Grazed pasture is notoriously variable in yield and nutritive value (Clark and Jans, 1995) due to components, some of which are unpredictable and/or uncontrollable, such as, climate, weather, soil fertility, drainage, herbage varieties and grassland management. Grass growth in Ireland can vary by up to 70% from year to year.

Sward characteristics that determine bite size are an important component of grass intake. Bite size is optimised with tall, dense, leafy swards, which are best achieved on rotational grazing systems (McGilloway and Mayne, 1996).

Adverse weather conditions e.g. "late springs" and drought often exacerbate these deficit periods.