The competitive ability of different New Zealand wheat (Triticum aestivum L.) cultivars and the economic impacts of weeds: A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Applied Science at Lincoln University

Abstract

Weeds are the leading biotic cause of potential yield losses in wheat production causing a reduction of up to 23% worldwide. Usage of chemicals and tillage to manage weeds have been widely adapted by growers since the “Green Revolution”. Due to increased usage of chemicals, resistance to herbicides has become a growing problem. This study investigated economic impact of weeds on wheat production and evaluated whether wheat cultivar choice can function as an effective integrated weed management strategy (IWM). In vivo field experiments quantified weed pressure; compared competitiveness of different wheat cultivars; assessed agronomic effects of weed interference and examined the performance of three contrasting herbicide regimes (T0: no herbicide applied after sowing; T1: pre-emergence herbicide; T2: pre- and post-emergence herbicide). Wheat biomass ranged from 26.2 T/ha, SE=1.94 (‘Graham’, T0) to 35.5 T/ha, SE = 2.12 (‘Voltron’, T2) and yield loss ranged from 0.27 T/ha SE = 0.13 ‘Whopper’ to 0.83 T/ha, SE = 0.08 ‘Graham’. Economic analyses projected the financial consequences of weed presence and highlighted the input requirements for weed management in autumn-sown New Zealand wheat. The greatest economic gain of a net return of $177.58 was found in the T1 treatment and across all cultivars. Together, these aims provided an integrated framework for understanding how cultivar choice influences weed suppression, crop performance, and profitability. This research used a combination of field experiments and economic analysis to evaluate the impact of weeds on wheat production and assess the potential of cultivar choice. Field and controlled-environment experiments were done to quantify weed pressure, compare crop competitiveness, and measure the agronomic effects of weed interference under standardised management conditions. Herbicide treatments representing common pre-emergent and post-emergent spray programmes were included to examine interactions between cultivar competitiveness and chemical control. Economic modelling was undertaken to project the financial impact of weed presence and estimate the input requirements for weed management The results show that weeds impose clear agronomic and economic costs in autumn-sown wheat, with early interference having the greatest influence on crop performance. Early-vigour cultivars consistently displayed stronger early competitiveness, reducing weed growth and improving crop outcomes as shown by seed yield results. Seed yield differed significantly among herbicide treatments (P < 0.001), with yields ranging from 13.58 t/ha under T0 to 14.41 t/ha under T2. Across cultivars, both T1 (pre-emergence) and T2 (pre- plus post-emergence) treatments consistently increased yield relative to T0 (no herbicide after sowing). ‘Graham’ yield increased from 13.58 ± 0.16 t/ha under T0 to 14.40 ± 0.11 t/ha under T1 and 14.41 ± 0.01 t/ha under T2. ‘Voltron’ increased from 13.79 ± 0.11 t/ha at T0 to 14.36 ± 0.13 t/ha at T1, with a slight reduction to 14.15 ± 0.18 t/ha under T2. ‘Whopper’ followed a similar pattern, increasing from 13.83 ± 0.20 t/ha at T0 to 14.21 ± 0.11 t/ha at T1 and 14.10 ± 0.05 t/ha at T2. Standard errors were small (0.01–0.20 t/ha), indicating high precision of treatment estimates. The advantages observed depended on environmental and management conditions. Competitive cultivars also strengthened the performance of pre-emergent herbicides, with ‘Whopper’ being inherently more competitive and lowering the need for additional inputs compared to ‘Graham’ which had a greater response to herbicide applications. Economic analysis indicated that systems combining competitive cultivars with well-timed herbicide programmes and the pre-emergence herbicide showed a positive net return compared to a pre-emergence and post-emergence application. Overall, the results demonstrated that cultivar choice can act as an effective IPM strategy, improve weed suppression, crop performance, and financial resilience. This thesis provides new insights into how weeds affect wheat production in the New Zealand context and demonstrated that cultivar choice can strengthen integrated weed management. Linking crop performance, herbicide efficacy, and economic outcomes in a single study lays the groundwork for grower friendly guidance for weed management decision making in order to improve weed suppression and profitability in New Zealand wheat systems.