Sexual reproduction of Eichhornia crassipes (Mart.) Solms in the Nile

In a study of some of the factors that are believed to affect fruit-setting in Eichhornia crassipes under Sudan conditions, the average number of flowers per inflorescence was twelve (range 4–26) while the average number of capsules per inflorescence was 1·5 (range 0–16). The problem of low fruit-setting was suggested to be basically due to the ecological factor of high temperature and low relative humidity. The flowers were preponderantly meso-stylic with the long and short-stylic ‘races’ absent or very rare. Recherches stir la biologic d'Eichhornia crassipes (Mart.) Solms dans le Nil Dans une étude sur quelques-uns des facteurs que l'on suppose actifs sur la mise à fruits de l’Eichhornia crassipes. dans les conditions du Soudan le nombre moyen de fleurs par inflorescence fut de 12 (s'étendent de 4 à 26), cependant que le nombre moyen de capsules par inflorescence fut de 1,5 (s'étendant de 0 à 16). Le probléme de la faible mise à fruit suggere qu'elle pourrait etre due a des facteurs écologiques: température élevée et faible humidity relative. Les fleurs étaient en majorité meso-stylées, les races à long style ou à style court étaient absentes ou tres rares. Generative Vermehrung von Eichhornia crassipes (Mart.) Solms. im Nil In einer Untersuchung über einige Faktoren, von denen angenommen wird, dass sie die Fruchtbildung von Eichhornia crassipes unter sudanesischen Bedingungen beeinflussen, war die durchschnittliche Anzahl von Blüten, 12 Blüten pro Inflorescenz (4–26), wáhrend die durchschnittliche Anzahl von Kapseln nur 1.5 Kapseln pro Inflorescenz war (0–16), Das Problem der geringen Fruchtausbildung wurde folgenden okologischen Faktoren zugeschrieben; hohe Temperatur und niedrige Luftfeuchtigkeit. Bei den Blüten wurden überwiegend Stempel miltlerer Länge gefunden, während die ‘Rassen’ kurzer und langer Stempel nicht oder nur sehr selten anzutreficn waren.     

Simulation of the persistence of atrazine in soil at different sites in Portugal

The effects of soil temperature and soil moisture content on the rates of degradation of atrazine, were measured in the laboratory in soils from different sites in Portugal. Persistence of atrazine was measured in the same soils in the field during the spring and summer of 1984, 1985, 1986 and 1987. Weather records from the different sites, measured during the periods of the field experi ments, were used in conjunction with appropriate constants derived from the laboratory data in a computer program to simulate persistence in the field. The model generally overestimated the ob served soil residues, particularly during the first 7–14 days after application. The fit from the model was good from day 14 to the end of the experiments.     

Weed control in conventional and herbicide tolerant winter oilseed rape (Brassica napus) grown in rotations with winter cereals in the UK

Two winter oilseed rape (Brassica napus) cultivars, tolerant to glyphosate and glufosinate, were compared with a conventional cultivar at three sites over 4 years, in 3‐year crop rotations in the UK. The winter oilseed rape was grown in Years 1 and 4, with winter cereals, which received uniform herbicide treatments, in the intervening years. The second winter oilseed rape treatments were applied to randomised sub‐plots of the original plots. Weed densities were recorded in autumn and spring and weed biomass was measured in summer. At most sites, there was only one application of glufosinate or glyphosate, whereas two products were often used on the conventional variety. The timing of glyphosate and glufosinate application was, on average, 34 days later than that of the conventional broad‐leaved weed control treatments. Overall weed control, across all sites and years, was not statistically different between the conventional, glyphosate and glufosinate treatments. However, glyphosate achieved higher control of individual weed species more frequently than the other treatments. Glufosinate and the conventional treatments were similar in performance. The treatments in Year 1 sometimes affected weed populations in the subsequent cereal crops and, in rare instances, those in the rape in Year 4. Carry‐over effects were small after most treatments. In general, weed survival was greater in the oilseed rape crops, irrespective of the treatment, than it was in the intervening cereal crops.     

Study on the behaviour of imazapyr in two Moroccan soils

Imazapyr owes its importance in Morocco to its success in controlling the perennial weed Solanumelaeagnifolium Cav., which infests the Tadla area. Persistence and mobility of imazapyr has been studied in two Moroccan soils from the Rabat area, with differing organic matter content (red and organic soils), under laboratory conditions at 75% of their field capacities and 25–28 °C. Residue analysis was performed on the basis of a bioassay test using lentil (Lens culinaris Medic.) as indicator species. The residual activity of imazapyr accounted for 69%, 25%, 50% and 62%, 46%, 66% of the initial activity for the red and organic soils at 1, 5 and 10 mg L^?1 respectively. The half-lives varied between 25 and 58 days for the red soil and 55 and 58 days for the organic soil. In the organic soil, imazapyr was highly mobile under the irrigation regime applied. Most of the activity was found in the first 3 × 75 mL of the effluents. A following biotest with the leached soil showed low remaining residual activity.     

