Optimal weed control under static and dynamic decision rules

Dynamic and static weed control decision rules are derived analytically and compared. The dynamic rule leads to increased farm profits and greater control of weeds and weed seeds than the static rule, while total herbicide use is unchanged. The magnitude of the differences is estimated for atrazine control of foxtail and cocklebur in corn production. Incorporating weed dynamics into weed control strategies increases farm profits between 1.0 and 1.4%.     

Herbicide resistance and the adoption of integrated weed management by Western Australian grain growers

Extension programs to encourage farmers to reduce reliance on herbicides by adopting integrated weed management (IWM) practices have met with limited success. Studies aiming to understand the factors that influence farmers' choices of integrated control practices have faced difficulties in variable specification, and have not achieved high explanatory power. Using data from grain growers in Western Australia, where herbicide resistance in major crop weeds is common, this study tests the applicability of a framework for the IWM adoption decision in which herbicide efficacy is assumed to be a potentially exhaustible resource. Farmers' perceptions of multiple techniques and other variables are aggregated using principal components, and used in logistic regressions to explain the intensity of use of IWM practices. Eighty‐six percent of growers were correctly classified according to use of multiple IWM practices. Herbicide resistance and expectations of the future availability of effective new herbicides were significant in explaining IWM adoption. IWM adoption and herbicide‐resistance management are shown to be information‐intensive and involving an intertemporal resource management decision.     

TOWARDS AN ECONOMIC FRAMEWORK FOR EVALUATING POTENTIAL BENEFITS FROM RESEARCH INTO WEED CONTROL

A dynamic economic model for estimating benefits from research into alternative tactics for weed control is developed, by treating weeds as renewable negative resources. Research benefits are evaluated as the change in steady-state profit resulting from research-induced changes in various parameters of the system. The model is illustrated by application to research aimed at the control of wild oats in wheat in Australia, whereby it was found that the magnitude of benefits depended not only on the level of research-induced change but also on demographic behaviour of the weed. Research leading to reductions in seedling survival, potential fecundity or recruitment gave higher benefits than the manipulation of weed competitiveness or seed mortality. Reduced seedling survival gave the highest benefits in most cases, but achieving such improvement by improving herbicide efficacy may have a high marginal cost given that the tactic is already well-researched. Thus it is concluded that research into reducing potential fecundity could be more worthwhile, since it also gave high benefits but is relatively unexplored.     

Determinants of herbicide use in rice production in the Philippines

This study identifies farm‐specific and market factors affecting the adoption of herbicides and the level of herbicide use by rice farmers in the Philippines. This requires the application of a modified version of Heckman's two‐step method to estimate a random‐effects double‐hurdle model for unbalanced panel data. The age of the farmer, household size, and irrigation use are significant determinants of the decision of farmers to adopt herbicides as an alternative to manual weeding, while economic variables such as the price of herbicides, total income, and access to credit determine the level of herbicide use. Determinants of both adoption and level are land ownership, farm area, and the method of crop establishment. These results are potentially relevant when designing policies to reduce excessive herbicide use or to encourage the adoption of alternative weed control methods such as integrated weed management.     

RESPONSES TO RISK IN WEED CONTROL DECISIONS UNDER EXPECTED PROFIT MAXIMISATION

Risk is an important characteristic of decisions about weed control in crops. In this paper it is shown that risk can affect weed control decisions even if the objective of the decision maker is to maximise expected profits: that is, even if the decision maker is ‘risk-neutral’ in the usual economic sense. This is shown for two decision frameworks: the optimal rate approach and the economic threshold approach. Empirical results are presented for control of ryegrass in wheat in Western Australia. It is found that, in general, risk reduces the optimal level of herbicide use under expected profit maximisation. Although individual sources of risk have a small impact on the optimal decision rules, combinations of uncertain variables can have a relatively large effect.     

AN OPTIMAL CONTROL MODEL FOR INTEGRATED WEED MANAGEMENT UNDER HERBICIDE RESISTANCE*

The presence of weeds which have developed resistance to chemical herbicides is a problem of rapidly growing importance in Australian agriculture. We present an optimal control model of herbicide resistance development in ryegrass, the weed for which resistance is most commonly reported. The model is used to select the optimal combination of chemical and non-chemical control measures taking account of the trade off between short term profits and the long term level of herbicide resistance. Results indicate that given the threat of resistance there are benefits from integrating a combination of chemical and non-chemical control measures. The optimal strategy is found to include a declining herbicide dosage as resistance develops, with compensatory increases in the level of non-chemical control.     

