To produce grass-fed lamb and beef, we've become students of the art and science of growing grass. In some respects, we farm grass that we harvest using sheep and cattle! I thought a brief description of how we take care of our grass might be of interest.
Grass plants (all forage plants, really) go through three phases of growth. Phase 1 includes germination and re-growth following grazing or harvest. In this phase, the plant does not have enough leaf area to photosynthesize sunlight and carbon dioxide into energy, so it draws on the carbohydrates stored in the seed or in the root system. From a grazing standpoint, Phase 1 plants are extremely nutritious, but because of the lack of leaf area, very low in quantity.
The plant will transition to Phase 2 when it has enough solar receptors (e.g., leaf area) to manufacture it's own energy. At this stage, the forage is both high in nutritional quality and plentiful. Phase 2 is a grass-farmer's goal!
In Phase 3, the plants are trying to complete their reproductive cycle. They may be producing seeds, in the process going through lignification (which means they are becoming more coarse and less palatable. The plants may have so many leaves that the lower ones begin to die for lack of sunlight. At this stage, the forage is plentiful but of low nutritional value. Phase 3 plants may also be called decadent - they are past their prime.
An annual plant is one that completes its life cycle all in one year - it germinates, grows, produces seeds, and dies. Most of California's unirrigated pastures are comprised of annual grasses and forbs (broad-leaf plants). A perennial plant stays alive from year to year. Most irrigated pasture plants are perennials, as were many of California's original native grasses.
In general, our goal as grass-farmers is to keep our forage plants in Phase 2 for as long as possible. We try to manage our livestock to remove just enough leaf material to take the plant to the bottom of Phase 2 - the plants should still have enough leaf area to manufacture their own energy (rather than draw on the energy stored in their roots). At the other end of the cycle, we want to graze these plants before they reach Phase 3, when they will not taste as good to the livestock (and when they will provide less nutrition).
As with every rule, there are exceptions to these principles. At certain stages of production, Phase 3 plants might be more desirable. When we wean our lambs, for example, we want to decrease the nutritional quality of the feed the ewes are consuming so that they'll stop producing milk. On the other hand, we want to keep our lambs on the best forage possible - they'll reach a finished weight much more quickly if they are on high quality forage.
Based on these principles, we can also develop a useful definition of over-grazing. Over-grazing happens one plant (and one bite) at a time. If a plant is grazed down to Phase 1, and then grazed again before it's had a chance to re-grow into Phase 2, we'll stress the root system. Repeated over-grazing will kill the plant because the root system can only store so much energy. In most cases, we don't want to over-graze. There are, however, some instances where over-grazing may be desirable. If we over-graze yellow starthistle, for example, we can reduce it's seed production by up to 90 percent. Repeated over-grazing of Himalayan blackberries will eventually kill the plants.
So far, most of this probably sounds fairly scientific. The art comes in when we introduce animals and humans into the equation. The weather, management timing, stage of production and other factors all enter into my decisions about when to move my animals onto fresh grass. On irrigated pasture, a grass or clover plant may fully recover and be ready to graze again in 25 days during May or June. In August, these same plants may be heat stressed - they might take 35-40 days to recover. Annual grasses typically germinate with the first substantial rainfall in October or November. If we graze these grasses in December, the cold weather and short days mean that they won't fully recover for 90-120 days. If we re-graze these plants too soon, we risk killing them.
As a self-described pasture geek, I enjoy tinkering with this system. I like trying to control undesirable plants like yellow starthistle with grazing alone - no chemicals! I also enjoy trying to grow the highest quality forage possible - and harvesting it with a biological system rather than a mechanical one! I feel like I've failed as a grass-farmer if I have to cut hay!