The genus Dorycnium L. consists of a number of species of perennial leguminous shrubs known to be relatively drought tolerant. Low rainfall areas (i.e <600 mm annually) of Australia under agricultural use are subject to periods of feed shortage during summer and autumn, and hence animal production is limited at this time. Dorycnium spp. have been identified as having the potential to be integrated into Australian grazing systems as a source of forage when little or no other feed is available. This project was established to further investigate a number of key issues related to the agronomic and forage characteristics of this potentially important genus. Research was undertaken into Dorycnium spp. to examine three important factors associated with the evaluation of a legume, seed germination characteristics, the nutritional value of the forage, and rhizobial associations. Dorycnium hirsutum Ser. accessions TAS1002 and TAS2001 were subjected to a range of germination experiments examining the level of pod maturity, harvest season, and the effect of pre-germination treatments. The seed coat of D. hirsutum was found to influence germination behaviour, with the use of pre-germination scarification treatments improving germination behaviour by increasing the percentage germination (PG) and lowering the mean time to complete germination (MTG) and percentage hard seed. Mechanical scarification of TAS2001 for 20 seconds was found to increase (P<0.05) PG from 86 to 96 %, lower the MTG from 6.0 to 2.7 days, and reduce the percentage hard from 13.6 to 1.9 % in relation to untreated seed. Mechanical and chemical scarification techniques were found to be the most effective in promoting rapid and uniform germination, were simple to apply and were repeatable. In general, inherent differences in seed lot germination characteristics were believed to be associated with the influence of environmental factors and the natural characteristics of selected accessions with indeterminate flowering. Established plots of D. rectum Ser., D. hirsutum and D. pentaphyllum Scop. were sampled along with an area of lucerne (Medicago sativa L.) on a regular basis throughout the spring/summer period of 2001/2002. Samples were analysed using near infrared reflectance spectroscopy (NIRS) and wet chemistry for crude protein (CP), neutral detergent fibre (NDF) and dry matter digestibility (DMD) and metabolisable energy (ME). Over the course of the sampling period forage of Dorycnium spp. generally displayed decreases in CP, ME, DMD and increases in NDF. Typical CP values ranged from 4 - 18 % of dry matter (DM), NDF 21 - 72 % of DM, DMD 32 - 75 %, and ME 4.1 - 11.0 MJ/Kg/DM. The nutritive value of Dorycnium spp. forage appeared to be influenced by environmental and developmental characteristics, with the growth stage identified as a useful tool for predicting forage quality. Although Dorycnium plants were of lower forage value than lucerne, their forage can provide livestock with an important source of nutrition in areas of low rainfall and during periods where there are feed gaps. Experimental plots of Dorycnium spp. at three Tasmanian sites were sampled every six weeks throughout the spring/summer period of 2002/2003 and analysed using a modified butanol-HCl method for condensed tannins (CT). The CT content of D. hirsutum was found to fluctuate from 3.2 to 16.6 % of the DM. Dorycnium rectum and D. pentaphyllum were found to contain CT levels of at least 7.7 and 6.8 % of DM respectively during the sampling period. The CT levels observed were considered to be high in general, with only D. hirsutum containing levels that may be considered to be low and possibly beneficial at certain stages of development. Increases in CT levels were associated with the initiation of flowering, and interactions between the environment and species were observed, although no common factor was identified as influencing CT levels. A glasshouse experiment was undertaken to assess the nitrogen fixing ability of the commercial Lotus corniculatus L. inoculant SU343 with Dorycnium spp. against a range of alternative inoculants. The host/rhizobia interactions of Dorycnium spp. along with six important pasture legumes and a range of inoculants was assessed. Strains WSM1284, WSM2323 and WSM2338, along with SU343 were found to be suitable inoculants for Dorycnium spp. examined. However, negative interactions between these inoculants and important pasture legumes were identified. Inoculant strains, WSM1284, WSM2323, WSM2338 and SU343 were selected to undergo evaluation under Tasmanian field conditions with D. hirsutum and D. rectum. In the field all strains were found to fix adequate amounts of atmospheric nitrogen. Inoculant SU343 was confirmed to be a suitable inoculant for D. rectum in terms of performance and commercial viability, however, WSM2338 and WSM1284 were found to be equally suitable. The inoculation of Dorycnium hirsutum with the Tasmanian isolate WSM2323, was found to be a significantly (P<0.05) superior strain to SU343 in terms of nitrogen fixation. The inoculation of D. hirsutum did not affect (P>0.05) plant DM production in the field. The ability of the inoculants to compete with a background population of root nodule bacteria was found to be of concern, and may have serious implications for the long-term performance from a single inoculation event. It was proposed that a combination of rhizobial strains may be more effective as a commercial inoculant rather than relying on the single L. corniculatus inoculant SU343.

