A.G. Thomas

Bio: A.G. Thomas is an academic researcher from Agriculture and Agri-Food Canada. The author has contributed to research in topics: Tillage & Weed. The author has an hindex of 3, co-authored 6 publications receiving 94 citations.

Relative contribution of genotype, seed size and environment to secondary seed dormancy potential in Canadian spring oilseed rape (Brassica napus)

Abstract: Summary Secondary seed dormancy has been linked to seedbank persistence of volunteer oilseed rape (Brassica napus) in western Canada. It has been suggested that there is a genetic component to secondary seed dormancy expression in oilseed rape, but little is known of its importance in relation to non-genetic factors. In a series of experiments we investigated the relative importance of genotype, seed size, time of windrowing and pre- and post-harvest environment on the expression of secondary seed dormancy. We found that genotype contributed between 44 and 82% to the total variation in secondary seed dormancy. A broad range in secondary seed dormancy expression was observed among 16 genotypes examined. Nevertheless, three-quarters of the genotypes investigated exhibited relatively high potential for the expression of secondary seed dormancy (back-transformed mean 71% dormant seeds). Seed size contributed 21% to the total variation, while the influence of seed maturity (harvest regime) on secondary seed dormancy expression was negligible. Despite diverging environmental conditions during the four growing seasons spanning these experiments, the influence of pre-harvest environment on seed dormancy expression was relatively small and ranged from 0.1% to 4.5%. Secondary seed dormancy potential decreased over time during seed storage. This decrease was greatest when seeds were stored at ambient temperatures and least when seeds were stored at −70°C.

Weed community response to contrasting integrated weed management systems for cool dryland annual crops

Abstract: Thomas AG, Legere A, Leeson JY, Stevenson FC, Holm FA & Gradin B (2011). Weed community response to contrasting integrated weed management systems for cool dryland annual crops. Weed Research 51, 41–50. Summary Contrasting approaches to integrated weed management (IWM) for prairie cropping systems were evaluated by measuring weed response to six IWM systems in a wheat–oilseed rape–barley–pea rotation at Saskatoon and Watrous, Saskatchewan, Canada. The six IWM systems (high herbicide/zero tillage; medium herbicide/zero tillage; low herbicide/zero tillage; low herbicide/low tillage; medium herbicide/medium tillage; no herbicide/high tillage) included various combinations of seeding rate and date, herbicide timing and rate, and tillage operations, in order to achieve similar weed management levels. Changes in weed communities were assessed over 4 years by monitoring species composition and abundance at various times during the growth season. Principal response curves indicated a gradual increase in Thlaspi arvense, Chenopodium album, Amaranthus retroflexus and Fallopia convolvulus in the no herbicide/high tillage system. Winter and early spring annuals and perennials increased in most systems, but particularly in the low herbicide/zero tillage and medium herbicide/zero tillage systems. Although five of the six IWM systems provided similar results, changes in weed communities would suggest that operations could be revised to improve the overall management of certain weed species and reduce seed return in the no herbicide/high tillage system and in systems with low herbicide inputs and zero tillage. This study confirms the potential of contrasting IWM systems under the challenging environmental conditions of the Canadian Prairies.

A risk-qualified approach to calculate locally varying herbicide application rates

Abstract: Summary Weed competition can decrease crop yield and profit. Herbicides are applied to reduce weed populations, minimize crop loss and maximize profit. Traditional practice is to apply herbicides at a uniform rate over an entire field. Complete knowledge of the weed distribution and appropriate instrumentation on the spraying equipment would allow the farm manager to apply the correct locally varying herbicide application rate. The locally variable rate would be greater in areas of high weed density and less where there are few weeds. A locally varying treatment would have both economic and environmental advantages. A major challenge facing farm managers is the unavoidable uncertainty in the spatial distribution of weeds in any particular field. This uncertainty in weed distribution influences the optimal locally varying herbicide rate. A mathematical model is presented to calculate the optimal herbicide application rate using geostatistical models of uncertainty in weed density combined with principles from decision making. Weed data from a 34-ha field near Saskatoon, Saskatchewan, Canada, illustrate the application of these tools. Weed control was achieved with a significant reduction in total herbicide use.

Seedbank response to integrated crop management systems

Abstract: Six integrated crop management systems varying primarily in terms of tillage and herbicide inputs were maintained for four years at two locations in the Moist Mixed Grassland ecoregion in Saskatchewan. The seedbank was sampled in each plot in the spring of 1997 prior to the initiation of the experiment and the spring of 2001 following the completion of the experiment to determine if cropping system had changed seedbank composition. Redundancy Analysis (RDA) was used to directly quantify the change in weed seedbank attributable to cropping system at each location. The RDA was constrained by cropping system for the 2001 data and year for the 1997 data. Initial spatial variation observed in the 1997 weed seedbank was removed from the analysis by using plot number and replicate block as binary covariables. The majority of species increased in the seedbank in all systems studied at both locations. The seedbank from the no herbicide high tillage systems was distinguishable from the other systems by large increases in lamb’s-quarters and stinkweed at both locations. While similar species tended to increase in all the systems with herbicides, the systems with lower levels of herbicides applied had larger increases in the seedbank than those with high herbicide inputs.

