Open Cut » Environment
This study was initiated by the Upper Hunter Mining Dialogue (UHMD) which comprises stakeholders from coal mining, agriculture, community and environment groups, and local and state government. The UHMD had questions around the sustainability and profitability of mine land which was rehabilitated to pasture. There was ongoing debate about these issues based on very limited data. Questions included the productivity and sustainability of rehabilitated mine pastures and the livestock production from these pastures. There was debate about rehabilitated pastures being more, or less productive than original or adjacent pastures which generally comprised native species. There were also questions about heavy metal residues contaminating soil, pasture and livestock.
Two study sites were identified near Singleton (HVO) and Muswellbrook (MAC) where rehabilitated mine pastures were compared with adjacent analogue native pastures. The Singleton site comprised 2 x 20 ha paddocks for each and was rehabilitated to pasture in the 1980s.The Muswellbrook site comprised 3 x 10 ha paddocks each and was rehabilitated to pasture in the late 1990s.
There were many management issues that needed to be resolved in relation to the set-up and operation of the trial, including: livestock type, stocking rates, grazing management, supplementary feeding, fertilisers and use of trace elements. To resolve these issues, discussions were held with both the UHMD Land Management Working Group and an Advisory Panel that was comprised of the graziers supplying cattle to the trial and industry and community members of the UHMD. It was decided in order to establish baseline data that the study would be as simple as possible with: (Angus) steers to be used rather than breeding cows; no use of fertiliser, supplementary feeding or minerals; and grazing management to be a simple rotation with cattle moved to a new paddock every three months (after weighing).
Two groups of cattle were run at each site. Initially 10 Angus steers per pasture type were run at each site giving a stocking rate of 1 steer per 3 ha at Muswellbrook and 1 steer per 4 ha at Singleton which was considered conservative. At the Singleton Rehab site the number of steers was increased to 15 (50% increase) in the second group in an attempt at increasing pasture utilisation, with hind sight stock numbers could have been increased further as pasture growth continued to exceed stock demand with an associated drop in feed quality. Group 2 stock numbers were again 10 at all other sites.
Similar independent studies of grazing performance on rehabilitated mine land were also conducted at Liddell Mine in the Hunter Valley and New Acland Mine in Queensland.
Soil testing identified no heavy metal toxicities although nickel levels were slightly high at the Muswellbrook rehabilitation site. Soil pH was near neutral on the rehab sites and slightly acidic on the analogue sites. Soil phosphorous was very low at both Muswellbrook sites and at the Singleton analogue site. Phosphorous was near adequate at the Singleton rehabilitation site which was attributed to a strong fertiliser history at the time of initial rehabilitation work and subsequent maintenance applications of fertiliser. Prior to the trial commencing in 2014, the last fertiliser application on the singleton rehabilitation site occurred in 2011. Sulphur was slightly low at both sites at Singleton and very low at both sites at Muswellbrook. All other soil nutrients were in a normal adequate range.
Plant analysis revealed no heavy metal toxicities in pasture samples. Forage feed value reflected growing conditions, plant species (some selective grazing was evident especially in native pastures) and plant growth stage (proportion of green to dead and leaf to stem ratio).
Pasture monitoring showed that while a few grass species dominated at each site there was a wide diversity of plant species present. The rehabilitated sites at Singleton and Muswellbrook had 107 and 87 plants species respectively while the analogue native pasture had 144 and 174 species respectively. The high species diversity in the rehabilitated pastures showed that many grass and herb species (mainly natives) had spread into these areas after sowing with introduced species. Although the sown grasses Rhodes Grass, Panic Grass, Kikuyu Grass and legumes dominated this pasture other species were present and could increase if growing conditions changed to favour their growth.
Pasture monitoring also showed that ground cover and number of weeds did not change during the study. It should be noted that a major increase in stock numbers or deterioration in growing conditions would be expected to impact both ground cover and weed spread as with any grazing enterprise.
