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Grasslands cover up to about 40% of the world's total land area (Suttie et al. 2005" Gibson 2009). These complex plant communities are generally comprised of a range of species including grasses, legumes, shrubs, and/or other forbs (Figure 17.1) (Allen et al. 2011), and are used primarily for grazing livestock such as cattle, sheep, and goats (Sanderson et al. 2004). The nomenclature associated with grasslands varies around the world and is largely influenced by the level of management needed to achieve some level of productive potential (Gibson 2009). For example, rangelands are generally comprised of indigenous vegetation that is sparingly grazed with little other management. In contrast, intensively managed pastures are likely to include improved species and agrochemical inputs, and are specifically devoted to the production of forage for harvest by either grazing or cutting. Furthermore, grazing is generally carefully managed to maintain satisfactory levels of plant persistence and animal productivity (Sindel 2006" Allen et al. 2011). For the purposes of this chapter, all plant communities that are grazed and have some level of management will be referred to as pastures.

Blady grass (Imperata cylindrica) is a major invasive weed that threatens agricultural systems, biodiversity and ecosystem function throughout tropical and sub-tropical to warm temperate regions around the world. It is able to reproduce both sexually via seeds and asexually via rhizomes, making it a difficult species to control or eradicate. It is adapted to a range of soil types, growing in poor acid soils with low fertility and organic matter, and is tolerant of fire and soil salinity. Once established, it is regarded as a strong competitor for nutrients, water and light. Although primarily a species of tropical and other warm environments, blady grass occurs in the New England region on the Northern Tablelands of New South Wales, Australia, in a cool temperature environment. Here, it occurs in patches, many of which appear to be expanding very little over time with few new patches being formed. The question arises as to how blady grass reproduces in this environment and what constrains its spread and distribution. However, there has been a lack of information about the ecology of blady grass in this region as well as more generally in Australia. Such information is critical for the design of effective future management programs. Therefore, this research focused on quantifying the size of the blady grass soil seedbank, assessing the longevity and germination of seeds, measuring flowering and growth responses to a range of environmental factors, and recording changes in rhizomes and their regrowth.

The soil seedbank is a vital mechanism for the establishment, persistence, and re-invasion of many weedy species. Through a germination study, the soil seedbank was explored within and outside blady grass patches in the New England region. The results showed no germinable seeds of blady grass to be present. Potential explanations for this absence of germinable seeds included the presence of seed dormancy, a lack of seed production in this cool temperate environment, and a short period of seed persistence under field conditions. Most seeds of other weed and pasture species were concentrated in the top 5 cm of the soil profile and there were more seeds at the edge and outside of blady grass patches than in the centre of the patch. These results indicate that blady grass has a strong competitive or allelopathic impact on the growth and re-production of pasture species.

In a series of laboratory and field experiments, the longevity and germination of seeds was found to vary according to storage and burial conditions. Seeds were not initially dormant and lost viability relatively rapidly (from 89% to 23% over 5 months) when stored at room temperature, while most seeds had either disappeared or were no longer able to germinate after 3 months of burial in the field. Seeds remained viable for a longer period of time when not buried. As a result, biological processes associated with soil moisture, such as germination and microbial decay, were thought to be responsible for the more rapid drop in seed viability when buried. The short life of seeds under field conditions is likely to constrain blady grass spread and persistence through the mechanism of seed dispersal. These results showed that regardless of their small size, light weight and ability to travel by wind over long distances, seeds may play a lesser role in the invasion of blady grass than rhizomes in cool temperate Australia.

Changes in temperature, day length, fertilizer, foliage management (cutting and burning), and water stress failed to stimulate flowering of blady grass, while several of the treatments had a large impact on the growth of the weed. The results showed that cooler temperatures, as experienced in the New England region, constrained growth relative to warmer temperatures as found in more tropical areas. In the high-altitude New England region, blady grass is likely to be most active in summer, and has mechanisms for obtaining light and responding well to increased soil fertility, suggesting it is likely to be highly competitive against summerdominant native pasture grasses. However, both cutting and burning reduced the growth of blady grass and may offer some potential for control of the weed. A lack of flowering and viable seed production may well be a contributing factor in the limited spread and distribution of blady grass.