Economic evaluation of a weed-activated sprayer for herbicide application to patchy weed populations

Spatial distribution of weeds in a crop is patchy. Traditional boom sprayers waste herbicide by applying it to areas where weed density is already low. A new technology, Weed Activated Spray Process (WASP), uses sensors to detect the presence of weeds and control spray nozzles accordingly. The economic benefits of this technology to extensive crop farmers in Western Australia are investigated using a model based on the economics of information. Existing technology is likely to reduce profits because the weed density at which it switches off spraying is too high. Even if sensitivity to low densities could be improved, likely benefits of pre-crop usage would still be very low or negative.     

“Resistance is futile”: estimating the costs of managing herbicide resistance as a first‐order Markov process and the case of U.S. upland cotton producers

A 2012 survey of upland U.S. cotton producers was analyzed to determine the factors contributing to changes in weed management costs (WMCs) after the identification of herbicide‐resistant weeds. An ordered probit regression estimated changes in WMC as a first‐order Markov process. The most important determinants of post‐resistance cost increases were initial WMCs, adoption of labor‐intensive remedial practices, and wick application of herbicides. Cultivation and mechanical/chemical‐intensive practices did not increase WMCs. Post‐resistance changes in WMC ranged between $85 and $138 ha^?1, depending on the practices adopted. WMCs increased by $88 ha^?1 when cost‐neutral practices were adopted. The in‐sample aggregate costs of managing herbicide resistance ranged between $25 and $53 million, depending on the types of adopted practices.     

The importance of seasonal variability and tactical responses to risk on estimating the economic benefits of integrated weed management

Seasonal variability is an important source of risk faced by farmers and, regardless of an individual's attitude to risk, there are options to tactically adjust production strategies as the outcomes of risk become known. The objective of this article is to measure the economic benefits of alternative approaches to managing weeds, one of the most serious production problems in Australian cropping systems. A bioeconomic model that combines weed biology, crop growth and economics is developed to value the effects of seasonal variability and the role of tactical responses and sequential decision making in determining an optimal integrated weed management strategy. This shows that there are substantial differences in the measured long‐term benefits from deterministic and stochastic simulations. It is concluded that, for research evaluation of technologies that involve complex biological and dynamic systems, ignoring the impacts of seasonal variability, responses to risk and sequential decision making can lead to an incorrect estimate of the economic benefits of a technology. In this case study of optimal weed management strategies in Australia, the size of the error is high.     

Economic contribution of French serradella (Ornithopus sativus Brot.) pasture to integrated weed management in Western Australian mixed‐farming systems: an application of compressed annealing*

Sowing phases of French serradella (Ornithopus sativus Brot.) pasture between extended cropping sequences in the Western Australian wheatbelt can sustain grain production through restoring soil fertility and reducing selective herbicide use. The objective of this article is to investigate the profitability of rotations involving this pasture under a variety of weed management scenarios to obtain greater insight into its value for mixed farming systems in this region. A stochastic search procedure, compressed annealing, is used to identify profitable sets of weed management strategies in a simulation model representing a large number of potential combinations of chemical and non‐chemical forms of weed control. In contrast to a continuous‐cropping sequence, the inclusion of a serradella phase in a rotation is profitable at high weed densities and with increasing levels of herbicide resistance. A single year of pasture in the rotation is optimal if resistance to Group A selective herbicides is present at the beginning of the planning horizon, but a three‐year phase is required if resistance to multiple herbicide groups is observed. Sowing a serradella pasture twice over a two‐year phase is also shown to be economically attractive given benefits of successive high weed kills.     

RESPONSES TO RISK IN WEED CONTROL DECISIONS UNDER EXPECTED PROFIT MAXIMISATION: COMMENT

Pannell's finding that risk reduces the optimal rate of herbicide use and raises the economic threshold for weed control is found to depend upon the form of the yield function. Pannell's results apply only to convex yield functions. For sigmoidal functions, such as the logistic, risk has the opposite effect over the concave portion of the function's range.     

