Investigation of tropical pasture species to improve dairying in the tropics : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, Manawatū, New Zealand
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2023
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Massey University
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Despite the ever-increasing demand for dairy products, dairy production in tropical regions is often lower than in temperate regions, due to a range of factors including the lower nutritive value of pastures. The availability of high-quality tropical pastures is a key requirement to increase the productivity of dairy cows as well as manage enteric methane emissions. Warm-season perennial grasses are the dominant forages in tropical and subtropical regions, and thus exploring their nutritive characteristics and evaluating their likely performance in existing dairy production systems, is imperative in the effort to improve dairy productivity. This thesis was undertaken to investigate tropical pasture species for improving dairying in the tropics via a modelling and simulation approach.
A dearth of data comparing the nutritive values of tropical pastures grown across different environments limits the selection of forages for livestock in the tropics. A database was constructed containing a total of 4750 records, with 1277 measurements of nutritive values representing 56 tropical pasture species and hybrid cultivars grown in 26 different locations in 16 countries, in order to compare the nutritive values and greenhouse gas production across different forage species, climatic zones, and defoliation management regimes. The average edaphoclimatic (with minimum and maximum values) conditions within this data set were characterized as 22.5°C temperature (range 17.5–29.30°C), 1253.9 mm rainfall (range 104.5–3390.0 mm), 582.6 m elevation (range 15–2393 m), and a soil pH of 5.6 (range 4.6–7.0). The data revealed spatial variability in nutritive metrics across bioclimatic zones and between and within species. The ranges of these nutrients were as follows: neutral detergent fibre (NDF) 50.9–79.8%, acid detergent fibre (ADF) 24.7–57.4%, crude protein (CP) 2.1–21.1%, dry matter (DM) digestibility 30.2–70.1%, metabolisable energy (ME) 3.4–9.7 MJ kg⁻¹ DM, with methane (CH₄) production at 132.9–133.3 g animal⁻¹ day⁻¹. The arid/dry zone recorded the highest DM yield, with decreased CP and high fibre components and minerals. Furthermore, the data revealed that climate, defoliation frequency and intensity, in addition to their interactions, have a significant effect on tropical pasture nutritive values and CH₄ production. Overall, hybrid and newer tropical cultivars performed well across different climates, with small variations in herbage nutritive value. Results revealed that greater pasture nutritive values and lower CH₄ production can be potentially achieved through the selection of improved pastures and subsequent management.
Subsequently, the suitability of three improved tropical perennial pastures: Chloris gayana ‘Rhodes grass cv. Reclaimer’ (RR), Megathyrsus maximus ‘Gatton Panic’ (GP), and Brachiaria ruziziensis x B. decumbens x B. brizantha ‘Brachiaria Mulato II’ (BM) for tropical dairy production was evaluated using their carbon assimilation, canopy structure, herbage plant-part accumulation and nutritive value parameters under irrigated conditions. A field experiment was conducted at Gatton Research Dairy (27° 54′ S, 152°33′ E, 89 m asl) Queensland, Australia, which has a predominantly subtropical climate. Photosynthesis biochemistry, canopy structure, herbage accumulation, plant-part composition, and nutritive value were evaluated. Photosynthesis biochemistry differed between pasture species. The efficiency of CO₂ assimilation was highest for GP and quantum efficiency was highest for BM. Pasture canopy structure was significantly affected by an interaction between pasture species and harvest. Forage biomass accumulation was highest in GP, while BM produced more leaves and less stem compared to both GP and RR. A greater leafy stratum and lower stemmy stratum depth were observed in the vertical sward structure of BM. Brachiaria Mulato II showed greater carbon partitioning to leaves, leaf: stem ratio, canopy, and leaf bulk density.
The BM also demonstrated greater nutritive value (Total digestible nutrients (TDN), ADF, NDF, neutral detergent insoluble protein (NDICP), starch, non-fibre carbohydrates (NFC), ME, mineral profile (Mg, P, K, Fe, Zn) and dietary cation-anion difference (DCAD)) for leaf, stem, and the whole plant. Overall, the observed greater quantum efficiency, leaf accumulation, and nutritive value of BM suggested that BM is an attractive forage option for dairying in pasture-based systems in tropical and subtropical climates.
