The response of littleseed canarygrass biotypes to isoproturon, pendim
ethalin, and diclofop-methyl was evaluated in India, in pot studies an
d the field during the winters of 1991 to 1999 and 1992 to 1993. Some
biotypes of littleseed canarygrass were resistant to isoproturon but c
ross-resistance to pendimethalin and diclofop-methyl was not confirmed
. The resistant biotype required a higher dose of diclofop-methyl than
the susceptible biotype. Variations in the response of littleseed can
arygrass biotypes were not due to isoproturon formulation, Resistant b
iotypes required 2 to 8 times more isoproturon than a susceptible biot
ype for the same level of control. Diclofop-methyl at 1.0 kg ai/ha app
lied at the 2- to 3-leaf stage of littleseed canarygrass in pot experi
ments and PRE pendimethalin at 1.5 kg ai/ha in field trials controlled
resistant biotypes. Field surveys of the affected areas revealed that
resistance in littleseed canarygrass is more prevalent in rice-wheat
rotations compared to other crop sequences. Control of littleseed cana
rygrass with isoproturon dropped from 78 to 21% from 1990 to 1993.