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Ivanka Stoimenova, Stefka Alexieva Institute of Soil Sciences and Agroecology "N.Poushkarov" Shosse Bankja 7, Sofia, Bulgaria The aim of this study was to determine weed control strategies in maize, affected by rate of organic and mineral fertilization. The aim of this study was to determine weed control strategies in maize, affected by rate of organic and mineral fertilization. A field experiment on Pelic Vertisol in the period 1994 - 1996 in Sofia district was carried out. Three years monoculture of maize randomized complete block design in four replications with plot size 200 m2 was investigated. The following scheme was used: Factor A - treated with organic fertilizers: a1 - without organic fertilizers, a2 - treated with cow manure (40 t ha-1) once for three years (in November 1993); Factor B - treated with N, P fertilizers: b1 - N0P0; b2 - N50P40; b3 -N70P60; Factor C - weed control: c1 - hand-weeding; c2 - atrazine 1500 g a.i. ha-1 +nicosulfuron 480 g a.i. ha-1. The maize field was not irrigated; cow manure and phosphorus fertilizer was applied during the main soil ploughing in the autumn and nitrogen fertilizer was applied with the last presowing cultivatin. The crop was sown between May 5 and 10. Maize density was 47000 plants ha-1, spaced 70 cm between the rows. The atrazine was applied pre-emergence of the maize for selective control of broad-leaved weeds and nicosulfuron was applied of 3-5 leaves of maize and 1-3 leaves of grasses weeds. The first hand-weeding of the control and herbicide variants was made in the growth stage "7-9 leaf", and was repeated in "tasseling" after determining the degree of infestation and the quantities of the dry biomass. In all trial treatments, according to the factors A, B and C, weed spp., and weed dry biomass was determined as number m-2 and g m-2. Maize yield in all variants of the trial were obtained in four replicatas complete factorial treatment arrangement. Data were subjected to analyses of variance and all main effects and interaction tested for significance. The growth of weed flora in combination with the slow development of crop habitus was positive influenced by the sufficient soil moisture due to the winter and spring rainfalls and the propitious condition of the meteorological elements in the period - "emergence to 7-9 leaf" of the maize vegetation, weeds from different biological groups infest the control variant (without herbicides). The main part of the weeds population belongs to the summer dicotyledonus followed by summer monocotyledonous, early spring and winter-spring ephemerals. In the control variant the degree of infestation and the quantities of the weed dry biomass depend on the norm of mineral fertilization. During the three years of investigation these two parameters were highest in the variants with the highest norms of fertilization and their values were lower in the variants without fertilization. The cow manure application in 1993 provoked an increase of the degree of weed infestation and the quantity of weed dry biomass in the variants with and without mineral fertilization. That was strongly manifested during the first year of investigation. During the above mentioned period "sowing - 7-9 leaf" the highest degree of infestation sowed Amaranthus retroflexus L. in hand weeding variants. The obtained of percent participation in the total degree of weed infestation varied from 43.1 to 65.2, depending on the norms of mineral and organic fertilization and precipitation. The effect of atrazine + nicosulfuron on weed flora varied and depended mostly on the precipitation in the period "sowing - 7-9 leaf. In phase "7-9 leaf" of maize vegetation the weed control by atrazine + nicosulfuron was better in years with 50-55 mm precipitation on the period "sowing - 7-9 leaf" (1994 and 1996) and it became less in the time of investigation more with 2-3 times rainfalls during the same period (1995). Weed control in treatment with herbicides depends mostly on organic fertilization. Independent of rainfalls the degree of weed infestation and the dry weed biomass was lower in plots without organic fertilization compared to the variants with cow manure application. Weed control in variants with herbicides depended on the norms of mineral fertilization. The best weed control was observed in treatments without mineral fertilization and that control was less in variants with mineral fertilization.  In the 1996 when the precipitation was 49.2 mm in combination with high effective temperature sum (ETS), was observed a highest weed control in all of the treatments, while in 1995 the low weed control level due to high precipitation. The herbicide application influenced more strongly on infestation degree in comparison to the quantity of the dry weed biomass in 1994. After manual weed cleaning in the end of "7-9 leaf" stage, the second infestation was mainly by Amarantus retroflexus L. Setaria glauca (L.) P. B. and Echinochloa crus-galli L. The small amount of rainfall water in 1996 during "7-9 leaf-tasseling" period leads to the best weed control in "tasseling" phase. In 1994 was obtained the less weed control as a result of the numerous weed seed population in the cow manure, while in 1995 the low weed control level due to high precipitation. The herbicide treatments increased the total yield independent of the kind of maize planting. The highest yield was obtained in the variants with organic and mineral fertilization (N70P60). During the experimental period the maize yield was as follows: In 1994 was obtained the highest yield as a result of the precipitation during "12-13 leafe-tasseling" phase - 95.2 mm; in 1995 the yield was poor due to the little ETS levels. From sowing to the phase 7-9 leaf the crop was infested by weeds from different biological groups with main representatives from the summer dicotyledonous, followed by early spring weeds, summer and winter-spring ephemerals. In control (no treated) variants during the phenophase "7-9 leaf-tasseling" weed infestation decreased considerably and in was realised of Amaranthus retroflexus and Setaria glauca. Significant differences in weed control was observed in atrazine + nicosulfuron depending on the meteorological condition, organic and mineral fertilization. The best weed control was determined in the variants no treated with cow manure. The herbicide treatments increased the total yield independent of the kind of maize planting. The highest yield was obtained in the variants with organic and mineral fertilization (N70P60). During the experimental period the maize yield was as follows: In 1994 was obtained the highest yield as a result of the precipitation during "12-13 leafe-tasseling" phase - 95.2 mm; in 1995 the yield was poor due to the little ETS levels. |