On a recent farm visit, the question of how often to clip pastures was raised by the farmer. He indicated that he wanted to maintain pasture quality, but to save on fuel costs, only wanted to clip when it was really needed. Of course, like most things, the answer was "it depends".
What does it depend on? First and foremost, it depends on how much forage is being rejected by the animals. That depends on how mature the forage is, how many times they have grazed the paddock since it was last clipped, and how much manure is building up in the paddock. If a significant amount of the forage is being rejected, this will decrease the intake of your livestock. Lactating dairy cows will tell you right away if they don't want to eat what's there or can't find what they need to eat - your milk tank shipment will drop. Other kinds and classes of livestock are more difficult to determine when the time is right for clipping. Of course, the decision to clip also may depend on your own personal tolerance for how the pastures look.
A management strategy that can help to reduce the need for clipping is smaller paddocks and more frequent moves. Just like people, when there is a limited quantity of food available, animals will graze more aggressively and will do a better job of harvesting - in other words, they are less selective about what they eat. Your paddocks are more likely to be grazed more evenly, and there will be less rejected forage - there will also be less waste due to trampling. Of course, this results in a need to move them more frequently, because they will run out of forage more quickly. The return on more management is less clipping, and less fuel and time spent doing so!
Source: Grazing Lands Conservation Institute Grazette, July 2008.