A full re-seed is not always possible due to factors such as stocking pressures or thin top soil not permitting deep cultivation. For thin or badly poached pastures, pasture rejuvenation should be considered a viable alternative to a complete reseed.
Weeds are low yielding and unpalatable, with very low response to nitrogen. You cannot afford to tolerate more than 30% weed grasses in your swards. It is the weeds that invade the pastures and gradually replace the existing sown grass ley that cause the most damage to productivity and this can lead to a huge reduction in yield. Pasture rejuvenation should not be considered for old pastures infected with creeping bent and other grass weeds as any new grass would struggle to compete.
Successful pasture rejuvenations occur when competition from existing sward is minimal, these conditions occur during early spring before the grass is growing too quickly or after second cut silage/early autumn when the soil temperature is still high.
First you need to assess the condition of the sward before deciding how to improve it. Your local Watson Seeds seed specialist will be happy to visit you and advise a course of action.