The re-sprouting and growth of rhizome fragments depended on their size, colour, and presence of terminal meristematic tissue. The results showed that rhizome fragments with a greater number of nodes had more shoot and rhizome regrowth, while there was significant interaction between the effects of node number, rhizome colour, and presence of tips on regrowth. Rhizomes from different sources also varied in weight and diameter and this affected the degree of change they underwent following soil burial. While the capacity of rhizomes to contribute to the spread of blady grass is likely to vary, they are clearly responsible for the slow short-distance spread of blady grass patches in the New England region and may also have been responsible for longer distance spread when the weed was first introduced to the region.

As far as can be determined, the patches of blady grass in the New England region that were included in this study have been in the same places for many years. How the local populations of blady grass were initially established is unknown. However, these studies support the hypothesis that these high elevation cool-climate populations of blady grass are 'relic' populations, that seeds do not play a significant role in the persistence of this species in the region, and that there is little evidence for long distance expansion under the current climatic conditions and land management regime. Whether climate change in future alters this situation remains to be seen. This research provides important information about the ecology of blady grass in the New England region that can help inform the design of effective and sustainable management strategies for its control.

Tropical pasture species are often grown in soils with available phosphorus (P) concentrations below their critical P requirements. However, little is known about how key soil traits such as pH and Phosphorus Buffering Index (PBI) influence the critical P requirements of these species. Two controlled-environment experiments were conducted to investigate the effect of different starting pH and PBI on the shoot yield and P acquisition of Digit and Desmanthus. In the first experiment, the two species were grown in low-P soil that was amended to achieve five soil pH treatments (4, 5, 6, 7 and 8). In the second experiment, the two species were grown in a low-P soil mix that contained varying combinations of a low-PBI soil and a high-PBI soil to achieve five soil PBI treatments (65, 145, 225, 305 and 385). Ten soil P treatments (0–120 mg P kg^–1 ) were prepared by adding KH₂PO₄ solution to the soil surface. The shoot yields of Digit and Desmanthus increased in response to the higher application rates of P in both experiments. In the soil pH experiment, both species were most productive in the pH 5–8 treatments. Critical external P requirements were lowest in the pH 7 treatment and increased at lower and higher pH levels. In the soil PBI experiment, critical external P requirements increased significantly with PBI. Nevertheless, critical internal P requirements remained relatively stable. Tissue P tests may therefore be a useful way to determine likely responses of tropical pasture species to P fertiliser application across a range of soil types.

Trifolium subterraneum is the most widely sown annual pasture legume in the P-deficient soils of southern Australia. Controlled-environment studies have demonstrated that variation exists between genotypes of this legume to acquire P and yield in low-P soils, and there appears to be a plant density effect on these traits. However, the magnitude of this effect is largely unknown. Two cultivars of T. subterraneum, that differ significantly for the aforementioned traits when using the same sowing rate, were grown to determine differences in shoot growth, P uptake and root traits with changing plant density. Microswards of both cultivars were grown at five plant densities and five P levels. Yield and P content of shoots and roots were determined after 5 weeks growth. Root samples were assessed for diameter, length and root hair length. Shoot dry mass of both cultivars increased in response to increasing P supply and increasing plant density. Differences between the cultivars for shoot yield were most pronounced at low plant densities and diminished as plant density increased. This response was particularly evident at lower soil-P levels, whereas maximum yield was relatively independent of plant density in the high-P soil. In contrast, differences between cultivars for root morphological traits such as specific root length were maintained regardless of plant density. The results demonstrate that plant density effects sward P-acquisition and hence shoot yield achieved in the P-deficient soil. Accurate screening for P-acquisition and shoot yield across the T. subterraneum genome therefore requires a uniform plant density comparable to densities observed in the field. The identification of T. subterraneum cultivars capable of improved growth in low-P soils would improve P-use efficiency in Australian soils which are often P-deficient and require annual applications of P fertiliser for high yields. This would consequently lead to greater resilience of the agricultural sector.

