Herbicide resistant weeds are driving implementation of integrated weed management (IWM). A new tactic to manage weeds is harvest weed seed control (HWSC), which targets weed seeds retained on the plant at crop harvest and either destroys, removes, or concentrates them. Research is limited on the effectiveness of HWSC in US cropping systems. For HWSC to be effective it is important to know when and how many seed are shed from a weed species in relation to crop harvest. Research was conducted to quantify when weed seed are shattered from 6 economically important weed species, four broadleaf (redroot pigweed, common ragweed, common lambsquarters, and common cocklebur) and two grass species (large crabgrass and giant foxtail). Results indicate that among summer annuals, broadleaf species retain larger proportions of their seed compared to grass species at the first opportunity for soybean harvest. As harvest was delayed, more seeds shattered from all species evaluated, indicating timely harvest is critical to maximizing HWSC effectiveness. Studies were conducted on grower fields in Virginia to evaluate the effectiveness of HWSC (field residue and weed seed removal). Results indicate that HWSC can significantly reduce populations of Italian ryegrass in wheat and common ragweed in soybean in the next growing season, but reductions were not observed for Palmer amaranth in soybean. Investigating IWM system for common ragweed control in soybean, HWSC was found to be less effective than soybean planting date (i.e. double cropping after wheat) at reducing common ragweed populations. However, the effectiveness of HWSC varied by location. If HWSC adoption were to become widespread, weeds could adapt by shedding seed earlier in the season. Research was conducted by growing Palmer amaranth populations from across the eastern US in a common garden. Currently there are differences in flowering time and seed shatter among Palmer amaranth populations based on the location of the maternal population, indicating potential for adaptation. This research demonstrates that HWSC is a viable option for weed management in US cropping systems but needs to be stewarded like any other weed management tool. / Doctor of Philosophy / Herbicide resistance in weeds is a growing problem in the US and around the world. Alternative methods of weed control must be adopted to maintain crop yields in the presence of herbicide-resistant weeds. Researchers and extension specialists strongly advise growers to adopt an integrated weed management (IWM) approach. Integrated weed management involves implementing multiple weed control tactics during a growing season. By using multiple methods of weed control within a given season the chances of weeds becoming resistant or adapting to any single tactic is reduced. Harvest weed seed control (HWSC) is a new tactic developed in Australia in response to herbicide resistance. HWSC targets weed seeds retained on the plant at crop harvest. In a normal crop harvest, the combine removes the grain and spreads crop residues (leaves, stalks, and other plant parts), including weed seeds, back across the field. When HWSC is implemented, weed seeds are destroyed (narrow windrow burning, cage mills) or concentrated and potentially removed from the field (chaff carts, direct bale, chaff lining). Thus, HWSC limits the number of weed seeds returned to the soil seed bank. There is limited research on HWSC and its integration with other tactics, in US cropping systems. For HWSC to be effective it is necessary for weed seeds to be retained on the mother plant in sufficient quantities at crop harvest. Research was conducted in Virginia to determine when weed seeds are shattered during the soybean growing season for 6 economically important weed species, four broadleaf (redroot pigweed, common ragweed, common lambsquarters, and common cocklebur) and two grass species (large crabgrass and giant foxtail). The broadleaf species retained >85% of their seed until the first opportunity for soybean harvest (mid-October). In the grass species, more seed shattered prior to soybean harvest with 50% of large crabgrass and 74% of giant foxtail seed being retained at the first opportunity for soybean harvest. When harvest was delayed seed continued to shatter and less was captured using HWSC. This research indicates broadleaf species are more suitable candidates for HWSC than grass species, among summer annuals. Further research on the ability of seed to germinate in relation to when seeds were shed was conducted on redroot pigweed, common ragweed and common lambsquarters. Results indicate that there are variable effects on germination of these species depending on when they were shed. HWSC was implemented on grower fields to assess the impact on weed populations of 3 weed species (Italian ryegrass, common ragweed, and Palmer amaranth). These experiments compared conventional harvest and HWSC (field residue and weed seed removal) when all other management strategies were the same within that field. Italian ryegrass tiller density in wheat varied by location but was reduced up to 69% in the spring following implementation of HWSC. By wheat harvest, HWSC reduced Italian ryegrass seed head density 67% at one location compared to conventional harvest. In soybean, common ragweed densities were reduced by 22 and 26% prior to field preparation and postemergence herbicide applications, respectively, in the HWSC plots compared to the conventional harvest plots. No differences were observed in common ragweed density by soybean harvest. No differences were observed with Palmer amaranth densities at any point during the soybean growing season. This research show that HWSC can reduce weed populations but is variable and additional research is still needed. IWM experiments were established across Virginia to compare soybean planting date (full season or double cropped), + cover crop (cereal rye/wheat or no cover), and + HWSC (field residue removal) to evaluate the best management strategy for common ragweed in soybean. Across all locations, double cropping soybean behind wheat had the greatest impact on common ragweed densities at the end of the first season. The impact of double cropping soybeans on common ragweed population is due to the emergence pattern of common ragweed; majority of common ragweed emerges prior to planting double cropped soybean (mid-June to early-July). HWSC was variable and only reduced common ragweed density at one of three locations. Widespread adoption of HWSC could place a selection pressure on weeds to shatter seed earlier in the season. A common garden experiment was conducted in Blacksburg, VA to assess Palmer amaranth populations collected from central Florida to southern Pennsylvania for differences in flowering time, time to seed shatter, and other phenotypic traits. Results indicate that latitude of the maternal population influences time to first flower with a 0.53 d reduction in flowering time for every degree north in latitude the maternal population was collected from. The strongest predictor of Palmer amaranth flowering time was emergence date/daylength. For every day emergence was delayed the time to first flower was reduced by 0.31 and 0.24 d for female and male plants, respectively. Time from emergence or first flower to first seed shatter was reduced by 0.48 or 0.17 d, respectively, for each day emergence was delayed. These results indicate that differences exist currently among Palmer amaranth populations and the selection pressure of HWSC could push these populations to flower and shatter seed early.

