Grazing Protection Research Articles

Facilitation and competition deconstructed: a mechanistic modelling approach to the stress gradient hypothesis applied to drylands

Facilitative interactions among species are key in plant communities. While experimental tests support the Stress Gradient Hypothesis (SGH) as an association between facilitation and stress, whether the shape of net effects along stress gradients can be predicted is controversial, with no available mathematical modelling approaches. We proposed a novel test, using a modification of the R* model to study how negative and positive partial effects of plant interactions in drylands combine along two common stress gradients. We modelled different interactions: competition for water and light, amelioration of soil infiltration and/or grazing protection, obtaining that intensity and importance of facilitation did not generally increase along stress gradients, being dependent on the interaction type. While along the water stress gradient net interactions became more positive, reaching a maximum and then waning again, various outcomes were observed along the grazing gradient. Shape variety was mainly driven by the various shapes of the partial positive effects. Under resource stress, additive interaction effects can be expected, whereas when including grazing, the effects were non-additive. In the context of the SGH, deconstructing the effect of positive and negative interaction in a pairwise mechanistic models of drylands does not show a unique shape along stress gradients.

Associational Resistance to Predation by Protists in a Mixed Species Biofilm.

Mixed species biofilms exhibit increased tolerance to numerous stresses compared to single species biofilms. The aim of this study was to examine the effect of grazing by the heterotrophic protist, Tetrahymena pyriformis, on a mixed species biofilm consisting of Pseudomonas aeruginosa, Pseudomonas protegens, and Klebsiella pneumoniae. Protozoan grazing significantly reduced the single species K. pneumoniae biofilm, and the single species P. protegens biofilm was also sensitive to grazing. In contrast, P. aeruginosa biofilms were resistant to predation. This resistance protected the otherwise sensitive members of the mixed species biofilm consortium. Rhamnolipids produced by P. aeruginosa were shown to be the primary toxic factor for T. pyriformis. However, a rhamnolipid-deficient mutant of P. aeruginosa (P. aeruginosa ΔrhlAB) maintained grazing resistance in the biofilm, suggesting the presence of at least one additional protective mechanism. P. aeruginosa with a deleted gene encoding the type III secretion system also resisted grazing. A transposon library was generated in the ΔrhlAB mutant to identify the additional factor involved in community biofilm protection. Results indicated that the Pseudomonas Quinolone Signal (PQS), a quorum sensing signaling molecule, was likely responsible for this effect. We confirmed this observation by showing that double mutants of ΔrhlAB and genes in the PQS biosynthetic operon lost grazing protection. We also showed that PQS was directly toxic to T. pyriformis. This study demonstrates that residing in a mixed species biofilm can be an advantageous strategy for grazing sensitive bacterial species, as P. aeruginosa confers community protection from protozoan grazing through multiple mechanisms. IMPORTANCE Biofilms have been shown to protect bacterial cells from predation by protists. Biofilm studies have traditionally used single species systems, which have provided information on the mechanisms and regulation of biofilm formation and dispersal, and the effects of predation on these biofilms. However, biofilms in nature are comprised of multiple species. To better understand how multispecies biofilms are impacted by predation, a model mixed-species biofilm was here exposed to protozoan predation. We show that the grazing sensitive strains K. pneumonia and P. protogens gained associational resistance from the grazing resistant P. aeruginosa. Resistance was due to the secretion of rhamnolipids and quorum sensing molecule PQS. This work highlights the importance of using mixed species systems.