Further studies of the enhanced biodegradation of some soil-applied herbicides

In field experiments to investigate the effect of repeated application of eight herbicides on their persistence in the soil, the rates of loss of metribuzin, metolachlor and isoproturon were shown to be unaffected by up to three previous doses of the same herbicide. With linuron, there was a small but significant enhancement of biodegradation by two or more previous applications. The persistence of monolinuron, propyzamide and metamitron was shorter in plots that had been treated once, twice or three times previously than in plots treated for the first time, but the effects were often small. In the case of napropamide, rates of loss were considerably increased by a single previous dose. In laboratory experiments with soil from the field plots, there was again no evidence of enhanced biodegradation of metribuzin, metolachlor or isoproturon. However, the rate of linuron degradation was enhanced by a single application. This effect was most marked after 12 weeks, but had disappeared by 57 weeks after initial treatment of the soil in the field. Considerable enhancement of monolinuron, propyzamide and metamitron degradation rates was also observed. The effects were again most pronounced after 12 weeks; however, with these compounds, they were still apparent more than 1 year later. Enhancement of napropamide biodegradation was most stable when assessed in laboratory incubations; the effects from a single pretreatment were still apparent more than 12 months later. Etudes complementaires de la degradation accélerée de certains herbicides appliques au sol Dans des essais de plein champ conduits pour etudier 1'influence d'applications répetées de 8 herbicides sur leur persistance dans le sol, les taux de perte de métribuzine, métolachlor et isoproturon n'ont pas été affectés jusqu'à 3 applications précédentes du même herbicide. Avec le linuron, il y a eu une faible mais significative augmentation de la dégradation aprés 2 ou 3 applications. La persistance du monolinuron, de la propyzamide et de la métamitrone éiait plus courte dans les parcelles déjà traitées une, deux ou trois fois que dans celles traitées pour la premiére fois, mais les effets étaient souvent faibles. Avec la napropromide, les taux ont été considérablement accrus aprés une seule application. Dans des essais de laboratoire avec du sol venant des parcelles de plein champ, il n'y a pas eu de nouveau d'accélération de la biodégradation pour la métribuzine, le métolachlor ou 1'isoproturon. Cependant, la dégradation du linuron a été renforcée par une seule application. Cet effet a été plus fort qu'aprés 12 semaines, mais a disparu 57 semaines aprés le traitement du sol au champ. Un accroissement important de la dégradation du monolinuron, de la propyzamide et de la métamitrone a étéégalement observé Les effets étaient de nouveau maxima aprés 12 semaines; cependant avec ces produits, ils étaient encore apparent une année plus tard. L'accé1ération de la biodégradation de la napropromide était la plus stable dans les observations de laboratoire; les effets d'une seule application étaient encore apparents plus de 12 mois plus tard. Weitere Untersuchungen zum beschleunigten Abbau von Bodenherbiziden In Feldversuchen zur Persistenz von 8 Herbiziden bei wiederholter Ausbringung ging bei Metribuzin, Metolachlor und Isoproturon nach 3-maliger Ausbringung desselben Herbizids die Verlustrate nicht zurück. Bei Linuron war der mikrobielle Abbau nach 2 oder mehr Vorbehandlungen nur leicht, aber signifikant erhöht. Die Persistenz von Monolinuron, Propyzamid und Metamitron war bei l-, 2- oder 3-maliger Vorbehandlung geringer als bei erstmaliger Behandlung, doch waren die Wirkungen oft klein. Bei Napropamid erhöhte sich die Verlustrate bereits nach einer Vorbehandlung beträchtlich. Auch in Laborversuchen mit Böden aus den Freilandparzellen konnte bei Metribuzin, Metolachlor und Isoproturon kein verstärkter mikrobieller Abbau nachgewiesen werden. Bei Linuron dagegen erhöhte sich die Abbaurate schon nach einmaliger Applikation. Diese Wirkung war nach 12 Wochen am deutlichsten ausgeprägt, verschwand jedoch nach 57 Wochen nach dem Behandlungstermin. Eine erheblich größere Abbaurate wurde auch bei Monolinuron, Propyzamid und Metamitron beobachtet, am deutlichsten nach 12 Wochen, aber auch noch nach mehr als l Jahr. Bei Napropamid war die Verstärkung der Biodegradation im Labor am bestRaUndigsten; die Wirkungen einmaliger Vorbehandlungen bestanden noch nach mehr als 1 Jahr.     

