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    Tiller dynamics and leaf growth processes of the perennial ryegrass cultivars "Ellett" and "Grasslands Ruanui" as influenced by environmental factors : a thesis in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Ph. D.), Institute of Natural Resources, College of Sciences, Massey University, Palmerston North, New Zealand
    (Massey University, 1999) Bahmani, Iman; Bahmani, Iman
    In New Zealand, perennial ryegrass (Lolium perenne) is considered the most important and cheapest feed source for dairy cows. The profitability of dairy farming is reliant on the quality, persistence and productivity of perennial ryegrass dominant pastures. However, research shows some 'modern' perennial ryegrass cultivars derived from the Mangere ecotype do not persist under intensive dairy cow grazing. Other research suggested that 'Ellett', a representative of 'modern' ryegrass cultivars, had a different perennation strategy from the older cultivar 'Grasslands Ruanui', used previously. The objectives of this study were (i) to compare the sward productivity, tiller dynamics and flowering behaviour of the perennial ryegrass cultivars 'Ellett' and 'Grasslands Ruanui' under different nitrogen and irrigation treatments and (ii) to determine the morphological and tillering responses of 'Ellett' and 'Grasslands Ruanui' ryegrasses when grown in different light environments. A field experiment at the No 5 Dairy of the Dairying Research Corporation (Hamilton, New Zealand) was conducted from April 1996 to April 1998 to assess the sward productivity of 'Ellett' and 'Grasslands Ruanui' ryegrasses, and their tiller dynamics and flowering behaviours under two levels of nitrogen and irrigation. This field experiment was rotationally grazed by dairy cows stocked at 3.2 cows per hectare. Results showed 'Ellett' had a greater tiller weight and lower tiller density than 'Grasslands Ruanui', which was reflected in higher average yields in spring (3220 vs 2788 kg DM/ha) and summer (2125 vs 1844 kg DM/ha) for 'Ellett' and 'Grasslands Ruanui', respectively. Because of different sward structures 'Ellett' tended to 'pull' more than did 'Grasslands Ruanui' during grazings in summer and autumn, but this did not appear to have greatly influenced the yield comparisons between cultivars. No significant differences in the proportions of white clover, weeds and other grass species were observed between 'Ellett' and 'Grasslands Ruanui' swards. 'Ellett' and 'Grasslands Ruanui' ryegrasses had similar herbage accumulation responses to nitrogen and irrigation; pre-grazing herbage mass, tiller weight and botanical composition were generally affected by these treatments. Overall, nitrogen fertiliser increased herbage accumulation and tiller density, but tiller weight responses were restricted to the second winter and autumn. Irrigation increased herbage accumulation and tiller density in summer and autumn, but did not significantly affect tiller weight. Seasonal tillering patterns for both cultivars were similar and were characterised by high tiller populations in summer followed by a rapid decline in early autumn and then a recovery or tillering 'flush'. This pattern was more pronounced in the second year than the first. 'Grasslands Ruanui' had a higher total tiller number than 'Ellett', regardless of nitrogen and irrigation treatments. From a sward stability diagram developed in Chapter 5, a population index (Pt/Po) was defined as reflecting population changes over time via changes in tiller birth and survival rates. 'Ellett' had a higher population index (Pt/Po) in winter (June 1997) than did 'Grasslands Ruanui' as 'Ellett' had a higher birth rate than did 'Grasslands Ruanui'. This suggests 'Grasslands Ruanui' was more dormant in winter than was 'Ellett'. The ryegrass cultivars when treated with nitrogen had different tiller turnovers in autumn (March 1997) and late spring/early summer (December 1997) giving rise to a significant cultivar x nitrogen interaction. The autumn (March 1997) population index of 'Grasslands Ruanui' (0.704 without nitrogen) decreased when treated with nitrogen (0.627) whereas the 'Ellett' population index (0.788 with nitrogen) increased, compared to the no nitrogen treatment (0.677). The opposite trend was observed between cultivars during the post-flowering period (December 1997) with a higher population index for 'Grasslands Ruanui' (1.129) compared with a lower population index for 'Ellett' (0.996) when both cultivars were treated with nitrogen. Spring differences in tiller population index between cultivars were linked to their flowering behaviour (see Chapter 5 and 6). 