Indirect facilitation drives species composition and stability in drylands

Dryland ecosystems are likely to respond discontinuously to gradual changes in environmental conditions. Direct facilitation between plants, whereby plants improve the local environmental conditions for others, has been shown to be a mechanism contributing to these discontinuous ecosystem transitions. Theoretical models describing dryland vegetation dynamics often consider a single plant species and one type of facilitation, namely direct facilitation. However, another type of facilitation–indirect facilitation–is widespread in dryland ecosystems as well; it is performed by plants protected against grazing, the nurses, when this protection extends to other plants growing in their neighbourhood that are deprived of such protection, the protegees. Little is known about the long-term effects of indirect facilitation on dryland dynamics. Here, we developed and analysed a theoretical model including two species–a nurse and a protegee–and indirect facilitation through grazing. We investigated the effects of indirect facilitation on species composition, species spatial clustering and the stability of dryland ecosystems. We showed that indirect facilitation through grazing enables the stable coexistence of the nurse and the protegee and allows the reversibility of the protegee extinction. Surprisingly, the strength of indirect facilitation affected neither the total nor the interspecific vegetation clustering. Our study highlights that spatially explicit grazing protection may affect species composition and the stability of dryland ecosystems and gives hints about how species interactions translate into spatial clustering.

Synergistic Interactions within a Multispecies Biofilm Enhance Individual Species Protection against Grazing by a Pelagic Protozoan.

Biofilm formation has been shown to confer protection against grazing, but little information is available on the effect of grazing on biofilm formation and protection in multispecies consortia. With most biofilms in nature being composed of multiple bacterial species, the interactions and dynamics of a multispecies bacterial biofilm subject to grazing by a pelagic protozoan predator were investigated. To this end, a mono and multispecies biofilms of four bacterial soil isolates, namely Xanthomonas retroflexus, Stenotrophomonas rhizophila, Microbacterium oxydans and Paenibacillus amylolyticus, were constructed and subjected to grazing by the ciliate Tetrahymena pyriformis. In monocultures, grazing strongly reduced planktonic cell numbers in P. amylolyticus and S. rhizophila and also X. retroflexus. At the same time, cell numbers in the underlying biofilms increased in S. rhizophila and X. retroflexus, but not in P. amylolyticus. This may be due to the fact that while grazing enhanced biofilm formation in the former two species, no biofilm was formed by P. amylolyticus in monoculture, either with or without grazing. In four-species biofilms, biofilm formation was higher than in the best monoculture, a strong biodiversity effect that was even more pronounced in the presence of grazing. While cell numbers of X. retroflexus, S. rhizophila, and P. amylolyticus in the planktonic fraction were greatly reduced in the presence of grazers, cell numbers of all three species strongly increased in the biofilm. Our results show that synergistic interactions between the four-species were important to induce biofilm formation, and suggest that bacterial members that produce more biofilm when exposed to the grazer not only protect themselves but also supported other members which are sensitive to grazing, thereby providing a “shared grazing protection” within the four-species biofilm model. Hence, complex interactions shape the dynamics of the biofilm and enhance overall community fitness under stressful conditions such as grazing. These emerging inter- and intra-species interactions could play a vital role in biofilm dynamics in natural environments like soil or aquatic systems.

Grazing of Nuclearia thermophila and Nuclearia delicatula (Nucleariidae, Opisthokonta) on the toxic cyanobacterium Planktothrix rubescens

During the last decades, the planktonic cyanobacterium Planktothrix rubescens became a dominant primary producer in many deep pre-alpine lakes. While altered physiochemical conditions due to lake warming seem to favour this cyanobacterial species, its dominance is partly attributed to factors conferring grazing resistance. The rigid structure of the cyanobacterial filaments and toxic secondary metabolites (e.g. microcystins) protect against diverse grazers. Nonetheless, species of the protistan genus Nuclearia (Nucleariidae, Opisthokonta) are able to overcome this grazing protection. Time lapse video documentation served as tool to record slow feeding processes of N. thermophila and N. delicatula. Three different feeding strategies could be distinguished: (i) Phagocytosis of small fragments, (ii) serial break-ups of cyanobacterial cells and (iii) bending and breaking of filaments. While observations revealed mechanical manipulation to be important for the efficient breakdown of P. rubescens filaments, the toxin microcystin had no pronounced negative effects on nucleariid cells. Growth experiments with N. thermophila/N. delicatula and different accompanying bacterial assemblages pointed to a pivotal role of distinct prokaryotic species for toxin degradation and for the growth success of the protists. Thus, the synergistic effect of nucleariids and specific bacteria favours an efficient degradation of P. rubescens along with its toxin.

