Dissertations / Theses on the topic 'Weed control'

Paraquat and diquat were effective against weeds immediately at 3 DAT. Glyphosate, sulfosate, and glufosinate exhibited activity against the weeds at 7 to 10 DAT. Paraquat provided the most complete weed control of most weeds at 10 to 16 DAT. Most of the diquat treated weed recovered and exhibited regrowth after 22 DAT. Glufosinate did not provide adequate control of most weeds at 22 DAT similar to diquat. Glyphosate and sulfosate were nearly equivalent at 0.50 and 2.0 lb AI/A against most weeds at most of the rating dates.

Additive weed competition experiments were undertaken to study the effect of lamb's-quarters (Chenopodium album) interference on direct seeded broccoli. Lamb's-quarters (3, 8, 10, 12, and 15 plants m⁻²) began to affect broccoli growth 28 to 36 days after seeding. Decreases in crop growth increased with weed density as time after seeding increased. Yield data were fitted to a rectangular hyperbolic model which indicated that even one lamb's-quarters plant m⁻² could reduce total yield by 18 to 20 percent and marketable yield (head >10 cm across) by 22 to 37 percent. Lamb's-quarters reduced total yield by reducing average head weight and not by lowering the number of heads per plot. On the other hand, the weed reduced marketable yield by reducing both the average head weight and the number of heads per plot. The feasibility of using liquid ammonium nitrate as a post-emergent weed control spray in cole crops was studied. The relative susceptibility of different weed species grown by themselves (1989) and with two crops (broccoli and onion; 1987) to ammonium nitrate (800 L ha⁻¹; 0, 7.5, 10, 15, 20 percent N) burning was investigated. The fertilizer controlled shepherd's-purse (Capsella bursa-pastoris), chickweed (Stellaria media), cudweed {Gnaphalium uliginosum), and redroot pigweed (Amaranthus retroflexus), but not lamb's-quarters, purslane (Portulaca oleracea), and annual bluegrass (Poa annua). Corn spurry (Spergula arvensis) varied in its tolerance. Although weed populations were reduced by 70 percent in 1987, the remaining weeds competed so strongly with the onion and broccoli that the crop plants did not reach a harvestable size. The large initial weed population (799 plants m⁻²), the large number of tolerant weeds present, and the possible recovery of some of the susceptible weeds may all have been factors responsible for crop failure. The effect of different shepherd's-purse densities (52 to 988 plants m⁻²) on the degree of ammonium nitrate (800 L ha⁻¹; 20 percent N) control in broccoli was also studied. The initial weed control achieved was reduced over time either because some weeds counted as dead had recovered or new plants were being recruited to the population through seed germination. Although the maximum density of shepherd's-purse plants that survived was 219 plants m⁻², these plants did not significantly reduce crop yield possibly because shepherd's-purse is not a very competitive species and all the surviving weeds had been damaged to varying degrees, further reducing their competitive ability. The relative susceptibility of various crop cultivars to ammonium nitrate (800 L ha⁻¹; 0, 10, 15, 20 percent N) burning was also studied. In 1987, the growth rates of 'Lunet' (Brussels sprouts), ‘SGI' (broccoli), 'Elgon' (cauliflower), and 'Matra' (cauliflower) initially decreased but the plants recovered; they were largely unaffected in 1988. The growth rates of 'White Lisbon' (onion), 'Emperor' (broccoli), and 'Early Marvel' (cabbage) were largely unaffected in either year. Although some cultivars had shown initial signs of lower growth rates, there was no decrease in crop yield. Leaf surfaces of tolerant and susceptible crop and weed species were examined by scanning electron microscopy to determine the basis of ammonium nitrate selectivity. Leaf surfaces of tolerant species were completely covered with a crystalline wax layer, while susceptible species had little or no epicuticular wax. Cellulose acetate was used to remove the epicuticular wax from cabbage leaves. The stripped leaves showed far greater ammonium nitrate retention and salt injury than unstripped leaves, demonstrating the importance of the epicuticular wax in providing protection against ammonium nitrate injury. Trichomes, observed on the leaf surfaces of some susceptible species, may further increase ammonium nitrate retention and, therefore, salt injury.
Land and Food Systems, Faculty of
Graduate

Efficacy of the grass herbicides showed that Select (clethodim) and BAS-620 (BASF Corporation) at rates as low as 0.1 lb AI/A were nearly comparable in controlling 2 leaf stage of growth watergrass or when applied a week later on 3-4 inch tall watergrass. Fusilade DX (fluazifop-p-butyl) was intermediate in controlling grasses and 0.188 lb AI/A was necessary to give equivalent control of larger grasses as compared to the 0.1 lb AI/A rate that gave acceptable control of smaller grasses. Poast (sethoxydim) at 0.188 lb AI/A gave acceptable control of small grasses but lower rates or later timed applications were not as efficacious.

Pendimethalin (Prowl7) and oxyfluorfen (Goal7) applied preemergence (PREE) caused minimal injury and gave very good weed control (>90%). Goal and sulfentrazone alone applied postemergence (POST) gave very good weed control at 6 WAT. The combination of Prowl followed by Goal or sulfentrazone gave complete control of all weeds. Goal and sulfentrazone applied POST following PREE treatments gave nearly complete weed control with good crop safety. Clomazone (Command7) caused significant crop injury and stand reduction when applied PREE. Metribuzin (Sencor7) applied POST completely reduced the crop stand and gave complete control of all weeds. Metolachlor (Dual7), dimethenamid (Frontier7), Sencor, flumetsulam, and imazamox generally did not provide acceptable control of Chenopodium desiccatum (narrowleaf lambsquarters) and Sonchus oleraceus (sowthistle). Bentazon (Basagran7), acifluorfen (Blazer7), and fomesafen (Reflex7) were not effective against narrowleaf lambsquarters but gave adequate control of the other weeds.