Harvest weed seed control (HWSC) methods concentrate, remove, or destroy weed seeds captured by the combine during harvest. Furthermore, chaff lining uses a chute fitted on the back of a combine to concentrate chaff and weed seed therein into a narrow line. Since chaff amount increases with crop yield, studies aimed to determine how varying crop yield and the associated chaff amount will affect chaff lining control of select weed species, while also examining subsequent crop performance. Objective 1 of this work focused on wheat chaff lining (WCL), and objective 2 studied soybean chaff lining (SCL). Weed species of interest included wild mustard (Sinapis arvensis L.) and Italian ryegrass (Lolium perenne ssp. multiflorum L. Husnot) in WCL and Palmer amaranth (Amaranthus palmeri S.) and common ragweed (Ambrosia artemisiifolia L.) subject to SCL. Each weed species was evaluated in separate experiments, and the SCL experiment included an additional factor of with and without a cereal rye cover crop treatment. Chaff lines mimicked harvest across a range of wheat and soybean yields, with equal weed seed additions (based on existing fecundity and seed shatter phenology data) to each chaff line. A conventional harvest (control) and an outside-the-chaff-line treatment were included, where total fecundity or weed seed rain occurring prior to harvest based on weed species were broadcast respectively. Inhibition of crop and weed emergence as a function of yield and the associated chaff amount was also investigated in the greenhouse. Crop yield across treatments at the field scale (accounts for both chaff lines and outside-the-chaff-line), was not affected in double-crop soybean following WCL and full-season soybean following SCL. Field scale wheat yield in WCL compared to conventional harvest was not different, increased, or decreased in 8, 3, and 1 site-years, respectively. WCL reduced total weed emergence over the combined double-crop soybean and winter wheat growing seasons by 43-54% at the field scale. SCL reduced common ragweed emergence in cereal rye by 64% and 85% in 2 of 3 locations across the soybean growing season. The cover crop did not reduce common ragweed emergence while it was growing, but residual mulch in soybean reduced emergence by 39%. No differences were observed in Palmer amaranth emergence during cereal rye growth, however cereal rye decreased total emergence by 41%. In 6 of 7 Palmer amaranth location-years, SCL decreased field scale weed emergence in soybean by 81%. These results indicate chaff may create an unfavorable environment for weed seed emergence. In both WCL and SCL, greater amounts of chaff caused larger reductions in weed emergence. Objective 3 focused on quantifying the above-ground biomass breakdown of soybean plants into chaff, straw, and seed fractions as they are processed and dispensed by various harvesters. Depending on HWSC system, chaff and straw residues may also be destroyed, removed, or concentrated. Therefore, chaff and straw nutrient composition was analyzed to evaluate the nutrient and economic consequences of HWSC. Our results show average soybean harvest index is 0.57:1. Furthermore, chaff and straw residues equal 13.4% and 68.5% of the seed weight, respectively. Using 5-year average fertilizer prices (2017 – 2021), replacement of N, P, K and S in chaff, straw, and the combination of both residues costs USD 1.58, USD 5.88, and USD 7.46, respectively. / Master of Science in Life Sciences / In conventional wheat and soybean production, the primary means of weed control is herbicides. If herbicide use is not diversified, a repeated selection pressure drives weeds to evolve resistance to such chemistries. Producers and researchers alike are constantly looking for new ways to combat weeds and herbicide-resistant issues. Originally developed to control nuisance weeds in Australia, harvest weed seed control (HWSC) offers promise in aiding our current herbicide resistance crisis. To further explain HWSC, it is important to know the harvesting mechanism. Many of the row crops including corn, soybean and wheat are harvested using a combine. Combines cut below or tear off plant material to capture the grain or seed which is processed via a threshing system and separated into three fractions: the seed, chaff, and straw. The grain is allocated to a storage bin and eventually removed from the field. In conventional harvest, the remaining crop residue is spread evenly behind the combine across the field to ensure a balanced distribution of organic matter, nutrients, and residue across the field. There is however more than chaff and