Tropical pasture legumes such as Desmanthus are expected to improve pasture productivity in the extensive grazing systems of Northern Australia. However, the soils in these areas are often hostile (e.g., hard-setting and nutrient-deficient), which reduces legume emergence and establishment. Furthermore, these soils are often not ameliorated with amendments such as gypsum or starter fertilisers before planting. A pot trial was conducted to investigate differences in the emergence and early growth of four Desmanthus species. The legumes were grown in three alkaline clay soils that were unamended or amended with either gypsum (1 t CaSO₄.2H₂O ha⁻¹ equivalent), a starter MAP fertiliser (12 kg P ha⁻¹ equivalent), or both gypsum and the starter fertiliser. Seedling emergence was recorded daily and shoot yield was determined after six weeks’ growth. Final seedling emergence (as a percentage of viable seeds) varied among the Desmanthus species (c.f. D. leptophyllus = 63%, D. pernambucanus = 68%, D. bicornutus = 85%, and D. virgatus = 86%). On average, across the treatments, gypsum increased seedling emergence by 15%, whereas the starter fertiliser had no effect. The shoot yields and shoot phosphorus content of the Desmanthus species generally increased in response to the starter fertiliser. The collective results demonstrated that there were differences in emergence and early growth among the four Desmanthus species, which indicates that Desmanthus cultivar selection may be important in the relatively hostile soils of Northern Australia. Gypsum was an effective amendment for seedling emergence, whereas the starter fertiliser was an effective amendment to increase legume productivity.

Knowledge of crop evapotranspiration is crucial for irrigation decision making. An appropriate, user-friendly and time-efficient means of inferring such information is therefore essential. In this study, a closed hemispherical chamber was instrumented, calibrated and deployed in the field for measuring actual evapotranspiration of a vital pasture species, Tall Fescue (Festuca arundinacea). The pasture crop coefficient (K_c) was calculated from the measured instantaneous evapotranspiration and reference crop evapotranspiration (ET_o) for a range of growth stages. Also the relationship between K_c and Normalized Difference Vegetation Index (NDVI) as measured using an active optical sensor was established. Using the FAO dual crop coefficient approach and the hemispherical chamber, a technique for partitioning evapotranspiration components was developed. The components of evapotranspiration in terms of basal crop coefficient (K_cb) and soil evaporation coefficient (K_e) were expressed relative to canopy NDVI and Leaf Area Index (LAI). A theoretical model for estimating transpiration was also developed by scaling up stomatal conductance to canopy level in a controlled glasshouse environment. The model was validated against the measured transpiration.

Pastures grown on P-deficient soils in temperate southern Australia use mixtures of grasses and legumes. The main legumes (Trifolium and Medicago spp.) are highly productive across a wide range of environments but have high 'critical' P requirements (i.e. the P supply needed for near-maximum yield) relative to the grasses with which they grow. Improving the P-efficiency of the most important legume (T. subterraneum), or developing the agronomic merit of alternatives that are already P-efficient (e.g. Ornithopus spp.) would deliver reductions in P fertiliser inputs, improve farm incomes, and achieve better use of scarce nutrient resources. Here we describe research to improve the P efficiency of T. subterraneum. Field and controlled-environment experiments, with various pasture legume species, have demonstrated that substantial differences in the nutrient foraging potential of their roots determines their requirement for P fertiliser. Three key root morphology traits ensure efficient P acquisition from low P soil: development of high root length, high specific root length and long root hairs. Ornithopus spp. deploy an "optimal" combination of these root traits, efficiently maximising soil exploration to capture more P and to yield well in low P soils. In contrast, Trifolium subterraneum develops long roots in response to low P but has low specific root length and short root hairs which limit its ability to explore soil for P. Within T. subterraneum, variation exists in specific root length and root proliferation. These key factors determine intra-specific variation in P acquisition with the best genotypes achieving twice the yield of the worst in low P soil. The short root hairs on T. subterraneum (0.2-0.4 mm) are a major factor limiting P acquisition efficiency. Wider studies of nutrient foraging root traits among genetically-allied Trifolium species from the Section Trichocephalum revealed substantial differences in propensity for root foraging (11-35 cm root/cm3 soil) and root hair length (0.2-0.5 mm) but, like T. subterraneum, no genotypes tested to date have root foraging traits in the optimal combinations achieved by Ornithopus spp. To drive further substantive change in the P efficiency in the key pasture legume, T. subterraneum, it will be necessary to break through apparent intra-specific 'boundaries' for specific root length and root hair length by identifying radical ecotypic outliers, inter-specific introgression or directed mutagenesis.

Tropical pasture legumes often lack persistence in the extensive grazing systems of northern Australia. However, their inclusion in these systems is expected to improve productivity through atmospheric nitrogen (N) fixation and increased pasture quality. Because the soils of northern Australia are often low in available phosphorus (P), it is expected that a greater understanding of legume P requirements and fertiliser application strategies will lead to better legume productivity and persistence. Numerous controlled-environment growth experiments were conducted to examine the P requirements of a range of tropical grasses and legumes. Subsequent experimentation focused on understanding the mechanisms behind P acquisition and identifying appropriate fertiliser application strategies for better legume growth. The results demonstrated that there are significant differences in yield potential and critical P requirements among tropical pasture species (including between grasses and legumes, and between different legume species). This indicates there is potential to select and use P-efficient species in soil that has inherently low P levels. The results also demonstrated that banded applications of P fertiliser can improve legume productivity in mixed pasture swards. These results have direct implications for the management of legumes and soil fertility in the extensive grazing systems of northern Australia.

