Plant Requirements vs. Animal Requirements

For plants to regrow, they must retain their growing points after grazing, and have energy reserves stored in the crown, stems and roots. For the animals, forage must be sufficient in both quantity and quality to meet their requirements for maintenance and production. Managers strive to balance these priorities over the course of a growing season. But situations may arise where one needs to be emphasized over the other for a time. For example, if feed requirements increase under conditions where the pasture is in a weakened state, the manager may choose to end grazing and purchase hay if he thinks that further grazing may compromise the health of the pasture. Alternately, if the pasture is in good condition, the manager might allow some slight overgrazing of the pasture, thereby avoiding the added cost of purchased feed. Anticipating the outcome of management decisions is an important skill to develop in designing and managing a rotational grazing program.

How Much of the Plant to Graze

A key decision for managers is how close to graze. The intensity of grazing in a paddock determines how much leaf area will remain after grazing and how many growing points are available to provide regrowth. As a general rule, tall-growing, bunch grasses should be grazed to a taller stubble height than low-growing or spreading grasses. The low-growing grasses typically have leaves and growing points at or very close to the soil surface to protect them from being overgrazed. As animals graze down a paddock, this increases the ratio of stem-to-leaf matter they consume, resulting in lower weight gain or milk production per animal. In an undergrazed paddock animals can be more selective in what they eat, but the result may be a poor utilization of pasture resource. Although meat or milk production per animal may be high when pastures are undergrazed, production per acre will be low. Managers usually take into account the stocking rate, and their own observations of plant height in the paddock to make decisions about how close to graze.

How Often to Graze

Rotational grazing is not a guarantee of good pasture management. Managers must look closely at the length of time the livestock are in a paddock, as well as the rest period between grazings. Based on this information, managers can calculate the number of paddocks needed to support the grazing system. A simple formula to calculate the number of paddocks needed is:
(length of grazing period + length of rest period) / length of the grazing period = number of paddocks required

The length of the grazing and rest periods are average estimates since these may vary throughout the year depending on weather conditions. During times of slow pasture growth the rest period will be longer; as pasture growth rate increases, the rest periods may be shortened. The most experienced managers have a concept of what the pasture height should be when livestock enter a paddock and when they exit it. As outlined in the previous bullet point, these heights are different for different forages, and sometimes for the same forage at different times of the year. The nutritional requirements of the animal and the grazing tolerance of the pasture will be the major factors that determine these heights.