Intraspecific facilitation by allelochemical mediated grazing protection within a toxigenic dinoflagellate population.

Dinoflagellates are a major cause of harmful algal blooms (HABs), with consequences for coastal marine ecosystem functioning and services. Alexandrium fundyense (previously Alexandrium tamarense) is one of the most abundant and widespread toxigenic species in the temperate Northern and Southern Hemisphere and produces paralytic shellfish poisoning toxins as well as lytic allelochemical substances. These bioactive compounds may support the success of A. fundyense and its ability to form blooms. Here we investigate the impact of grazing on monoclonal and mixed set-ups of highly (Alex2) and moderately (Alex4) allelochemically active A. fundyense strains and a non-allelochemically active conspecific (Alex5) by the heterotrophic dinoflagellate Polykrikos kofoidii. While Alex4 and particularly Alex5 were strongly grazed by P. kofoidii when offered alone, both strains grew well in the mixed assemblages (Alex4 + Alex5 and Alex2 + Alex5). Hence, the allelochemical active strains facilitated growth of the non-active strain by protecting the population as a whole against grazing. Based on our results, we argue that facilitation among clonal lineages within a species may partly explain the high genotypic and phenotypic diversity of Alexandrium populations. Populations of Alexandrium may comprise multiple cooperative traits that act in concert with intraspecific facilitation, and hence promote the success of this notorious HAB species.

Grazing Protection Influences Soil Mesofauna in Ungrazed and Grazed Riparian and Upland Pastures

The influence of grazing protection caused by streambank fencing on soil mesofauna density is unknown. Our objective was to determine if grazing protection (ungrazed vs. grazed), location (upland vs. riparian pasture), and seasonal (spring vs. fall) treatment effects associated with streambank fencing had a significant influence on soil mesofauna distribution and density. We collected five intact soil cores (0–5 cm depth) in June and October of 2012 from within four treatments consisting of ungrazed and grazed riparian and upland pastures associated with streambank fencing along an 800-m reach of the Lower Little Bow River in southern Alberta, Canada. Soil mesofauna were extracted and densities of Acari (mites) taxa, Collembola (springtails) taxa, and other mesofauna were determined. Grazing protection resulted in a significant (P ≤ 0.05) negative response of Astigmata mite densities for the upland pasture, and a positive response for Oribatida mites and total Collembola, and Hypogastruridae and Onychiuridae springtails for both pastures. Location and season had a significant influence on Acari and Collembola taxa, but the effects were dependent on interaction effects. We conclude that grazing protection influenced certain soil mesofauna in pastures associated with streambank fencing, and this may influence decomposition of soil organic matter, nutrient cycling, and soil structure in associated pastures.

Plant species richness responses to grazing protection and degradation history in a low productivity landscape

Questions: Does species richness and abundance accumulate with grazing protection in low productivity ecosystems with a short evolutionary history of grazing, as predicted by emerging theory? How do responses to grazing protection inform degradation history?

Soil nutrient losses in an altered ecosystem are associated with native ungulate grazing

