Improving high-country native pastures

Improving native pastures involves a balance of management and inputs for plant persistence and animal production. To develop some guidelines for native pastures in Victoria, a trial was established in the spring of 2016 in hill country in the Tambo Valley, east Gippsland. The aim was to demonstrate how grazing management and soil fertility could be used to improve pasture production and feed quality of native pastures, while minimising soil erosion.

A site at Reedy Flat (Figure 1) had a southerly aspect and was dominated by microlaena (Microlaena stipoides).

The second site at Connors Hill had a northerly aspect and was dominated by red grass (Bothriochloa macra) (Figure 2).

At each site, two areas were monitored: a control area (farmer’s usual practices) and a treatment area (grazing management and the application of phosphorus fertiliser). Ground cover, species composition, feed on offer, nutritive value and soil fertility were monitored in both areas. Summer measurements were critical, as this is the time of year when ground cover is traditionally at its lowest.

Fertility

From previous research, we know that native species respond to increased soil fertility. The target level for both sites was an Olsen P of 12mg per kg. The Reedy Flat site had an initial Olsen P of 9.6 mg/kg and the Connors Hill site an Olsen P of 4.4mg per kg. The soils at both sites were acid, with soil pH (CaCl2) of 4.8. In 2017, the Reedy Flat treatment area had 232kg per hectare of single superphosphate applied (equivalent to P at 20.5g per hectare). The Connors Hill treatment area had 295kg per hectare of single superphosphate (equivalent to P at 26kg per hectare) and a follow-up application of 200kg per hectare of single superphosphate (equivalent to P at 8kg per hectare) in 2018, because of lower fertility.

In 2019, the Reedy Flat treatment area had an Olsen P of 8.9mg per kg compared with a decline in the control area to 6.7mg per kg. The fertiliser application at Connors Hill increased the Olsen P to 10mg per kg.

Grazing management

The treatment areas were rotationally grazed with cattle to maintain ground cover, and to limit the amount of bare ground for weeds to germinate and reduce opportunities for soil loss. When feed on offer reached 1800 kgDM per hectare, the control and treatment areas were grazed. Stock were removed from the sites when herbage mass was around 1200 kgDM per hectare, but no less than 1000 kgDM per hectare.

The aim was to decrease competition from annual species, including weeds, through high pasture utilisation. Grazing was excluded over the summer (mid-November to mid-March) to allow native seed germination. The seed bank from native pastures is very limited and reliant on the current year’s seed set.

Grazing was only allowed in this period if significant summer storms (and therefore significant native pasture growth) were experienced.

Despite the area experiencing extremely dry conditions during the spring and summer of 2019, careful grazing management maintained very good ground cover. At Reedy Flat, ground cover was maintained at more than
90 per cent and the Connors Hill site recorded 81 per cent at the end of summer. However, because of the low rainfall, microlaena at the Reedy Flat site struggled to get above 1600 kgDM/ha, and the amount of feed on offer decreased due to moisture stress. As a result, during extended dry periods, the recommendation is to commence grazing microlaena pastures at 1600 kgDM/ha rather than 1800 kgDM per hectare.

Applying the right amount of fertiliser to native pastures and grazing at 1800 kgDM per hectare can maintain ground cover, mitigating the likelihood of soil erosion and weed infestation. The greatest benefits of this strategy were observed, and are more likely to occur, on northwest-facing native pastures.

Feed quality

At both sites, application of fertiliser resulted in a higher legume component of the pasture (54 to 60 per cent), which increased pasture quality. Crude protein and metabolisable energy were 25 per cent higher on the treatment areas than on the unfertilised control areas. The legume is important in terms of livestock production. However, its increase may have also lead to a slight decline in ground cover over the summer months.

Seasonal conditions

The area’s long-term average annual rainfall is 700.8mm at Connors Hill and 780mm at Reedy Flat with the majority falling in September, October, November, December and February. Rainfall in this area has been well below average since the trial started, and has only recently received decent rains.

Persistence

Perennial native grasses need to be carefully managed because seedling recruitment events are rare.

Basal cover is a measure of the presence of a species and is commonly used to measure plant persistence. When measurements are taken at the plant base, they are less affected by grazing, seasonal conditions and phenological development. A good perennial grass pasture may only have a total basal cover of five per cent for all species. At the Reedy Flat site, the basal cover of microlaena and wallaby grass was monitored (Figure 3).

Figure 3 shows a decrease in the basal cover of microlaena in the treatment paddock from four per cent in the summer of 2017 to 2 per cent at the end of summer in 2019. The decline in 2019 was most likely due to the dry seasonal conditions. The increase in plants in both treatment and control areas in 2018 was related to the summer rainfall. In December 2017, 115.5mm was recorded (compared with a long-term average of 62.2mm), and a further 68.5mm was recorded in January 2018 (which is equal to the long-term average).

At the Connors Hill site, basal cover of microlaena, wallaby grass, red grass, windmill grass and spear grass was monitored. In the treatment area, there was no significant change in the basal cover. Figure 4 shows that the red grass component in the control area was initially much higher than in the treatment area, and so the changes over time are more notable than the actual proportion. In the control area, there was a decrease from 15 per cent in the summer of 2017 to 7 per cent in the summer of 2019. This decline was most likely due to the dry seasonal conditions.

The increase in red grass basal cover in 2018 in the control area was related to the summer rainfall. In December 2017, 111.5mm was recorded (compared with the long-term average of 70.1mm). In January 2018, 102.3mm was recorded (compared with the long-term average of 61.8mm).

At the Connors Hill site, a slight increase in the amount of red grass, in both frequencies and basal measurements, was recorded. This was likely due to increased cover by the plants that are there, rather than to recruitment. It will be interesting to see if this trend continues.

This project is supported by the East Gippsland Catchment Management Authority, through funding from the Commonwealth Government’s National Landcare program and Agriculture Victoria.

Key points

• Rotationally graze native pastures to a minimum herbage mass (1200 kgDM/ha).
• Fertiliser application can increase the legume component and nutritive value of the pasture without reducing the native grasses.
• In spring, apply extra grazing pressure to ensure that clover does not smother native grasses.
• In summer, avoid long rest periods during the growing season because they result in standing dead material but reduce grazing pressure in summer, allowing seed set.
• In late summer, retain plant litter to minimise bare ground.