Browsing by Autor "Scott A. Dalzell"

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Effect of tree density on competition between Leucaena leucocephala and Chloris gayana using a Nelder Wheel trial. I. Aboveground interactions
(CSIRO Publishing, 2018) Anibal Nahuel Alejandro Pachas; H. M. Shelton; Christopher J. Lambrides; Scott A. Dalzell; G. J. Murtagh
Silvopastoral systems with the tree legume leucaena (Leucaena leucocephala (Lam.) de Wit) and grass pastures are widely used for ruminant feeding in subtropical and tropical regions. Different densities and planting configurations of leucaena will influence relative yields of both species because of intra- and interspecific competition. With the aim to describe the effects of competition between leucaena and Rhodes grass (Chloris gayana Kunth), a Nelder Wheel trial with 10 different leucaena tree densities (100–80 000 trees ha–1) growing with and without Rhodes grass was established in a subtropical environment at Gatton, south-east Queensland, in November 2013. From 2014 to 2016, the biomass of leucaena (six harvests) and Rhodes grass (seven harvests) was measured by using allometric equations and the BOTANAL sampling procedure over 742 and 721 days, respectively. No complementary or facilitative aboveground interactions were observed between the leucaena and Rhodes grass components of the pasture system. Increasing leucaena tree density resulted in greater aboveground intra- and interspecific competition. Average maximum individual tree yield (38.9 kg DM tree–1 year–1) was reached at 100 trees ha–1 without grass competition and was reduced by 60% with grass competition. Rhodes grass biomass yield was negatively affected by shading from the leucaena canopy, with negligible grass yield at tree densities =8618 trees ha–1. Therefore, there was effectively no grass competition on individual tree yield at higher leucaena densities. Accordingly, edible leucaena biomass per unit area was positively related to log10 leucaena density (R2 = 0.99) regardless of grass competition, reaching 21.7 t DM ha–1 year–1 (2014–15) and 27 t DM ha–1 year–1 (2015–16) at the highest leucaena density of 80 000 trees ha–1. By contrast, the yield of Rhodes grass was linearly and inversely correlated with log10 tree density (R2 = 0.99). Practical implications for the design and management of commercial leucaena–grass pastures are discussed.
Effect of tree density on competition between Leucaena leucocephala and Chloris gayana using a Nelder Wheel trial. II. Belowground interactions
(CSIRO Publishing, 2018) Anibal Nahuel Alejandro Pachas; H. M. Shelton; Christopher J. Lambrides; Scott A. Dalzell; G. J. Murtagh; Craig Hardner
Leucaena (Leucaena leucocephala (Lam.) de Wit subsp. glabrata (Rose) Zarate) in combination with grass pasture is one of the most persistent, productive and sustainable grazing systems used in Queensland, Australia. Nevertheless, a better understanding of the competitive interactions that determine the proportions of leucaena and grass components is needed to optimise the design and management of the hedgerow pasture system. In a water-limited environment, belowground interactions between species are especially influential. Accordingly, the aim of this study was to determine the effect of leucaena plant density and Rhodes grass (Chloris gayana Kunth) competition on root distribution, evapotranspiration, patterns of soil-water use and the resulting water-use efficiency (WUE) of the leucaena and grass components. Results showed that although leucaena had deeper roots than Rhodes grass, the majority of fine roots of both leucaena and Rhodes grass were in the upper 1.5 m of the soil profile suggesting a high level of competition for water resources. A major factor favouring Rhodes grass was that its root abundance was 8–10 times greater than leucaena, allowing it to compete more effectively for water resources and limit the lateral spread of leucaena roots. Higher cumulative evapotranspiration values were recorded from leucaena grown with Rhodes grass than from leucaena grown in absence of grass. However, this difference was negligible at the highest leucaena density owing to the reduced yield of grass caused by shading and increased water uptake of leucaena. The findings of this study also confirmed the hypothesis that at low tree densities, leucaena–grass pasture will have higher WUE (13.8 kg DM mm–1) than sole leucaena, but this difference was reduced with increments of leucaena density. Highest WUE (65.9 kg DM mm–1) occurred at highest leucaena density with or without grass.