'Ellett' had a higher percentage of reproductive tillers than did 'Grasslands Ruanui', regardless of environmental factors, and these differences were stronger in the second year (1997/98), despite there being no clear differences in tiller turnover between 'Ellett' and 'Grasslands Ruanui' during the pre- and post-flowering periods. Moreover, 'Ellett' had a tendency to produce more reproductive tillers in response to nitrogen whereas 'Grasslands Ruanui' favoured vegetative tillering, especially after the establishment year. Due to an acceleration of leaf area expansion, as tiller number increased in response to nitrogen, the half-life of ryegrass tillers was reduced for the tillers tagged in September 1996 and June 1997. Irrigation increased total tiller number for both cultivars in January 1997 and February 1998 but tiller populations declined in March 1998 with 'Grasslands Ruanui' losing more tillers than 'Ellett' (see Chapter 5). Reproductive tiller number averaged across cultivars increased with irrigation in November and December 1996 but decreased in October 1997. Both ryegrass cultivars had similar responses to irrigation with respect to reproductive and vegetative tillering, except in December 1996 and January 1997, when 'Grasslands Ruanui' had more vegetative tillers than did 'Ellett' (see Chapter 6). To characterise the morphological traits affecting vegetative tillering of 'Ellett' and 'Grasslands Ruanui' ryegrasses, an experiment during vegetative growth with different light environments (control and shade treatments, see Chapter 4), was carried out at INRA, Lusignan, France from February to May 1998. To study the relationship between leaf size and tillering, the morphogenesis of 'Ellett' and 'Grasslands Ruanui' were compared with short- and long-leaved perennial ryegrasses ('SL' and 'LL', respectively) resulting from divergent selection in France based on lamina length. Results from this experiment, after imposing thirty three days of light treatment, showed that ryegrass genotypes had different leaf morphology and tillering. 'Ellett' had longer leaves, higher tiller weight and lower tiller number than 'Grasslands Ruanui' regardless of light environments. Differences between genotypes in leaf length were attributable mainly to higher leaf elongation rate in the two long-leaved genotypes ('Ellett' and 'LL') compared with short-leaved types ('Grasslands Ruanui' and 'SL'), as leaf elongation duration (LED) did not differ significantly between genotypes. 'Grasslands Ruanui' maintained greater site filling and a similar leaf appearance rate to 'Ellett' in both light environments, offering an explanation for the higher tiller number per plant for 'Grasslands Ruanui' than for 'Ellett'. However, 'LL' had a higher tiller number per plant than 'SL', because of a higher leaf appearance rate, although this difference decreased during the experiment under the control treatment as 'SL' tended to have a higher site filling ratio than 'LL'. When shaded, differences in tiller number between 'LL' and 'SL', were more mediated by the leaf appearance rate than site filling. With regard to the relationship between leaf growth characteristics and tillering, these results show genotypes with high LER and long lamina length, even though associated with reduced site filling in both light environments, do not necessarily have low tiller number per plant. To describe post-flowering tillering in a dense canopy of 'Ellett' and 'Grasslands Ruanui' ryegrasses, before and after removal of the seed-head, a second controlled light experiment was carried out at INRA, Lusignan, France (see Chapter 6), from October 1997 to July 1998. Results of this experiment after imposing the different light treatments (near full light and shading) for forty five days showed that 'Ellett' had a higher proportion of reproductive tillers than did 'Grasslands Ruanui' during the summer of the establishment year, regardless of light environment (34 % vs 25 %, respectively). These results were in agreement with previous field observations (Chapter 6). Shading reduced the proportion of reproductive tillers produced by both cultivars compared with the full light treatment. Vegetative tiller weight of both cultivars was similar under both light treatments whereas shading reduced reproductive tiller weight by 40 % for 'Ellett' and 43 % for 'Grasslands Ruanui' compared with full light. From the results obtained in field and controlled light experiments, it was concluded that an explanation of the tiller dynamics pattern for 'Ellett' and 'Grasslands Ruanui' could be related to their morphological characteristics (see Chapter 7 and 8). Thus, to assist with the understanding of the seasonal tillering patterns of 'Ellett' and 'Grasslands Ruanui' ryegrasses in different environments, a conceptual approach was proposed, using the morphological ceiling leaf area (MCLA) as an agronomic indicator regulating tiller density.
