Master of Science / Department of Agronomy / Phillip W. Stahlman / Several weed species infest sunflower fields, but herbicidal options for broadleaf weed control are limited. In recent years, imazamox and tribenuron herbicides have been registered for POST use in imidazolinone-tolerant and tribenuron-tolerant sunflowers, respectively. Objectives of this study were to 1) investigate the effects of soil nitrogen level on Palmer amaranth control with imazamox in imidazolinone-tolerant sunflower and 2) evaluate crop response and weed control efficacy of single and sequential applications of tribenuron at two rates and the effectiveness of preemergence herbicides followed by postemergence tribenuron in tribenuron-tolerant sunflower. Greenhouse experiments were conducted in Manhattan, KS and field experiments were conducted near Hays, KS in 2007 and 2008. For the first objective, treatments consisted of a factorial arrangement of three soil nitrogen levels (28, 56, and 84 kg/ha) and two imazamox rates (26 and 35 g ai/ha) in a RCBD. Palmer amaranth growth rate increased with increasing soil nitrogen level. In all experiments, plants grown at the highest soil nitrogen level exceeded the maximum recommended plant height (7.6 cm) by >35% at the time of imazamox application. Generally, imazamox rates did not differ in control effectiveness at the 56 kg/ha soil nitrogen level, but the higher 35 g/ha rate was superior to the lower rate at the 84 kg/ha soil nitrogen level because of greater weed size. For the second objective, tribenuron was applied singly at 9 and 18 g/ha, sequentially in all combinations of those rates, and singly at those rates following PRE herbicide treatments. In general, tribenuron at 18 g/ha applied with methylated seed oil adjuvant before weeds exceeded 10 cm in height provided excellent control of most species with insignificant injury to the crop. The need for supplemental PRE herbicides for weed control in tribenuron-tolerant sunflower depends on weed species present and their size at the time of tribenuron application.

