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Alternative pasture species to perennial ryegrass/white clover suitable for use under irrigation in the Midlands of Tasmania

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Caiger-Watson, BM ORCID: 0000-0003-0994-6057 2021 , 'Alternative pasture species to perennial ryegrass/white clover suitable for use under irrigation in the Midlands of Tasmania', PhD thesis, University of Tasmania.

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Abstract

The Midlands region in Tasmania is experiencing a major expansion of irrigated agricultural activity. Frequency of frost days, extreme temperatures, waterlogging and salinity, challenge the performance of pasture species grown for livestock production in the region. Irrigation expansion in this medium to low rainfall zone, enables intensification of grazing as pasture production increases. Two of the major species sown for intensive livestock grazing in southern Australia are perennial ryegrass (PRG; Lolium perenne L.) and white clover (WC; Trifolium repens L.).
This research reviewed the literature aiming to identify complementary, or better suited species to PRG/WC, capable of maintaining a productive perennial feedbase under irrigation in the region. Key measures of suitability were opportunity for early grazing due to rapid establishment and accelerated regrowth after defoliation during establishment. Grass species likely to be suitable included PRG, phalaris (PA; Phalaris aquatica L.) and coloured brome (CB; Bromus coloratus Schreb.). Clover species included WC, stoloniferous red clover (SRC; Trifolium pratense L.) and Talish clover (TC; Trifolium tumens Steven ex. M. Bieb.).
Rapid establishment and recovery after defoliation of grass-based pastures is promoted by increasing frequency of nitrogen (N) fertiliser applications. Applications of fertilizer N prior to or at planting on bare soils, is unlikely to be fully utilised by soil microorganisms, or immature plant roots. Under irrigation, leaching of N fertiliser is more likely than leaching of organic N sources in the soil nutrient pool. To mitigate leaching losses, initiation of pasture plant root development from sources of organic N is a potential solution.
Clover was not generally thought to contribute N to pastures until they are established. Primarily decaying leaves, stems, stolons, nodules and roots of mature clovers contribute to the soil organic N pool. It is now accepted that exudate transfer from clover roots to grass roots, occurs before established plants decay. During establishment clover supports grass growth as a consequence of co-habitation. Immature clover roots exude amino acid (AA) derived N enabling attraction of N fixing rhizobia and unavoidable sharing of nutrient with grass roots. Interestingly apart from AA, the soil adjacent to plant roots is mainly N free.
Initial experimental research explored pasture defoliation intervals and cutting heights, using controlled pot experiments. Field experiments then compared species performance during establishment. Based on these results SRC was identified as a species with considerable potential. Grass regrowth after defoliation when sown without SRC and with SRC was studied. Results showed grass dry matter (DM) yield increased significantly (P<0.0001) when SRC was sown as a companion, in comparison with grass sown as a monoculture. A rate response experiment was sown to explore mechanisms that may contribute to the observed increase in grass yield. Root interactions during establishment were suspected to be a mechanism of advantage to grasses sown with clovers.
The AA profiles and concentration were measured in grass roots sown without and with clovers in the field and in pots. Grasses sown with clover were found to differ in AA profiles and concentrations in comparison with grasses sown without clover. Amino acid profiles and concentrations were then measured in PRG grass roots sown without or with seven or three clover plants. The PRG component DM yield from plants harvested at 35 days after sowing (DAS) was significantly (P<0.05) higher when sown with seven clover plants in comparison to three clover plants or monoculture sowings. A rate response experiment with increasing numbers of clover plants (one to eight) sown with decreasing numbers of PRG plants (eight to one) measured AA in the grass roots. Results showed PRG shoot and root biomass kg DM/ha increased 70% and 66% respectively when sown with clovers, compared to without clovers at all rates. If N from AA is transferring from clover roots to grass roots between emergence and 62 DAS as proposed in this research, reliance on synthetic N is diminished during establishment. Importantly, species rather than the rate of clover to grass plants sown affected AA profiles, concentration and transfer.
To extend the value of the physical research findings, modelling using the biophysical pasture growth model DairyMod aimed to examine the influence of Midlands climate, soil type and management on dairy pasture production. Stoloniferous red clover was parameterised and validated with an R2 0.73, mean prediction error (MPE) 1.29, model efficiency (MEF) 0.65 and variance ratio (VR) of 0.80 using field and pot data comparisons with biophysical modelling. Grass/clover performance under rainfed, full and restricted (limited water allocation) irrigation, historically 1986-2015 and using climate futures Tasmania (CFT) predictions 2015-2044 were then modelled. Simulations examined DM yield, intake and N mineralisation of PRG and PA sown as monocultures or with WC or SRC.
Modelling suggested PRG, PA,WC and SRC grass clover mixes were suited to maintain a perennial feedbase under irrigation historically and in the future. Inclusion of clovers with grasses generated increases in DM yield and intake annually under rainfed, irrigated or restricted irrigation scenarios. Annual N mineralisation modelled under irrigation at Cressy in the Midlands was 22% and 34% greater when SRC was sown with PRG or PA respectively, in comparison to the same grasses sown with WC. This modelled increase in N mineralisation can be partially explained by increased biomass supporting a greater N contribution from SRC.
Both experimental and modelled outcomes support sowing of binary mixes of grass/clover, in preference to grass monocultures under irrigation during establishment. Acceleration of grass regrowth during establishment by the inclusion of clover, and prevention of synthetic N losses by replacement with clover organic N exudates, are positive outcomes originating from this research. Future research should explore the potential to commercialise soluble micro- organic exudates (MOE™), to apply to soils pre- and post-seedling emergence. The potential of clover root AA derived N uptake by a grass companion, is suggested as an option for the reduction of inadvertent nutrient leaching into the environment during pasture establishment.

Item Type: Thesis - PhD
Authors/Creators:Caiger-Watson, BM
Keywords: Red clover, exudates, amino acids, mineralisation
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Copyright 2021 the author

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