Summary 1. Protected natural areas often inherit legacies of altered habitat and predator–prey dynamics. Extirpation of top predators and ecosystem engineers modifies the density and distribution of the remaining species, with potentially significant ramifications for ecological condition. Land managers need, among other things, information on how populations of large grazing animals affect their habitat when key components are missing from the ecosystem. 2. Our objective was to examine correlations between intensive grazing by Cervus elaphus (red deer or North American elk) and soil processes in a protected natural area with a history of predator reduction and habitat modification. We used 12 exclosures constructed in 1994 to compare soil carbon and nitrogen, nitrogen cycling, bulk density, soil moisture and soil microbial characteristics in grazed and ungrazed areas. Samples were collected in 1998 and in 2010. 3. Soil carbon (C) concentration was 25% lower in grazed areas than in exclosures; soil nitrogen (N) concentration was 23% lower. From 1998 to 2010, the mean percentage change for soil C and N concentrations was +14% and +27%, respectively, in exclosures and )22% and )24%, respectively, in grazed sites. 4. Soil bulk density in grazed areas was 25% greater than in ungrazed areas; soil moisture in grazed areas was 15% lower than in ungrazed areas. 5. Physical soil attributes, soil nutrient stocks and soil microbial biomass were significantly correlated, exhibiting linkages that can accelerate changes in ecosystem function. Plant N concentrations suggest soil nutrient losses were not related to substitutions for N-rich forage species. 6. Synthesis and applications. Intensive grazing was correlated with drier more compacted soils and nutrient loss, suggesting increasing N limitation and reduced plant biodiversity. Recovery of soil nutrients and soil structure appears possible in the studied montane riparian zone if vegetation receives grazing protection. Cervus elaphus density and distribution should be controlled in the short term to restore soil structure, hydraulic conductivity and nutrients. Active management could be obviated in the longer term through restoration of predators and ecosystem engineers.

The impact of cattle and goats grazing on vegetation in oak stands of varying coppicing age

The effects of cattle and goats grazing on oak shoot growth and herbaceous vegetation in three oak forest stands with different coppicing age (1, 4 and 7 yrs after the clear cutting) were investigated. In April 1997, an experimental area was chosen with three forest stands, which were clear cut in 1996 (CL1996), 1993 (CL1993), and 1990 (CL1990). All stands were grazed by cattle and goats after they were clear cut. In each forest stand, five 10m×10m paired plots were located, which represented grazed and protected patches. Herbage biomass within protected and grazed plots was measured four times each year (spring: May–June, summer: July–August, autumn: September–October, and winter: November–December). Behavioural observations on grazing animals were conducted in the same periods. In both protected and open plots the height and basal diameter of all oak shoots on 5 preselected stumps were measured at the end of five growing periods from 1997 to 2001. All forest stands carried a similar amount of available herbage (averaged over forest stands and growing season, 2614kg/ha). Grazing animals removed on average 1057kg/ha throughout the growing period. Cattle mainly consumed herbage (97% of bites) while goats consumed a mixture of oak browse (41% bites), herbaceous species (34% bites), and other woody species browse (25% bites). The height, diameter and volume of oak shoots were affected by grazing. The three forest stands had similar shoot heights in the protected plots in 2001 after 5 years of grazing protection. The volume of oak shoots of the grazed plots were 146.7cm3 for CL1996, 232.9cm3 for CL1993, and 239.1cm3 for CL1990 in 2001 (i.e. 5, 8, and 11 years grazing after the clear cuttings, respectively). The protected plots carried greater volumes of oak shoots, CL1996: 496.0cm3 (few months grazing before protection), CL1993: 690.0cm3 (4 years grazing before protection), and CL1990: 344.0cm3 (7 years grazing before protection). In conclusion, almost half of the herbaceous vegetation was removed through grazing but it had no positive effect on the growth rate of oak shoots. It seems that goats, which selected almost half of their diet from oak shoots, are responsible for that relative negative growth of shoots. However, cattle grazed only herbs and may therefore control herbaceous vegetation.

Grazing by Kangaroos Limits the Establishment of the Grass Trees Xanthorrhoea gracilis and X. preissii in Restored Bauxite Mines in Eucalypt Forest of Southwestern Australia