Thesis (MScAgric)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: Ripgut brome (Bromus diandrus Roth.) is a weed that causes great problems in the most wheat and grain producing areas and also in livestock practices. Until recently (1995) there were no registered chemicals for the management of ripgut brome in wheat, in South Africa. After the registration of sulfosulfuron and iodosulfuron + mesosulfuron for the management of ripgut brome in wheat, these two herbicides were widely used and in the case of wheat monocultures, it was used repeatedly. During the last few years, reports of ripgut brome that were suspected to be resistant to these chemicals, increased. With the development of herbicide resistance it is of great importance to investigate methods to confirm resistance and also to control it. The goal of this study was firstly, to confirm resistance in ripgut brome and secondly to compare growth and development of resistant ripgut brome popualtions to that of susceptible ripgut brome populations. The dormancy of Bromus seed was also investigated as were effective methods to break seed dormancy. Lastly, quicker methods to confirm resistance were investigated. A short summary of the experiments follows. In the first experiment the degree of resistance of three different ripgut brome populations were determined, by using the pot spray method. One population each of B. pectinatus and B. rigidus were also included in the study. The seed of the Bromus populations were germinated after which it was planted in plastic pots and were placed in the glasshouse until the three to four leaf stage. Subsequently the plants were treated with the following four herbicides: sulfosulfuron, iodosulfuron + mesosulfuron, imazamox and haloxyfop-R methyl ester, at seven concentrations namely, the recommended dosage, one quarter, one half, twice, four times and eight times the recommended dosage. After six weeks the percentage survival and the dry mass of the plants were determined. Results showed that the three ripgut brome populations had different degrees of resistance to sulfosulfuron and iodosulfuron + mesosulfuron, varying from no resistance to moderate resistance to strong resistance. There was no resistance to imazamox and haloxyfop-R methyl ester. The B. rigidus population exhibited strong resistance or tolerance (natural resistance) to the two sulfonylureum herbicides. In the second experiment the seed of the same Bromus populations were germinated and planted in plastic pots that were filled with three litres of river sand to determine the growth, development and seed production of the plants. The number of leaves for each plant as well as the plant height were measured weekly, until the plants became reproductive. The two resistant populations grew much faster than the susceptible population and they also produced taller plants. The susceptible population produced more leaves, but seed production was delayed considerably. This probably relates more to the plant’s adaptation to their enviroment, than to adaptation due to resistance. The susceptible population was collected from a natural environment, while the others were collected from wheat fields. In the third experiment the seed dormancy of the Bromus populations was investigated. The effect of different treatments on the dormancy of the seed was also investigated. The treatments that were applied were gibberrellic acid, fumigation with ammonia gas and an ammonia treatment combined with a cold treatment. Seed dormancy in all populations was short-lived and the cold treatment was an effective way of stimulating fresh seed to germinate. The last experiment was performed to develop a quicker method for the evaluation of resistance in Bromus spp. In this experiment the petridish method was investigated. Only sulfosulfuron and iodosulfuron + mesosulfuron were used, because resistance to them was proven earlier. Different concentrations of the herbicides were applied to the dishes with the seed and were exposed to a cold treatment before being placed in a germination chamber. The seed in al the treatments germinated and it was decided to let the seed grow for two weeks in the petri dishes to observe whether the herbicides may have a detrimental effect on the growth of the small seedlings. After two weeks there were no differences between treatments and the experiment was terminated. The study showed that resistance is present in some of the Bromus populations and that there are biological differences between populations with different degrees of resistance. However, the fact that the susceptible population comes from a completely different environment than the other populations, complicate matters and further studies are required to obtain a clearer picture. / AFRIKAANSE OPSOMMING: Predikantsluis (Bromus diandrus Roth.) is ‘n onkruid wat in die meeste koringen garsproduserende gebiede, asook in sommige vee praktyke, probleme veroorsaak. Tot redelik onlangs (ongeveer 1995) was daar in Suid-Afrika geen middels geregistreer wat predikantsluis in koring kon beheer nie. Nadat sulfosulfuron en iodosulfuron + mesosulfuron vir