The prospects of sustainable crop production technologies in East Timor were discerned with relevant case studies and experimental data. An overview of the agricultural development in East Timor with particular emphasis on the traditional farming and cropping systems was presented complemented by the discussion on the aspects of agricultural mechanization and technological change and their socioeconomic ramifications on food security. Empirical data from tillage trials, established both in East Timor and New Zealand, were gathered and discussed in the quest for a better understanding of tillage effects on soil structure and crop production environment. The agro-climatic zones of East Timor provide a well-defined set of ecological boundaries upon which further collaborative research work can be developed. Given land resources as one of the major capital investments in agriculture development, the drive towards improvement and technical change in agriculture should be directed in a balanced combination, whenever appropriate, between technologies of land-saving (hybrid seeds, irrigation, and drainage) or labour-saving (mechanization, herbicides, varieties and cropping techniques) characteristics. Moreover, the justification for acquiring an improved technology for traditional farmers, to some extent, needs to conform to the features of their subsistence mode of farming. The emphasis in technology dissemination, therefore, will have to shift from communication to education. Experimental results of this study on the effects of tillage, and no-tillage in Particular as a form of conservation tillage, on the edaphic changes affecting cropping environment generally concur with the findings known in the literature. Organic carbon levels are generally restored with cropping in East Timor. In addition, soil bulk density and crop grain and biomass yield were not affected by tillage treatments. Soil compaction was significantly affected by tillage as shown by data from the Palmerston North experiment. Soil aggregate stability in the 0-10 cm topsoil was similar under all the tillage treatments. Manual tillage (MT) had the greatest number of soil aggregates on sieve after a 30-minute wet-sieving (68.3%) followed by no-tillage (NT) (65.1), permanent pasture (PP) (62.6) and conventional tillage (CT) (56.5). Similarly, the top 0-10 cm soil under MT had significantly larger macroporosity (16.4%) than CT (9.23), NT (11.5), and PP (10.6). MT and CT significantly reduced the total C whereas N levels were significantly decreased by tillage (CT, MT and NT) compared to permanent pasture at the top 0-10 cm soil layer. Barley grain and biomass were unaffected by tillage whereas potato tuber yield and biomass were significantly less under no-tillage. Conventional tillage significantly increased water runoff but produced less leachate compared to no-till and permanent pasture. Total soil sediment loss was significantly lower under PP (95.8 kg/ha) and NT (132.9) compared to CT (3556.7) and MT (4652.2). pH of water runoff was significantly reduced under tillage treatments compared to that from permanent pasture whereas nitrogen losses were unaffected. There are at least four major public policy components that will play vital roles in the development of sustainable crop production technologies in East Timor: (i) Agricultural research and development (ii) Agricultural extension (iii) International and regional networking (iv) Shift of policy focus. The policy approach needs to be decentralized and broad-based and conservation agriculture should be promoted as opposed to conventional production agriculture. Three major areas for the future research agenda include: (i) Integrated Farming Systems (ii) Soil tillage and erosion (iii) Applied science and technology. The last component may cover disciplines such as: food policy analysis, farm machinery selection and testing, soil testing and mapping, land evaluation and GIs, bio-energy technologies, improved local seed varieties, adaptive fodder crops for improved grazing and pasture management, appropriate agro-forestry and soil and water conservation technologies and cash crop initiatives.