straw being dispersed. Weeds present in the field at harvest whose seed is retained at crop maturity and at an elevation above the combine header height will inherently end up inside the combine. HWSC are methods intended to capitalize on the combine capturing weed seeds during harvest. Many HWSC approaches to managing weed seed exist, including destruction, removal and concentration of weed seed. Most of this research focuses on only one method of HWSC, chaff lining. Chaff lining utilizes a chute fitted onto the back of the combine and concentrates weed seed and the chaff fraction only into a narrow line behind the combine. Although not directly known, chaff may inhibit future weed emergence within the line due to a mulching effect, intraspecific competition, a greater degree of rotting and increased seed predators. The chute is inexpensive to construct, and there are no additional labor requirements at harvest making it an appealing HWSC option. There is a limited amount of research on chaff lining in North American cropping systems making it a prime HWSC candidate for this thesis. We were curious if chaff lining could benefit wheat and soybean farmers and if crop yield and the associated chaff amount deposited in chaff lines would have any impact on crops planted and weeds placed in lines. Our results indicate chaff lining does not cause field scale yield consequence in double-crop and full-season soybean following wheat and soybean chaff lining, respectively. The effect of wheat chaff lining on wheat field scale yield was variable, but only caused a yield decrease in 1 of 12 experimental locations. Reductions in weed emergence in chaff lining systems compared to conventional indicate chaff may alter the environment to be unfavorable for weed seed emergence. The final objective of this thesis investigates the economic cost of nutrient loss among HWSC systems. Using average fertilizer prices, the cost to apply N, P, K and S concentrated or lost during HWSC in chaff, straw, and the combination of both residues is USD 1.58, USD 5.88, and USD 7.46, respectively.

Sustainable weed control in managed agricultural systems requires the judicious use of multiple weed control tactics and prevents over-reliance on any one tactic. In this context, sustainable weed management plays a critical role in the mitigation of one of agriculture's most pressing problems- herbicide resistance. Research conducted in Virginia sought to explore the effects of integrating multiple weed management tactics in corn and cool-season turfgrass. Additionally, research was conducted to confirm nicosulfuron and glyphosate herbicide resistance in Virginia johnsongrass and elucidate the molecular mechanisms conferring those resistances. Rye and hairy vetch cover crop residues, combined with reduced rates of preemergence herbicide and postemergence glyphosate applications, were shown to provide sufficient weed control and corn yield. Cover crop type or residue level did not augment weed control in corn production systems, but the use of glyphosate was essential for late-season weed control. Rye and vetch biculture as a cover crop increased corn yield compared to rye cover crop alone. In cool-season turfgrass, the addition of reduced preemergence herbicide rates to corn gluten meal, an organic herbicide product, reduced crabgrass 25%. Moreover, control was dependent on herbicide choice. Herbicides applied at half of recommended labeled rates or less did not control crabgrass at a commercially-acceptable level, regardless of corn gluten meal addition. In field experiments, Virginia johnsongrass expressed resistance to nicosulfuron and glyphosate. Glyphosate at 0.88 kg ae ha-1 controlled johnsongrass 65%. Nicosulfuron at 0.14 kg ai ha-1 controlled the same population 10%. Greenhouse experiments confirmed differential sensitivity of putative herbicide-resistant johnsongrass seedlings to nicosulfuron and glyphosate when compared to a susceptible population. Herbicide resistance was not conferred via target-site mutation. Five ALS-gene site mutations were confirmed absent in Virginia johnsongrass, while three others were located in coding regions that could not be elucidated in johnsongrass. Further investigations showed glyphosate resistance was not conferred via reduction in herbicide absorption or translocation. The susceptible johnsongrass caused an increase in a polar metabolite at Rf = 0.17 with concomitant reduction in glyphosate over time. Although the mechanism is not clear, these data suggests that glyphosate resistance in johnsongrass may be associated with differential metabolism. / Ph. D.

