Perennial grasses are there for the long haul – if you look after them!

David Orr, formerly Queensland Government
Peter O’Reagan, Department of Agriculture and Fisheries

Annual grasses like button and flinders grass come and go. But perennials like desert bluegrass (Bothriochloa ewartiana) are there for years, producing forage in even the worst seasons. These species are really the backbone of every beef enterprise but we know surprisingly little about their ecology. For example, how long do perennial grasses actually live? How are they affected by grazing management and variable rainfall? When are new plants recruited to replace those that die?

Long-term work by David Orr at the Wambiana grazing trial near Charters Towers has provided some answers to these questions. This work involved establishing permanent quadrats in the moderate (MSR) and heavy stocking rate (HSR) paddocks at the trial. These quadrats were then visited at the end of the wet season each year from 1998 to 2010, and the position and size of every perennial plant recorded. This allowed the progress and fate of every tussock and seedling to be followed over 12 years. Soil samples were also collected in 1998 and 2008 and germinated in a greenhouse to measure soil seed banks.

Rainfall over this time was very variable; there were four very good seasons at the start and another three good seasons at the end of the 12 years but there was also a severe drought from 2001 to 2006. This sequence of rainfall years, together with the very different stocking pressures in the two treatments, gave some very interesting results.

  1. Desert bluegrass was indeed very long lived and trial data suggests that tussocks can easily live up to 30 years with moderate stocking. This contrasts with species like wiregrass and hairy panic (Panicum effusum) which survive for only a few years. Interestingly, black speargrass (Heteropogon contortus) was also relatively short lived compared to desert bluegrass.
  2. Desert bluegrass is also far more drought tolerant than most other species: during the dry years of 2001 to 2006, the majority of tussocks survived (60%) while virtually every plant of most other grass species died. Golden beard grass (Chrysopogon fallax) was also long lived and 50% of its tussocks survived the drought.
  3. Although desert bluegrass is long lived and relatively drought tolerant, its survival was strongly influenced by grazing. Overall, mortality through the dry years was far higher under heavy stocking than under moderate stocking: by 2010, 55% of the original desert bluegrass plants from 1998 had survived under moderate stocking compared with only 25% in the heavy stocking treatment. The desert bluegrass plants at moderate stocking were also far larger than the latter plants.
  4. Although many of the desert bluegrass tussocks survived over the whole 12 years, surprisingly, virtually no recruitment of new plants occurred under either moderate or heavy stocking. There was also virtually no seed of this species in the soil seed bank – in both of the years that soils samples were taken, no seedlings emerged.

What do these findings mean for management? First, they show that species like desert bluegrass are very long lived and can withstand some very severe droughts. They really are the species that you can depend upon. However, if they are grazed too heavily during dry years they will eventually die. The bad news is that once they are gone, it takes a very long time for them to return. There seems to be very little viable seed in the soil and it appears that very specific conditions are necessary for it to germinate and for new plants to establish. If there are only a few adult plants left to provide seed then any recovery will naturally take even longer.

The plants that die will probably be replaced by annuals and less productive perennials. These species can supply useful feed, but only in years when there is plenty of rain. The result will be a forage resource that cannot be relied on in dry times—and it is almost impossible to produce beef economically when that happens.

The bottom line is: your perennial grasses are like good friends: look after them and you have them forever in good times and in bad. Treat them badly and they are almost impossible to get back!

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