Land Theme Report
Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Ann Hamblin, Bureau of Rural Sciences, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06748 5
Accelerated erosion and loss of surface soil (continued)
Pressures (continued)
Domestic grazing animal density in the Intensive Land-use Zone [L Indicator 1.2A]
The amount of animal grazing pressure that can be sustained without causing an increase in bare soil and hence the potential for erosion varies greatly with the type of pasture, animal species and production needs, and the season. Good grazing management requires careful attention to the pasture and animal condition, with adequate alternative or supplementary feeds available or alternative grazing arrangements (such as agistment, or sending animals elsewhere during drought).
A study was undertaken to calculate the impact of grazing animals on agricultural lands in the Intensive Land-use Zone (ILZ) (Unkovich and Valentine unpublished study commissioned for this report). The results provide a rough estimate of the relative impact of grazing animals across fluctuating seasonal conditions and varying animal numbers, over the period 1983-1997. This temporal study covers a sufficient length of time to encompass both good and poor seasons in different parts of the country. There are no comparable figures for the period 1997-1998 to 2000 because the ABS now only undertakes an agricultural census every five years, and the next is due in 2002.
At the beginning of the period sheep numbers were very high (Figure 6). By the end of the 1990s the sheep numbers in the ILZ had fallen by 30% compared with the numbers just before the crash in wool prices in 1987. The effect on overall grazing intensity, calculated as dry sheep equivalents per hectare (DSE/ha) was more pronounced in the latter part of the 1980s than in the 1990s, when cattle prices improved and some areas replaced sheep with cattle.
Figure 6: Changes in sheep and cattle numbers in the Intensive Land-use Zone, 1990-1998.
Source: ABS (2001)
The total area used for grazing has fluctuated as the result of combinations of seasonal variations and prices, but much of the fluctuation was in improved pasture areas where grazing is rotated with cropping. Native pastures and rough grazing make up over 75% of the total grazed area of the ILZ (97-110 million hectares) whereas improved pastures cover 25-30 million hectares.
Figure 7: Total area used for grazing and dry matter production (improved and unimproved) in the ILZ, 1983-1998.
Source: ABS (2001)
In 1983-84, which was a very good season following a very pronounced drought in eastern Australia, a record 18.5 million hectares were planted to winter grain crops, and a drop in pasture area resulted. However, grain production area then fell back to 14-15 million hectares until the latter half of the 1990s, reaching 20 million hectares in 1997 to 2000, while sheep numbers showed a progressive decline (Figure 9).
Figure 8 shows the spatial distribution of the long-term average dry matter production of combined native, improved rain fed and irrigated pastures, mapped by Statistical Local Areas (SLAs), for the period 1983-1997. Figure 9 shows the calculated stock densities, as dry sheep equivalents (DSEs). Note there are some SLAs in Victoria that have not been included, because the statistics relating to these areas are unreliable.
Figure 8: Mean and range (standard error) of dry matter production from grazed lands in statistical local areas within the Intensive Land-use Zone, 1983-1997.
Source: Unkovich 2000 (unpublished)
Figure 9: Mean and range (standard error) in dry sheep equivalents of domestic stock in statistical local areas within the Intensive Land-use Zone, 1983-1997.
Source: Unkovich and BRS (unpublished)
The maps indicate that, while the distribution of higher stock densities is roughly aligned with areas of higher biomass production, there are local differences, particularly in the irrigation and high rainfall coastal areas. High stocking rates characterise irrigated pastures in dairy areas, but numbers of dairy cows have fallen everywhere except Victoria, where numbers have risen rapidly from 1 million to 1.4 million since 1991 and now constitute over half the national herd. These changes have resulted from industry restructuring, pricing differentials and the establishment of processing plants. They reflect the increased intensification of large-scale, irrigated and shedded dairy farms in Victoria.