Preemergence herbicide treatments metolachlor (Dual®) and pendimethalin (Prowl®) gave better than acceptable weed control ( >85 %) of prostrate and tumble pigweeds, puncturevine, common purslane, and groundcherry in cantaloupes. Preplant incorporated treatments provided less than adequate control of pigweeds and groundcherry. Bentazon (Basagran®) applied postemergence gave good control of pigweeds but groundcherry control was marginal. Napropamide (Devrinol®), trifluralin (Treflan®), and DCPA (Dacthal (D) caused cantaloupe stand reduction and injury. Bensulide (Prefar®) and Basagran® were safe when applied on cantaloupes.

The sequence of preemergence (PREE) herbicide metolachlor (Dual II®) followed by postemergence (POST) herbicide mixture of primisulfuron plus prosulfuron (Exceed®) provided season-long near complete weed control in sweet corn. Preplant incorporated (PPI) treatments of dimethenamid (Frontier®), EPTC plus safener (Eradicane®), and herbicide mixture FOE 5043 plus metribuzin (Axiom®, Bayer) provided effective weed control for most of the season. Similar effective weed control was observed for PREE treatments of pendimethalin (Prowl®), Frontier, and Axiom.

Bensulide (Prefar®), clomazone (Command®), sulfentrazone (FMC), and halosulfuron (FMC) treatments applied preemergence (PREE) gave very good weed control of prostrate pigweed (Amaranthus blitoides), lambsquarters (Chenopodium album), and common purslane (Potulaca oleracea) at 5 weeks after treatment (WAT). Bentazon (Basagran ® and halosulfuron applied postemergence (POST) alone were marginally effective at less than 85% against the pigweed species at 2 WAT and controlled lambsquarters and common purslane. POST treatments following PREE treatments were highly effective to control most weeds. Watermelon injury was acceptable for Command and halosulfuron treatments. Basagran caused slight injury when applied POST on the watermelons. Carfentrazone was not effective against the weeds present in this test site and was safe on the crop. The greatest number of marketable watermelons were harvested from plots having treatments that provided effective weed control. Command plus Prefar PREE followed by Basagran POST and Prefar PREE followed by halosulfuron POST treated watermelons yielded high numbers of marketable fruit.

Clomazone (Command®), bensulide (Prefar®), sulfentrazone, and halosulfuron treatments applied preemergence (PREE) provided very good control of prostrate pigweed (Amaranthus blitoides), lambsquarters (Chenopodium album), and common purslane (Portulaca oleracea) at better than 90% at 5 weeks after treatment (WAT). Halosulfuron was effective in controlling all weeds better than 90% at 7 WAT Carfentrazone was not effective against most of the weeds present in the test but appeared to be safe on cantaloupe. Postemergence (POST) treatments alone did not provide acceptable control of pigweeds but controlled lambsquarters and common purslane at 2 WAT. Halosulfuron and bentazon (Basagran®) applied POST following PREE treatments controlled most of the weeds better than 90% through 7 WAT. Cantaloupe yields were highest with good weed control provided by PREE treatments followed by POST herbicide applications. Basagran at 0.50 lb /A injured cantaloupe after applications but yields were not affected compared to the untreated check. Command, sulfentrazone, and halosulfuron caused cantaloupe injury after PREE applications. Basagran caused substantial crop injury after POST applications.

Field experiments were conducted from 1989 to 1991 to evaluate several sulfonylurea herbicides, glyphosate and clopyralid for the control of bunchberry and other hexazinone tolerant weeds in lowbush blueberry. Broadcast applications of chlorosulfuron, metsulfuron and glyphosate reduced bunchberry densities at all application dates, though crop damage and subsequent yield reductions were unacceptable. Glyphosate was very effective in controlling a large number of plant species when applied as a spot spray treatment. Tribenuron and DPX R9674 were effective in suppressing bunchberry stem densities at all application dates, without major adverse effects on blueberry, and also controlled a large number of hexazinone tolerant weeds when applied as a spot spray treatment. Clopyralid, at rates as low as 100 g a.i. ha$ sp{-1}$, was very effective as a broadcast treatment for the control of tufted vetch, although problems with crop tolerance and yield reductions were evident in some instances. Clopyralid did not control a large number of hexazinone tolerant species when applied as a spot spray treatment.

Onions treated at the 2-leaf stage of growth with the 3rd leaf just beginning to emerge with postemergence herbicides bromoxynil (Buctril®) and oxyfluorfen (Goal®) exhibited slight injury at 11 days after treatment (DAT) but had recovered to show no injury at 1 month after treatment (MAT). Annual yellow sweetclover (Melilotus officinalis) was the predominat weed in the test site and early ratings showed that Goal at 0.25 lb a.i. /A and Goal plus Buctril gave marginally acceptable control at 80 %. Buctril alone did not control clover. At 1 MAT, the clovers had recovered from the initial injury and the level of control had declined to become unacceptable.