Evaluating combined land conservation benefits from perennial pasture: lucerne (Medicago sativa L.) for management of dryland salinity and herbicide resistance in Western Australia*

The inclusion of perennial pasture phases in cropping rotations has been widely promoted throughout Australia for reducing the incidence of dryland salinity. To a lesser extent, they have also been promoted to enhance the management of herbicide‐resistant weeds. No previous economic analysis of perennial pasture has considered both of these benefits. This study combines a dynamic linear programming model to estimate the magnitude of salinity‐related benefits and a complex simulation model to assess the economics of herbicide‐resistance management. We present a case study of the perennial pasture lucerne (Medicago sativa L.) in the Wheatbelt of Western Australia, where the weed annual ryegrass (Lolium rigidum Gaudin) is resistant to multiple herbicide groups. Sequences incorporating lucerne are the most profitable land use at the standard set of parameter values if (i) annual ryegrass is resistant to all selective herbicides, (ii) the water table is so shallow (approximately < 3.5 m deep) that frequent rotation with perennials is required to avert soil salinisation, (iii) sheep production is highly profitable, or (iv) there is a combination of less extreme cases. The value of perennial pasture is sufficient under these circumstances to overcome its high establishment cost and the displacement of multiple years of crop. Consideration of dryland salinity and herbicide resistance are about equally important in evaluating the economics of lucerne; neither should be neglected.     

Applying search theory to determine the feasibility of eradicating an invasive population in natural environments

The detectability of invasive organisms influences the feasibility of eradicating an infestation. Search theory offers a framework for defining and measuring detectability, taking account of searcher ability, biological factors and the search environment. In this paper, search theory concepts are incorporated into a population model, and the costs of search and control are calculated as functions of the amount of search effort (the decision variable). Simulations are performed on a set of weed scenarios in a natural environment, involving different combinations of plant longevity, seed longevity and plant fecundity. Results provide preliminary estimates of the cost and duration of eradication programs to assist in prioritising weeds for control. The analysis shows that the success of an eradication program depends critically on the detectability of the target plant, the effectiveness of the control method, the labour requirements for search and control, and the germination rate of the plant.     

Financial comparisons of under‐vine management systems in four South Australian vineyard districts*

Conventional viticultural practice in Australia and elsewhere involves removal of under‐vine vegetation using herbicides or cultivation. Concerns over the long‐term effects of herbicides on soil health, evolution of resistant weeds and possible impacts on human health motivate the search for alternative weed management options. Industry‐supported trials on commercial vineyards in four South Australian regions investigated standard practices of straw mulch and bare earth created with herbicides, compared to under‐vine cover crops, focusing on soil health attributes (soil carbon, soil microbiological processes, etc.) and grape yields in 2016 and 2017. Measured yields with the Control (herbicide) treatment were combined with published district grape prices and yields over the 12‐year (2006–2017) period, defining multivariate distributions of gross revenues ($/ha). Assuming all treatments produce grapes of equal quality and price as the Control, our results showed median per‐hectare gross margins greater than the Control in the Barossa district, lower than Control in Riverland, and mixed results in Langhorne Creek and Eden Valley. Multi‐year risk profiles, based on decadal whole‐farm (50 ha) cash flows for each treatment, were calculated using Monte Carlo analysis, based on historical yield and price distributions. These risk profiles showed the under‐vine treatments may result in major differences in long‐term vineyard financial viability.     

Estimating the economic cost of weeds in Australian annual winter crops

An analysis of the annual costs of weeds in seven winter crops across Australia demonstrated that the most important 15 weed species cause substantial annual costs in both financial and economic terms. Using survey data captured over the 1998–1999 growing season, the financial cost of these weeds in seven crops was estimated to be AU$1,182 million. The main components of this cost were herbicides (AU$571 million), the competitive effects of residual weeds (AU$380 million), and tillage (AU$206 million) while weed contamination of grain was a minor cost (AU$25 million). Across all regions, the most economically important weeds were annual ryegrass, wild oats, and wild radish, although there were regional differences in importance. An economic surplus analysis determined the annual economic cost of weeds in annual winter crops to be AU$1,279 million. This surplus loss represented 17% of the gross value of Australian grain and oilseed production in 1998–1999. Australian grain producers incurred a major loss, with a reduction in producer surplus of AU$1,047 million. Australian grain consumers had a large consumer surplus loss (AU$229 million), while international consumers suffered a small loss and international grain producers gained a small producer surplus from the higher grain prices.     