Dans le cadre de l’évaluation multicritère des systèmes en protection intégrée contre les adventices (PIC-Adventices), cette étude a eu pour objectif d’étudier l’impact de la combinaison de pratiques alternatives à l’usage d’herbicides sur les émissions de N2O par les sols. L’étude a été conduite sur quatre systèmes de culture, un système de référence en agriculture raisonnée (S1) et trois systèmes PIC-Adventices (S2, S3, S5), sur le site expérimental Dijon-Epoisses (47°20'N, 5°2'E). La mise en place de dispositifs de mesure à haute résolution, alliant la méthode des chambres d’accumulation de gaz en surface du sol aux analyses de gaz par infrarouge, a permis d’estimer l’intensité des émissions de N2O pour les sols argilo-calcaires caractéristiques de la plaine dijonnaise, avec des flux moyens compris entre -6 et 257 g N-N2O ha-1 jour-1. Cette intensité s’est avérée étroitement liée à certains paramètres environnementaux (température, part de porosité occupée par l’eau, teneur en azote), et certaines pratiques agricoles, induisant des différences significatives entre les systèmes. Les émissions de N2O ont notamment été affectées par le mode de gestion des sols et la nature des cultures constituant la succession de chaque système. L’absence de travail du sol depuis 2008 dans le système S2 s’est traduite par une amplification des émissions de N2O d’un facteur multiplicatif d’au moins six par rapport aux autres systèmes pour l’année 2012. Par ailleurs, ces émissions équivalentes à 0.7 t C-CO2 ha-1 pourraient avoir compensé la séquestration de carbone additionnelle induite par le travail réduit du sol pendant la période de mesure. De la même manière, la dégradation des résidus de culture légumineuse, dans les systèmes S3 et S5 ont respectivement stimulé les émissions de N2O en 2011 et en 2012, en comparaison avec le système de référence S1. Par ailleurs, le suivi mensuel de la taille des communautés microbiennes du sol a également permis de mettre en évidence une variabilité temporelle liée aux pratiques culturales (travail du sol, récolte), en plus de la variabilité saisonnière. En revanche, dans cette étude les variations de taille des communautés microbiennes nitrifiantes et dénitrifiantes ne sont pas apparues comme un facteur d’explication de la variabilité des émissions de N2O, quel que soit le système. Enfin, l’approche par la modélisation des émissions de N2O sur les quatre systèmes, à l’aide des modèles NOE et DNDC, a permis d’identifier les principaux facteurs de régulation de ces émissions. De surcroît, le modèle NOE a bien identifié le système de culture S2, sans travail du sol, comme étant le système le plus émetteur de N2O, conformément aux observations de terrain. Cette étude renforce donc la pertinence de la modélisation pour estimer et analyser les émissions de N2O dans différents systèmes de culture. / In the context of the multicriteria evaluation of Integrated Weed Management (IWM) cropping systems, our study highlights the impacts of the combination of alternative agricultural practices to herbicide reliance on N2O emissions from soils. The study was located at the INRA experimental site of Dijon-Epoisses (47 ° 20'N, 5 ° 2'E) and considered 4 cropping systems, a reference system designed to maximize financial returns (S1) and three IWM systems (S2, S3, S5). The setup of high resolution measuring devices, combining the non-steady state chamber method with infrared gas analysis, allowed to estimate the intensity of N2O emissions from the calcareous clay soil, which is specific of Dijon plain, with average fluxes laying between -6 and 257 g N2O-N ha-1 day-1. The intensity of fluxes was closely related to environmental parameters (temperature, water filled pore space, inorganic N amount) and particular agricultural practices, leading to significant differences between systems. N2O emissions were notably affected by soil management and nature of crops. The conversion of S2 system to no-till in 2008 increased the N2O emissions, fluxes being at least 6 times more important than those emitted by the other cropping systems, during the year 2012. Moreover, these emissions, equivalent to 0.7 ha-1 C-CO2 ha-1, could have offset the soil additional carbon sequestration induced by reduced tillage during the studying period. Similarly, the degradation of legume crop residues in the S3 and S5 systems respectively enhanced N2O emissions in 2011 and 2012, by comparison with the S1 reference system. Elsewhere, the monthly monitoring of the soil microbial communities sizes has also helped to highlight temporal variability induced by the agricultural practices (tillage, harvest) as well as the seasonal variability. However, in this study the fluctuations of nitrifying and denitrifying communities sizes observed did not explain the variability of N2O emissions, whatever the considered cropping system. Finally, the modeling of N2O emitted from the 4 cropping systems, using NOE and DNDC models, allowed to identify the mains factors regulating the emissions. In addition, the NOE model accurately identified the S2 system, without tillage, as the most N2O emitter, in accordance with field observations. Thus, our study reinforces the relevance of modeling to estimate and explain N2O emissions in different cropping systems.