Seasonal weed response to integrated crop management systems

Abstract: Weed plant density was monitored to determine the effect of various integrated crop management systems on weed population levels and evaluate the efficacy of the various management practices. The management systems studied primarily varied tillage and herbicide inputs. Weed plant density was determined in spring, prior to in-crop management and after incrop management in each year of the study. Also, weeds were counted after in-crop management prior to the establishment of the management systems. Multivariate analyses were used to determine the response of the weed communities to the management systems. Principal response curves were used to illustrate seasonal fluctuations and trends in weed densities. Stinkweed, lamb’s-quarters, wild buckwheat, redroot pigweed, wild mustard and Russian thistle are best controlled with in-crop herbicide applications. These species increased over time in the no herbicide high tillage system and low herbicide systems. Winter annuals, perennials and early spring annuals were found to increase in the lower herbicide zero-tillage systems. These species tended to be found in high densities in the spring and not effectively controlled in-crop.

Recent Weed Control, Weed Management, and Integrated Weed Management

Abstract: Integrated weed management (IWM) can be defined as a holistic approach to weed management that integrates different methods of weed control to provide the crop with an advantage over weeds. It is practiced globally at varying levels of adoption from farm to farm. IWM has the potential to restrict weed populations to manageable levels, reduce the environmental impact of individual weed management practices, increase cropping system sustainability, and reduce selection pressure for weed resistance to herbicides. There is some debate as to whether simple herbicidal weed control programs have now shifted to more diverse IWM cropping systems. Given the rapid evolution and spread of herbicide-resistant weeds and their negative consequences, one might predict that IWM research would currently be a prominent activity among weed scientists. Here we examine the level of research activity dedicated to weed control techniques and the assemblage of IWM techniques in cropping systems as evidenced by scientific...

Germ banking: bet-hedging and variable release from egg and seed dormancy.

Abstract: Many species produce eggs or seeds that refrain from hatching despite developmental preparedness and favorable environmental conditions. Instead, these propaguleshatchinintervalsoverlongperiods. Such variable hatch or germination tactics may represent bet-hedging against future catastrophes. Empiricists have independently recognized these approaches in diverse species. Terms such as seed banking, delayed egg hatching, and embryonic diapause have been used to describe these tactics, but connections between fields of study have been rare. Here we suggest a general term, germ banking, to incorporate all previous terms, unifying many seemingly disparate biological strategies under a single definition. We define the phenomenon of germ banking and use several biological examples to illustrate it. We then discuss the different causes of variation in emergence timing, delineate which constitute germ banking, and distinguish between germ banking and optimal timing of diapause. The wideranging consequences of germ banking are discussed, including modification of the age structure of a population, the alteration of microevolutionary dynamics, the migration of alleles from the past, the maintenance of genetic and species diversity, and the promotion of species coexistence. We end byposing questions to direct future research.

A decade of herbicide-resistant crops in Canada

Abstract: This review examines some agronomic, economic, and environmental impacts of herbicide-resistant (HR) canola, soybean, corn, and wheat in Canada after 10 yr of growing HR cultivars. The rapid adopti...

Persistence of seeds from crops of conventional and herbicide tolerant oilseed rape (Brassica napus)

Abstract: A series of rotation experiments at five sites over four years has explored the environmental and agronomic implications of growing herbicide tolerant oilseed rape and sugar beet. This paper reports on the population dynamics of volunteer rape (Brassica napus). The experiments compared four winter oilseed rape (WOSR) cultivars: a conventional cultivar (Apex) and three developmental cultivars either genetically modified (GM) to be tolerant to glyphosate or glufosinate, or conventionally bred to be tolerant to herbicides of the imidazolinone group. Seed losses at harvest averaged 3575 seeds m(-2) but ranged from less than 2000 up to more than 10000 seeds m(-2). There was a rapid decline in seed numbers during the first few months after harvest, resulting in a mean loss of seeds of 60%. In subsequent seasons, the seedbank declined much more slowly at four of the five sites (ca 20% per year) and the models predicted 95% seed loss after approximately 9 years. Seed decline was much faster at the fifth site. There were no clear differences between the four cultivars in either the numbers of seeds shed at harvest or in their subsequent persistence. The importance of the persistence of GM rape seeds, in the context of the coexistence of GM and non-GM crops and the role of good management practices that minimize seed persistence, are discussed.

Strategies and tactics for herbicide use reduction in field crops in Canada: A review

Abstract: There are many economic and health reasons for reducing pesticide use in Canada. Herbicide use on field crops is by far the most common pesticide use in Canada. This paper is a review of four topics related to herbicide use reduction on field crops in Canada: (1) broad strategies and (2) specific tactics for herbicide use reduction; (3) factors affecting adoption; and 4) research approaches for improving the implementation of herbicide use reduction. Numerous tactics exist to use herbicides more efficiently and herbicides can sometimes be replaced by non-chemical weed control methods. Many of these tactics and methods have been investigated and demonstrated for use on field crops in Canada. However, herbicide use reduction is fundamentally dependent upon preventative strategies designed to create robust cropping systems that maintain low weed densities. Diverse crop rotation forms the basis of preventative strategies as it inherently varies cropping system conditions to avoid weed adaptation. There is evi...