John G. McIvor

How season of grazing and herbivore selectivity influence monsoon tall‐grass communities of northern Australia

. The hypothesis that season of defoliation and herbivore selectivity may be as important as level of use in determining plant community response to grazing was tested in a monsoon grassland in northern Australia. Plots, dominated by the tussock grasses Themeda triandra and Chrysopogon fallax, were grazed by cattle at low, medium and high rates of utilization in either the early wet, late wet or dry seasons. Effects of grazing on species composition were greatest in the early wet season when high rates of utilization significantly reduced the proportion and occurrence of Themeda and increased the proportion of forbs. Grazing in the dry season had no significant effect on composition. At medium and high levels of utilization in the early wet season, the pasture responded negatively to defoliation, only partially compensating for plant tissue lost to herbivory. The negative response to defoliation carried over to the next wet season when these same medium and high-grazing treatments produced only 80 % and 60 % growth, respectively, of that in treatments grazed at low levels of utilization or those grazed during the dry season. The frequency of Themeda was still lower, and that of annual grasses and non-leguminous forbs higher, in plots that had been grazed at a high rate of utilization for just eight weeks in the early wet season two years previously. Species richness and diversity were also significantly affected by this grazing disturbance. If species composition is to be maintained in these grasslands then stocking rates must be set at low levels to cope with the combined effect of undercompensation in response to defoliation in the wet season and strong dietary preferences for grazing sensitive species.

How land condition alters plant-animal relationships in Australia's tropical rangelands

Abstract Changes in vegetation composition and land condition that occur in response to grazing in natural grassland systems may alter plant and animal production, but the relationships are poorly understood. This study examined the effect of a change in land condition on herbage and animal production in two tropical tallgrass regions of northern Australia. The contrasting land condition classes, selected along fence-lines separating paddocks with different grazing histories, were defined as: State I. Dominated by palatable tussock perennial grasses; State II. Less palatable tussock perennial grasses, annual grasses and forbs as the major species. State II produced considerably less herbage than State I. Cattle grazed the two land condition classes to achieve comparable utilisation rates over the range 5–60% utilisation in a number of 8 week seasonal grazing periods over 2 years. At both Charters Towers and Katherine, steer growth was greater in State II than in State I, most rapid in the early wet season and least in the late dry season, and greatest at low utilisation rates. Differences in liveweight gain due to land condition class were greatest at the lightest levels of utilisation. Owing to the differences in pasture productivity between land condition classes, comparable levels of utilisation were achieved by imposing contrasting stocking rates. Steer growth was higher from State II land at low stocking rates while at high stocking rates State I produced better liveweight gains. The greater liveweight gain by steers grazing State II at low stocking rates can be explained by better diet quality which was reflected in higher levels of faecal nitrogen. The proportion of C3 species (forbs, native legumes, shrubs, eucalypt leaves) in the diet of animals grazing State II vegetation was about twice that of steers in State I. These C3 species were considerably higher in nitrogen than the C4 grasses at the end of the wet season. At high stocking rates, animals in the State II treatment had less available feed on offer, resulting in lower liveweight gains compared with State I. The likelihood of further deleterious changes to land condition and productivity occurring should management attempt to maximise production from State II land are high.

Grazing Management in Tropical Savannas: Utilization and Rest Strategies to Manipulate Rangeland Condition

Abstract Grazing management is important for sustaining the productivity and health of rangelands. However, the effects of grazing management on herbage growth and species composition in the tropical savannas of northern Australia are not well known. In this eight-year study the influences of utilization rate and resting pastures from grazing on vegetation dynamics were measured at three sites in northeast Queensland, Australia. The sites had high, medium, and low soil fertility, and there were two land condition classes (States I and II) at each site. Severe drought occurred during the first four years, but above-average rainfall was received in the second half of the study. High utilization rates reduced biomass, perennial grass basal area, and ground cover. The reduction in biomass was due to both higher consumption and decreased primary production. State I condition plots at the high and medium soil fertility sites were initially dominated by decreaser perennial grasses, but these declined at all utilization levels, particularly the high rate. They were largely replaced by exotic perennial grasses. At the low fertility site there were no exotic grasses, and the decreaser grasses increased in all treatments, with the increases greatest in plots with low utilization or medium utilization plus resting. In the State II condition plots at the high and medium fertility sites, low or moderate utilization, led to an increase in both decreaser and exotic perennial grasses; with high utilization the decreaser perennial grasses declined and were replaced largely by exotic perennial grasses. This study clearly demonstrated that either conservative stocking with year-round grazing or a grazing system that includes some wet-season resting will help maintain land in a desirable state or help facilitate the transition from a less desirable ecological state to one more desirable for pastoral production and rangeland condition.

