Gulf pastures after the 2019 flood event

The response of grass pasture systems across the north west Gulf has been variable following the monsoonal rains and the February flood in early 2019. In some areas, severe soil erosion occurred as a result of the fast moving floodwaters, while in others, the rain provided some recovery opportunities for drought-stressed perennial pastures.

The flooding resulted in:

• Mass germination of Mitchell grass seedlings and the opportunity to improve productivity of Mitchell grass pastures.
• Fast moving floodwater exposing Mitchell grass crowns and roots increasing the risk of tussocks being pulled out or weakened by cattle grazing.
• Large areas of pastures buried in silt due to relocation of topsoil into downstream areas.
• Mass germination of less palatable and short-lived forbs and broadleaf weeds.
• An increase in undesirable pasture species; e.g. Asbestos grass, particularly in flood out areas.
• An increase in woody weeds and woody natives.
• Emerging weed infestations in previously unaffected areas.

Depending on the severity of the situation, a range of strategies may assist with rangeland recovery during the coming wet season.

Bluegrass browntop plains

In the Gulf Bluegrass browntop plains (Figure 1), the dominant perennial pasture species include Gulf bluegrass (Dichanthium fecundum), hoop Mitchell (Astrebla elymoides), silky browntop (Eulalia aurea) and bull Mitchell, (Astrebla squarrosa). These species are known to be quite tolerant of flooding and water inundation. They have responded reasonably well depending on the condition of these pastures leading into the wet season.

Buffel grass

Buffel grass (Cenchrus ciliaris) that was underwater for three to five days may have drowned. Drowning of tussocks depended on depth, turbidity and temperature of the flood waters. However, some tussocks that appear to be dead may well produce new tillers. It is important to monitor these tillers when assessing grazing pressure as removal of these tillers will effectively prevent these regrowing tussocks ability to respond to future rain events.

In addition, new plants should have established from seed, or will establish in the 2019/2020 wet season. Long-standing and well managed Buffel grass pastures will have a good soil seedbank despite grazing. Buffel grass will set seed during all phases of pasture growth, i.e. very young plants will set seed early in the season which helps replenish soil reserves even in tough years.

Weeds and other undesirables

The emergence and movement of weeds and undesirable pastures is an unfortunate outcome of flooding. Asbestos grass (Cenchrus basedowii; Figure 2) is an annual or short-lived perennial species commonly found in Gulf pastures. Dispersal of the seed occurs via wind, overland flow of flood waters and attachment to animals and vehicles. Given the extent of the February flooding, Asbestos grass may well be expanding into new areas.

Good land management practices, such as monitoring stocking rates and wet-season spelling of pastures, will assist with increasing competition from desirable pasture species.

The emergence of new woody weed infestations is also highly likely and the spread of prickly acacia (Acacia nilotica) is of significant concern. Conditions following the February floods were ideal for the spread and establishment of such species. Priority treatment of invasive weeds will be critical to prevent the establishment of new infestations.

Assisting landscape recovery

Erosion, loss of native pastures and the depletion of soil seed reserves, has been significant across flooded regions and how long these severely impacted pasture systems will take to recover is unknown.

Some low cost management options for recovery of these degraded areas include:

• wet-season spelling
• forage budgeting for the season ahead, after the wet season (usually April/May)
• very light stocking
• exclusion of livestock from severely impacted areas.

High-cost options include mechanical interventions to prepare a seedbed and planting suitable sown pastures into flood-impacted areas, usually in January after break-of-season rain.

This is not necessarily going to be a practical or cost effective option for everyone as it is challenging to apply across large areas and success is seasonally dependent. Check which grasses and legumes are suitable and if unsure, seek advice from Department of Agriculture and Fisheries pasture agronomists.

Surviving grass tussocks and newly established grasses post-flooding will need time to recover, establish and set seed in order to secure a well-stocked seedbank for following years.

Wet season spelling and light grazing pressure are critical for the recovery of native pastures. Data from the long term Wambiana grazing trial shows that little or no recovery occurs with wet season spelling unless stocking rates are at an appropriate level. Managing stocking rates will thus accelerate pasture recovery of pastures and encourage any surviving seedlings to grow to maturity during this upcoming wet season. Recovery may initially be slow but will accelerate over time as tussocks expand, produce more seed and better species return.

General recommendations:

• Wet season spelling is important for pasture recovery but to be effective, stocking rates need to be carefully managed.
• For country that was badly damaged with a lot of erosion and tussock roots exposed, a full wet-season spell for successive years is needed to maximise seed setting to rebuild the soil seedbank. Paddocks that recovered well after the flood, should be utilised for grazing over the wet season.
• For country where tussocks are recovering, spell at least until the pasture is “ahead of the cattle” i.e. the grasses will hold their own when the cattle are introduced.
• Manage and move livestock based on pasture availability.
• Establish pasture monitoring points that are easily accessible and monitor seedling and tussock development regularly.
• Undertake forage budgeting and understand short- and long-term carrying capacities. When stocking rates are too high, animal performance generally declines and overall production is impacted.
• Cattle that are grazing pastures down to the stem will ultimately go hungry if that is all that remains. When animals are forced to eat a higher proportion of stem, diet quality and resulting animal growth will be reduced as stem has less than a quarter of the leaf protein content; this is the case irrespective of the stage of growth i.e. green or hayed-off pasture.
• Leave a minimum of 15-20 cm residual grass stubble height at the end of the dry season for optimum plant health and to enable a quick response following rain.

Remember, the removal of pasture cover at the end of the dry season increases runoff and decreases water infiltration. More rainfall into the soil equals more pasture growth which in turn increases the carrying capacity for the next dry season. Land condition can be slow to change and improving it relies on both break of season cover and wet-season spelling.