Abstract This study examined factors affecting germination, survival, and growth of the grass trees Xanthorrhoea gracilis and X. preissii on newly rehabilitated bauxite mine pits in the jarrah forest of southwestern Australia. Grazing by kangaroos (Macropus fuliginosus) was the major factor in reducing survival and growth of both species during the first 2 years. Provision of artificial grazing protection increased survival and growth (plant mass) of both species by 3‐fold. Grazing by native vertebrates has not previously been identified as affecting mine restoration in Western Australia. Initial germination rates from sown seeds of X. preissii at eight replicate sites ranged from 25 to 64% with a mean of 42%. Corresponding figures for X. gracilis were 5–42% with a mean of 17%. Germination of X. gracilis was greater on heavier, moister soils, but X. preissii germinated better on sandier soils. High levels of initial germination did not ensure high survival. Plants of both species grew bigger and survived better on the lighter, sandy soils. Xanthorrhoea seedlings located in the depressions created by the ripping process grew larger than seedlings on the slopes of the riplines. The presence of plants of other species did not have a significant effect on survival. However, these plants facilitated the growth of both species when artificial grazing protection was unavailable. Plants of other species reduced the growth rates of Xanthorrhoea seedlings where artificial grazing protection was provided. Artificial shade by itself had no significant effect on growth of either species. In rehabilitated bauxite mines in the jarrah forest, the provision of grazing protection is recommended to ensure successful establishment and early survival of Xanthorrhoea spp.

Impact of violacein-producing bacteria on survival and feeding of bacterivorous nanoflagellates.

We studied the role of bacterial secondary metabolites in the context of grazing protection against protozoans. A model system was used to examine the impact of violacein-producing bacteria on feeding rates, growth, and survival of three common bacterivorous nanoflagellates. Freshwater isolates of Janthinobacterium lividum and Chromobacterium violaceum produced the purple pigment violacein and exhibited acute toxicity to the nanoflagellates tested. High-resolution video microscopy revealed that these bacteria were ingested by the flagellates at high rates. The uptake of less than three bacteria resulted in rapid flagellate cell death after about 20 min and cell lysis within 1 to 2 h. In selectivity experiments with nontoxic Pseudomonas putida MM1, flagellates did not discriminate against pigmented strains. Purified violacein from cell extracts of C. violaceum showed high toxicity to nanoflagellates. In addition, antiprotozoal activity was found to positively correlate with the violacein content of the bacterial strains. Pigment synthesis in C. violaceum is regulated by an N-acylhomoserine lactone (AHL)-dependent quorum-sensing system. An AHL-deficient, nonpigmented mutant provided high flagellate growth rates, while the addition of the natural C. violaceum AHL could restore toxicity. Moreover, it was shown that the presence of violacein-producing bacteria in an otherwise nontoxic bacterial diet considerably inhibited flagellate population growth. Our results suggest that violacein-producing bacteria possess a highly effective survival mechanism which may exemplify the potential of some bacterial secondary metabolites to undermine protozoan grazing pressure and population dynamics.

Phenotypic plasticity in the green algae Desmodesmus and Scenedesmus with special reference to the induction of defensive morphology

Organisms belonging to the green algal genera Desmodesmus and Scenedesmus are characterized by a high degree of flexibility allowing them to be true cosmopolitans and to withstand harsh conditions. The environmental conditions determine which phenotypes are being produced and one of the most dangerous situations for the algae is when the organisms are confronted with strong grazing pressure from numerous zooplankton organisms. An overview is given of several aspect of an induced defense in many non-spiny Scenedesmus and some spiny Desmodesmus that may form typical protective eight-celled colonies to avoid mortality from numerous grazers. The morphological response does seem to be linked with a herbivorous zooplankton chemical cue, rather than to a more general animal excretory product. The grazing activity of small sized grazers is reduced, but not of large Daphnia. However, arguments are given why grazing protection of colonial Scenedesmus is probably more efficient under natural than under carbon/light limited laboratory conditions. Finally, a life-history cycle of Desmodesmus and Scenedesmus is presented in which biological aspects such as the anti-grazer response and sexual reproduction are being included.

