efficacy used in low doses was tested. For investigations several herbicides recommended for weed control in cereals were selected. The results of experiments indicate on possibility of reducing of herbicides' doses by about 25 to 3 7.5%, and still obtaining good effects ofweed control without significant decrease of yielding of cultivation plants. The dose is depended upon sensitivity of weed species, growth stage of weeds and their infestation, density of crop and the weather conditions. Low doses of herbicide can be applied where weeds are in early growth stage, where there is low infestation ofweeds, and weeds show susceptibility to used herbicides, moreover under correct crop density and good weather conditions.

Zea mays originate from warm and tropical region of the world and needs high sum of effective temperature throughout vegetative period. On the Polish market varieties of maize differ in early maturing, provenance and temperature requirements. The foreign varieties react stronger to the sulfonylurea herbicides (nicosulfuron, rimsulfuron, foramsulfuron + jodosulfuron) than Polish maize varieties under unfavorable weather conditions. The trials were conducted in field conditions using 8 maize varieties. The aim of this experiment was evaluation of the influence of sulfonylurea herbicides on foreign and Polish varieties of maize under different weather conditions. The reaction of foreign maize varieties on herbicides was compared with reaction of Polish maize varieties in the years 2001-2002. Four early maturing varieties of maize were testing: AVENUE (FAO 220)-FR, JANNA (FAO 190)-USA, WIARUS (FAO 220)-PL, FIDO (FAO 220)-PL. It was proved that Polish maize varieties showed a higher toleration towards sulfonylurea herbicides. The later maturing varieties KRÓL (FAO 280)-PL, IMAN (FAO 300)-PL, KANZLER (FAO 300)-FR, MAGISTER (FAO 280)-CH responded in a similar way. In 2002 there were no differences in plant development between foreign and Polish maize varieties.

One of the negative aspects of the intensive use of herbicides is related to the selection of resistant biotypes (Gasquez 2001). Of all biotypes resistant to herbicides, 93 species do not respond to sulfonylurea herbicides (ALS-inhibiting herbicides). The acetolactate synthase (ALS) enzyme is the first step in biosynthesis of a branched chain amino acids (valine, leucine, isoleucine). In Poland the problem of resistance to sulfonylurea herbicides has been discussed since 2001 (Rola and Marczewska 2002). Resistance tests of Apera spica-venti biotypes were conducted in the greenhouse conditions. Chlorsulfuron was applied at the four-leaf stage of development at rates ranging from 11.25 to 360 g/ha. In confirmation of resistance to chlorsulfuron as identified in biological tests, the chemical analyses were performed. The analyses investigated the influence of different doses of chlorsulfuron on free amino acids content in the aboveground part of resistant and susceptible Apera spica-venti. The analyses were carried out applying high performance liquid chromatography method (HPLC). The resistance of the biotype was confirmed in amino acids analysis. In the resistant biotype followed the increase of valine, leucine and isoleucine concentration in comparison with untreated plants and those susceptible to chlorsulfuron biotype.