Spelling paddocks between grazings is a common feature of pasture systems in the Northeast. An important question for intermittent grazing management, more commonly called rotational grazing, is how long should a pasture be left before it is grazed again? The number of days between grazings of the same paddock is called the rotation length. Adjusting the rotation length is an integral part of a successful pasture system because of its effects on pasture quality, the amount of pasture produced each year and the ability of plants to survive in the sward (e.g. alfalfa is not suited to continuous stocking but ryegrass is). Changing the length of rotation is referred to as “speeding up” (i.e. reducing the number of days between grazings) or “slowing down” (i.e. lengthening the spelling interval).
The selection of a rotation length is usually dependent on the rate of pasture growth, the post-grazing residual, residual dry matter (RDM) and the number of paddocks available. In general rotation lengths should be shortest when the rate of pasture growth is high and longest when pasture is growing slowly. If growth rates are 70 lb DM/acre/day the time to regrow sufficient pasture to pre-graze levels is half that if growth rates are 35 lb DM/acre/day. However, while pasture growth rates are important they should not be considered apart from plant and animal requirements, and the time of year.
The post-grazing residual can affect the rotation length in two ways. First, if pastures are being grazed very short the rate of pasture regrowth is reduced, particularly if this occurs over a short period at successive grazings, because plant root reserves are depleted and relatively little leaf remains for photosynthetic activity to generate plant energy requirements.
Second, rotation length is usually lengthened if the post-grazing residual (or height) is low because it takes animals physically longer to graze down to this level. That is, bite size is reduced as the height of pasture declines and the animal must take more bites (and hence longer) to harvest the same amount of pasture each day than if a lax grazing residual is adopted (see Article 6). It follows that weight gains and milk production are usually greatest when a quick rotation with lax grazing residuals, that allows animals to mainly select high quality leaf at each grazing, is adopted.
The number of paddocks is often the primary factor used by farmers to determine rotation length, but in practice it should receive less attention than the rate of pasture growth, animal feed requirements and the time of year. Setting the rotation length solely on the number of paddocks available may result in the grazing interval being too long during the spring-early summer and too short during summer and fall. The reason for this is outlined in the following example.
Consider a farm with 30 paddocks, and a herbage growth rate of 70 lb DM/acre/d in spring. If the target residual is 1800 lb DM/acre, then pastures will regrow to about 3900lb DM/acre (1800 + (70 x 30)) in a 30 day round (grazing interval) and 6000 lb DM/acre if 2 days grazing per paddock was adopted. From a pasture quality point of view it would be better to graze orchard grass pastures when herbage mass is between 2500 and 3500 lb DM/acre. This means that the farmer should consider taking paddocks out of the rotation (i.e. to drop paddocks), adopting 12 hour grazings per paddock (in which case a 15 day rotation length or round is achieved), or a combination of these options. The point is that the number of paddocks is not a satisfactory way to determine rotation length, although subdivision provides the mechanism by which rotational grazing is implemented.
Rotation length can serve two contrasting purposes. One is to ensure that paddocks are grazed at the correct interval so that pasture quality is maintained. This is particularly true during the spring-early summer when rotation lengths are often too long. The second purpose is to transfer pasture through time, particularly during the autumn. By slowing the rotation down and reducing the post-grazing residual animal intake swill be reduced and the amount of pasture available can be rationed over a longer period of time. Stock piling fall pasture is one example of this.
In general a quick rotation (as short as 10-15 days) should be adopted when spring herbage growth is at its peak. It may be also necessary to drop paddocks from the grazing rotation after the first or second round is completed. These can be made into silage or hay and then returned to the grazing round. Keep an eye on pastures, not only those being grazed but also on those that may not be grazed for 10-20 days and the amount of regrowth that has occurred on paddocks already grazed in the rotation. The state of pastures (e.g. seedhead development beginning or poor regrowth) may indicate that the rotation length should be altered during the round. For this reason it is not possible to give recipes on rotation lengths during the season (e.g. Voison system). Effective grazing management is achieved when the farmer understands the principles for setting the rotation length for his/her own property, not when a figure prescribed by a textbook or magazine article is adopted.