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Stocking and pastures

Stocking rates

The stocking rate (head of cattle per unit area or animals per hectare) is perhaps the single most important factor in grazing management. It will influence the persistence of pastures and animal productivity and performance. Determining the appropriate stocking rate for a given area can be difficult. In particular, climatic variability makes it difficult to forecast the capacity of an area to carry a certain head of cattle.The more variable the rainfall, the more fluctuation in the optimum stocking rate. For much of the tropical savannas, quality rather than quantity of dry-season feed is the major limiting factor.

Cattle grazing on native pastures in the Northern Territory

Cattle grazing on native pastures in the Northern Territory
Photo: NT DPIF

Lower stocking rates can be more sustainable as animals and pastures can maintain condition and burning is more effective. If the stocking rate is too heavy, pasture species composition declines, weeds invade, soil is exposed to erosion and the country is unable to carry hot fires. Animal health and condition is poorer and their vulnerability to drought increases.

Stocking rates also vary significantly within the very large paddocks that exist on properties in northern Australia. Cattle tend to heavily graze areas where the pasture composition is best. This results in a change in the composition of pasture species over time, and less desirable species eventually dominate. There are also other concerns relating to localised heavy grazing such as soil erosion and weed invasion. While there are several ways of addressing the tendency for cattle to graze unevenly over an area, it remains one of the major management concerns for graziers in the tropical savannas.

Methods to adjust stocking

Adjusting total stock numbers is one way, but often this is not financially viable. Opportunistic spelling, in which stock numbers within a paddock are drastically reduced, is one approach that allows pastures and land condition to fully recover. However, this is only a viable option in very good years when there is plenty of feed about.

The distribution of watering points also influences where cattle graze within an area as pasture condition tends to deteriorate with proximity to water. So effective stocking rates can be reduced by placing new watering points in distant regions, thus encouraging a more even distribution of stock (Partridge 1999: 15).

Fencing is another, albeit expensive, means of controlling cattle numbers and distribution. Stock access to "sweet" country such as river frontages or areas with exceptionally good pasture can therefore be controlled, and degradation of these more valuable areas minimised.

Finally, fire can be used to entice herds to graze more evenly, as cattle graze indiscriminately over the 'green pick' of recently burnt areas. Pasture condition is thus improved because preferred pasture species are not overgrazed. It is important that recently burnt pasture be left free of grazing until the grasses have reached a height of around 15cm; heavy grazing immediately after a fire can result in a decline of desirable pasture species, and can expose the soil to erosion.

Pasture condition

Pasture condition refers to the species composition of a pasture community and the condition of the plants themselves. The most valuable grasses for grazing are perennials, plants whose life spans are longer than one growing season. Valuable perennial grasses for cattle grazing include Mitchell grass (Astrebla sp.) which copes well with relatively high stocking rates and maintains its nutrient value over the dry season, bluegrass and ribbon grass/ golden beard grass (Chrysopogon fallax).

Annuals such as Flinders grass (Iseilema sp.) can provide some valuable feed at certain times throughout the year, however they are much less resilient under heavy grazing than perennials. (Partridge 1999: 8) Which is not to say that perrenials are indestructible. With continuous and heavy grazing, the tops and roots of these grasses become smaller and generally less productive, and they will eventually be killed, allowing less desirable species and weeds to invade. Animal production as a result will decline.

Grass growth

The majority of grass growth in the tropical savannas occurs during the first month or two of the wet season. Cattle gain most of their weight at this point in the year, as their feed intake is at its highest. However the large amount of pasture grass available does not necessarily translate into nutritious feed. In fact the opposite occurs.

While early wet season pastures are fairly nutritious, available soil nutrients to be taken up are quickly diluted into the mass of plant growth. The result, which occurs by about mid-February, is a "green desert" in which there is a good deal of plant bulk but poor quality herbage. Phosphorous is especially important at this time as cattle are gaining weight.

Supplementary feeding is one way in which producers redress this problem, allowing cattle to increase their intake of plant bulk by up to 40 per cent (Partridge 1999: 35). The other is pasture improvement, in which legumes such as Caribbean stylos (Verano and Amiga), shrubby stylos (Seca and Siran) and Wynn cassia are sown. Higher levels of protein and nitrogen, and better digestibility of the leaves of these plants enable the cattle to gain more weight. There are some risks associated with this approach, in particular a loss of desirable native pasture species which may occur because of the higher stocking rates.

Other introduced pasture species such as gamba grass and buffel grass can greatly improve productivity. Many of these species however can invade local ecosystems and out compete native grasses, so they need to be carefully managed and controlled.

The amount of grass growth at the end of the wet season, as well as species composition, should be used as an indicator for the carrying capacity of an area over the dry season, since it is unlikely that any new growth will occur. Producers should consider as a bonus any winter rains, since these are very unreliable. If there is a lack of palatable species or a problem with weeds, the grazier may also factor in fuel levels required to carry a fire at the end of the dry season, and limit stock levels accordingly to allow fuel to accumulate.

Fire management

Fire is an extremely important management tool in the tropical savannas, and is second only to stocking rate management in its capacity for maintaining healthy pastures. Compared to other options it is also relatively cheap and easy to apply on a large scale. The judicious use of fire can be beneficial in several different respects. It can even out the impacts of preferential grazing and encourage stock to move into underutilised areas. It also can be instrumental in pasture species composition and nutrient quality of feed.

Weed control

One of the most significant applications of fire is in the control of weeds and woody plants. A decrease in the use of fire across much of northern Australia has seen a "thickening up" of country under the latter. These woody trees can outcompete pasture grasses for light and moisture, decrease the carrying capacity of country and make mustering extremely difficult. The most effective burns for the removal of these trees are hot, late dry-season fires, although these are also the most dangerous and difficult to control. Producers are then advised to burn firebreaks around the targeted area early in the dry to ensure that fires later in the season can be controlled.

The absence of fire in many parts of the savannas has also been blamed for the incursion of exotic shrubs such as rubbervine, mesquite, chinee apple and prickly acacia. These exotic weeds cause similar problems to their native counterparts, effectively decreasing the productivity of country. The other way that graziers use fire is to mitigate against late dry season wildfires which can be extremely destructive. Burning carried out earlier in the dry is much safer, and reduces fuel available for later fires.

Overall there is still a lot to be learnt about the interaction of fire and grazing, and the impact of both of these on broader ecology. While it is generally accepted that fire is of benefit, questions remain about optimum frequency and timing of fires to achieve certain ends.