Abstract The negative impact of invasive plant species on biodiversity and ecosystem functions, such as productivity and nutrient cycling has been deemed a global epidemic. To address this worldwide concern, information is needed on how the invasion process happens and how to control an existing invasion. The main aim of the research presented in this thesis was to develop a better understanding of the interacting role different mechanisms play in facilitating invasion and then link this understanding to the design of more effective control strategies. This aim is significant because traditional weed control strategies are not working. The estimated cost of controlling weeds in Australia is $1.4 billion per year in agricultural landscapes. Despite this substantial investment, invasive weed species are estimated to continue to cost the agricultural industry $2.2 billion per year in loss of yield. Current control strategies tend to focus on killing or removing an invasive plant species directly with the application of herbicides and/or mechanical removal. These strategies have proven ineffectual because the plant communities that assemble after management often remain dominated by the same invader or another. In this thesis, I use a combination of empirical and modelling techniques to investigate how disturbance regimes and competitive interactions between invasive plants and native plants can be manipulated to improve the efficacy of restoration efforts. To do this, I use the model scenario of the invasion of Eragrostis curvula (African lovegrass), an invasive grass species introduced into Australia in the early 1900s from South Africa. This species has now spread into every Australian state and territory (chapter 2). I specifically focus on two mechanisms: (1) disturbance, i.e. cattle grazing, and (2) competitive interactions. In chapter 3, I examine connections between dominance and competitive differences among African lovegrass and several functionally similar native grass species in a pasture community. To test the displacement hypothesis, I used a glasshouse competition trial to investigate interactions between African lovegrass and two non-persisting native grass species (Themeda australis and Bothriochloa decipiens) with manipulations of resources, neighbour density, and establishment order. To test the partitioning hypothesis, I compared in situ water use patterns among African lovegrass and two coexisting native grass species (Aristida calycina and Aristida personata) based on the assumption that water is the most limiting resource in this system. The key finding of this chapter is that competition can have important, but contingent, impacts on dominance. Competitive differences appear to partially contribute to abundance patterns after establishment, but may be relatively unimportant during the establishment phase where disturbance appears more critical. In chapter 4, I provide evidence that the identification of mechanisms that led to an invasion, while crucial for the development of effective preventative measures and understanding the invasion process, may not be necessary for the design of more effective control strategies. To examine the effects of different control strategies on African lovegrass and the resultant community, I established a large factorial field-trial with a split-plot design. I manipulated grazing, soil nutrient levels and the presence of the invader. The most common control strategy (removing the causal disturbance and killing the invasive grass), based implicitly on traditional equilibrium models, was not an effective option for restoring a desirable native community. Instead, this strategy led to the dominance of a secondary invader. The most effective control strategy was based on alternative stable states models and involved maintaining grazing, and increasing the palatability of the invader with fertilizers. The key finding of this chapter is that novel approaches for control, which consider the dynamics of the invader-dominated system, are needed. In chapter 5, I investigate the benefits of explicitly incorporating actions that manipulate disturbance (natural or imposed) into control efforts. To do this, I first developed a process model that described the dynamics of an invader whose establishment is preferentially favoured by disturbance. I then couched this model in a decision theory framework, a stochastic dynamic program, and applied a case-study of another invasive plant species, Mimosa pigra (a perennial legume shrub and pan-tropical weed). The key finding of this chapter is that strategies should not only focus on existing invader-dominated sites, but should also protect sites occupied by native species from disturbances that facilitate invasion. The research discussed in this thesis makes three key contributions to a better understanding of the invasion process and the design of more effective control strategies: 1) the search for one key mechanism is not sufficient because multiple mechanisms can interact or shift in importance to facilitate different stages of invasion, 2) a novel approach is needed to restore a more desirable native community because the dynamics of the invader-dominated system can differ from the historical native community, and 3) control efforts should be broadened in focus to include protection of the integrity of native communities from disturbances that facilitate invasion.