The study undertaken was carried out in the montane tussock grassland region which in the South Island comprises some six million acres. Field trials were established at Mt Somers, Mid Canterbury. This study attempts to elucidate some of the factors connected with the sulphur nutrition of grasses. For the particular trial area it was hoped to determine four key aspects ; 1. Whether the response of the grass to nitrogen is governed by the sulphur level available. 2. A determination of what level of nitrogen was likely to be provided by clover fixation and transferred to the grass component under improved grassland conditions in the area. 3. What the optimum level of sulphur for the association is, allowing adequate sulphur for maximum possible nitrogen fixation as well as for utilization by the grass of the nitrogen transferred. 4. The effects of nitrogen and sulphur on plant chemical composition and any possible relationships between plant and soil as infuenced by these factors. Following a review of literature, a summary of experimental work is presented, with results, discussion and conclusions.

This thesis is presented in the form of a series of papers some of which have already been published. It is prefaced by a review of research on the agronomy and physiology of wheat yield in New Zealand, and concluded with a general discussion, Other relevant papers in which the author was a contributor may be found in the Appendix. / Between 1972 and 1976 a series of field experiments were conducted to investigate the factors limiting grain yield in wheat (Triticum aestivum L). Spike population restricted yield when it was below about 500 spikes/m² but once it exceeded 700/m² grain yield declined due to a reduction in the number of grains per spikelet. Irrigation, high sowing rate, and nitrogen fertiliser generally increased spike number but the effects on the number of grains/spikelet were variable, apparently depending on processes related to leaf area index. The semi-dwarf Karamu outyielded New Zealand wheats when spring-sown because it produced more spikelets/spike and more grains/ spikelet than the other cultivars. The superior grain set of Karamu was because more of the distal florets within each spikelet produced grain than in the other cultivars. The results are discussed in relation to cultural practices and plant breeding.

Variables for the accelerated ageing (AA) test, methods for reducing fungal contamination during the AA test, using the conductivity test as a vigour test, the effect of seed size on seed vigour and the relationship between laboratory test results and field perfonnance in selected Brassica spp were investigated. In the first experiment, three seed lots of turnip rape hybrid (B. rapa x campestris), turnip (B. campestris) and forage rape (B. napus); and seven seed lots of Asian rape (B. napus), six seed lots of Asian kale (B. oleraceae var. alboglabra L.) and five seed lots of choisum (B. rapa var. pekinensis) with germinations above 90% were aged at two different temperatures (41 and 42°C ± 0.3°C) and three ageing times (24, 48 and 72 ± 15 minutes). The second experiment was divided into three sections. In the first, the same seed lots and species were aged at one temperature (41°C) and time (72 h), but either 40 ml of saturated salts; KCl (83%RH), NaCl (76%RH), NaBr (55%RH); or distilled water (96%RH) were used as the ageing solutions. In the second, one turnip rape hyprid seed lot was aged at three temperatures (41, 42 and 45°C) and two times (72 and 96h), again using the three saturated salts and distilled water as ageing solutions. In the third, three turnip rape hybrid seed lots and three Asian kale seed lots were surface sterilised (1 % sodium hypochlorite) prior to ageing at one temperature (41°C) and time (72 h). In the third experiment, the same species and seed lots used in experiment one at their original seed moisture content (SMC) were tested for conductivity after soaking in deionised water for 4, 8, 12, 16, 20 and 24 h. They were then re-tested after the SMC had been adjusted to 8.5%. In the fourth experiment, three seed lots of forage rape and three seed lots of Asian kale were graded into three seed size categories; large (retained on a 2.0 mm screen), medium (retained on a 1.7 mm screen) and small (passed through a 1.7 mm screen). Graded seeds were then tested for standard germination, AA (41°C/48 h) and conductivity (measured at 16 and 24 h). In the final experiment, the relationships