Impacto do cultivo de azevém tetraploide sobre azevém diploide resistente a herbicidas

Abstract

Competition between the tetraploid with natural population of the diploid ryegrass may be a tool to reduce resistant plants in the cultivated rangeland. The objective of this work was to evaluate the opportunity and the impacts of the use of tetraploid ryegrass genotypes in substitution to the diploid, in the crop-livestock systems of Rio Grande do Sul. To meet the objectives of present work, were done four studies, aiming: 1) evaluate the fitness cost, phenological development, yield components, and competitive ability, between diploid and tetraploid ryegrasses; 2) quantify the effect of rates of post-emergence herbicides on selectivity, and application of herbicides in different phenological development under the control of tetraploid ryegrass; 3) determinate the seeds longevity of diploid and tetraploid ryegrasses buried in different depths and periods; 4) measure the crossing between diploid and tetraploid ryegrasses to evaluate the physiological quality and viability of the seeds from the crossing, percentage of survival and inheritance of glyphosate resistance, as well as the level of ploidy from progenies. The results showed that the tetraploid ryegrass have a longer cycle than the diploid, due to the longer tillering period. The tiller number, leaf area, root dry mass and the number of seeds were higher to the tetraploid ryegrass, that also had a higher competitive ability than the diploid. The herbicides bentazon, 2,4-D, saflufenacil and carfentrazone-ethyl were selective to the tetraploid ryegrass regardless of the rate used. Glyphosate, diuron + paraquat, clethodim and haloxyfop showed satisfactory control to tetraploid ryegrass. Glufosinate had unsatisfactory control in tillering and stretching stages, however was good when was sprayed in flowering stage. Diploid ryegrass seeds got primary dormancy, avoiding the germination immediately after buried, while tetraploid seeds showed no dormancy, remaining in the seed bank in a quiescent stage. Both genotypes have potentially emergent seeds in the fourth month after burial, when the environmental conditions (temperature, radiation and moisture) are adequate for the ryegrass development. Whatever of the genotype tested and the period after burial, seeds buried at greater depth showed superior longevity in the soil seed bank. Progeny from the susceptible tetraploid♂ x glyphosate resistant diploid♀ crossing exhibited germination problems, with only one triploid plant being detected from the 22 plants evaluated. However, the progeny from the crossing between glyphosate resistant diploid♂ x tetraploid susceptible♀, did not show germination problems, but 46% of the plants died quickly after the seedling stage, with triploid (10%) and pentaploid (25%) seedlings identified. No glyphosate resistant plants were detected in susceptible tetraploid♀ x resistant diploid♂ progeny, however in susceptible tetraploid♂ x diploid resistant♀ progeny, was observed a reduction of 47% of glyphosate resistant plants compared to cross-fertilization progeny resistant to glyphosate.