Les plantes envahissantes menacent la biodiversité ainsi que les activités humaines. Afin de les maîtriser, la pulvérisation d'herbicides est une méthode fréquemment employée en Amérique du Nord. Cette approche ne fait pas toujours consensus et est même parfois interdite ou restreinte, ce qui justifie le recours à d'autres options. Les alternatives peuvent toutefois s'avérer rares, comporter d'importantes limitations ou sont peu documentées. Cette étude vise à tester l’efficacité de méthodes permettant de maîtriser et de prévenir les invasions de roseau commun (Phragmites australis), l'une des plantes envahissantes les plus problématiques sur le continent nord-américain, tout en limitant au minimum l'utilisation d'herbicides. Le potentiel de quatre méthodes de lutte aux petites populations de roseau bien établies a d'abord été évalué : l’excavation avec enfouissement sur place, le bâchage, l’excavation avec enfouissement sur place combinée au bâchage, et la fauche répétée. Les résultats ont montré que l'excavation avec enfouissement sur place, avec ou sans bâchage, a entraîné une élimination presque totale des populations visées, ce qui est comparable ou supérieur à l'effet généralement obtenu avec la pulvérisation d'herbicide. Le bâchage avec des toiles opaques, maintenues pendant un an, a pour sa part entraîné une maîtrise partielle du roseau, suggérant qu'une application prolongée serait nécessaire pour l'éradication de la plante. La fauche répétée, exécutée à raison de cinq fauches par été pendant deux ans, a fourni une efficacité mitigée. Les résultats suggèrent néanmoins que la fauche pendant plusieurs années contribue à affaiblir la plante, ce qui pourrait aider à son confinement. Une méthode additionnelle a été expérimentée afin de traiter les tiges éparses de roseau tout en limitant les risques d'effets hors cibles, soit le badigeonnage manuel d’herbicide. Suite à ces tests, les résultats ont montré une diminution importante de la densité des tiges, ce qui suggère que la méthode est efficace afin d'éliminer les repousses après un traitement initial, et pourrait également être employée sur de jeunes populations clairsemées. L'effet d'un ensemencement préventif de plantes herbacées sur l'établissement de semis de roseau a également été étudié, suite à des traitements sur de vastes parcelles de sol nu. Les résultats suggèrent que la méthode est efficace afin de limiter la propagation du roseau par semences et qu'un suivi périodique suite à l'intervention serait suffisant afin de maintenir l'effet préventif. / Invasive plants threaten biodiversity as well as human activities and in North America, attempts to control them often rely on the spraying of herbicides. This approach does not always achieve consensus and is sometimes forbidden or restricted, encouraging managers to consider other options. Alternatives can however prove to be rare, involve serious limitations or be poorly documented. This study aims at testing the efficiency of methods for the control and prevention of invasions by common reed (Phragmites australis), one of the most problematic invasive plants on the North American continent, all while reducing herbicide use to a minimum. The potential of four methods for the control of small, well established stands of common reed has first been evaluated: excavation and on-site burial, covering with heavy-duty tarps, excavation and on-site burial combined with covering with heavy-duty tarps, and repeated cutting. Results have shown that excavation with on-site burial, followed or not by covering with heavy-duty tarps, led to an almost complete elimination of the populations targeted, which is comparable or superior to the effect typically obtained with herbicide spraying. Covering common reed with opaque heavy-duty tarps, held in place for one year, lead to partial control of the plant, suggesting that holding the tarps in place for a longer period would be necessary to foresee the eradication. Cutting, repeated five times per summer for two years, gave mixed results. The results suggest, however, that cutting during multiple years contributes to weakening the plant, which could help to its confinement. An additional method, the hand-wicking of common reed stems with herbicide, has been tested to treat scattered stems while limiting off-target effects. Following these tests, the results have shown an important reduction of the density of reed stems, which suggests that the method is effective to control common reed regrowth after an initial treatment, and could be used on young stands. The effect of seeding herbaceous plants on the establishment of common reed seedlings has also been evaluated, following the sowing of vast plots of bare soil. The results suggest that the method is an efficient way of limiting the propagation of common reed through seed dispersal. Results also suggest that periodic follow-ups to eliminate seedlings would be sufficient to maintain the preventive effect.

Crop rotation may be used to prevent the continuous selection of particular weed species adapted to one crop type. This might be useful for weed management, economy in herbicide applications and promoting biodiversity. Common simple crop sequences might be diversified by introducing perennial forage crops. Impacts of such perennial crops on weeds were studied with four approaches : 1) Large-scale weed surveys in 632 fields in western France showed that weed species composition differed most strongly between perennial alfalfa crops and annual crops. Comparisons of fields before, during and after perennial alfalfa suggested that community composition varies in a cyclic way during such crop rotations. Several weed species problematic in annual crops were suppressed during and after perennial crops, but the appearance of other species led to equal or even higher plant diversities. 2) A 3-year field experiment with contrasting crop management options allowed an investigation of the underlying mechanisms for this: The absence of soil tillage reduced weed emergence but increased the survival of established plants. The permanent vegetation cover and frequent hay cuttings reduced weed growth, plant survival and seed production. 3) Greenhouse experiments testing the regrowth ability of individual plants after cutting showed strong differences between species and functional groups. An two-factorial experiment suggested that the negative impacts of cutting and competition on weed growth were mainly additive. 4) Special measurements of weed seed predation in the field experiment showed positive correlations with vegetation cover, indicating that this ecosystem service may be particularly fostered by perennial crops. Consistent preferences of seed predators for certain weed species indicates that seed predation may be another cause of the observed weed community shifts.

