Dormancy, germination and ageing of Lolium multiflorum seeds following contrasting herbicide selection regimes

Lolium multiflorum Lam. is an annual weed that has developed resistance to diclofop-methyl herbicide. Knowledge of the relative fitness components of resistant and susceptible biotypes is important to predict and manage resistance. Here, we use different L. multiflorum selections obtained from a long-term experiment, in which the herbicide dose (0, 280, 560 and 1120 g a.i. ha−1) and gene flow chances from Festuca rubra, naturally resistant to the herbicide, were managed. The relationship among herbicide selection history, the level of resistance achieved, and seed dormancy, germination rate and ageing is evaluated. The current resistance level of the different selections was not related to the herbicide dose of selection. The level of association between herbicide resistance and germination varied with the selection treatment and after-ripening time. There was a general positive relationship between hours to 50% germination and herbicide dose of selection, which was independent from the presence of F. rubra. Two L. multiflorum selections with highest resistance level diverged in their response to alternating temperature (i.e. seed dormancy level). While one of them displayed a high germination rate and high ageing rate, the second one displayed a low ageing rate during the evaluated period, similar to the control selection. Most arguments on how weeds evolve increasing fitness on agricultural fields are centered on dose responses and rarely on the effects of herbicide selection on other traits. Our results provide evidence that directional selection for other characteristics apart from resistance, may result from herbicide control causing adaptive phenological change.