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    Environmental and management factors as determinants of pasture diversity and production of North Island, New Zealand hill pasture systems : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD), Pastures and Crops Group, Institute of Natural Resources, Massey University, New Zealand
    (Massey University, 1999) Nicholas, Philippa Karen
    Hill pasture systems are inherently variable due to both environmental (e.g. rainfall, temperature, altitude, slope, aspect and microtopography) and management (e.g. stock type, stocking rate, grazing behaviour and soil fertility) factors. Fertiliser application and grazing pressure are the two main tools used for hill pasture management, as hill pastures are non-arable and the success of oversowing into existing pastures has been limited. One aim of pasture management is to increase the percentage of desirable species (e.g. L perenne and T. repens) by changing composition rather than the addition of new species. Pasture botanical composition affects production directly through the productive capabilities of species present, but it is hypothesised that the number of species present play a role in pasture productivity and stability of hill pasture systems. A field survey and two glasshouse experiments were performed on hill pasture swards to identify the effects of imposed management and environmental factors on botanical composition. The relationship between species diversity and productivity was also investigated. Ten field survey data sets were collected from two research farms. These data sets reflected different management history, climate, season and time (28 year time lapse). Information collected for each data set included botanical composition, Olsen P, hill slope, standing green biomass, and species growth rate over a one month period. The results of the survey indicated that the same species were present on all sites surveyed, but the abundance of those species changed. For example, L. perenne, A. capillaris and T. repens were most abundant on the high fertility sites, A. capillaris was the dominant species on the low fertility sites, flatweeds were more abundant on the dry sites, and Muscii spp. were more abundant in spring than summer. There was no direct relationship between species diversity and pasture production, but factors such as hill slope, fertility and season appeared to play a role in a more complex, undefined relationship between species diversity and productivity. The first glasshouse experiment involved the application of two simulated management factors (i.e. defoliation height and treading) to hill pasture turves. The turves were removed from three hill country farmlets that had different management conditions imposed on them for 20 years. The abundance of A. capillaris, L. perenne, A. odoratum, Poa spp. and T. repens increased with the tall defoliation height, which was a positive effect, as was the increase in abundance of T. repens with treading. L. perenne and H. lanatus abundance decreased whilst treading was occurring, which was a negative effect. The second glasshouse experiment involved the application of a simulated environmental factor (i.e. moisture deficit and excess moisture) and its interaction with a management factor (i.e. treading) on the same turves. The abundance of H. lanatus, Poa spp., T. repens and other legumes decreased under the moisture deficit treatment. L. perenne abundance was unaffected. The abundance of C. cristatus, A. odoratum, F. rubra and L. perenne decreased under the excess moisture treatment, all others increased. T. repens abundance was increased with a combination of treading and excess moisture. Functional groups were developed as part of the objectives of this research programme, to simplify the system being studied. In response to the management and environmental factors applied to the turves, the functional groups were described as being increasers, decreasers or static. The results of the turves experiments were used to validate the definition of the functional groups. For example, the high fertility responsive grass functional group was more abundant on the high fertility turves and L. perenne, which is also responsive to high fertility conditions, was found to be in a functional group of its own because of its ability to recover from treading. A. capillaris, which like low fertility tolerant grasses was abundant on low fertility sites, was separated into a group of its own because of its great abundance and dominance of the sward. No definitive relationship between species diversity, production and stability of production was observed in the turves experiments. That A. capillaris was particularly dominant in all swards may have significantly influenced the relationship. As with the field survey, however, all species were observed on all turves, and just the abundance of those species changed. The results of this experimental work showed that pasture composition can be altered to a more desirable (leafy green with legumes and adapted to the environment in which they are occurring) form with the use of management factors such as fertiliser application, defoliation height and treading. That composition was changed without a change in the number of species present, suggested that such composition changes are reversible. The work also highlighted the importance of an environmental factor, that cannot be controlled by land managers (i.e. soil moisture), and its interaction with management practices in maintaining a desirable and stable pasture composition.
