No abstract

The effects of combinations of different fertiliser rates and grazing methods applied to phalaris-based pastures on an acid, saline, yellow sodosol on the Dundas Tablelands of western Victoria (mean annual rainfall 623�mm) were measured from 1997 to 2000. The objective was to help identify management systems that improve phalaris growth and persistence, water use, and animal production, and thereby the productivity and sustainability of grazing systems. Pastures were either set stocked with low [mean 6.4 kg phosphorus (P)/ha.year] or high (mean 25 kg P/ha.year) fertiliser rates, or rotationally grazed with high fertiliser (mean 25 kg P/ha.year). Rotational grazing was implemented as either a simple '4-paddock' system (fixed rotation length), or a more intensive system where rotation length varied with pasture growth rate. Unreplicated paddocks of volunteer pasture (dominated by onion grass and annual grass weeds) receiving an average of 8 kg P/ha.year were also monitored. All treatments were stocked with spring-lambing Merino ewes. Stocking rate was an emergent property of each treatment, and was driven by pasture quality and availability. Total pasture herbage accumulation ranged from 7150 to 9750 kg DM/ha.year and was significantly lower on the set-stocked, low-fertility treatment than on all other treatments. A significant treatment.day effect in the spline analysis of herbage mass was explained by a trend toward higher pasture mass in the rotationally grazed treatments than set-stocked treatments from the break of season until mid-spring. Rotational grazing led to significantly higher phalaris herbage accumulation than set stocking (mean 3680 v. 2120 kg DM/ha.year), but significantly lower subterranean clover herbage accumulation (1440 v. 2490 kg DM/ha.year). Despite the stronger growth of deep-rooted phalaris in the rotationally grazed treatments, maximum soil water deficits at the end of summer differed only slightly between treatments, with the difference between driest and wettest treatments amounting to only 14 mm. Summer growth of phalaris was apparently insufficient to generate significant differences in soil water extraction at depth, even when phalaris content was increased by rotational grazing, and re-wetting of the soil profile occurred at a similar rate for all treatments. Rotationally grazed treatments supported higher stocking rates than set-stocked treatments at high fertiliser rates (mean 14.9 v. 13.7 ewes/ha), but apparent losses in pasture feeding value due to lower legume content under rotational grazing meant that there were few significant differences between treatments in lamb production per hectare. The experiment showed that grazing method can have a substantial and rapid effect on pasture botanical composition. There are clear opportunities for producers to use temporal and spatial combinations of set stocking and rotational grazing to manipulate herbage mass and pasture composition within broad target ranges for achieving both animal production (e.g. high per-head animal performance) and sustainability (e.g. persistence of perennial grasses) objectives. Rigid application of either set stocking or rotational grazing imposes limitations on both pasture and animal production, and neither grazing method will optimise system performance under all conditions. The experiment also demonstrated that management and land-use changes that have much greater potential to increase water use than those examined here will be needed to ensure the sustainability of pasture systems in the high rainfall zone of western Victoria.

The defoliation tolerance of cultivars of four temperate perennial pasture grasses, perennial ryegrass (Lolium perenne, cv. Yatsyn1), phalaris (Phalaris aquatica cv. Australian), tall fescue (Festuca arundinaceae cv. Demeter) and cocksfoot (Dactylis glomerata cv. Porto), was determined under controlled conditions over a period of 12 weeks. Undefoliated plants were compared with defoliated plants, where only half of one leaf was left intact at the initial defoliation, and leaf regrowth was harvested every 3-4 d. The growth responses measured were plant tiller number, dry weight, relative leaf regrowth rate, root:shoot ratio, sheath:stem ratio and specific leaf weight. All species showed morphological adaptations that potentially increased their ability to tolerate defoliation (e.g. increased allocation to shoot at the expense of roots and lower specific leaf weights) but cocksfoot was found to be the most defoliation-tolerant and perennial ryegrass the least. The adaptation that favoured cocksfoot most strongly was high sheath:stem ratio, which, it is proposed, allowed it to maintain photosynthesis and a level of carbon supply sufficient to support regrowth throughout the experiment. The strategy of perennial ryegrass which favours leaf growth and leads to rapid leaf turnover rates made it particularly susceptible to defoliation under the conditions of this experiment. This highlights the likely importance of defoliation-avoidance responses in explaining the wellknown grazing resistance of this species. Phalaris and tall fescue showed responses that were intermediate between the other two species. The importance of defoliation-avoidance mechanisms and implications for grazing management are discussed.

A survey of 71 sites was conducted in the dairying districts of south-western Victoria in October 1994 to determine factors associated with nitrogen (N) fixation in white clover based pastures. Twenty-eight factors (environmental, microbiological, management, soil, and pasture) were considered in relation to 2 indicators of N fixation by white clover, %Ndfa (percentage of total plant N derived from the atmosphere, as determined by the 15N natural abundance method) and kg of N fixed per tonne herbage dry matter (legume and non-legume). On light-textured soils (sandy loams), soil potassium, rhizobia numbers, total soil N, and density of the nematode Pratylenchus sp. accounted for 72% of variation in %Ndfa. On medium-textured soils (clay loams), crude protein of perennial ryegrass and plant-available phosphorus in the soil accounted for 30% of variation in %Ndfa. The amount of N fixed was influenced by different factors, again depending on soil texture. Soil chemical properties accounted for 31% variation on the medium-textured soils with nematode density, pasture quality, and soil chemical properties accounting for 77% of variation on light-textured soils. Amounts of N fixed per tonne herbage dry matter produced averaged 8.2 kg on the light-textured soils and 7.3 kg on the medium-textured soils. Average %Ndfa values were 67% and 60% on light- and medium-textured soils, respectively. Effects of soil texture on N fixation were attributed to the different cation exchange and water-holding capacities of the soils and highlight the importance of considering soil type in N fixation studies.

The relationships between productivity and plant species diversity were assessed using data from the Sustainable Grazing Systems (SGS) 10 national experiment sites. Each site applied up to 7 different management treatments. Sites varied in the amount of data available for analysis. Plant species diversity was assessed in terms of both the total and native species present at each site. More than 200 plant taxa were recorded over the period of the experiment, about one-third of which were native. In the majority of cases, the native species present within a treatment remained there throughout the experiments, even when fertilised and oversown with introduced species. The number of native species increased by 1 or 2 over the 3–4 years of each experiment where grasslands were less intensively used (average herbage mass >2 t DM/ha), but decreased in more heavily grazed treatments. Native grasses made much greater contributions to herbage mass than other native species. Of the more than 70 native plant species found, the most numerous were broadleaf species, which tended to be more variable under management treatments. As total species richness increased, there was a tendency for pasture productivity to be less, for the mean standing herbage mass to be less and for seasonal growth to be less stable. This depended upon experiments and tended to apply at the sites with higher annual pasture growth rates. All treatments had >10 plant species within the small (about 1.5 ha) paddocks used and larger paddocks often had many more. It was apparent from the high plant species richness at each site (about 25–100 species) that many more niches exist in these paddocks than the number of species usually sown in a pasture. Opportunities to redesign pasture mixtures to exploit more of the resources available obviously exist. Understanding of the relationships among management practices, productivity and plant species diversity is limited, but will become important as we seek more sustainable grazing systems. The studies analysed here suggested that where the herbage mass was maintained between 2 and 4 t DM/ha then species were maintained and productivity was optimised.