The impact of conservation tillage on maize yield and input demand: the case of smallholder farmers in north‐west Ethiopia

This study analyses the economics of conservation tillage (CT) with respect to its effect on maize yield and chemical fertiliser, herbicide, and female and male labour demand. We estimate production and input demand functions using seemingly unrelated regressions on plot‐level cross‐sectional farm household data collected in the north‐west of Ethiopia. A two‐step control function is applied to address potential endogeneity bias due to the inclusion of the CT adoption decision as an explanatory variable. Our results show that CT increases maize yield and chemical fertiliser demand. Additionally, the results show that the adoption of CT reduces female and male labour required for crop production. However, this is achieved through the increased use of herbicides, which might have an undesirable health and environmental effects.     

A stochastic dynamic programming framework for weed control decision making: an application to Avena fatua L.

This paper develops a stochastic multi-period decision model to analyse a continuous wheat cropping system infested by wild oats (Avena fatua L.), in southern Australia. The multi-period solutions is obtained by employing a dynamic programming model in conjunction with a bioeconomic simulation model. An empirically estimated dose response function is used to derive the optimal herbicide rate. Uncertainties due to environmental effects on the performance of herbicide and crop yields are modelled and optimal decision rules derived. The results indicate that substantial economic gains can be realised if herbicide dose decisions are taken by considering future profit effects of current decisions, as opposed to the more common approach of only considering the current-period effect.     

The economic costs of weeds on productive land in New Zealand

Here we review published research on the costs of weeds to New Zealand’s pastoral, arable and forestry sectors, and propose an alternative dynamic approach for future research. The studies reviewed had little in common methodologically, often contained guesswork, or were outdated. Their aggregation resulted in a conservative estimate of the cost of weeds to New Zealand’s agricultural economy of $1658 million (2014 NZD). To address deficiencies in previously used methodologies, a dynamic approach is developed and applied to a case study on giant buttercup in dairy pastures. This approach accounts for probable temporal changes in both the geographic extent of the weed and in producer prices and indicates annuitized costs (over the period 2012–2030) of $166 million, $259 million and $592 million for rates of spread of 144, 60 and 20 years for giant buttercup to invade all dairy regions in New Zealand. Comparing the aggregate cost of all weeds to the three productive sectors estimated from the historical data with these ‘dynamic’ estimates for the one species in dairy pasture, indicates that the historical data provide a substantial underestimate of the true aggregate cost of weeds to New Zealand’s agricultural economy.     

A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops

Weed control in the U.S. Midwest has become increasingly herbicide-centric due to the adoption of herbicide-tolerant (HT) crops in the 1990s. That integrated weed management (IWM) practices, including ecological and mechanical controls, are scarcely used is concerning. IWM would be a more sustainable form of farming for two reasons. First, it would reduce the negative health and environmental externalities associated with herbicide use. Second, it would reduce the selection pressure on weed populations and the development of weed resistance to some herbicides, thereby reducing the uncertainty of the long-term effectiveness of herbicidal weed control. In this context, we develop an economic framework to clarify the interplay among the different market failures that either contribute to the herbicidal ‘lock-in’ or make it problematic. We then analyse the evidence for and perceptions of these market failures based on twenty-four semi-structured interviews with farmers and experts conducted in 2017, as well as on discussions in the academic literature. To this end, we put into perspective the possible self-reinforcing effects in the adoption path of HT crops, such as increasing farm size, changes in farm equipment, increasing incentives for simplified crop rotations, and the loss of practical knowledge of IWM practices.     

Integration of conservation agriculture in smallholder farming systems of southern Africa: identification of key entry points

A component-omission experiment based on the principle of conservation agriculture (CA) was established on smallholder farms for three seasons in Murehwa and Hwedza districts, Zimbabwe; Barue district in Mozambique; Balaka district and Chitedze Research Station in Malawi, and Monze district in Zambia to identify strategies for improving crop productivity and livelihoods for smallholder farmers. The objective of the experiment was to evaluate the effect of tillage, residue retention, fertiliser application and weed control on maize yield. In addition, the study analysed possible combinations of these factors that could provide a sustainable entry point for intensification through CA. Results showed that fertilisation had the strongest effect on crop yield in both tillage systems; adequate fertilisation is therefore key to success in CA. Retention of crop harvest residues increased yield in no-tillage systems; no-tillage without residues depressed yield by 50% when compared with yields of conventional tillage. A step-wise integration of CA into the smallholder farming systems is proposed as a possible strategy to avoid new constraints on smallholder farms. If resources are limiting, farmers may apply all principles on small areas to overcome the initial demand in resources (labour, fertiliser and residues), and once productivity is raised, they can expand.