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    An evaluation of a dairy systems study of the effects of contrasting spring grazing managements on pasture and animal performance : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Ph. D.), Pastures and Crops Group, Institute of Natural Resources, College of Sciences, Massey University, Palmerston North, New Zealand
    (Massey University, 1999) Bishop-Hurley, Gregory John; Bishop-Hurley, Gregory John
    Traditionally, the emphasis in dairying systems in New Zealand has been on maintaining pasture quality in late spring through increased grazing pressure and occasionally topping. Recent studies have reported an increase in summer and autumn herbage production by allowing some reproductive development during spring, followed by a period of hard grazing at the time of anthesis when seed heads are immature and still palatable (late control), through effects on tiller population and size. The objectives of this study were to (i) evaluate whether the benefits of late control can be measured within the management constraints of a self-contained spring calving dairy production system, (ii) investigate the conditions under which late control spring grazing management can be implemented, and (iii) investigate the options available for the use of additional feed over spring and summer assuming late control spring grazing management is effective. A dairying systems study at No 4 Dairy Unit, Massey University was set up in which two 20-paddock perennial ryegrass/white clover dominant farmlets of 45 hectares were each stocked with 120 spring calving Friesian cows in October 1993 and run for three lactations until May 1996. With the exception of spring grazing management and spring supplement feeding the farmlets were balanced. The first treatment, designated early control (EC), involved strict control of grazing throughout the spring and summer with average pasture cover targeted at approximately 2000 kg DM ha-1 and a post-grazing residual of approximately 1500 kg DM ha-1. Pastures in the second treatment (late control - LC) were allowed to develop some reproductive growth through October and November for removal in December. Average pasture cover target was 2700 kg DM ha-1, with a post-grazing residual of approximately 2000 kg DM ha-1 over spring. Average pasture cover was reduced to 2000 kg DM ha-1 in December by grazing to lower residuals while at the same time removing paddocks from grazing for immediate conservation. Bayesian smoothing provided an alternative to analysis of variance (ANOVA) for those variables where both treatments and/or all replicates were not measured at the same point in time, and for large data sets and produced mean values close to those that would be produced by conventional analysis methods without the need to group arbitrarily. The development of a dynamic rising plate meter calibration equation which accounts for seasonal differences in pasture density allowed clearer definition of herbage mass estimates from rising plate meter measurements. Mass per unit height values showed a distinct seasonal pattern reflecting changes in the sward. The bulk density of pasture in the summer was found to be twice that in the winter. Overall, there was no extended period of difference between early control and late control in either pasture production or animal production during the three years of the trial. However, large differences in animal performance would not be expected considering the marginal differences in pasture production achieved. While treatment differences in average pasture cover and pre- and post-grazing cover were achieved over late spring in all three years, the pasture cover differences required for the late control treatment were not achieved, and as a consequence the response in animal performance was smaller than the results of previous small-plot and paddock-scale experiments suggested. The results of the trial showed good internal consistency between production components and good control of variability was achieved in this large systems trial, providing an objective basis for evaluation. A number of the variables (clover contents and tiller densities) measured during the trial suggest the potential for contrast in system performance between early control and late control. The ability of the system to buffer changes contributed to the difficulties in achieving treatment specifications. Systems research of this type needs to include tight specifications and control of pre- and post-grazing pasture cover in addition to average pasture cover. More flexibility in stocking rate or use of supplements may be needed to establish spring pasture cover contrasts in future studies. A whole farm simulation model (UDDER) was used to investigate alternative management strategies for utilising grazed and conserved herbage, after modifications to achieve effective matching between predicted and measured levels of pasture production and animal performance. The level of milk production predicted by UDDER was not achieved in the field over three years using the same inputs, possibly due to the inability of the model to cope with the limitations of colder/wet winters and wetland dairy farming. The adjustments made to the parameters of UDDER were in general successful, allowing daily and annual milksolids production to be modelled. However, herbage intake was insensitive to higher spring pasture covers and resultant increase in allowance. For most of the year UDDER predicted the herbage intake of lactating cows to be at or near their potential. Early control and late control base models were used to evaluate alternative management strategies for using the extra herbage accumulation generated under the late control management, including feeding conserved forage during summer to lactating cows or during winter to dry cows, stocking rate (2.6, 2.8 and 3.0 cows ha-1) and the level of conservation (none versus increased). The loss of quality associated with conservation meant that conserving and adding silage back into the system did not increase milksolids production or gross margin, particularly when UDDER predicted that no real feed shortage existed. However, in practice conserving herbage reduces the risk associated with poor growing years. The low stocking rate policy was the best for early control, although the stocking rate policy with 2.8 cows ha-1 and conserved supplements being fed back to lactating cows in summer was similar. The latter policy was