Competition by herbs as a limiting factor in shrub invasion in grassland: a test with different growth forms

We tested the hypothesis that seedling establish- ment, the critical stage in the invasion of grassland by shrubs, is limited by competition with perennial grasses in seasonally wet/dry savannas. We placed seeds of two invasive exotic shrubs - Cryptostegia grandiflora, a woody vine, and Acacia nilotica, an arborescent legume - into pots with a wide range of existing above- and below-ground herbaceous biomass provided by either a tussock or a stoloniferous perennial grass. We also imposed different levels of watering frequency (5, 10 and 21 d), nutrient addition (+ and -) and grass clipping intensity (no clipping, clipped to 5 cm and clipped to 25 cm). There was no effect of any treatment on shrub seedling emer- gence or survival and all of the seedlings that emerged sur- vived the 90-d growing period. Herbaceous competition also failed to have an effect on biomass accumulation in shrub seedlings. More frequent watering significantly increased above- and below-ground biomass accumulation for both shrub species and nutrient addition significantly increased Crypto- stegia biomass accumulation. Based on these results, we ques- tion the proposition that reduction in competition by herbs via livestock grazing has been a significant factor in determining the rate or pattern of exotic shrub increase in the seasonally wet/dry tropics. We also question the suitability of the two- layer soil moisture hypothesis as a basis for management practices to control the ingress of woody species into grasslands and open savannas.

Principles and guidelines for managing cattle grazing in the grazing lands of northern Australia: stocking rates, pasture resting, prescribed fire, paddock size and water points - a review

Beef cattle grazing is the dominant land use in the extensive tropical and sub-tropical rangelands of northern Australia. Despite the considerable knowledge on land and herd management gained from both research and practical experience, the adoption of improved management is limited by an inability to predict how changes in practices and combinations of practices will affect cattle production, economic returns and resource condition. To address these issues, past Australian and international research relating to four management factors that affect productivity and resource condition was reviewed in order to identify key management principles. The four management factors considered were stocking rates, pasture resting, prescribed fire, and fencing and water point development for managing grazing distribution. Four management principles for sound grazing management in northern Australia were formulated as follows: (1) manage stocking rates to meet goals for livestock production and land condition; (2) rest pastures to maintain them in good condition or to restore them from poor condition to increase pasture productivity; (3) devise and apply fire regimes that enhance the condition of grazing land and livestock productivity while minimising undesirable impacts; and (4) use fencing and water points to manipulate grazing distribution. Each principle is supported by several more specific guidelines. These principles and guidelines, and the supporting research on which they are based, are presented.

An economic assessment of the impact of grazing land condition on livestock performance in tropical woodlands

Beef cattle grazing is the dominant economic use of the tropical woodlands of northern Australia. Land condition has declined over a large part of the region as a result of over-utilisation of pastures through poor management of grazing, fire and vegetation. While often assumed to be the case, there is little empirical evidence to support a link between deteriorating land condition and reduced economic outcomes for livestock production. A model of a representative livestock enterprise near Charters Towers in northern Queensland is used in conjunction with a simulation of 100 trials that is consistent with long-term rainfall in the region to examine the relationship between stocking rates, animal production and economic outcomes. The present study supports a view that ecological and economic outcomes of grazing management do have some general linkages. However, the linkages are less direct than suggested by common definitions of resource degradation and simple stocking rate models. Key economic parameters include the reproductive performance of breeding herds, the level and duration of supplementary feeding required to meet seasonal feed shortages under different land condition and stocking rate regimes, and additional capital invested in larger herds when stocking rates are increased. An overgrazing spiral is hypothesised whereby there is a short-term economic incentive to exploit the natural capital of land in good condition by grazing at relatively high stocking rates. Should land condition deteriorate, a much lower stocking rate is warranted. In an extreme case of a shift to very poor land condition, there is limited scope for profitable production. The 100 year mean values of many production variables for good and moderate condition land for the range of stocking rates trialled are similar. The inherent variation in climatic conditions in the woodlands region and the ability and skills of managers may lead to the actual differences in these measures passing un-noticed before major land resource problems become apparent.

Dormancy and germination characteristics of herbaceous species in the seasonally dry tropics of northern Australia

Abstract This study investigated changes in dormancy and germination over 8 months for 23 common species (annual and perennial grasses, legumes and other dicotyledons) from herbaceous communities in northern Australia. Seeds were exposed to three storage treatments: relatively constant laboratory conditions, an oven with fluctuating temperatures similar to those found on the soil surface (25/60°C), or exposed on the soil surface at Townsville. There were wide ranges of initial levels of dormancy (9–100%), rates of change of dormancy and response to the different storage conditions showing that species with several types of dormancy characteristics are able to coexist in these communities. The general trend in dormancy levels was a decline with time with the rate of decline greatest for seeds exposed on the soil surface and least for those stored in the laboratory. The species were divided into groups based on dormancy levels in seeds on the soil surface during the late dry and mid wet seasons. The dormancy characteristics of the groups were related to the ecology of the species in the groups. There was an approximately linear increase in germination rate (i.e. a decrease in the number of days to 50% of final germination) over time for all storage treatments; rates for seeds on the soil surface increased more rapidly than those of seeds in laboratory and oven samples.