La rotation de cultures peut être utilisée pour empêcher la sélection continue d’espèces adventices adaptées à un type de culture. Elle pourrait favoriser la gestion des adventices, l’économie d’herbicides et la biodiversité. Les successions de cultures simples d’aujourd’hui pourraient être diversifiées par des cultures fourragères pérennes. Les impacts des ces cultures sur les adventices ont été étudié utilisant quatre approches : 1) Des relevés d’adventices sur 632 champs dans l’ouest de la France ont montré que la composition spécifique varie le plus entre des cultures fourragères pérennes et des cultures annuelles. Une comparaison des champs avant, pendant, et après des cultures fourragères pérennes a suggéré que la composition des communautés varie d’une manière cyclique pendant ces rotations. Plusieurs espèces problématiques dans des cultures annuelles ont été supprimées pendant et après les cultures pérennes, mais l’apparition d’autres espèces a produit une diversité de plantes comparable, voire supérieure. 2) Une expérimentation au champ de trois ans avec des modes de gestion contrastés a permis d’étudier les mécanismes sous-jacents: L’absence de travail du sol a réduit la levée des adventices, mais a augmenté la survie des plantes adultes. Le couvert végétal permanent et les fauches fréquentes ont réduit la croissance, la survie des plantes et la production de graines. 3) Des expérimentations sous serre analysant la croissance poste fauche de plantes individuelles ont montré des différences importantes entre espèces et groupes fonctionnels. Une expérimentation à deux facteurs a suggéré que les impacts négatifs de la fauche et de la compétition sur la croissance des adventices ont été additifs. 4) Des mesures spéciales de prédation de graines d’adventices sur l’expérimentation au champ ont montré des corrélations positives avec le couvert végétal et la prédation, indiquant une importance particulière de ce service écosystémique dans des cultures pérennes. La préférence des graines de certaines espèces montre que la prédation de graines peut être une autre cause des changements de communautés d’adventices. / Crop rotation may be used to prevent the continuous selection of particular weed species adapted to one crop type. This might be useful for weed management, economy in herbicide applications and promoting biodiversity. Common simple crop sequences might be diversified by introducing perennial forage crops. Impacts of such perennial crops on weeds were studied with four approaches : 1) Large-scale weed surveys in 632 fields in western France showed that weed species composition differed most strongly between perennial alfalfa crops and annual crops. Comparisons of fields before, during and after perennial alfalfa suggested that community composition varies in a cyclic way during such crop rotations. Several weed species problematic in annual crops were suppressed during and after perennial crops, but the appearance of other species led to equal or even higher plant diversities. 2) A 3-year field experiment with contrasting crop management options allowed an investigation of the underlying mechanisms for this: The absence of soil tillage reduced weed emergence but increased the survival of established plants. The permanent vegetation cover and frequent hay cuttings reduced weed growth, plant survival and seed production. 3) Greenhouse experiments testing the regrowth ability of individual plants after cutting showed strong differences between species and functional groups. An two-factorial experiment suggested that the negative impacts of cutting and competition on weed growth were mainly additive. 4) Special measurements of weed seed predation in the field experiment showed positive correlations with vegetation cover, indicating that this ecosystem service may be particularly fostered by perennial crops. Consistent preferences of seed predators for certain weed species indicates that seed predation may be another cause of the observed weed community shifts. / Fruchtfolgen können dazu dienen, die kontinuierliche Selektion von Unkrautarten zu verhindern, die an eine bestimmte Kultur angepasst sind. Dies könnte dem Unkrautmanagement, der Einsparung von Herbiziden, und der Biodiversität dienen. Heutige, sehr einfache Furchtfolgen könnten durch mehrjährige Futterkulturen diversifiziert werden. Die Einflüsse solcher mehrjähriger Kulturen auf Unkräuter wurden in vier Ansätzen untersucht : 1) Vegetationsaufnahmen auf 632 Feldern in Westfrankreich zeigten, dass die Unkrautzusammensetzung zwischen mehrjährigen Futterkulturen und einjährigen Kulturen stark variiert. Der Vergleich von Feldern vor, während und nach mehrjährigen Futterkulturen legte nahe, dass die Pflanzengemeinschaft während solcher Fruchtfolgen zyklisch variiert. Mehrere problematische Unkrautarten wurden während und nach den mehrjährigen Kulturen zurückgedrängt. Das Auftauchen anderer Arten führte jedoch zu einer gleichbleibenden oder leicht erhöhten Pflanzenvielfalt. 2) Ein dreijähriger Feldversuch mit verschiedenen Bearbeitungsoptionen ermöglichte es, die zugrunde liegenden Mechanismen zu untersuchen: Die fehlende Bodenbearbeitung hat das Auflaufen der Unkräuter reduziert und das Überleben der adulten Pflanzen erhöht. Die permanente Vegetationsbedeckung und die häufigen Heuschnitte haben das Wachstum, das Überleben und die Samenproduktion vermindert. 3) Gewächshausexperimente zum Nachwachsen von Unkrautpflanzen nach Heuschnitten zeigten große Unterschiede zwischen verschiedenen Arten und funktionellen Gruppen. Ein Experiment mit zwei Faktoren lässt vermuten, dass die negativen Effekte der Schnitte und der Konkurrenz auf das Unkrautwachstum sich addieren. 4) Spezielle Messungen der Prädation von Unkrautsamen auf den untersuchten Feldern zeigten positive Korrelationen mit der Vegetationsbedeckung, was auf eine besondere Wichtigkeit dieser Ökosystemdienstleistung in ausdauernden Kulturen hindeutet. Die Präferenz von bestimmten Samenarten deutet darauf hin, dass Samenprädation ein weiterer Grund für die beobachteten Änderungen der Unkrautgemeinschaften sein kann.