the best for late control. At the high stocking rate the flexibility of the system was reduced. In general, milksolids production and gross margin were higher for late control than early control, provided the increase in herbage accumulation rate associated with lax spring grazing management (late control) was factored in. Since a search of the literature failed to identify a model capable of predicting the response of pastures to late control spring grazing management, an attempt was made to develop a tiller-based model to allow the late control system to be investigated further. The model developed estimates equilibrium tiller density based on size or mass of ryegrass tillers at the environmental ceiling leaf area based on daily levels of photosynthetically active radiation. However, there was insufficient detailed sward data available to provide conclusive evidence for the validity of the tiller model. Despite the lack of consistent treatment differences obtained from the trial and the difficulties experienced when modelling late control management alternatives, this project has provided a comprehensive data set and considerable insight into the dairy production system. Late control spring grazing management can potentially increase the overall productivity of the seasonal dairying systems of New Zealand. In practical terms the main requirement is for a change in conservation management over the spring period, with no other direct costs involved. However, the timing limitations which are an inevitable consequence of rotational grazing systems restrict the opportunity to impose late control management with the rigorous timing that component research suggests may be necessary. During the course of this trial spring grazing management on dairy farms has tended towards that of late control management, with farmers operating grazing systems with higher average pasture covers through the spring with the aim of improving per hectare production through per cow performance.
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    Ecology of pastoral communities in a heterogeneous environment : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Pastoral Science at Massey University, Palmerston North, New Zealand
    (Massey University, 2000) López, Ignacio Fernando; López, Ignacio Fernando
    A group of studies was conducted to examine environmental variables and pasture components and their relationships in heterogeneous hill country pasture. Four studies were conducted in relation to the hill country grassland ecosystem of New Zealand. 1. The effects of long-term fertiliser-stocking rate and hill country slope category (LS Low slope, 0-12° MS Medium slope, 13-25° HS High slope, ?25°) on soil physical and fertility attributes and pasture production were examined. Field treatments, high fertility-high stocking rate (HH) and low fertility-low stocking rate (LN), have been applied to paddocks since 1975. Soil samples were taken from the slope categories of the two field treatments (microsites) and physical and fertility features were analysed. Dry matter production through the year was also measured from these units. The soil attributes that explained the largest percentage of the differences between microsites were water holding capacity (WHC), water conductivity (Kunsat), slope, soil compressibility (SC), bulk density (BD), Olsen-P, soil total nitrogen (Total-N) and soil rebound after compression (SR). Slope led to greater differences between soil features of microsites than fertiliser and stocking rate history. Dry matter production increased with increasing Total-N, Olsen-P, WHC and SC, and decreasing slope, Kunsat, BD and SR. 2. The presence of plant functional groups, species segregation and their relationship with soil features were analysed. The relationship between field condition and plant functional group was also examined. The evaluation was conducted in the same sites as the first study. The pasture botanical composition for each microsite was measured through the year and plant functional groups determined. The relationship between the presence of plant species and the soil attributes WHC, Kunsat, slope, SC, BD, Olsen-P, Total-N and SR (from the first study) and plant functional groups were studied, as well as the field condition-plant functional groups relationship. Seven functional groups were determined. High fertility grasses and Lolium perenne (Lp) were associated with LS and high availability of resources, while low fertility species were segregated to HS. Groups of species such as Agrostis capillaris (Ac) were indifferent to environmental changes. Functional groups proved to be good indicators of soil development. Field condition and plant functional groups were complementary concepts in grassland dynamic analyses. 3. Sheep grazing behaviour was examined in relation to slope category and plant species selection. The study was conducted in the same microsites as studies 1 and 2. Transects with marked tillers of Anthoxanthum odoratum (Ao), Ac and Lp were placed in the slope categories as follows: Ac and Lp in LS; Ac, Ao and Lp in MS; and Ac and Ao in HS. The evaluation was carried out during 4 weeks in each of Summer, Autumn, Winter and Spring, and records of grazed and ungrazed tillers were analysed. Pasture growth rates were calculated through the year. During Spring sheep grazed mainly the LS. With decreasing availability of pasture, sheep enlarged their grazing areas towards the HS. Species selection was only present during Winter when pasture availability was low. In Winter sheep also grazed in all slope categories and selected Lp over of Ac but showed no selection for Ao. 4. Ecotype segregation and plant phenotypic plasticity were examined. Plant material was collected from the extremes of the environmental gradient analysed in studies 1, 2 and 3 and grown in glasshouse conditions under five levels of phosphorus and three of nitrogen in the soil. The plants in each pot were cut on three occasions and total dry matter was calculated. Height, plant architecture, plant horizontal expansion and leaf growth were analysed for Cynosorus cristatus (Cc), Holcus lanatus (HI), Ac, Ao and Lp. Morphological and physiological differences were present between genotypes of Lp whereas only physiological genotypic differences existed in Ao and Cc. Consistent differences were not found between HI genotypes. Thus, Ao, Cc and Lp showed ecotype differentiation. Ac genotypes showed high plasticity with no ecotype differentiation.