An exploratory field study provided results of postemergence herbicide weed control efficacy and broccoli tolerance. Pyridate (Tough®), clopyralid (Stinger®), and oxyfluorfen (Goal®) did not cause any crop stand reduction compared to bentazon (Basagran®) that completely reduced the broccoli stand. Tough and Goal at the lower rates tested caused marginally acceptable broccoli injury. Goal effectively controlled pigweed species (Amaranthus sp.), groundcherry (Physalis wrightii), and purslane (Portulacç oleracea). Tough gave good control of pigweed and purslane but not groundcherry. Stinger was safe on broccoli and marginally controlled groundcherry. In a second field study, Tough and Goal were evaluated for cheeseweed control. Goal marginally controlled cheeseweed at all rates tested and caused marginally acceptable injury at the two lowest rates. Tough was relatively safe at the lower rates but did not adequately control the cheeseweed.

Exploratory field studies conducted in broccoli showed that clomazone (Command®) and isoxaben (Gallery®) were extremely phytotoxic to broccoli when applied preemergence (PREE) on the soil surface after planting. Both offered good weed control of the weeds present. Napropamide (Devrinol®) caused moderate crop injury and marginally acceptable weed control.

The addition of an adjuvant, Agridex, to halosulfuron or Basagran7 (bentazon) did not increase crop injury significantly compared to treatments without Agridex. The addition of Agridex to halosulfuron slightly improved morningglory (Ipomoea hederacea) control compared to without the use of an adjuvant. Bentazon at 1.0 lb/A plus Agridex gave very good morningglory control at 92%. At 2 WAT on 20 Aug, cantaloupe injury decreased for halosulfuron and bentazon treatments. A second application of halosulfuron at 0.05 lb/A did not cause additional crop injury.

The soil applied herbicides EPTC plus safener (Eradicane7), metolachlor (Dual)7, dimethenamid (Frontier7), pendimethalin (Prowl7), and fluthiamide/metribuzin (Axiom7) applied at planting time all provided very good weed control of pigweeds (Amaranthus spp.), grasses, and puncturevine (Tribulus terrestris). Most of the soil applied followed by POST combination treatments gave very good control (>90%) of lambsquarters (Chenopodium spp.), pigweeds, and groundcherry (Physalis wrightii). POST treatments bentazon (Basagran7), dicamba (Clarity7), and diflufenzopyr plus dicamba (Distinct7) applied alone were not as effective as combinations with soil applied herbicides.

Halosulfuron at rates ranging from 0.05 to 0.10 lb AI/A with no adjuvant added to the POST application spray did not cause any injury to watermelons. Halosulfuron did not appear to cause significant crop injury earlier in the season to reduce marketable fruit yield at harvest. Halosulfuron was highly effective against London rocket but did not control purslane or groundcherry. Weed control efficacy was improved significantly when Latron CS-7 or Activator-90 was added to halosulfuron at either 0.05 or 0.075 lb AI/A. LI-700 did not improve the activity of halosulfuron over the treatments without an adjuvant.

Two field studies were conducted to evaluate metolachlor (Dual®) for preemergence weed control in spinach to provide support to gain registration through the IR-4 program. Dual® at 1.0 to 1.5 lb a.i./A gave acceptable control of London rocket, black mustard, lambsquarters, and knotweed. Nettleleaf goosefoot, cheeseweed, and yellow sweetclover control was not acceptable. In one test, dimethenamid (Frontier® or SAN -582H, Sandoz) controlled London rocket, lambsquarters, knotweed, and goosefoot at 0.25 lb ai. /A and did not control cheeseweed or sweetclover. Spinach was not injured by Dual® or Frontier®.

Preplant incorporated and/or preemergence herbicide treatments including metolachlor (Dual®), EPTC with safener (Eradicane®), cyanazine (Bladex®), pendimethalin (Prowl®), and tank-mix combinations provided good (88 %) to excellent ( >98 %) weed control of prostrate and tumble pigweeds and purslane in sweet corn.

Season-long near complete weed control in sweet corn was achieved with preemergence (PREE) herbicide applications of pendimethalin (Prowl®), metolachlor (Dual®), or thiafluamide/metribuzin (Axiom®) followed by postemergence applications of bentazon (Basagran®) or dicamba (Banvel®). Basagran applied alone POST gave very good control ( >93 %) of lambsquarters (Chenopodium album) and purslane (Portulaca oleracea) but did not adequately control tumble pigweed (Amaranthus albus). Prowl applied alone PREE gave acceptable control of most weeds. POST applications of prosulfuron/primisulfuron (Exceed®) caused moderate corn injury by shortening internodes and overall plant height and slight foliar chlorosis.

The postemergence (POST) herbicide treatments did not cause any crop stand reduction following applications. Bentazon (Basagran®) at 0.50 lb AI /A caused marginally acceptable injury on the cantaloupe leaves. At 2 weeks after treatment (WAT), the amount of injury decreased and cantaloupe treated with Basagran at 1.0 lb AI/A showed marginally acceptable injury symptoms. Halosulfuron (Monsanto) at 0.05 to 0.10 lb AI/A caused slightly more injury (10 to 17 %) with increasing rates. Basagran at 1.0 lb Al/A gave good control ( >90 %) of morningglory and was marginal in controlling morningglory at 0.75 lb AI/A Halosulfuron at 1 WAT was marginal in controlling morningglory but improved to give acceptable control at 2 WAT. Fewer and smaller plants were removed by hand-hoeing from Basagran and halosulfuron treated plots compared to the untreated check.