Biomass requirements from natural pastures for livestock grazing and soil protection in the Eastern African highlands

The problem of seasonal shortages of herbage production from natural pastures in the Ethiopian highlands was investigated. This was done by comparing the available biomass amounts on the pastures with biomass amounts required for livestock grazing and for protecting land slope from soil erosion within a given slope limit at different times of the year. It was observed that the pattern of biomass availability and the requirements for livestock grazing and soil protection vary seasonally, as grazing is concentrated in different parts of the watershed at different times. It was further observed that from March to September feed supplementation is necessary since the available biomass quantities from natural pastures and crop residues on slopes above 3 % are generally inadequate to meet livestock and soil protection requirements. However, with changes in the grazing management to regulate grazing pressure, herbage production on these slopes may be increased to adequately meet livestock needs while protecting the soil from erosion. Herbage production could further be improved through fertilizer application, provision of feed supplements during times of shortfall in biomass, and/or by leaving animal dung on the pasturelands instead of collecting it for fuel and other uses. The cost of feed supplements relative to the cost of restoring degraded land as a result of allowing grazing to exceed the safe limits should be investigated. UNISWA Jnl of Agric Vol 8 1999: pp 23-30

Reconstitution de la végétation à la suite de mesures de protection des sols en terrains montagneux de Tunisie centrale

Recover of vegetation after soil conservation measures in mountain areas of central Tunisia. Sampling sites with a similar soil type, slope and degree of exposure, within areas were selected along the mountain ridge Djebel Kbar in central Tunisia, gouvemorate Sidi Bouzid. Five of the sites had been subjected to various types of soil conservation treatments (e.g. construction of long banks, terraces and pits ; grazing protection, pine plantation establishment) between 5 and 35 years earlier, and another five sites had not been conservateci except grazing protection for the same time période, i.e. from 5 to 35 years. All species of vascular plants were identified, and the number of individuals of each species was counted in each of 9 plots (1 m²) at each sampling site (5 soil conservation areas and four control areas). Biomass of the 25 most frequent species was estimated. Ground cover in % of alfa grass (Stipa tenacissima) was estimated in large plots (10 m x 10 m). The recovery of the plant cover over time was described in terms of character species representing four important stages of succession. About 50 species were identified in the small plots. About 60 plant individuals were present in each small plot. However, plants were much more abundant in the more recently treated areas (about 140) owing to the high frequency of annuals (50-70 % vs 20-40 % in the older ones). The mean biomass per plant was 6,3 g and the range was 0,2-28 g. Approximately 10 species accounted for 80 % of the total biomass, which was 250 g/m². The percent of the ground covered with the alfa grass was considerably higher in the older areas (30 %) than in the more recently established area (5 -10 %). Consequently herbs and grasses were less abundant in the older areas. As the sampling areas increased in age as concerns conservation and protection, plant succession resulted in gradual shift from ruderal, overgrazed vegetation with a high proportion of annuals over to a dwarf shrub -and herb -dominated vegetation type, toward a stage dominated by shrub vegetation. Apparently, these changes were more the result of grazing protection than of the technical conservation measures.

Plant community patterns and soil moisture regime in gypsum grasslands of north central Mexico

Plant community patterns and seasonal changes in soil moisture were evaluated at grazed and ungrazed sites on semi-arid gypsum plains in northern San Luis Potosi, Mexico. Muhlenbergia purpusii was dominant on exposed gypsum soils with indurated surface crusts, while Bouteloua chasei was dominant on exposed gypsum soils with cryptogamic surface crusts. Shallow fine-calcareous alluvial surface soils with gypsum sub-soils were dominated by desert shrubland species. Soil surface characteristics apparently prevent shrubs from establishing on exposed gypsum soils. Gypsum sub-soils are favorable for wide-ranging shrub species as long as a shallow, alluvial surface layer is present. The grassland dominants are slow-growing endemics which appear to be excluded from alluvial surface soils by shrub competition and by the longer period of extreme soil moisture stress in alluvium. Gypsum grassland communities did not show reduction in basal area or invasion by shrubs under heavy grazing pressure, while desert shrubland showed no reversion to perennial grassland under grazing protection. Cryptogamic crust cover was reduced by grazing in all three community types. Grazing decreased mean moisture content in the shrubland soil, possibly by increasing evaporative loss from the surface horizons and forcing the shrubs to use water stored at depth. It increased moisture content in the Muhlenbergia grassland soil, apparently by lowering consumptive use loss.