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    Legume abundance in New Zealand summer-moist and summer-dry hill country pastures: a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy, Institute of Natural Resources, Massey University, Palmerston North, New Zealand
    (Massey University, 2003) Hepp, Christian
    The introduction of legumes has been a central factor in hill country pasture improvement in New Zealand since they are able to fix atmospheric nitrogen to contribute to pasture production and also improve pasture quality. However, at present legume content of hill swards has dropped to generally very low levels which will likely affect medium-long term sustainability of these pastoral ecosystems. This thesis is focussed on determining the relative importance of management and environmental factors that affect legume abundance in hill country swards. A series of field experiments were undertaken between May 2000 and April 2002 in contrasting summer-moist (Ballantrae; 40°19'S, 175°50'E) and summer-dry (Waipawa; 40°00'S, 176°23'E) hill country areas, including north aspects and south aspects at Waipawa, to evaluate the relative impacts of cutting height (targets of 3 or 7 cm), grass suppression by application of selective herbicide, soil-P status (high or low), autumn nitrogen application (0 or 50 kg N/ha), defoliation management (cutting, and rotational grazing with sheep), on legume abundance. A glasshouse experiment involved the effects of companion grasses and defoliation on individual white clover and subterranean clover plants grown in pots. The suppression of grass competition caused the most marked changes in short-term sward composition, with an increase in legume abundance at all sites. Increasing soil-P status increased legume abundance, especially where white clover was present as a major botanical component. Altering pasture cutting height or applying nitrogen in the autumn had comparatively minor effects on legume abundance. Defoliation management (i.e. rotational grazing with sheep compared with cutting) played a secondary role in affecting legume abundance of swards, at least in the short term. Residual effects of grass suppression resulted in an increase in legume abundance, which persisted for at least one year after treatment and affected legume species composition in the drier Waipawa north aspect. Pastures in the summer-moist areas showed a high degree of resilience and reverted quickly to the original grass dominance. Inter-annual and site contrasts in soil moisture patterns highlighted the relevance of soil moisture as a key factor in determining legume abundance and production, mainly in the summer-dry areas. In the glasshouse trial, severe defoliation had very strong effects, reducing plant size, leaf area, vegetative stem development and branching of individual legume plants. Moreover, root competition from grasses was seen to significantly limit clover shoot growth, this being more important than the shading effect. From these experiments, it is suggested that the relative importance of factors affecting legume abundance in hill pastures is: soil moisture (if limiting, e.g. summer-dry hill country) > grass suppression > soil-P status > defoliation management > nitrogen application. However, some of these factors have been noted to interact, and relationships can acquire higher levels of complexity. This study highlighted the relative importance of factors that influence legume abundance of hill country swards and showed that pasture composition can be dramatically modified and legume content increased, but also that these changes can be short-lived. Summer-dry hill country pastures strongly rely on highly variable non-manageable seasonal and inter-annual soil moisture profiles, and legume abundance is difficult to predict and less stable in time than in summer-moist conditions. However, a high potential of legume abundance is achievable with adequate soil moisture in summer, even at low soil-P status. In these dryland areas, the combination of limiting soil moisture, close-to-ground defoliation, and selective grazing, is likely to severely limit white clover growth and spread, and be aggravated by low soil-P status, as well as subterranean clover growth and seed set, therefore compromising legume abundance in the following season. High tiller population density, combined with a high frequency and intensity of legume defoliation, will put legumes under stress from severe grass competition, ultimately affecting legume abundance. This is likely to be the case for many intensively set stocked hill country systems managed with sheep.