Les systèmes de culture en Protection Intégrée de Cultures contre les adventices (systèmes PIC-adventices) sont d’un grand intérêt pour réduire significativement l’utilisation des herbicides en les substituant par une combinaison complexe de différentes techniques culturales. L’objectif principal de notre travail a été de contribuer au volet environnemental de l’évaluation multicritère des systèmes PIC-adventices mis en place depuis 2000 sur le domaine expérimental de l’INRA à Bretenière (21). La variabilité des propriétés physiques et hydrodynamiques qui interviennent dans le transfert de l’eau dans le sol a été étudiée dans les 5 systèmes de culture présents sur le site (1 système de référence et 4 systèmes PIC-adventices). Le premier volet du travail a consisté à étudier la rétention de l’eau dans la couche du sol travaillée dans les 5 systèmes de culture. La rétention de l’eau a été comparée dans des échantillons de sol dont la structure a été soit « conservée », soit « remaniée » afin d’identifier la part de la variabilité liée aux caractéristiques physiques intrinsèques du sol de celle induite par les pratiques culturales. Les résultats ont montré que le sol du système de culture avec suppression totale des herbicides, associant une teneur élevée en argile, une teneur moyenne en C organique et un travail superficiel intensif, présentait les valeurs les plus élevées de rétention de l’eau dans la couche de sol travaillée. Le second volet a porté sur l’étude de la variabilité de la conductivité hydraulique proche de la saturation, K(h), aux échelles de l’année culturale et du profil du sol. Pour cette étude, 3 systèmes PIC-adventices (système de culture en semis direct et systèmes de culture avec travail du sol modéré à intensif) ont été caractérisés pendant 2 ou 3 années culturales consécutives, en considérant 3 profondeurs de sol incluant la couche du sol travaillée et l’horizon sous-jacent. Les résultats n’ont montré aucune variabilité de K(h) à l’échelle verticale. Par contre, ils ont mis en évidence les effets à long terme des systèmes PIC-adventices sur les valeurs de K(h). Enfin, le dernier volet a étudié la dynamique de l’eau dans le sol de 2 systèmes PIC-adventices contrastés (système avec conduite du sol en semis direct et système sans herbicide avec travail du sol intensif) en suivant une approche d’hydraulique agricole et par le biais de la caractérisation in situ du potentiel matriciel de l’eau dans le sol et de la mesure des quantités d’eau collectée au moyen de lysimètres à mèche. L’approche utilisée a permis la description de la dynamique de l’eau du sol pendant la période de drainage, la validation du fonctionnement des lysimètres à mèche et le calcul de la surface de sol équivalente qui contribuait au transfert de l’eau dans ces dispositifs. Ces premiers résultats prometteurs nécessiteront néanmoins d’être validés au cours de prochaines campagnes de mesure. L’ensemble des résultats recueillis constituent une première évaluation du fonctionnement physique et hydrodynamique du sol des systèmes PIC-adventices présents sur le dispositif expérimental. La base de données constituée permettra ultérieurement la poursuite du travail engagé en modélisant le transfert de l’eau, de solutés, de pesticides ou d’autres polluants dans le sol de ces systèmes PIC-adventices. / Integrated weed management (IWM)-based cropping systems are of great interest due to their significant reduction of herbicide inputs by using a complex combination of agricultural techniques. The main objective of our work was to contribute to the environmental dimension of the multi-criteria evaluation of IWM-based cropping systems which were set up since 2000 on the experimental domain of the National Institute for Agronomical Research (INRA) to Bretenière (France). The variability of physical and hydrodynamic soil properties involved in the transfer of water through the vadose zone was studied for the 5 cropping systems present on the experimental site (1 reference cropping system and 4 IWM-based cropping systems). The first section of our study consisted in studying the water retention of the tilled soil layers for the 5 cropping systems. Water retention values were compared between undisturbed and repacked soil samples in order to distinguish the part of the variability due to the intrinsic soil properties from the one induced by the agricultural practices. Our results show that the soil from cropping system without any herbicides, associating high clay content, average organic C content and intensive superficial tillage, may increase soil water retention in the tilled soil layer. The second section carried on the study of the annual and vertical variability of the near-saturated hydraulic conductivity, K(h). For this study, 3 IWM-based cropping systems (no-tilled cropping system and moderate to intensive tilled cropping systems) were characterized during 2 or 3 consecutive years and considering 3 depths including the soil tilled layer and the underlying no-tilled layer. The results showed no vertical variability of K(h). However, they highlighted the long-term effect of IWM-based cropping systems on K(h). Finally, the third section studied the soil water dynamics of 2 contrasted IWM-based cropping systems (no-tilled and zero herbicide cropping systems with intensive tillage) by using ,a hydraulic agricultural approach and by the means of the in situ characterization of soil matric potential and the measure of the amounts of water collected with wick lysimeters. This approach allowed the description of the water dynamics during the drainage season, the validation of the wick lysimeters functioning and the calculation of the surface contributing to water flows collected by the wick lysimeters. These promising results will need to be validated during forthcoming campaigns of measurement. All data collected provided constitute a first evaluation of the soil physical and hydraulic functioning of IWM-based cropping systems present on the experimental site. The established database will allow later the pursuit of work by modeling the water transfer and the transport of solutes, pesticides or other pollutants through the vadose zone of these IWM-based cropping systems.