Resistance to diclofop-methyl in Lolium multiflorum: selection history and tradeoffs between resistance and growth.

GUNDEL, P.E.; MARTINEZ GHERSA, MARIA ALEJAND; GHERSA, C. M.; VILA AIUB, M.M.; DIEGO OMAR FERRARO

Durban, South Africa

Congreso; IV International Weed Science Congress.; 2004

Lolium multiflorum is an outbreeder with great variability in seed persistence in the soil bank, seedling emergence rate, and time of pollen release among different populations. This species has developed resistance to diclofop-methyl. There are uncertainty corcerning fitness responses of herbicide resistant and susceptible biotypes, in part, due to inadequate information as to their relative performance in different environments. The objective of this study was to evaluate 1) the effect of selection history on the level of herbicide resistance and 2) the tradeoffs between herbicide resistance and growth. 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 g ai.ha-1 (2x), 1120 g ai.ha-1 (label dose, x), 560 gai.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 were evaluated in two subsequent years. The LD50 varied between 32.86 gr a.i/ha and 772.30 gr a.i/ha. 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.