If you are using herbicides to control weeds then it is best not to use the same herbicide year after year lest you eventually make the local populations of specific weeds resitant to the herbicide and then a major problem for everone around you.
You will notice that herbicide products are assigned a herbicide grouping from A through to Z. This relates to the different biochemical modes of action of the various herbicides.
Resistance to group A herbicides is likely to arise with repeated use where as this is highly unlikely with Group Z herbicides. So it is best to use herbicides from different groupings at different times when targeting a particular weed.
If targeting herbaceous weeds you might use glyphosate, Garlon(triclopyr), Kamba(dicamba), KambaM(dicamba + MCPA) or MCPA(MCPA) at different times or together in the same mixture. Glyphosate is a general systemic herbicide where as the others are selective broad leaf, or herbaceous plants other than grasses, herbicides.