La depredació de llavors pot causar pèrdues significatives de llavors de males herbes en sistemes agrícoles i, per tant, pot contribuir al control de les arvenses. Actualment, el coneixement existent sobre la identitat i la contribució relativa dels diferents depredadors de llavors, i dels factors que limiten aquesta depredació és escàs. Aquest estudi té com a objectiu contribuir a augmentar aquest coneixement pel cas específic del cereals d’hivern a les zones semi-àrides del nord-est espanyol. Tradicionalment, aquesta zona ha estat de secà i caracteritzada per un intens treball del sòl. Però, amb el temps, una superfície creixent d’aquesta zona ha anat transformant-se al regadiu; i a la restant zona de secà, la sembra directa ha anat creixent en superfície. Per aquest motiu, es va estudiar l’impacte d’aquestes dues transformacions en la depredació de llavors de males herbes. L’estudi ha mostrat que en les àrees de secà, les formigues recol·lectores de l’espècie Messor barbarus estan contribuint d’una manera substancial al control de males herbes al emportar-se grans quantitats de llavors de males herbes durant els mesos de primavera i estiu. El conreu del sòl a l’estiu va fer decréixer les taxes de depredació i va provocar l’enterrament de la majoria de les llavors que es trobaven a la superfície del sòl, impedint, d’aquesta manera, la continuació de la depredació. Igualment, el conreu del sòl també va fer disminuir la densitat de nius de formigues als camps conreats en comparació amb els camps de sembra directa. L’expansió de l’àrea en sembra directa hauria de donar lloc a unes taxes elevades de control natural de les males herbes en una àmplia zona. Per contra, el reg per inundació va eliminar completament les formigues recol·lectores i va portar casi a la completa pèrdua d’aquest servei de l’ecosistema. Encara que caràbids i ratolins eren presents als marges dels cultius, les taxes de depredació a l’interior dels camps van ser extremadament baixes. Les causes d’aquesta falta de depredació encara no són conegudes i haurien de ser estudiades en el futur. Les densitats de nius de formigues recol·lectores varien enormement entre camps i, per tant, s’espera que les taxes de depredació variïn en conseqüència. Les causes d’aquesta variabilitat no van poder ser identificades. La densitat de nius de M. barbarus no es van poder correlacionar amb les característiques del sòl més comunes, amb paràmetres topogràfics ni amb practiques de maneig, excepte amb el nombre d’anys des de l’adopció de la sembra directa. La densitat de nius de M. barbarus va ser màxima després de 11 – 12 anys de sembra directa. A part d’això, no es van poder formular recomanacions per incrementar les densitats de nius en aquelles zones en les que són baixes. L’èxit de la depredació de llavors de males herbes com a servei de l’ecosistema també depèn de l’habilitat dels depredadors de respondre d’una manera directament denso-dependent a densitats creixents de llavors. La resposta a diferents densitats de llavors per part de ratolins granívors va ser investigada en camps de blat del nord-est d’Alemanya. Els ratolins van respondre a densitats creixents de llavors d’una forma directament denso-dependent i per tant, s’espera que puguin ser capaços de controlar d’una manera efectiva els rodals de males herbes. Les respostes a densitats creixents de llavors per part de ratolins i formigues recol·lectores en les condicions del nord-est de l’estat espanyol estan essent investigades actualment. Es possible que les formigues recol·lectores puguin, ocasionalment, destruir llavors de cultiu. Tanmateix, les pèrdues de rendiment causades per M. barbarus van ser negligibles en la majoria dels casos (0.4% del rendiment) i poden ser explicades per la densitat de nius, la mida d’aquests i el nombre d’anys que el camp porta en sembra directa. Ocasionalment, es van registrar pèrdues de rendiment més altes (9.2% del rendiment). Les causes d’aquestes pèrdues han d’esser estudiades en detall en el futur. Aquest estudi exemplifica la fortalesa i la vulnerabilitat d’un servei del ecosistema. A les zones de secà del nord-est espanyol, s’estan donant, d’una forma natural, altes taxes altes de depredació de llavors de males herbes que contribueixen substancialment al control d’aquestes herbes. Tanmateix, aquest servei es pot perdre fàcilment tal com il·lustren l’absència de depredació de llavors en les àrees regades a manta i la resposta de les formigues recol·lectores a un excessiu treball del sòl. Les densitats de nius de formigues recol·lectores existents podrien ser preservades limitant el nivell de pertorbació del sòl. En regions semi-àrides, on la producció de cereals és marginalment rendible degut a l’escassetat d’aigua, la preservació del control natural de les males herbes dut a terme per les formigues recol·lectores és necessària per preservar la sostenibilitat del sistema
La depredación de semillas puede causar pérdidas significativas de semillas de malas hierbas en sistemas agrícolas y, por lo tanto, puede contribuir al control de dichas hierbas. Actualmente, el conocimiento existente acerca de la identidad y contribución relativa de los depredadores de semillas, y de los factores que limitan esta depredación es escaso. Este estudio tiene como objetivo contribuir a incrementar dicho conocimiento para el caso específico de los cereales de invierno en las zonas semi-áridas del noreste español. Tradicionalmente, esta zona ha sido de secano y caracterizada por un intenso laboreo del suelo. Sin embargo, la superficie de regadío ha ido incrementándose en la zona y, en la zona de secano restante, la siembra directa también ha ido en aumento. Por este motivo, se estudió el impacto de estas dos transformaciones en la depredación de semillas de malas hierbas. El estudio ha mostrado que, en las zonas de secano, las hormigas granívoras de la especie Messor barbarus están contribuyendo de una forma sustancial al control de malas hierbas, al llevarse grandes cantidades de semillas de malas hierbas durante los meses de primavera i verano. El laboreo del suelo en verano redujo las tasas de depredación de semillas y provocó el enterramiento de la mayoría de las semillas presentes en la superficie del suelo, lo que impidió, en gran medida, la continuación de la depredación. De la misma forma, el laboreo del suelo también disminuyó la densidad de nidos de hormigas en los campos cultivados en comparación con los campos de siembra directa. La expansión del área en siembra directa debería dar lugar a tasas elevadas de control natural de malas hierbas en una amplia zona. Contrariamente, el riego por inundación eliminó por completo a las hormigas granívoras y llevó a la casi completa desaparición de este servicio del ecosistema. Aún cuando carábidos y pequeños roedores estaban presentes en los márgenes de los cultivos, las tasas de depredación en el interior de los campos fueron extremadamente bajas. Las causas de esta falta de depredación son aún desconocidas y deberían