between laboratory tests for the six species (each consisting of three seed lots), field emergence from three sowings, and cold room emergence were evaluated. Both time and temperature influenced post-AA germination. Increasing the ageing period from 48 to 72 hours at 41°C, and 24 to 48 hours at 42°C resulted in decreased mean germination percentage for all species but not always clear separation of seed lots. While there were sometimes few differences between ageing at 41°C and 42°C, the former is preferred because it is already the temperature used for other species. For Asian rape, choisum and turnip, the previously recommended testing conditions of 41°C/72 h provided good seed lot separation, but for Asian kale and turnip rape hybrid, AA testing at 41°C/48 h provided better results. Seed moisture content after ageing ranged from 29-37% depending on species. Fungal growth on seeds during the ageing period appeared to reduce post-ageing germination in some seed lots . Substituting saturated salts for distilled water did not stress seed lots in the AA test, due to the lowered RH%, the exception being seed lots 1210 and 1296. For forage and Asian species, seed lot germination mostly remained above 90% when aged for 72 h at lowered RH%. Increasing the ageing duration from 72 to 96 hours resulted in some decreases in post-AA germination but no clear separation of seed lots. Surface sterilising the seeds prior to the AA test resulted in a lower incidence of contaminant fungi which was associated with a lower percentage of abnormal seedlings. The conductivity test was mostly able to identify vigour differences among forage and Asian vegetable brassica seed lots. Differences in conductivity readings were observed among seed lots in all species. Increasing the period of imbibition resulted in increased conductivity from most seed lots but radicle emergence occurred after 16-20 h of imbibition. Variation was observed in the time to reach 95% maximum of the imbibition curve for most species. Conductivity readings at 16 h would avoid possible influences of radicle emergence on results. Adjusting the SMC to 8.5% resulted in reduced variation in conductivity among replicates of seed lots, due to a reduction in imbibition damage. Seed size had a significant effect on both post-AA germination and conductivity results. In forage rape, large size seeds had higher post-AA germination cf. medium cf. small size seeds. In Asian kale, large size seeds had higher post-AA germination compared with small size seeds. For both forage rape and Asian kale, large size seeds had lower conductivity readings cf. small size seeds. The correlation analyses demonstrated significant relationships between AA testing and field emergence parameters (percentage emergence, emergence index and emergence rate). Significant relationships were also observed between conductivity testing and these field emergence parameters. Based on the correlation analysis, AA testing at 41°C/48 hand/or 42°C/48 h could be recommended to be used as an AA test for turnip and Asian rape; and 41°C/48 hand/or 41°C/72 h for Asian kale and choisum. Based on the correlation analysis, conductivity testing at 16 h can be used to predict the field emergence potential of forage and Asian vegetable seed lots. Vigour tests were consistently able to provide better indicators of field perfonnance than the standard germination test, although these relationships did vary with the different field sowings.

The permanent pasture based on a grass-clover association and utilised in situ by grazing stock is the basis of New Zealand's chief primary industries. Although white clover is the main legume species used for this purpose its production and persistence in the drier eastern areas of the South Island often leaves much to be desired. In such situations subterranean clover may provide a useful alternative. Subterranean clover has been a very useful species in the past and will probably continue to play an important role in the future, provided the most climatically adapted cultivars are grown. The aim of the field trials was to evaluate the productive performance of several subterranean clover cultivars in Canterbury. Because of the big influence of seed yield on subsequent production and persistence in annuals, this aspect was also investigated. The subterranean clover cultivars Geraldton, Yarloop, Woogenellup, Clare, Mt. Barker and Tallarook were evaluated under cutting in swards at two sites in Canterbury.

