No abstract

The dynamics of Heteropogon contortus and Stylosanthes scabra cv. Seca populations were studied in a subset of treatments in an extensive grazing study conducted in central Queensland between 1988 and 2001. These treatments were 4 stocking rates in native pasture and 2 of these stocking rates in legume oversown and supplement/spring burning treatments. For the 1999–2000 summer, population data for H. contortus in 5 of these native pasture and supplement/burning treatments were compared with those for an additional burnt treatment. Seasonal rainfall throughout this study was below the long-term mean and mean annual pasture utilisation ranged from 24 to 61%. Increasing stocking rate from 5 to 2 ha/steer in native pasture reduced H. contortus plant density. Increasing stocking rate reduced seedling recruitment as a result of its effect on soil seedbanks. Seedling recruitment was the major determinant of change in plant density, although some individual H. contortus plants did survive throughout the study. Burning in spring 1999, particularly at light stocking rate, promoted seedling recruitment above that in both unburnt native and legume oversown pasture and resulted in increased H. contortus plant density. In the legume oversown treatments, S. scabra cv. Seca density increased rapidly from 15 plants/m2 in 1988 to 140 plants/m2 in 2001 following a lag phase between 1988 and 1993. This increased S. scabra density was associated with an eventual decline in H. contortus plant density through reduced seedling recruitment. It was concluded that H. contortus population density is sustainable at stocking rates of 4 and 5 ha/steer (30% pasture utilisation) and that spring burning at light stocking rate can promote H. contortus populations. Increasing densities of S. scabra need to be managed to prevent its dominance.

The population dynamics of the palatable, perennial grasses Bothriochloa ewartiana (Domin) C.E.Hubb. (desert Mitchell grass), Chrysopogon fallax S.T.Blake (golden beard grass) and Heteropogon contortus (L.) P.Beauv. ex Roem. & Schult. (black speargrass), were studied in an extensive grazing study conducted in a eucalypt woodland within the Aristida–Bothriochloa pasture community in central Queensland between 1994 and 2000. Treatments were three grazing pressures based on light, medium and heavy utilisation of forage available at the end of summer and two timber treatments (trees intact and trees killed). Seasonal rainfall throughout this study was generally favourable for plant growth with no severe drought periods. Grazing pressure had a greater overall impact on plant dynamics than timber treatment, which had minimal impact. Grazing pressure had a large impact on H. contortus dynamics, an intermediate impact on B. ewartiana and no impact on C. fallax. Fluctuations in plant density of both B. ewartiana and C. fallax were small because both species were long lived with low levels of seedling recruitment and plant death, whereas fluctuations in H. contortus density were relatively high because of its relatively short life span and higher levels of both recruitment and death. Heavy grazing pressure increased the recruitment of B. ewartiana and H. contortus in some years but had no impact on that of C. fallax. Heavy grazing pressure reduced the survival of the original plants of both B. ewartiana and H. contortus but not of C. fallax. For H. contortus, the size of the original plants was larger where trees were killed than where trees were left intact and plants of the 1995 seedling cohort were larger in 1998 at heavy compared with those at light and medium grazing pressure. Grazing had a minor negative impact on the soil seed bank of H. contortus. Populations of all three species remained stable throughout this study, although the favourable seasonal rainfall experienced and the short duration of this study relative to the life span of these species may have masked longer term, deleterious impacts of heavy grazing pressure.

An extensive grazing study was conducted between 1988 and 2001 in a Heteropogon contortus (black speargrass) pasture in central Queensland. The study was designed to measure the effects of stocking rate on native pasture, native pasture with legume oversown, and native pasture with animal diet supplement/spring burning on pasture and animal production. Summer rainfall throughout the study was below the long-term mean. Mean annual pasture utilisation ranged from 13% at 8 ha/steer up to 61% at 2 ha/steer. Increasing stocking rate treatments reduced total pasture yields while total yields in legume oversown treatments were similar to those in native pasture at the same stocking rate. When spring burning was possible, total yields were reduced in the subsequent autumn. Increasing stocking rate in native pasture tended to reduce H. contortus and Bothriochloa bladhii, increased the composition of intermediate species, such as B. decipiens and Chloris divaricata, and also changed the frequencies of a range of minor species. Oversowing legumes resulted in Stylosanthes scabra cv. Seca increasing from <1% of pasture composition in 1988 to 50% in 2000 and was associated with a reduction in H. contortus and changes in the frequencies of some minor species. Stocking rates heavier than 4 ha/steer resulted in annual pasture utilisation greater than 30% and were unsustainable because they reduced total yield and resulted in undesirable changes in species composition. It was concluded that pasture production was sustainable when stocking rates were maintained at 4 ha/steer, which equates to 30% annual pasture utilisation, and through the judicious use of spring burning.