ser estudiadas en el futuro. Las densidades de nidos de hormigas granívoras varían enormemente entre campos y, por lo tanto, se espera que las tasas de depredación también varíen en consecuencia. Las causas de dicha variabilidad no pudieron ser identificadas. Las densidades de nidos de M. barbarus no se pudieron correlacionar con las características del suelo más comunes, con parámetros topográficos ni con las prácticas de manejo del cultivo, exceptuando en número de años desde la adopción de la siembra directa. Las densidades de nidos de M. barbarus fueron máximas después de 11-12 años de siembra directa. A parte de esto, no se pudieron formular recomendaciones para incrementar las densidades de nidos en aquellas zonas en las que son bajas. El éxito de la depredación de semillas de malas hierbas como servicio del ecosistema depende también de la habilidad de los depredadores para responder de una forma directamente denso-dependiente a densidades crecientes de semillas. La respuesta a diferentes densidades de semillas por parte de roedores granívoros fue investigada en campos de trigo del noreste de Alemania. Los roedores respondieron a densidades crecientes de semillas de una forma directamente denso-dependiente, por lo que se espera que puedan ser capaces de controlar de una forma efectiva, los rodales de malas hierbas. Las respuestas a densidades crecientes de semillas por parte de roedores y hormigas granívoras en condiciones del noreste español están siendo investigadas en la actualidad. Es posible que las hormigas granívoras puedan, ocasionalmente, destruir semillas de cultivo. Sin embargo, las pérdidas de rendimiento causadas por M. barbarus fueron insignificantes en la mayoría de casos (0.4% del rendimiento) y pueden ser explicadas por la densidad de nidos, su tamaño y el número de años de siembra directa del campo. Ocasionalmente, se registraron pérdidas de rendimiento más elevadas (9.2% del rendimiento). Las causas de estas pérdidas deben ser estudiadas en más detalle en el futuro. Este estudio ejemplifica la fortaleza y la vulnerabilidad de un servicio del ecosistema. En las zonas de secano del noreste español se están dando, de forma natural, altas tasas de depredación de semillas de malas hierbas, que están contribuyendo sustancialmente al control de las malas hierbas. Sin embargo, este servicio puede perderse fácilmente tal como muestran la ausencia de depredación de semillas en las áreas regadas a manta y la respuesta de las hormigas granívoras a un excesivo laboreo del suelo. Las densidades de nidos de hormigas existentes podrían ser preservadas limitando el grado de perturbación del suelo. En regiones semi-áridas, donde la producción de cereales es marginalmente rentable debido a la escasez de agua, el mantenimiento del control natural de las malas hierbas por parte de las hormigas granívoras se hace necesario para preservar la sostenibilidad del sistema.
Seed predation can cause significant losses of weed seeds in agricultural systems and can, thus, contribute to weed control. Knowledge on the identity and relative contribution to weed control by various seed predators, and on factors limiting seed predation is currently lacking. This study aimed at casting light on these aspects for the specific case of winter cereal fields in semi-arid north-eastern Spain. This area is traditionally managed without irrigation and with tillage. However, an ever increasing proportion of the arable land is being irrigated and the remainder of the rain fed land is managed without tillage. The impact of tillage and irrigation on weed seed predators and seed removal rates were, therefore, studied. The study showed that in the rain-fed area, Messor barbarus harvester ants are contributing substantially to weed control by removing large quantities of weed seeds during spring and summer. Tillage during summer decreased predation rates and buried most of the weed seeds located on the soil surface, thus preventing further seed removal. Tillage also decreased the number of harvester ant nests compared to no-till fields. The expansion of the area that is managed with minimum and no-till should result in high natural weed control level over a large area. In contrast, inundative irrigation completely eliminated harvester ants, and led to the almost complete loss of this ecosystem service. Although carabid beetles and rodents were present in the field edges, predation rates in the field interior were extremely low. Causes for the lack of seed predation are still unknown and should be further investigated. Densities of harvester ant nests varied enormously between fields; concomitant weed seed predation rates are expected to vary accordingly. Causes for this variability could not be identified. Harvester ant nest density did not correlate with the most common soil characteristics, topographic variables or crop and management practices, with the exception of the number of years of no-till. Harvester ant density was highest after 11-12 years of no-till. Other than that, no recommendations could be formulated to increase nest densities in those areas were they are low. Success of weed seed predation as an ecosystem service also depends on the ability of predators to respond in a direct density dependent way to increasing seed densities. The density dependent response of granivorous rodents to seed patches with varying density was investigated in winter cereal fields of north-eastern Germany. Rodents responded in a direct density dependent way to increasing seed densities and are, therefore, expected to effectively control weed patches. The density dependent response by harvester ants and granivorous rodents in cereal fields in NE Spain are currently being investigated. It is feasible that harvester ants occasionally destroy crop seeds and cause crop damage. Yield loss caused by M. barbarus was, however, negligible (0.4 % of yield), and was explained by nest density, nest size and number of years without tillage. Based on these results, damage caused by harvester ants was more than offset by the benefits with regard to weed control. However, occasional higher yield losses (max. 9.2%) were recorded and the conditions leading to higher losses should be investigated further. This study exemplifies both the strength and vulnerability of an ecosystem service. High weed seed predation by harvester ants is occurring naturally in rain-fed cereals in north-eastern Spain and contributes substantially to weed control. However, this service can easily be lost as illustrated by the absence of seed predation in the flood irrigated areas and the response of harvester ants to excessive tillage. Existing densities of harvester ant nests could be preserved by limiting the level of soil disturbance. In semi-arid regions, cereal production is marginally cost effective due to limited water availability and, therefore, preserving natural weed control by harvester ants is needed in order to preserve the sustainability of the system.