Development versus Preservation in the Snake River Birds of Prey Conservation Area

The proposed Birds of Prey Conservation Area (BPCA) contains approximately 800,000 acres of mostly public land along the Snake River in southwestern Idaho. It is the most concentrated nesting site for birds of prey-or raptors-in North America, including prairie falcons and peregrine falcons, hawks, eagles, and owls. It also provides habitat for jackrabbits and ground squirrels, the raptors' principal prey, as well as forage for domestic livestock. In addition, the area has a potential use in irrigated agriculture. Bird protection, grazing, and irrigated agriculture came into direct conflict with each other in 1978 when the secretary of interior requested the Bureau of Land Management (BLM) to assess allocation of these acres as a reserved natural area. This conflict provided the impetus for our analysis. The federal Desert Land Act of 1877 and Carey Act of 1906 permit withdrawal of these lands from the public domain for agricultural development. Locally, withdrawal means irrigated production of primarily potatoes and sugar beets. These crops replace the natural vegetation and, therefore, compete with grazing. Irrigated agricultural development also destroys natural ground cover for rabbits and squirrels, thereby creating competition with bird protection. The result is an interesting and unusual, perhaps unique, political alliance. Raptor protectors and grazing interests are usually fierce opponents. The latter fear that the birds kill young livestock. In this case, however, the ranchers' usual opposition to the birds is subordinated to their fear of total loss. Meanwhile, their livestock keep the range vegetation short, enabling raptors to find prey more easily. As a result raptor protectors and livestock organizations readily united to confront their common opponent, irrigated agricultural development.' The analytical problem confronting BLM takes the classical form of two mutually exclusive alternatives, development (irrigated agriculture) versus preservation (grazing and raptor protection). The problem is complicated by the uncertain nature of irrigated agricultural development and by the nonmarket raptor protection values. We consider this problem, proceeding from a conceptual model to an empirical estimation to conclusions and policy implications.

Beef production from native pastures sown to fine-stem stylo in the Burnett region of south-eastern Queensland

At Gayndah, south-eastern Queensland, a native Heteropogon contortus pasture, sown to fine-stem stylo (Stylosanthes guianensis var. intermedia), and invaded by red natal grass (Rhynchelytrum repens), was grazed by weaner steers from June 1 in three treatments : heavy and light put-and-take grazing for five years from 1971, and set-stocked at 1.37 animals ha-1 for two years from 1974. Weight gains in the put-and-take treatments were not significantly different. The mean annual liveweight gain was 167 kg animal-1 at a mean equivalent stocking rate of 1.47 animals ha-1. Over the same period unsown native pasture, cleared of timber, gave a gain of 62 kg animal-1 at 0.62 animals ha-1. In all seasons except summer, weight gains declined linearly with stocking rate and in 1972-73, with a mean equivalent stocking rate of 2.66 animals ha-1, the annual gain was almost halved. When equivalent stocking rates were 0.9, 0.9, 1.8 and 1.2 animals ha-1 in winter, spring, summer and autumn, the respective gains were 4, 73, 65 and 45 kg animal-1. The set-stocked treatment had a mean annual gain of 147 kg animal-1. At another site 116 km north-west of Gayndah, two paddocks of Heteropogon contortus and fine-stem stylo were set-stocked with weaners over four years. One paddock had four applications of superphosphate of 250 kg ha-1. The mean annual liveweight gains were significantly different, being 154 and 143 kg animal-1 in the fertilized and unfertilized paddocks at mean stocking rates of 0.83 and 0.74 animals ha-1, respectively. In a grazing protection experiment the density of fine-stem stylo declined exponentially with an accumulation of pasture dry matter in spring and summer. Heavy continuous grazing, an annual hay cut and an accidental fire all increased the density of fine-stem stylo. Management options to maintain the density of fine-stem stylo and the relative importance of the legume and grass to animal production are discussed.