Managing large variations in herbage production, resulting from highly variable seasonal rainfall, provides a major challenge for the sustainable management of Astrebla (Mitchell grass) grasslands in Australia. A grazing study with sheep was conducted between 1984 and 2010 on an Astrebla grassland in northern Queensland to describe the effects of a range of levels of utilisation of the herbage at the end of the summer growing season (April–May in northern Australia) on the sustainability of these grasslands. In unreplicated paddocks, sheep numbers were adjusted annually to achieve 0, 10, 20, 30, 50 and 80% utilisation of the herbage mass at the end of the summer over the ensuing 12 months. Higher levels of utilisation reduced both total and Astrebla spp. herbage mass because of the effects of higher utilisation on Astrebla spp. and this effect was accentuated by drought. The tussock density of Astrebla spp. varied widely among years but with few treatment differences until 2005 when density was reduced at the 50% level of utilisation. A major change in density resulted from a large recruitment of Astrebla spp. in 1989 that influenced its density for the remainder of the study. Basal area of the tussocks fluctuated among years, with increases due to rainfall and decreases during droughts. Seasonal rainfall was more influential than level of utilisation in changes to the basal area of perennial grasses. Drought resulted in the death of Astrebla spp. tussocks and this effect was accentuated at higher levels of utilisation. A series of three grazing exclosures were used to examine the recovery of the density and basal area of Astrebla spp. after it had been reduced by 80% utilisation over the preceding 9 years. This recovery study indicated that, although grazing exclusion was useful in the recovery of Astrebla spp., above-average rainfall was the major factor driving increases in the basal area of perennial grasses. Spring values of the Southern Oscillation Index and associated rainfall probabilities were considered to have potential for understanding the dynamics of Astrebla spp. It was concluded that Astrebla grassland remained sustainable after 26 years when grazed at up to 30% utilisation, while, at 50% utilisation, they became unsustainable after 20 years. Results from this study emphasised the need to maintain the population of Astrebla spp. tussocks.

Rainfall variability remains a major challenge to sustainable grazing management in northern Australia with perennial grasses the key to the stability of the resources that maintain a sustainable grazing industry. This paper describes the dynamics of five perennial grasses – Bothriochloa ewartiana (Domin) C.E. Hubb., Chrysopogon fallax S.T. Blake, Aristida spp., Panicum effusum R. Br. and Heteropogon contortus (L.) P. Beauv. ex Roem. & Schult. in relation to three grazing strategies – moderate stocking at long-term carrying capacity, heavy stocking and rotational wet season spelling. The research was conducted in permanent quadrats on the predominant land type in an extensive grazing study in an Aristida-Bothriochloa pasture in north Australia between 1998 and 2010. Summer rainfall was above average for two periods – 1998 – 2001 and 2008 – 2010 with drought and below-average rainfall from 2002 to 2007. Low rainfall affected the dynamics of all grasses by reducing survival and basal area through its effect on plant size; this impact was most noticeable for the shorter-lived Aristida spp., P. effusum and H. contortus. The impact of grazing was greatest on the long-lived B. ewartiana and C. fallax; this effect was accentuated by the 2002–07 drought. Heavy grazing during this period further reduced the survival and size of B. ewartiana in comparison with the moderate stocking and rotational spell treatments. In contrast, the survival of C. fallax was reduced in the moderate stocking and rotational spelling treatment during drought, relative to that under heavy grazing. The density of B. ewartiana declined even under moderate grazing and despite two sequences of above-average rainfall because seedling recruitment failed to offset mature plant death. Results from this study emphasised the importance of maintaining the existing populations of key long-lived species such as B. ewartiana through good management. These results also supported the overall findings from the grazing study indicating that stocking at the long-term carrying capacity is sustainable in managing for climate variability.

Seasonal changes in the quantitative floristics at a wide range of Astrebla grassland sites in south-western Queensland were monitored between 1972 and 1980 with a wheel point apparatus. Changes in the floristics were measured in terms of both relative abundance and basal cover. A large increase in the relative abundance of perennial grasses, particularly Aristida latifolia, Astrebla spp. and Dichanthium sericeum, occurred between 1972 and 1976. This increase was at the expense of annual grasses and forbs which declined in both relative abundance and number of genera present. The relative abundance of perennial grasses declined between 1978 and 1980 and this was associated with a large increase in the forbs such as Daucus glochidiatus and Plantago spp., particularly at southern sites. The contribution of annual grasses to botanical composition remained low throughout the period. Total basal cover differed between years although these differences were not significant. As perennial grass, particularly Astrebla spp., was the major vegetation component of total basal cover, changes in the latter were associated mainly with changes in the basal cover of Astrebla spp. Changes in the contribution of individual species to total basal cover were related to changes in the relative abundance of those species. Changes in botanical composition in Astrebla grassland may be influenced more by trends in seasonal rainfall than by grazing pressure.

A grazing study conducted between 1979 and 1983 assessed the seasonal trends of ewe productivity in Astrebla grassland in south- western Queensland. This study was designed originally to compare productivity on two pastures with different compositions, however, these differences in pastures composition were not achieved. Large differences in liveweight, wool growth and reproductive performance occurred between years in response to differences in pasture growth resulting from large variation in the seasonal incidence of rainfall. Rainfall effective for plant growth, both forbs and grasses, resulted in a high quality diet which resulted, in turn, in increased sheep productivity. Reproductive performance was particularly sensitive to the quality of the ewes diet around the time of lambing through the effect of diet quality on lamb survival and growth rate. It is suggested that the effect of rainfall on reproductive performance is pasture.