In the Northern Great Plains (NGP), weed management within organic systems remains a challenge. Experiments were conducted at two distinct sites in North Dakota to investigate effects of deep mulch no-till (NT) on soil quality indices, weed densities, and weed seedbank densities. We hypothesized that alfalfa mulch no-till and arbuscular mycorrhizal fungi (AMF) inoculant would be associated with reductions in weed densities and improvements to soil quality and vegetable yield. NT treatments were associated with reductions in weed densities and time required for weeding, with improvements in soil quality, such as increased AMF biomass, and yield for snap pea, onion, beet, and butternut squash compared to tilled treatments. Our findings suggest deep mulch no-till using alfalfa residue may be a viable option for small-scale organic vegetable producers in the NGP. Additional research is required to determine costs associated with sowing, harvesting, baling, and applying alfalfa mulch compared to tilling.

The distribution of weed species and the herbicides and cultural practices used to control weeds in Arizona cotton fields were surveyed in 1995 and 1996. The most common weeds were purple nutsedge, bermudagrass, annual morningglory, Palmer amarnath, Wright groundcherry, common purselane, yellow nutsedge and Johnsongrass. The average statewide cost for hand weeding in 1995 was reported as $27.87 per acre in addition to other weed control costs. Statewide, most growers used preemergence herbicides before or at planting and used pre- and post-emergence herbicides later in the season. Most of these applications were broadcast applications suggesting that many of the postemergence herbicide applications were layby applications. Preemergence herbicides (usually applied preplant incorporated) such as Treflan, Prowl, and Prometryn were more commonly used than postemergence herbicides. Statewide, few growers banded preemergence herbicides or used electro- hydraulic quick-hitch guidance systems and in-row weeding tools with their cultivators.

Field trials were conducted to evaluate the effect of quizalofop-ethyl on quackgrass plants in a potato cropping sequence. Fall and summer applications were compared for their quackgrass control potential. Season-long quackgrass control was obtained with quizalofop-ethyl at 96 g/ha following summer application. An increase in the rate of quizalofop-ethyl did not further improve control. Yields with quizalofop-ethyl at 96 g/ha were similar to standard treatments sethoxydim and fluazifop-butyl at recommended rates. Quackgrass control following a summer application was not maintained through to the following season. Fall applications did not result in adequate control of quackgrass the following season at any of the quizalofop-ethyl rates tested.

No observable injury was evident by any herbicide treatment on any of the twelve sweet corn varieties during the season. Overall, pendimethalin (Prowl®) treatments applied preemergence (PREE) provided very good control ( >87 %) of all weeds rated. Metolachlor (Dual®), EPTC plus safener (Eradicane®), dimethanamid (Frontier®), and cyanazine ( Bladex®) treatments gave good control ( >80 %) of pigweeds ( Amaranthus sp.) and purslane (Portulaca oleraceq) with annual yellow sweetclover (Melilotus ocf`icinalis) not adequately controlled. All treatments except Bladex alone gave good control of volunteer sudangrass.

Several registered and exploratory herbicides were effective for broadleaved weed control when applied preplant incorporated or preemergence in cantaloupes. Bensulide (Preface), clomazone (Command®), cyanazine (Bladex®), dimethanamid (Frontier®), ethafluralin (Curbit®), metolachlor (Dual ®), pendimethalin (Prowl®), trifluralin, dithiopyr (Rohm and Haas), and thiazopyr (Rohm and Haas) gave marginally acceptable control of most weeds. Crop injury was observed for some treatments of Frontier, Prowl, napropamide (Devrinol®), and Command. As the season progressed, groundcherry (Physalis wrightii) was not adequately controlled by any treatment.

All herbicide treatments, Prefar, Frontier, Dual Magnum, Valor, and Prefar combined with Dual Magnum or Frontier caused less than 10% injury on cantaloupes. Frontier at 0.75 lb AI/A, Dual Magnum at 1.0 lb AI/A, Valor at 0.03 and 0.05 lb AI/A controlled weeds similar to Prefar. Prefar at 4.0 lb AI/A combined with Frontier controlled tumble pigweed (94%), narrowleaf lambsquarters (95%), Wright’s groundcherry (97%), and horse purslane (94%). None of the preemergence herbicide treatments controlled purple nutsedge.