Variables for the accelerated ageing (AA) test, methods for reducing fungal contamination during the AA test, using the conductivity test as a vigour test, the effect of seed size on seed vigour and the relationship between laboratory test results and field perfonnance in selected Brassica spp were investigated. In the first experiment, three seed lots of turnip rape hybrid (B. rapa x campestris), turnip (B. campestris) and forage rape (B. napus); and seven seed lots of Asian rape (B. napus), six seed lots of Asian kale (B. oleraceae var. alboglabra L.) and five seed lots of choisum (B. rapa var. pekinensis) with germinations above 90% were aged at two different temperatures (41 and 42°C ± 0.3°C) and three ageing times (24, 48 and 72 ± 15 minutes). The second experiment was divided into three sections. In the first, the same seed lots and species were aged at one temperature (41°C) and time (72 h), but either 40 ml of saturated salts; KCl (83%RH), NaCl (76%RH), NaBr (55%RH); or distilled water (96%RH) were used as the ageing solutions. In the second, one turnip rape hyprid seed lot was aged at three temperatures (41, 42 and 45°C) and two times (72 and 96h), again using the three saturated salts and distilled water as ageing solutions. In the third, three turnip rape hybrid seed lots and three Asian kale seed lots were surface sterilised (1 % sodium hypochlorite) prior to ageing at one temperature (41°C) and time (72 h). In the third experiment, the same species and seed lots used in experiment one at their original seed moisture content (SMC) were tested for conductivity after soaking in deionised water for 4, 8, 12, 16, 20 and 24 h. They were then re-tested after the SMC had been adjusted to 8.5%. In the fourth experiment, three seed lots of forage rape and three seed lots of Asian kale were graded into three seed size categories; large (retained on a 2.0 mm screen), medium (retained on a 1.7 mm screen) and small (passed through a 1.7 mm screen). Graded seeds were then tested for standard germination, AA (41°C/48 h) and conductivity (measured at 16 and 24 h). In the final experiment, the relationships between laboratory tests for the six species (each consisting of three seed lots), field emergence from three sowings, and cold room emergence were evaluated. Both time and temperature influenced post-AA germination. Increasing the ageing period from 48 to 72 hours at 41°C, and 24 to 48 hours at 42°C resulted in decreased mean germination percentage for all species but not always clear separation of seed lots. While there were sometimes few differences between ageing at 41°C and 42°C, the former is preferred because it is already the temperature used for other species. For Asian rape, choisum and turnip, the previously recommended testing conditions of 41°C/72 h provided good seed lot separation, but for Asian kale and turnip rape hybrid, AA testing at 41°C/48 h provided better results. Seed moisture content after ageing ranged from 29-37% depending on species. Fungal growth on seeds during the ageing period appeared to reduce post-ageing germination in some seed lots . Substituting saturated salts for distilled water did not stress seed lots in the AA test, due to the lowered RH%, the exception being seed lots 1210 and 1296. For forage and Asian species, seed lot germination mostly remained above 90% when aged for 72 h at lowered RH%. Increasing the ageing duration from 72 to 96 hours resulted in some decreases in post-AA germination but no clear separation of seed lots. Surface sterilising the seeds prior to the AA test resulted in a lower incidence of contaminant fungi which was associated with a lower percentage of abnormal seedlings. The conductivity test was mostly able to identify vigour differences among forage and Asian vegetable brassica seed lots. Differences in conductivity readings were observed among seed lots in all species. Increasing the period of imbibition resulted in increased conductivity from most seed lots but radicle emergence occurred after 16-20 h of imbibition. Variation was observed in the time to reach 95% maximum of the imbibition curve for most species. Conductivity readings at 16 h would avoid possible influences of radicle emergence on results. Adjusting the SMC to 8.5% resulted in reduced variation in conductivity among replicates of seed lots, due to a reduction in imbibition damage. Seed size had a significant effect on both post-AA germination and conductivity results. In forage rape, large size seeds had higher post-AA germination cf. medium cf. small size seeds. In Asian kale, large size seeds had higher post-AA germination compared with small size seeds. For both forage rape and Asian kale, large size seeds had lower conductivity readings cf. small size seeds. The correlation analyses demonstrated significant relationships between AA testing and field emergence parameters (percentage emergence, emergence index and emergence rate). Significant relationships were also observed between conductivity testing and these field emergence parameters. Based on the correlation analysis, AA testing at 41°C/48 hand/or 42°C/48 h could be recommended to be used as an AA test for turnip and Asian rape; and 41°C/48 hand/or 41°C/72 h for Asian kale and choisum. Based on the correlation analysis, conductivity testing at 16 h can be used to predict the field emergence potential of forage and Asian vegetable seed lots. Vigour tests were consistently able to provide better indicators of field perfonnance than the standard germination test, although these relationships did vary with the different field sowings.