The DairyMod-SGS, a mechanistic biophysical pasture model was parametrised and robustly validated for the prediction of the growth of the three tropical pastures (BM, GP, and RR), aiming to use the model as an effective tool to explore the likely performances of newer species under different edaphoclimatic and agronomic management practices. The model was calibrated using measurements of biomass components, canopy structure, and carbon assimilation collected from the field experiment at the Gatton Research Dairy Farm. Subsequently, the model was tested extensively using the published and unpublished data (16 data sets, 32 experiments, 14 different locations across South America, North America, Australia and Africa) to ensure that the parameterised model performed well and was reliable across a diverse set of environments and management practices. In the model parameterisation stage, the model predicted the above-ground biomass with good agreement for all tropical pastures with a high R² of 0.92, 0.98, 0.74 and low RMSE of 341, 583, 848 kg DM ha⁻¹ for BM, GP, and RR, respectively. The model agreement was good for the validation data with R² of 0.86, 0.80, 0.87 and RMSE of 954.5, 790.5, and 633.2 kg DM ha⁻¹ for BM, GP, and RR, respectively. The predicted leaf and stem partitioning was relatively poor, and the model also struggled to simulate realistic pasture growth in Mediterranean and desert environments (R² < 0.50). The present study has improved the robustness and accuracy of DairyMod-SGS in relation to tropical pastures and indicated that the model can be successfully used for investigating the likely performance of improved tropical pastures under a broad range of conditions.
The validated DairyMod-SGS pasture model was applied to simulate the long-term pasture production of three improved pastures (BM, GP, and RR) in major dairying regions of Sri Lanka under three management scenarios: 1) rainfed pasture production system under the industry average nitrogen (N) fertiliser rate (Yₜᵂᴺ = yield over time under water and N limitation); 2) rainfed pasture production system under non-limiting N fertilisation (Yₜᵂ = yield over time with no N limitation but water limitation); and 3) potential pasture production system under non-limiting N and irrigation (Yₜ = yield over time with no N and water limitations). Simulations were carried out for 16 sites across Sri Lanka (8 sites in the dry zone (DZ), 5 sites in the intermediate zone (IZ), and 3 sites in the wet zone (WZ)) over a 30-years period (1980–2010). The model simulated that the long-term pasture production greatly varied between climate zones, pasture species and management scenarios. Overall, the Yₜᵂᴺ scenario showed a seasonal cycle following the rainfall pattern, with a reduction in growth rates in dry seasons (May to September). Growth rate and herbage accumulation were greater in GP at Yₜᵂᴺ, and BM at Yₜᵂ and Yₜ, while RR always showed the lowest growth rate. The variability of pasture growth between climate zones was highest in DZ (May to September) whereas the variability between species was lowest in RR. Pasture accumulation of both BM and GP outperformed their standard cultivars currently grown in Sri Lanka. In general, the pasture accumulation under Yₜᵂ increased (doubled) the growth rate, while the Yₜ scenario substantially increased (nearly tripled) the growth rate. Overall, the finding of this thesis suggested that all three pasture cultivars tested (BM, GP, and RR) are suitable for growing across major dairying regions in Sri Lanka, and that appropriate fertiliser and irrigation management can greatly intensify the herbage accumulation. In particular, BM appeared to be promising in terms of nutritive value, and agronomic and physiological traits. However, regional edaphic conditions, and the management ability of the farmers with respect to inputs and defoliation management, will ultimately determine the performance of improved cultivars such as those used in this thesis. There is a need for extension activities to support farmers in the management required to achieve best performance (yield, nutritive value and persistence) from these species.
Future research is recommended to: 1) validate the model under Sri Lankan conditions, using both unimproved and improved cultivars, to determine its suitability in estimating tropical pasture yield; and 2) use additional models to evaluate the performance of improved tropical pastures in the context of the whole farm system, to identify the likely impact on dairy production and economic return.
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Pasture plants, Varieties, Nutrition, Dairying, Tropical conditions, Dairy cattle, Feeding and feeds, Tropics, Sri Lanka