Onions treated with bromoxynil (Buctril7) or oxyfluorfen (Goal7) at the time when the first true leaf was emerging were not injured. No significant onion crop stand reduction occurred from any of the postemergence (POST) treatments. Onion height was not affected by any of the POST treatments through the season. A single application of Goal or Buctril offered up to 7 WAT of very good weed control with excellent crop safety. Onions treated at the typical 2-leaf stage of growth with Buctril or Goal exhibited no significant crop injury. Delayed and reduced control of knotweed (Polygonum aviculare) could have contributed to the decreased onion yield in the herbicide treated onions compared to the handweeded check. Onions in the untreated check were significantly reduced compared to Goal treated onions or the handweeded check.

At 4 weeks after treatment (WAT), all preemergence (PREE) treatments were completely safe on cantaloupes. At 1 WAT of postemergence (POST) applications, marginally acceptable melon injury (11 to 19%) was observed. At 6 WAT, crop injury increased significantly for both halosulfuron and bentazon. Halosulfuron (POST) following bensulide (PREE) caused minimal crop injury. The pigweeds were marginally controlled when POST treatments followed PREE herbicides. Tumble pigweed (Amaranthus albus) was more difficult to control than prostrate pigweed (A. blitoides). Halosulfuron gave good control of nutsedge (Cyperus rotundus) at 6 WAT.

Most herbicides applied alone preemergence (PREE) caused minimal crop injury (<10%) when furrow irrigated. Pendimethalin (Prowl) applied at 0.5 lb AI/A plus bensulide (Prefar) at 6.0 lb AI/A under sprinkler irrigation in Tolleson caused crop injury that was highly unacceptable and the crop stand was severely reduced. Onion yields were significantly reduced for the Prowl plus Prefar treatments. Prowl at 0.25 or 0.5 lb AI/A alone provided very good (>90%) weed control of all weeds. Combination treatments of Prowl plus other herbicides provided very good weed control but did not offer enhanced control of weeds already controlled by Prowl alone. The combinations of ethofumesate (Nortron) with metolachlor (Dual) or dimethenamid (Frontier) gave improved weed control compared to when either was applied alone.

Pendimethalin (Prowl7) applied preemergence (PREE) at 0.25 to 0.50 lb AI/A caused no observable injury and did not affect yields of onions that were furrow irrigated . Prowl applied PREE at 0.50 lb AI/A caused significant crop stand and yield reduction compared to lower rates or the untreated check under sprinkler irrigation. Prowl applied preplant incorporated (PPI) at rates ranging from 0.25 to 0.75 lb AI/A did not significantly injure onions or cause a significant yield reduction. Combination treatments of Prowl plus bensulide (Prefar7) applied PREE did not cause any measurable crop height or stand reduction compared to the standard treatment or untreated check. Prowl at 0.25 lb AI/A plus Prefar at 4.0 lb AI/A adequately controlled cheeseweed, yellow sweetclover, sowthistle, and London rocket.

Oxyfluorfen (Goal®) herbicide at 0.125 to 0.25 lb. a.i./A applied postemergence (POST) to onions at the 3-leaf stage of growth effectively controlled London rocket, yellow sweetclover, and prostrate pigweed with marginal injury to onions. Earlier POST applications on 1- and 2-leaf onions caused injury and some stand reduction. Bromoxynil (Buctril®) herbicide at 0.25 lb. a.i./A applied early POST gave generally good weed control but did not adequately control sweetclover. Buctril® applied in clear weather did not injure 1-leaf onions but caused severe injury on 2- and 3-leaf onions when applied during cloudy weather conditions. Buctril® plus pendimethalin (Prowl®) tank-mix combination applied POST provided good control of London rocket, sweetclover, and prostrate pigweed; however, onion injury was severe due to applying Buctril® in cloudy weather. Buctril® and Goal® effectively controlled weeds present in the onions but timing of POST applications was critical with respect to onion size and weather conditions to minimize injury.

Field and greenhouse studies were conducted in 2007 and 2008 to evaluate the foliar efficacy of saflufenacil on horseweed (Conyza canadensis (L.) Cronq.). In the field, saflufenacil applied alone at the lowest rate (25 g/ha) resulted in less control than all other herbicide treatments that included saflufenacil. The addition of glyphosate to 25 g/ha of saflufenacil increased the level of control over either herbicide applied alone. However, the addition of glyphosate to 50 g/ha of saflufenacil or greater was not beneficial because saflufenacil alone provided at least 95% control. Overall, horseweed height at the time of herbicide application had very little effect on the efficacy of saflufenacil applied alone or in combination with glyphosate. Application variables can enhance the foliar activity of saflufenacil. In the greenhouse, saflufenacil combined with glyphosate provided greater control than saflufenacil applied alone on both glyphosate-susceptible and -resistant horseweed populations. Regardless of horseweed population or glyphosate, saflufenacil had greater activity when crop oil concentrate rather than nonionic surfactant was used as the adjuvant. Decreasing light level within 24 hours of herbicide application resulted in greater saflufenacil activity. Applying saflufenacil in a pH 5 spray solution resulted in greater activity than pH 7 or pH 9. Although effects from saflufenacil applied under different temperatures were evident in early timings, there were no lasting effects on the efficacy of saflufenacil. Saflufenacil had significant activity on both glyphosate-susceptible and -resistant horseweed. Under certain conditions when complete control of horseweed is not achieved, such as low application rates, large target weeds, and varying environmental conditions, application variables including glyphosate tank-mixtures, crop oil concentrate, low spray solution pH, and low light level may increase the level of horseweed control from saflufenacil.