The aim of this study was to evaluate risk management strategies and decision support tools that might be useful to dryland farmers in southwest Queensland to improve their decision making. This topic was chosen because there has been little previous work done to examine the sources of risk faced by farmers in that area, the practical risk management strategies employed by these farmers, or their interests in and attitudes towards risk management. This study adopted an action-learning approach to present farmers with opportunities to use various tools that might help to manage the range of risks affecting their farm management. The study was designed to test farmers’ interests in existing tools, or the potential for developing new tools, to assist dryland farmers in southwest Queensland improve their risk management. The thesis is presented in three parts including an introduction to the study and an extensive review of the relevant literature on decision making and risk management (Part I), an overview of the area in southwest Queensland where this study was focused and the various research methods used in this study (Part II). Part III comprises four chapters reporting results and presents the conclusions from the study. The thesis also reviews the advantages and disadvantages mentioned in the literature about decision support systems (DSS) in Australian agriculture and examines some programming and simulation models that can be applied to risk management in agriculture. The research methods used in this study included a literature review, interviews, focus group discussions, an ‘expert’ survey, training workshops for farmers, and evaluation techniques. The observations and reflections from the preliminary inquiries identified soil moisture management and crop choice as the critical issues concerning dryland farmers in southwest Queensland when dealing with crop production risks. Those discussions suggested possibilities for developing a decision support tool to help farmers in the study area assess their planting options. In developing the options for a decision support tool for planting decisions, a series of workshops was conducted with farmers in the study area, while some observations of farmers who attended similar workshops in adjacent areas are also reported. These workshops provided the opportunity for participants to experience some existing risk management and decision support tools. They were also designed to collect inputs to develop an appropriate decision support tool for crop planting decisions. A ‘Key to dryland planting decisions’ for farmers in southwest Queensland was developed and the personal experiences and lessons that the author has learnt through the course of this research are reported. The thesis enhances the understanding of farmers’ attitudes to risk, contemporary risk management strategies, and decision supports tools used in agriculture. This research has contributed to knowledge in the following ways. It has presented a methodological framework for doing research of this type. This study has identified the different sources of risk faced by dryland farmers in southwest Queensland and the practical risk management strategies they employ. The research has introduced those farmers to some existing risk management and decision support tools. The research has contributed more specifically to improving their management decisions by developing a decision support tool that could help dryland farmers in southwest Queensland make better informed and more appropriate planting decisions in the very uncertain and risky conditions with which they have to cope.

Chicory (Cichorium intybus L.), a valuable forage for ruminant livestock in temperate regions, appears highly degradable in the rumen. Fundamental reasons for the rapid breakdown of chicory cell walls in the rumen were studied. Cell walls were isolated from laminae and midribs of chicory (cv. Grasslands Puna II) leaves. The walls, which, except for the walls of xylem tracheary elements in vascular bundles, were non-lignified, were fractionated progressively with 50 mM CDTA, 50 mM Na2CO3, 1 M KOH, 4 M KOH, 4 M KOH + 3.5% H3BO3, and hot water. The polysaccharides were similar to those in nonlignified walls of other dicotyledons, but with high proportions of pectic polysaccharides (67% of the total wall polysaccharides in the laminae). These included homogalacturonans (HGs, 50% of the total wall polysaccharides in laminae) and rhamnogalacturonan I (RG I). In contrast, the proportions of cellulose, xyloglucans, heteroxylans and glucomannans were low. The locations of different pectic polysaccharides were determined using the monoclonal antibodies JIM5 and JIM7 against HGs with low and high degrees of methyl esterification, respectively, LM6 against arabinan and LM5 against galactan. All primary walls were labelled with all the antibodies used. However, the middle lamella, tricellular junctions and the corners of intercellular spaces were labelled with JIM5 and JIM7, but not with LM5. The middle lamella was labelled with LM6, but not the corners of intercellular spaces. These results support the involvement in cell adhesion of HGs with low degrees of methyl esterification. A preparation of endopolygalacturonase (endo-PG) was used to investigate cell adhesion, and its effect on forage particle breakdown was determined using weight loss, chemical analysis and immunofluorescence labelling. The preparation dramatically reduced particle size. Cell separation was accompanied by a loss of HGs with low degrees of methyl esterifcation from the middle lamella and corners of intercellular spaces. A consequential loss of cell adhesion evidently caused leaf breakdown. The degradation of fresh chicory leaves by rumen bacteria was investigated by measuring weight loss, monosaccharide release and immunocytolabelling. Two bacteria, the pectolytic Lachnospira multiparus D32 and the cellulolytic Fibrobacter succinogenes S85, effectively degraded chicory. Pectic polysaccharides were degraded faster than other wall polysaccharides, with uronic acid released faster and more completely than neutral monosaccharides. The preponderance of non-lignified primary walls and abundance of pectic polysaccharides may account, in part, for the rapid degradation of forage chicory in the rumen. The HGs in the middle lamellae and corners of intercellular spaces probably have a role in cell adhesion, and their degradation is probably responsible for the rapid reduction in the particle size of chicory leaves in the rumen.