predikantsluisbeheer in koring geregistreer is, is die twee middels op groot skaal, en in die geval van koring monokultuurstelsels, aanhoudend toegedien. Gedurende die afgelope paar jaar is berigte ontvang dat beheer van predikantsluis met die middels nie meer so doeltreffend is nie, moontlik as gevolg van onkruiddoderweerstand wat ontwikkel het. Met die ontstaan van onkruiddoderweerstand is dit belangrik om praktyke en maniere te vind om weerstand vinniger te bevestig en doeltreffend te bestuur. Die doel van hierdie studie was eerstens om weerstand in predikantsluis te bevestig en tweedens om die groei en ontwikkeling van plante afkomstig van vermoedelike weerstandbiedende predikantsluis populasies te vergelyk met plante uit ‘n vatbare populasie. Die saadproduksie en dormansie van die saad is ook ondersoek asook effektiewe metodes om dormansie te breek. Laastens is ondersoek ingestel na ‘n vinniger manier (petribakkie metode) om weerstand te bevestig. Hieronder volg ‘n oorsig oor die vier eksperimente wat uitgevoer is. In die eerste proef is die mate van weerstand van drie verskillende predikantsluis populasies bepaal, deur van die gewone potspuit metode gebruik te maak. Daar is ook een populasie elk van Bromus pectinatus en vermoedelik Bromus rigidus ingesluit in die studie. Die sade van die verskillende populasies is toegelaat om te ontkiem en daarna is dit in plastiese potjies geplant en in ‘n glashuis geplaas totdat die drie tot vier blaarstadium bereik is. Die plante is daarna gespuit met die volgende vier middels: haloksifop-R-metielester (Gallant Super), imasamoks (Cysure), iodosulfuron + mesosulfuron (Cossack) en sulfosulfuron (Monitor), teen sewe konsentrasies elk, nl. teen die aanbevole dosis, asook teen een kwart van, een helfte van, twee keer, vier keer en agt keer die aanbevole dosis. Na ses weke is die persentasie oorlewendes en die droëmassa van die plante bepaal. Resultate het getoon dat die drie predikantsluis populasies verskillende grade van weerstand teen die twee sulfonielureums (sulfosulfuron en iodosulfuron + mesosulfuron) toon, dit wil sê van geen tot matig tot sterk weerstandbiedend. Daar is egter geen weerstand teen haloksifop-R-metielester (Gallant Super) en imasamoks (Cysure) waargeneem nie. Die B. rigidus populasie het sterk weerstand of toleransie (natuurlike weerstand) teen die sulfonielureum middels getoon. In die tweede proef is saad van dieselfde Bromus populasies ontkiem en oorgeplant in plastiese potte gevul met 3 liter riviersand om die groei en ontwikkeling en saadproduksie van die plante te evalueer. Die aantal blare per plant en hoogte van die plante is weekliks bepaal totdat die plante reproduktief geraak het. Hierna is die metings gestaak om te voorkom dat die saadproduksie van die plante benadeel word. Die resultate het getoon dat die twee weerstandbiedende predikantsluis populasies vinniger groei as die vatbare populasie en ook langer plante vorm, terwyl die vatbare populasie vinniger en meer blare vorm, maar langer neem om saad te vorm. Hierdie waarnemings hou egter waarskynlik meer verband met die oorsprong van die populasies as met die graad van weerstandbiedendheid. Die vatbare populasie is versamel in natuurlike veld vêr van enige landerye terwyl die ander populasies almal uit graanlande afkomstig is. In die derde proef is saaddormansie van die Bromus populasies ondersoek. Daar is ook ondersoek ingestel na verskillende behandelings om dormansie te breek. Die behandelings wat toegepas is, is ‘n gibberelienesuur behandeling teen verskillende konsentrasies, beroking met ammoniak vir verskillende tye en ‘n ammoniak behandeling tesame met ‘n koue behandeling. Die resultate het getoon dat saaddormansie van die Bromus populasies van korte duur is, maar dat kouebehandeling effektief is om ontkieming van vars saad te stimuleer. Die vierde proef is uitgevoer om vas te stel of daar vinniger evaluasiemetodes is vir die evaluasie van weerstand in Bromus spp., deur van die petribakkie metode gebruik te maak. In hierdie proef is slegs die middels iodosulfuron + mesosulfuron (Cossack) en sulfosulfuron (Monitor) gebruik, omdat daar ‘n mate van weerstand teen hulle waargeneem is in die eerste proef. Die middels is teen verskillende konsentrasies in petribakkies gevoeg, tesame met die sade en toe blootgestel aan ‘n kouebehandeling voordat dit in ‘n ontkiemingskabinet geplaas is vir ontkieming. Die sade in al die behandelings het ontkiem en daar is besluit om die saailinge uit die ontkiemingskabinet te haal en vir twee weke te laat groei sodat daar bepaal kon word of die middels ‘n effek op die groei van die plantjies het. Na twee weke kon geen verskil in die groei van die plantjies waargeneem word nie en die proef is beëindig. Die studie het getoon dat daar wel weerstand in sommige van die Bromus populasies voorkom, en dat biologiese verskille voorkom tussen predikantsluis populasies met verskillende grade van weerstand. Die feit dat die vatbare populasie uit ‘n heeltemaal verskillende omgewing kom as die ander populasies, maak definitiewe afleidings moeilik. Daar sal opvolgstudies uitgevoer moet word om van die onduidelikhede op te klaar.

