Effect of grazing rotation interval and defoliation residual height on the botanical composition, yield and quality of hyper-diverse pastures : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Agricultural Science at Massey University, Manawatū, New Zealand
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Date
2024
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Massey University
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Abstract
Pastoral farming systems in New Zealand currently heavily rely on Lolium perenne L. (perennial ryegrass)-Trifolium repens L. (white clover) (relatively simple) pasture mixtures. However, there are problems with seasonal growth fluctuations, creating periods of feed deficit and surplus. Further, these species result in low nitrogen (N) use efficiency, leading to high N concentrations in urine, N leaching and nitrous oxide (N₂O) emissions. Pressure on these farming systems has grown to improve productivity, efficiency and mitigate environmental impacts. It has been suggested that diverse pastures (those including mixtures of grass, legume and herb species) may provide improved yield, seasonal distribution of pasture supply and quality through the inclusion of species with different characteristics (i.e. heat and drought tolerance, deeper rooting). However, the success of a given pasture mixture is largely dependent on management. Whilst there is a plethora of literature looking into defoliation management of diverse pastures, there is a dearth of studies looking at hyper-diverse pastures (those including >9 species from at least 3 different functional groups). The objectives of this research were to examine how differing defoliation management (i.e. residual height and defoliation interval) impacted the botanical composition, yield and quality of hyper-diverse pasture mixtures. This study indicated that, whilst a large number of species were sown (18 species in the dairy mixture and 19 species in the sheep mixture), less than half were observed to remain present throughout the study period of 7 months (November 2022 to May 2023). Of those present, there were only several that dominated (i.e. perennial ryegrass, white clover, Plantago lanceolata L. (plantain) and Trifolium pratense L. (red clover), and they were not sensitive to any residual height or interval treatments, over the period of this study. The reversion of the hyper-diverse mixtures to a smaller number of dominant species within a short timeframe poses the question of whether it would have been easier to create a simpler pasture mixture at the outset. However, the survival and dominance of species was undoubtedly driven by edaphic conditions, along with the prevailing climate at time of sowing and throughout the experimental period, and interactions between these and the defoliation management. Overall, there was a general trend where a longer defoliation interval (9 weeks) reduced total yield, whilst shorter intervals (3 and 6 weeks) increased yieldsin association with higher residual heights. There were no significant differences in nutritive value parameters, but there were trends in the data which showed that higher residual heights and longer rotations resulted in increased lignin and neutral detergent fibre and decreased metabolisable energy for these pastures during May. However, regardless of treatment, the nutritive value parameters showed that these hyper-diverse pasture mixtures are capable of maintaining high animal performance. More research needs to be undertaken before clear recommendations are able to be made regarding likely suitable situation-specific mixtures, and these will vary with region and management system (e.g. dairy, sheep, and beef). Further research should focus on how to achieve successful establishment of a large number of species in mixture, and then to define the optimal defoliation management that will maintain diversity over a longer time period. Overall, the results of this study indicated that hyper-diverse pasture mixtures can be defoliated under a range of residual heights and intervals whilst maintaining yield and quality, although with less diversity, suggesting that these mixtures have the potential to be valuable in New Zealand pastoral systems.