Relatively few areas have been overstocked during the past decade. However, SLAs that have had less than 1000 kg/ha of plant biomass per DSE per year are assumed to have been at risk of overgrazing. Some SLAs have had occasional years at risk, generally associated with a drought period. In New South Wales and Queensland, for example, a number of SLAs dropped below the 1000 kg/DSE value only in 1994, which was a very dry year in those regions.
Vertebrate pest herbivore pressure in the ILZ [L Indicator 1-2B]
Data on feral and native pest vertebrate populations have not been collected regularly in the ILZ in the period 1995-2000. However, the National Task Force on Rabbit Calicivirus has provided very interesting data on the change in rabbit distribution, numbers and impact on vegetation during this period across the whole continent (Neave 1999). Estimates on kangaroo numbers are computed annually for setting quotas for culling the most abundant species (Pople and Grigg 1999), but are acknowledged to be subject to both over and under estimates.
The most challenging problem has been to allocate the estimated numbers to specific locations that could be related to the administrative boundary-based data that are available for domestic animals, and the likely grazing lands that feral and native herbivores will inhabit. Table 4 summarises the occurrence of vertebrate herbivores that were considered for inclusion in the study. Non-domestic herbivores added an estimated 57 million DSEs to the domestic sheep and cattle total of 350 million DSEs in 1999.
| Region | DSEs |
|---|---|
|
Intensive Land-use Zone (south-western, southern and eastern agricultural regions) |
8.6 million |
|
Western Extensive Land-use Zone (excluding far north) |
7.3 million |
|
Central Australia and 'Top End' (northern Australia) |
16.3 million |
|
Eastern Extensive Land-use Zone (excluding Cape York) |
28 million |
A Rabbit, pig, goat, horse, eastern grey kangaroo, western grey kangaroo, red kangaroo, other macropods.
These estimates put the combined grazing effect of non-domestic herbivores at less than 10% of domestic stock in nearly all areas of the ILZ. However, there are local areas and seasons where this may build up to a larger proportion (Caughley et al. 1987).
Implications for the ILZ
From this analysis it would appear that most areas in the ILZ have not been overgrazed during the past decade, except where prolonged drought has caught farmers in the difficult situation of managing destocking during periods of negative income. The irony of this situation is that, from an environmental point of view, vegetation and land cover tend to increase when total grazing pressure is reduced by low commodity prices and diversification out of grazing enterprises. Good rainfall seasons have enhanced, rather than caused, the increase in cover in many regions.
There has been some substitution of cattle for sheep following the drop in wool prices, but on the whole adjustments have been made in the degree to which more marginal land within each farm or SLA has been used or not. In many broadacre farming districts there are some land areas of low productivity that are simply not used when seasons are poor or stock numbers are low.
The expansion of the cropping area during the past decade has been less than has been widely supposed, although there have been small regional increases in the area of industrial and horticultural crops. More frequently, diversification of enterprises has been used to increase productivity or profitability; for example, by moving to a higher-value product such as producing cross-bred sheep for prime lamb rather than pure bred Merino-wool sheep, oilseeds, grain legumes and hard wheats rather than Australian Standard White wheat, and barley. While these adjustments have improved profits it has not altered land use or land management practices that may affect erosion significantly.
While the impact of grazing animals on erosion has therefore been less than in the 1970s and 1980s over much of the ILZ, the low water-use efficiency of many pastures has not improved significantly, because the pasture composition is still dominated by annual short-rooted species such as rye-grass and clovers. This issue is examined in more detail in the section 'Secondary salinity and acidity'.
One of the big issues that remains is the degree of uncertainty and lack of consistency from year to year with many items in the ABS agricultural statistics. The most difficult issue of all relates to the very poor information that can be gleaned from these statistics on the total amount of grazing land. The lack of reliable, consistent statistics on the land area used for grazing, its condition, and its species composition seriously hampers the interpretation of the likely impact of grazing on catchment water use, erosion and water quality.