Thesis (MSc)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: The high cost of herbicides for weed control in crop fields in the Western Cape is a major cause of reducing farmers' net income. As chemical weed control became more difficult and expensive, it became necessary to focus on the technique of reducing weed impact, which does not only involve herbicide usage. Aspects such as tillage method, sowing date of crops, crop rotation, weed ecology and germination requirements, amongst other, may play a role in reducing weed impact. The first experiment was done to determine whether temperature and light had an effect on the germination of six selected weed species, namely Arctotheca calendula, Avena fatua, Bromus diandrus, Emex australis, Lolium temulentum and Raphanus raphanistrum. Seeds of the aforementioned weeds were collected from Langgewens during 2000 and stored at room temperature before being used in this study. The seeds were germinated in a germination cabinet at three temperature regimes namely 5 "C /15 "C, 10°C / 15°C and 10°C / 20°C. Most seeds showed a positive germination response at the 10°C / 15°C treatment, except for Emex australis and Lolium temulentum, which was believed to germinate throughout the year under favourable conditions. Among the germinated weed species, Avena fatua germinated best with a cumulative value of 90% as compared to Raphanus raphanistrum, which germinated least with a cumulative value of 12%. The second experiment was done to evaluate the effect of three growth regulators, namely gibberellic acid, hydroxylamine (auxin), and kinetin (cytokinin) to break dormancy and enable simultaneous germination of the six weed species mentioned above. Weed seeds were germinated in a germination cabinet at 20 °C using the test solutions of the aforementioned growth regulators. The germination was assessed after 3, 7, 10 and 14 days of incubation and the tetrazolium test for the viability of seeds was done for ungerminated seeds. The result obtained showed that no chemical/concentration proved to be successful in stimulating the germination of all species tested. As for example a high concentration of hydroxylamine increased germination of Emex australis to nearly 100% and inhibited germination of Raphanus raphanistrum to less than 12.5% at all hydroxylamine concentrations. The third experiment was conducted with the aim of determining the competitiveness of the six weed species mentioned above when grown together with wheat in order to decide when weed control will be cost-effective. An additive series experiment was conducted in a glasshouse. The influence of weed species on wheat plant height, tiller number at different growth stages, vegetative dry mass and gram mass was determined by using different varying weed densities. Weed densities, through reduction in number of tillers, reduced wheat grain yield. By increasing the density of wheat this competitive effect could be reduced. Different weed densities caused a significant difference in wheat plant height, tiller number, dry mass and grain mass. The results showed that species had a significant effect (p < 0.05) on wheat plant height during tillering, stem elongation and heading growth stages. Total wheat plant above-ground dry mass was significantly (p < 0.05) reduced by an increase in weed density. / AFRIKAANSE OPSOMMING: Die hoë koste van onkruiddoders vir onkruidbeheer in graangebiede van die Wes- Kaap is een van die hooffaktore wat graanprodusente se netto inkomste verlaag. Weens moeiliker en duurder chemiese onkruidbeheer en om die impak van onkruide te verlaag, het dit noodsaaklik geword om op die tegnieke te fokus wat nie die gebruik van chemikalieë insluit nie. Faktore wat in rol kan speel sluit bewerkingspraktyke, saaidatum van gewasse, wisselbou, ekologie en ontkiemingsvereistes van onkruide in. Die eerste eksperiment is gedoen om te bepaal of temperatuur en lig 'n effek het op die ontkieming van ses gekose onkruide, naamlik Arctotheca calendula, Avenajatua, Bromus diandrus, Emex australis, Lo/ium temulentum en Raphanus raphanistrum. Sade van die voorgenoemde onkruide is in 2000 op Langgewens versamel en by kamertemperatuur geberg voordat dit in hierdie studie gebruik is. Die sade is in 'n ontkiemingskabinet geïnkubeer by drie temperatuurreekse naamlik 5 oe / 15 oe, 10 oe / 15 oe en 10 oe / 20 oe. Die meeste sade het 'n positiwe ontkiemingsreaksie getoon na die 10 oe / 15 oe behandeling, met die uitsondering van Emex australis en Lolium temulentum, wat oënskynlik heeljaar salontkiem onder gunstige omstandighede. Avena fatua het die hoogste ontkiemingspersentasie oor alle spesies getoon met 'n kumulatiewe waarde van 90% en Raphanus raphanistrum die minste met 'n kumulatiewe waarde van 12%. Die tweede eksperiment is gedoen om die invloed van drie groeireguleerders, naamlik gibberelliensuur, hidroksielamien (ouksien) en kinetin (sitokinien), op die opheffmg van saadrus te ondersoek en om ook die gelyktydige ontkieming van bogenoemde ses onkruidspesies moontlik te maak. Onkruidsade is in 'n ontkiemingskabinet by 20 oe ontkiem deur gebruik te maak van toetsoplossings van bogenoemde groei-reguleerders. Die ontkieming van die sade is na inkubasie periodes van 3, 7, 10 en 14 dae geëvalueer en die tetrazoliumtoets VIr saadkiemkragtigheid is toegepas vir onontkiemde sade. Die verkreë resultate het getoon dat geen chemiekalie / konsentrasie die ontkieming van alle spesies suksesvol kon stimuleer nie. Hoë konsentrasies hidroksielamien het die ontkieming van Emex australis tot byna 100% verhoog en het die ontkieming van Raphanus raphanistrum geïnhibeer tot minder as 12.5% by alle hidroksielamien konsentrasies. Die derde eksperiment is uitgevoer met die doelom die kompetisievermoë van dieselfde ses onkruidspesies te bepaal as dit saam met koring groei en om te besluit wanneer onkruidbeheer koste effektief sal wees. 'n Additiewe reeks eksperiment wat uit vier herhalings bestaan het, is in plastiekpotte in 'n glashuis uitgevoer. Die invloed van die onkruidspesies op koring planthoogte, halmgetal by verskillende groeistadiums, droë massa en graanmassa is bepaal deur onkruiddigthede te gebruik. Onkruide het koring opbrengs verlaag deur die vermindering van halmgetal en hierdie kompeterende effek kon verminder word deur 'n toename in die plantestand van koring. Verskillende onkruiddigthede het 'n betekenisvolle invloed op koring planthoogte, halmgetal, droë massa en graanmassa gehad. Die resultate het getoon dat spesies 'n betekenisvolle effek (p< 0.05) op koring planthoogte, stem verlenging en aarverskyning gehad het. Die totale bogrondse droë massa van koring is betekenisvol (p< 0.05) verminder deur 'n verhoging in onkruiddigtheid.

