EFFECTS OF APPLIED HERBICIDES ON CROP PRODUCTIVITY AND ON WEED INFESTATION IN DIFFERENT GROWTH STAGES OF SUNFLOWER (Helianthus annuus L.) / LOS EFECTOS DE LOS HERBICIDAS APLICADOS CONTRA LA INFESTACIÓN POR LAS MALAS HIERBAS DE GIRASOL (Helianthus annuus L.) EN DIFERENTES ESTADIOS DE DESENROLLO Y PRODUCTIVIDAD / EFFETS DE L'APPLICATION D'HERBICIDES SUR L'ENHERBEMENT A DIFFERENTS STADES DE DEVELOPPEMENT DU TOURNESOL (Helianthus annuus L.) ET SUR SA PRODUCTIVITE

The level of weed infestation directly affects the intensity of competitive relationship between sunflower crops and weeds. The greatest damage is caused by annual, broad-leaf and invasive weeds, such as L., L. and L. Suppression of these weeds is difficult because of deficiency of adequate herbicides and because in years with dry springs, such as in 2009, the use of herbicides gives no results. This two-year study dealt with the observed effects of pre-emergence flurochloridone + s-metolachlor herbicides on the distribution of weeds in different stages of sunflower development. Weed infestation was estimated over the sunflower developmental stages, , in the periods when weeds were removed and when the total fresh and dry biomass of the weed species was determined. Simultaneously, in order to monitor the effects of weeds in dependence on herbicides application and the duration of competition, the sunflower plant height was measured. At the end of the life cycle, the yield and the oil content of the sunflower seeds were established. The total fresh weed biomass changed in dependence on the sunflower developmental stages and was always lower on the herbicide-treated area. This affected the sunflower plant height, yield and the oil content, which were higher, on the average, in the variants with herbicide applications (70.4 cm, 2959.7 kg ha and 42.0%) than in the variants without herbicides application (57.4 cm, 2711.1 kg ha and 40.1%). Weed suppression in the sunflower crop has to be done with adequate herbicides and in due time in order to suppress a significant reduction in morphological and yield parameters. El nivel de infestación por malezas afecta directamente la intensidad de las relaciones de competencia entre los cultivos de girasol y las malas hierbas. Los mayores daños se sufren por las malas hierbas anuales, de hojas anchas e invasivas, tales como L., L. y L. A eliminación de estas malezas es difícil debido a la falta de herbicidas adecuados y la ausencia de efectos cuando se aplican herbicidas en años con primaveras secas como fue en el 2009. Este estudio de dos años trata de los efectos observados de aplicar en pre-emergencia el herbicida flurochloridone+ metolachlor en malas hierbas distribuidas en cultivos de girasol a diferentes etapas de su desarrollo. La infestación de malezas se calcula sobre las etapas del desarrollo del girasol, es decir, en los períodos cuando las malas hierbas son eliminadas y cuando el total de biomasa fresca y seca