Even on the best-managed grassland, output declines year by year as weed species take hold. In the second year, for example, it is not unusual to see a drop in yield of 10% to 15%. The forage quality also declines because the nutritional value of weed species is lower than that of specially developed pasture species.
The best solution is to reseed with a new ley, but that is not always possible nor desirable. If so, overseeding with an appropriate mixture, such as GrassMax, is a relatively low-cost way to revitalise your sward.