3 pp. / Revised / The most effective weed control practice in alfalfa is maintaining a healthy crop and dense stand. Cultural practices that promote a vigorous stand can reduce the need for chemical weed control. Cultural practices that promote a healthy stand reduce the need for chemical weed control, but herbicides are sometimes necessary even in well-managed alfalfa.

A test was conducted on established alfalfa to evaluate the efficacy of 10 herbicide treatments for the control of shepardspurse (Capsella bursa). Control ranged from 13 to 99 percent.

A test was conducted to compare EPTC applied preplant to alfalfa as a 10% granule and as a emulsifiable concentrate metered into the irrigation water. Observations and measurements on weed control and phytotoxicity were recorded EPTC appeared to be more active in controlling weeds and injuring seedling alfalfa when applied to 3.0 lb./acre as a granule rather than the same rate applied as a water run.

Two successive applications of Poast Plus (60 oz. product /A, 0.47 lb. sethoxydim /A), Ultima 160 (46 oz. product /A, 0.47 lb. sethoxydim /A), and Prism (34 oz. /A, 0.25 lb clethodim/A) were made on May 31 and June 27, 1996 to control bermudagrass (Cynodon dactylon) in a commercial alfalfa field. A crop oil concentrate was added to all herbicide solutions. Applications were made two days after the irrigations that followed removal of the cuttings. The bermudagrass density was high with extensive stolon development; the grass formed almost a complete mat in most of the plots. Percent control was visually estimated on June 27 (prior to second application) and on July 25, 28 days following the second application and after cutting of the alfalfa. Percent control was 84.5, 71.2, and 65% for Poast Plus, Ultima, and Prism, respectively, after one application. Percent control was 87.5, 72.5, and 90.5% for Poast Plus, Ultima, and Prism, respectively, after two applications. A single application of Poast Plus resulted in good bermudagrass control but single applications of Ultima or Prism resulted in marginal control. The second application of Prism significantly improved bermudagrass control but the second application of Poast Plus or Ultima did not improve control. The Poast Plus formulation of sethoxydim appeared to be superior to the Ultima 160 formulation of sethoxydim.

Summer weeds including purple nutsedge are of economic concern to alfalfa growers in western Arizona. Application rates of Cadre 2 ASU, a new sulfonylurea herbicide chemistry currently registered for experimental use in peanuts, for purple nutsedge control in a roadway bordering established alfalfa were examined in a two year duration, replicated field study. Fair to good (35- 65%) purple nutsedge control was obtained when Cadre was applied at the 3 oz/acre rate to a severe initial nutsedge infestation (80- 100%). Control was most effective when Cadre was applied in late summer compared to early spring, and repeat split applications were necessary under the high weed pressure observed in this study.

Summer weeds such as nutsedge are of economic concern to alfalfa growers in western Arizona. A two year replicated evaluation of the effectiveness of granular norflurazon herbicide for purple nutsedge control was conducted on an established alfalfa field in La Paz County. Zorial Rapid 80 WP and Evital 5G herbicides were tested for their effectiveness at controlling purple nutsedge when applied following hay harvest but prior to irrigation in early spring and late summer. Zorial 80 WP was applied at 2.0 lb a. i. /acre. Evital 5G was applied in spring 1996 at application rates of 1.0, 1.5, 2.0, and 3.0 lb a. i. /acre. Split applications were made the following summer to four plots for a total of 2.0, 3.0, and 4.0 lb a. i. /acre/year. In 1996, purple nutsedge control resulting from a single application of Evital 5G at 2.0 lb a. i. /acre was 41, 82, and 35% at 35, 63, and 99 DAT, respectively. However, Zorial 80 WP applied at 2.0 lb a. i. /acre provided poor purple nutsedge control due to poor soil incorporation since the liquid was unable to penetrate the dense nutsedge foliage. The treatments were reapplied in spring and summer of 1997.

Late summer grassy weed control is a questionable practice since it reduces alfalfa hay tonnage during summer slump, and the reduction in hay feed quality caused by these weeds in horse hay is questionable. A field experiment was conducted at the September alfalfa cutting to examine the feed quality of grassy and broadleaf weeds found in western Arizona hay fields at this time which corresponds with annual summer slump. These weeds included bermudagrass, junglerice (watergrass), Mexican sprangletop, Johnsongrass, purple nutsedge, and common purslane. Since hay cut during this period is used primarily for dry dairy cow and horse hay this study examined the suitability of alfalfa hay infested with these summer weeds as a feed for these animals. Based on this study, horse owners could benefit financially if they waited until late summer when hay prices slump, and purchase off-grade alfalfa hay containing less than one half grassy summer weeds for an economical, nutritious feed source.