Annual grass weeds can provide significant competition to an establishing sown tropical perennial grass pasture. At least two years of grass weed control prior to sowing is required to reduce the weed seed bank. Pre-emergent herbicides used in summer cereals, such as atrazine or s-metolachlor with metcamifen seed safener, may reduce this preparation time. Two controlled-environment experiments were conducted to assess the potential for these pre-emergent herbicides to be used with several tropical perennial grasses. Experiment 1 tested the effect of metcamifen (400 g L⁻¹ a.i. at 0–2× label rate) on the emergence and vigor of Chloris gayana, Dichanthium aristatum, Digitaria eriantha and Panicum coloratum, with Sorghum bicolor as the control. Experiment 2 tested the effect of s-metolachlor (960 g h⁻¹ a.i.) with metcamifen-treated or untreated seed, and atrazine (1800 g h⁻¹ a.i.) on the emergence and early growth of the grasses. Metcamifen did not inhibit emergence or vigor of the grasses. Without metcamifen seed treatment, smetolachlor reduced the growth of the tropical perennial grasses by 47–100%, while it had no such effect on S. bicolor. In contrast, there was no effect of atrazine on shoot yields of the grasses, nor of s-metolachlor when D. aristatum, D. eriantha and P. coloratum seed had been treated with metcamifen. The collective results indicate that the herbicide safener metcamifen does not reduce the viability of tropical perennial grass seed and provides some protection against s-metolachlor, albeit not complete protection at the rates used in our study. Atrazine did not affect emergence or early growth of the grasses.

Extensive grazing systems often receive minimal fertiliser due to the risk associated with using relatively expensive inputs. Nevertheless, nutrient applications are known to improve pasture productivity, and the benefit of applying fertiliser is being more widely accepted. Two tropical pasture mixes (Digit/Desmanthus and Rhodes/Centro) were established in plastic boxes containing phosphorus (P) responsive soil to investigate shoot yield and P fertiliser recovery. The grasses and legumes were planted in separate rows, and three P treatments were applied along with the seed ('BOTH low-P' had 2kg P ha⁻¹ banded below both components, 'BOTH high-P' had 12kg P ha⁻¹ banded below both components and 'LEGUME superhigh-P' had 12kg P ha⁻¹ banded below the legume only). The P applied below the legumes was labelled with ³²P-radioisotope tracer. When P fertiliser was applied below both components, the grasses consistently out-yielded the legumes (avg. legume content=29%). Preferential fertiliser application below the legumes increased the average legume content of the two pasture mixes to 66%. Legume tissue P derived from applied P fertiliser increased from 20% to 77% as the P application rate was increased. However, total recovery of applied P by the legumes was relatively low in each of the treatments (≤7% of applied P). These collective results demonstrate that a preferential application of P fertiliser can benefit legume productivity, with applied P being a significant proportion of plant tissue P. Although only a small proportion of applied P was recovered within the seven-week growth period, it is expected that this fertiliser application at planting will remain beneficial for a large proportion of the growing season following pasture establishment.

Grasses generally dominate the pastures of northern Australia. This may be associated, in part, with varietal differences in critical phosphorus (P) requirements that influence the competitive ability and persistence of the legume component. However, the effect of plant competition on shoot yield responses to soil P supply remain unquantified in tropical pasture swards. Micro-swards of Premier Digit and Progardes Desmanthus were grown, both as monocultures and mixed plantings, in soil amended with five rates of P fertiliser to determine the influence of sward conditions on shoot yield and tissue P. The shoot yield and tissue P concentrations of both species increased in response to soil P supply, with the shoot yield of Progardes Desmanthus in mixed plantings representing between 33–47% of the total yield of Digit and Desmanthus combined. The critical external P requirements of Progardes Desmanthus were generally equal to or lower than that of Premier Digit, yet both species competed effectively for applied P. Therefore, Premier Digit and Progardes Desmanthus may be suitable companion pasture species for establishment in the low-P soils of northern Australia.