Cattle were tested for major mineral and heavy metal deficiency or toxicity. Inconsistent deficiency in selenium and copper was found. Major minerals were in the normal range and no heavy metal toxicity was identified.
Steers grazing on the rehabilitated mine site pastures were found to gain more weight, have better condition (fat cover) and be worth more money than steers grazing on the analogue native pastures. At the Muswellbrook site where soil fertility and pasture availability were similar, the steers grazing rehabilitated pasture were found to have an advantage of 46kg/head and 68kg/head for groups one and two respectively. At Singleton, steers grazing rehabilitated pastures were found to have an advantage of 167kg/head and 97kg/head for groups one and two respectively. Considering that Singleton group two had 50% more stock grazing the rehabilitated pasture, this means that the weight gain per hectare was more than double that achieved on the analogue site.
Normal indicators of sustainability used in grazing enterprises can and should be applied to rehabilitated pastures. During the study, ground cover, and weeds did not change reflecting favourable stocking rates and seasonal conditions. As with any grazing enterprise, if conditions changed then management should respond by adjusting stocking rates and grazing management. The study protocols meant that management options such as supplementary feeding or use of fertilisers were not used and stock numbers were not adjusted to reflect seasonal pasture growth and availability. In the long term soil nutrients must be replaced in any farming system where nutrients are exported in product. In a grazing system nutrient loss due to sale of cattle must be considered.
This project received a great deal of community and media interest which confirms the importance of the topic and the potential to increase recognition of the value of grazing on rehabilitated mine pastures. To realise this potential value of production and public recognition, it is important that a good job is done on all mine rehabilitation to pasture. Any short cuts or poor quality work will reflect badly on the whole industry. There is potential to increase (cattle) production from existing rehabilitated mine pastures by adopting appropriate pasture management strategies especially improving soil fertility and grazing management. Increased pasture growth can also help increase soil carbon with associated improvement in soil health.
It isn't possible to state whether the two sites monitored in this study were broadly representative of all mine rehabilitation to pasture in the Hunter Valley. A review of mine rehabilitation practices and results, focussing on rehabilitation returned to pasture in the Hunter Valley, would help put this project into perspective and answer questions concerning the representativeness of the two study sites.
There are a range of management practices which could improve future mine rehabilitation to pasture. Research into pasture species and variety selection, ground preparation and establishment techniques could potentially identify more productive and cost effective pasture options.
Observation and experience working on these sites has identified various procedures and management options which could improve outcomes achieved when rehabilitating mine sites to pasture for grazing.
- Have a clear objective to sow pasture for grazing with adjacent woodland for shelter and habitat allowing each area to be managed differently within the broader landscape.
- Landform and slope should be compatible with stock movement and management. Steep, broken ground, exposed rocks, logs and holes are a hazard to stock (cattle and horses), people, vehicles and machinery. Steep ground increases erosion risk and can interfere with livestock movement and grazing habit.
- Soil depth and fertility is critical to establishment and growth of pastures. A greater depth of topsoil, improved moisture holding characteristics, chemical, biological and physical soil fertility will all help pasture growth. Many of these features will be enhanced by increasing soil carbon levels which can be helped by having strong pasture growth.
- Grazing management is important for success and production from all pastures. New permanent pastures should not be grazed until established but then need appropriate grazing to encourage a desired mix of pasture species. Lack of grazing can lead to a few species dominating pastures and undesirable pasture characteristics. This has been a problem in mine rehabilitation where it has not been possible to graze for some years after the pasture is sown. Overgrazing would also be a problem leading to loss of some pasture species and exposure of bare ground increasing erosion risk.
Selection of new and improved pasture species and varieties will further improve production potential. The pastures monitored in this study comprised old varieties which were largely chosen at the time to provide ground cover and reduce soil erosion. Newer species and varieties have potential to be more productive and of higher feed value resulting in potentially greater livestock production from rehabilitated mine pastures.