Despite being different cultivars of the same plant species (Zea mays L.), maize and sweet corn have contrasting P fertiliser recommendations in New Zealand, that are reflected in different target Olsen P values of 10-15 mg P/kg soil for optimum maize growth and 26-35 mg P/kg soil for optimum sweet corn growth. Three key hypotheses were developed in this study to explain why these differences may exist: i) maize and sweet corn differ in their responsiveness to P fertiliser i.e. maize is more internally P efficient and requires less P than sweet corn to grow, ii) both cultivars differ in external P efficiency i.e. their ability to take P up from soil iii) both cultivars differ in external P efficiency because they have different root system structure. Two field experiments evaluated the growth and yield responses of maize and sweet to different rates of P fertiliser application. The first experiment was conducted in Hawke's Bay (2001-02) and second in the Manawatu (2002-03) with P application rates of 0, 100 and 200 kg P/ha in the Hawke's Bay and 0, 15 and 70 kg P/ha in the Manawatu. Both experiments were conducted on soils of low available P status. The Olsen P test values of 13 mg P/kg soil in the Hawke's Bay and 11 mg P/kg soil in the Manawatu were far below the recommended values for sweet corn (25-35 mg P/kg soil). In both experiments and across all P treatments maize produced significantly higher dry matter yields than sweet corn during all sampling stages. In the Hawke's Bay experiment at 100 days after sowing (DAS), the maize (87719 plants/ha, 20.9 t/ha) produced 43% more dry matter than sweet corn (71124 plants/ha, 14.6 t/ha), whereas, in the Manawatu experiment (140 DAS), maize (71124 plants/ha, 15.2 t/ha) had a 39% higher dry matter yield than sweet corn (71124 plants/ha, 10.9 t/ha). In both the field experiments, the sweet corn fresh cob yield of 27 and 28 t/ha in the Hawke's Bay and the Manawatu regions and maize grain yields of 16 and 10 t/ha, respectively, were within the range of the reported commercial yields for each region. In both experiments, the P fertiliser application raised the soil P status (Olsen P test values) but caused no significant increases in either maize or sweet corn yields (total dry matter, sweet corn fresh cob or maize grain). Commercially viable yields of both cultivars were able to be achieved without P fertiliser application with Olsen P soil test in the range of 10-15 mg P/kg soil. Sweet corn reached harvestable maturity at 115 DAS in the Hawke's Bay and 140 DAS in the Manawatu experiments. By this time maize had produced 4-6 t/ha more total dry matter yield than sweet corn, yet maize and sweet corn had achieved similar total P uptake (32-37 kg P/ha at 100 DAS in the Hawke's Bay and 18-19 kg P/ha at 140 DAS in the Manawatu). At silking (after 75 DAS in the Hawke's Bay and approximately 110 DAS in the Manawatu), both cultivar's total leaf P concentrations (0.21-0.25%) were within the sufficiency range values for maize crops in New Zealand (0.18-0.33 %). Maize, however was more internally P efficient growing more dry matter per unit P taken up, which was more noticeable in the drier season. Fertiliser P application increased P uptake with both cultivars under moist conditions in the Hawke's Bay experiment (2001-02). However, the dry conditions in the Manawatu (2002-03) limited P uptake as well as restricted dry matter yields with both cultivars. Further, there were no significant differences between maize and sweet corn P uptake efficiency (kg P/kg root) despite significant differences in the root system structure (biomass) for both cultivars at all stages, which lead to different temporal patterns of P uptake. The lack of maize yield response to fertiliser P in both field experiments is consistent with the New Zealand recommendations for growing a maize grain crop (because soil Olsen P was in the range of 10-15 mg P/kg). However, the lack of sweet corn yield response in both field experiments does not support the New Zealand recommendations for growing sweet corn (which assume optimal Olsen P values are 26-35 mg P/kg).