The treatments that included 2,4-D, mecoprop, and dicamba effectively controlled burclover and malva in the dormant bermudagrass turf. The addition of carfentrazone to the hormonal herbicides (Speedzone*) appeared to enhance malva control earlier than the other treatments. Surge* containing sulfentrazone with hormonal herbicides gave the highest degree of malva control. Fluroxypyr (Spotlight*) was not effective against burclover and showed moderate control of malva in this test.

Three isolates of Curvularia belonging to Curvularia tuberculata (isolates 93-020 and 93-022) and Curvularia oryzae (isolate 93-061) were obtained from diseased Cyperus difformis, Cyperus iria, and Fimbristylis miliacea, respectively, in the Philippines in 1993. Under greenhouse conditions, these fungal isolates caused high mortality and significant plant dry weight reduction in C. difformis, C. iria, and F. miliacea when sprayed at the rate of 1 x 108 spores/m3. Cross-pathogenicity of the isolates was demonstrated in three other sedge weed species. C. difformis, C. iria, and F. miliacea were killed but C. rotundus was resistant. Most of the thirteen rice varieties tested were resistant to the fungal isolates. The order of decreasing pathogenicity to rice was C. oryzae (93-061), C. tuberculata (93-020), and C. tuberculata (93-022). The infection process of C. tuberculata and C. oryzae was similar. Spore germination was polar for C. tuberculata and bipolar for C. oryzae. Germ tube growth was random and branching. Appressoria were formed preferentially over epidermal cell wall junctions on sedge hosts and over stomatal apertures in rice. Complex infection cushions were observed only on sedge hosts. Infection hyphae developed inter- and intracellularly, causing epidermal cell walls to separate and mesophyll cells to shrink and collapse. The vascular bundles were not invaded. Colonization of susceptible weeds was rapid and conidiophores emerged from the stomatal aperture between 96 to 120 hours post inoculation (HPI). Resistance to C. tuberculata and C. oryzae in C. rotundus and rice was expressed as a delay in appressorial formation, inhibition of fungal growth after penetration, and lack of sporulation.

One potential alternative to chemical weed control is the use of microwave radiation, a particular form of indirect thermal weeding. Absorption of microwave radiation causes water molecules within the tissue to oscillate, thereby converting electromagnetic energy into heat. This technique is rapid, versatile and effective, as the electromagnetic waves heat the plant tissue and destroy cellular integrity. The objective of this research was to evaluate the potential use of dielectric heating for weed management. Ten weed species representing monocots and dicots were selected for this study: southern crabgrass, dallisgrass, yellow nutsedge, fragrant flatsedge, false green kyllinga, common ragweed, field bindweed, henbit, white clover, and pitted morningglory. There was a lag or warm up period between energizing the magnetron and actual microwave radiation production. To eliminate the gap between electric power supplied to magnetron and actual microwave radiation produced, a conveyer was used. Overall injury to grasses, sedges and broadleaf weeds was higher at each dose when weeds were treated by microwave radiation while moving on a conveyer in comparison to being stationary. Grasses showed slightly more tolerance to microwave treatments in comparison to broadleaf weeds. Older weeds (8 to 10 weeks old) showed more tolerance to microwave treatments in comparison to younger weed plants (4 to 6 weeks old). Microwave radiation was able to control a range of weed species, although larger weeds were more likely to regrow after treatment. Ambient temperature had a significant effect on injuries caused by microwave radiation to target weeds, with control increasing as the air temperature increased. Weed control using microwave radiation required more energy when weeds were treated at 13 C compared to 35 C. More energy was needed at lower air temperatures to raise the plant canopy temperature from ambient levels to beyond the biological limit. Microwave radiation at lower doses caused greater injury to common chickweed and yellow woodsorrel than bermudagrass, suggesting the potential for selective weed control in certain situations. A custom built microwave applicator provided similar control of emerged weeds as the contact herbicides diquat and acetic acid.
Ph. D.

Harvest weed seed controls (HWSC) destroy weed seeds that are retained by the plant at crop harvest, which would typically be spread by the harvester along with other field residues. HWSC exploits coincidental maturity between crops and weeds, so an experiment was designed to collect weed seeds as they shatter throughout the growing season and through a simulated harvest delay. This experiment monitored four economically important broadleaf species and two grass species in a soybean (Glycine max (L.) Merr.) field. Results indicated that broadleaf species shattered seed at rates accelerating through the growing season, while grass species shattered more seed early in the growing season. Field experiments in organic and conventional winter wheat (Triticum aestivum L.) fields infested with Italian ryegrass (Lolium perenne L. ssp. multiflorum (Lam.) Husnot) compared two HWSC techniques to grower-standard weed management programs in each system, including both no-till and full-till standard treatments in the conventional system. Italian ryegrass populations were monitored, and wheat yield was measured both before and after HWSC application. In both organic and conventional cropping systems, HWSC treatments did not provide better Italian ryegrass control than the grower-standard treatments. The conventional program including tillage boosted Italian ryegrass populations. These results suggest that HWSC treatments did not enhance Italian ryegrass control compared to grower-standard practices in either the organic or conventional systems. Additionally, broadleaf weeds may retain enough seeds to be viable targets for HWSC. Incorporating best practices, such as a timely crop harvest, is key for understanding and optimizing HWSC.
Master of Science