RESISTANCE TO DICLOFOP-METHYL IN LOLIUM MULTIFLORUM: SELECTION HISTORY AND TRADEOFFS BETWEEN RESISTANCE AND GROWTH

GUNDEL, P.E.; MARTÍNEZ-GHERSA, M.A.; GHERSA, C.M., VILA-AIUB, M.M. & D.O FERRARO.

Durban, Sudáfrica.

Congreso; 4th International Weed Science Congress; 2004

Continuous use of herbicide has promoted the appearance of resistant populations in Lolium multiflorum. There is uncertainty corcerning fitness responses of herbicide resistant and susceptible biotypes.  A population of Lolium multiflorum was grown during four years and subjected to 8 selection histories: grown as a monoculture (L) or in mixture with a related species (F, Festuca rubra) under four levels of herbicide. F. rubra is naturally resistant to the herbicide, and potentially capable of crossing with L. multiflorum. Plants were sprayed every year with diclofop-methyl herbicide at 2240 gr ai ha-1 (2x), 1120 gr ai ha-1 (label dose, x), 560 gr ai ha-1 (0.5 x) or left unsprayed (control, 0x). Herbicide lethal dose 50 (LD50) was calculated for each selected population using a standard probit approach. Vegetative and reproductive biomass outputs of plants under field conditions and without herbicide pressure were evaluated in two subsequent years. The LD50 varied between 32.86 gr ai ha-1 and 772.30 gr ai ha-1. Cluster analysis of the survival rates showed three different groups according with the increasing herbicide-resistance level exhibited (0xL, 0xF, 1xF – 1xL, 2xF, 0.5xL – 0.5xL, 2xL). Vegetative biomass production was similar among L. multiflorum populations but differed between years. However, reproductive biomass showed both, year and population (resistance level) effects, with no significant interaction. Within each year the group with intermediate resistance level exhibited the lowest reproductive biomass values.