Pasture management in semi-arid tropical woodlands: dynamics of perennial grasses

The effects of a range of pasture management options (introduced legumes and grasses, superphosphate, timber treatment, cultivation before sowing and stocking rate) on the basal cover of perennial grasses were measured from 1982 to 1991 at two sites, ‘Hillgrove’ and ‘Cardigan’, near Charters Towers, in north-east Queensland. Colonisation and survival of eight native and exotic grasses were followed in permanent quadrats in a subset of treatments. Overall, there were significant changes in total basal cover of plots between years and with tree killing, but no significant differences in sown pastures, fertiliser or stocking rate. Basal cover increased when defoliation levels were less than 40% but increases were smaller at higher levels of defoliation and basal cover often declined when defoliation was greater than 60%. Basal cover declined when growing seasons were 50% survival). Annual survival rates increased with plant age, were higher in good growing seasons than in poor seasons, were higher for large plants than small plants, and were lower at high defoliation levels than where defoliation was less severe. The differences between species in ability to colonise and survive, and the small influence of management compared to seasonal effects on survival, are discussed to explain species performance in pastures.

Buffel grass and climate change: a framework for projecting invasive species distributions when data are scarce.

Invasive species pose a substantial risk to native biodiversity. As distributions of invasive species shift in response to changes in climate so will management priorities and investment. To develop cost-effective invasive species management strategies into the future it is necessary to understand how species distributions are likely to change over time and space. For most species however, few data are available on their current distributions, let alone projected future distributions. We demonstrate the benefits of Bayesian Networks (BNs) for projecting distributions of invasive species under various climate futures, when empirical data are lacking. Using the introduced pasture species, buffel grass (Cenchrus ciliaris) in Australia as an example, we employ a framework by which expert knowledge and available empirical data are used to build a BN. The framework models the susceptibility and suitability of the Australian continent to buffel grass colonization using three invasion requirements; the introduction of plant propagules to a site, the establishment of new plants at a site, and the persistence of established, reproducing populations. Our results highlight the potential for buffel grass management to become increasingly important in the southern part of the continent, whereas in the north conditions are projected to become less suitable. With respect to biodiversity impacts, our modelling suggests that the risk of buffel grass invasion within Australia’s National Reserve System is likely to increase with climate change as a result of the high number of reserves located in the central and southern portion of the continent. In situations where data are limited, we find BNs to be a flexible and inexpensive tool for incorporating existing process-understanding alongside bioclimatic and edaphic variables for projecting future distributions of species invasions.

Resting pastures to improve land condition in northern Australia: guidelines based on the literature and simulation modelling

Pasture rest is a possible strategy for improving land condition in the extensive grazing lands of northern Australia. If pastures currently in poor condition could be improved, then overall animal productivity and the sustainability of grazing could be increased. The scientific literature is examined to assess the strength of the experimental information to support and guide the use of pasture rest, and simulation modelling is undertaken to extend this information to a broader range of resting practices, growing conditions and initial pasture condition. From this, guidelines are developed that can be applied in the management of northern Australia’s grazing lands and also serve as hypotheses for further field experiments. The literature on pasture rest is diverse but there is a paucity of data from much of northern Australia as most experiments have been conducted in southern and central parts of Queensland. Despite this, the limited experimental information and the results from modelling were used to formulate the following guidelines. Rest during the growing season gives the most rapid improvement in the proportion of perennial grasses in pastures; rest during the dormant winter period is ineffective in increasing perennial grasses in a pasture but may have other benefits. Appropriate stocking rates are essential to gain the greatest benefit from rest: if stocking rates are too high, then pasture rest will not lead to improvement; if stocking rates are low, pastures will tend to improve without rest. The lower the initial percentage of perennial grasses, the more frequent the rests should be to give a major improvement within a reasonable management timeframe. Conditions during the growing season also have an impact on responses with the greatest improvement likely to be in years of good growing conditions. The duration and frequency of rest periods can be combined into a single value expressed as the proportion of time during which resting occurs; when this is done the modelling suggests the greater the proportion of time that a pasture is rested, the greater is the improvement but this needs to be tested experimentally. These guidelines should assist land managers to use pasture resting but the challenge remains to integrate pasture rest with other pasture and animal management practices at the whole-property scale.