Measurement and simulation of the movement of thiazafluron, clopyralid and metamitron in soil columns

Adsorption and leaching of the herbicides thiazafluron (1,3-dimethyl-1(5-trifluoromethyl-1,3,4-thiadiazol-2-yl)urea), metamitron (4-amino-4,5-dihydro-3-methyl-6-phenyl-1,2,4-tri azin-5-one) and clopyralid (3,6-dichloropicolinic acid) were studied in one sandy and two silty-clay soils. Equilibrium adsorption coefficients (K_d) were measured using a batch equilibration procedure, and mobility was studied in repacked columns of the soils under fluctuating saturated/unsaturated flow conditions. Breakthrough curves (BTCs) were consistent with an inverse relationship between leaching and adsorption with greater mobility of the weakly-adsorbed clopyralid than the more strongly adsorbed thiazafluron or metamitron. The BTC data were used to evaluate the LEACHP simulation model. Following model calibration with respect to hydrological parameters and some of the herbicide degradation rates, the best fits between predicted and observed data were with the less adsorptive and highly mobile clopyralid. In general, the model gave acceptable predictions of the timing of the concentration maxima and the shapes of the BTCs, although earlier breakthrough than that observed was predicted with the less mobile herbicides, thiazafluron and metamitron, in the silty-clay soils. For metamitron, the total amounts leached were not predicted accurately, suggesting more rapid degradation of the herbicide in the soil columns than in the kinetic studies performed in a 1:1 soil:solution ratio shaken system.     

Enhanced degradation of some soil-applied herbicides

In a field experiment involving repeated herbicide application, persistence of simazine was not affected by up to three previous doses of the herbicide. With propyzamide, there was a trend to more rapid rates of degradation with increasing number of previous treatments. Persistence of linuron and alachlor was affected only slightly by prior applications. In a laboratory incubation with soil from the field that had received four doses of the appropriate herbicide over a 12–month period, there was again no effect from simazine pretreatments on rates of loss. However, propyzamide, linuron and alachlor all degraded more rapidly in the previously treated than in similar untreated soil samples. Propyzamide, linuron, alachlor and napropamide degradation rates were all enhanced by a single pretreatment of soil in laboratory incubations, whereas degradation rates of isoproturon, metazachlor, atrazine and simazine were the same in pretreated and control soil samples.     

Effects from simulated rainfall on shoot-zone uptake of herbicides

In glasshouse experiments, atrazine, simazine, lenacil and linuron applied to the soil surface were phytotoxic to turnip seedlings which had emerged from a depth of 1–5 cm when 3 mm artificial rainfall was applied at the time of seedling emergence or shortly afterwards. When rainfall was applied on 2 or 3 consecutive days, the herbicides were in general more phytotoxic. As the delay between emergence and commencement of surface watering increased, however, the response of the seedlings decreased. Studies with [¹⁴C]-atrazine showed that at emergence the seedlings contained a small amount of herbicide which increased considerably with surface watering. Relatively high concentrations were attained when rainfall was applied while the seedlings were small, but as the interval between seedling emergence and rainfall increased, the same uptake resulted in lower shoot concentrations, Atrazine extraction from the soil showed little movement from the surface 1.0 cm, suggesting uptake via the stem. The observed responses of the other three herbicides are explained by assuming the same pattern of uptake as that recorded for atrazine.     

Change in the Concentration of Employment in Computer Services: Spatial Estimation at the U.S. Metro County Level

ABSTRACT This article models the concentration of computer services activity across the U.S. with factors that incorporate spatial relationships. Specifically, we enhance the standard home‐area study with an analysis that allows conditions in neighboring counties to affect the concentration of employment in the home county. We use county‐level data for metropolitan areas between 1990 and 1997. To measure change in employment concentration, we use the change in location quotients for SIC 737, which captures employment concentration changes caused by both the number of firms and the scale of their activity relative to the national average. After controlling for local demand for computer services, our results support the importance of the presence of a qualified labor supply, interindustry linkages, proximity to a major airport, and spatial processes in explaining changes in computer services employment concentration, finding little support for the influence of cost factors. Our enhanced model reveals interjurisdictional relationships among these metro counties that could not be captured with standard estimates by state, metropolitan statistical area (MSA), or county. Using counties within MSAs, therefore, provides more general results than case studies but still allows measurement of local interactions.     

The efficiency of herbicide incorporation into soil with different implements

The distribution of trifluralin in soil was measured following its incorporation with the rotavator, rotary power harrow, reciprocating harrow, spring-tined harrow, tandem disc cultivator and drag harrow. A single pass with the rotavator gave an even distribution In the top 5 cm of soil and similar results were obtained by cross cultivations with the rotary power harrow, reciprocating harrow, spring-tined harrow and disc cultivator. A single cultivation with the spring-tined harrow, reciprocating harrow and rotary power harrow left much of the herbicide close to the soil surface as did both a single and cross cultivation with the drag harrow. With the tandem discs, although a single pass did incorporate the herbicide, a cross cultivation was necessary to ensure that incorporation was even enough across the treated area. A single pass of the rotavator gave a uniform mixing throughout its working depth, whereas cross cultivations with each of the other implements gave a relatively uniform mixing to about half of their working depth.