摘要:

SummaryExperiments to monitor weed seedling emergence from various soil depths were begun in 1989. Recently shed seeds were buried in either narrow bands at a range of depths in the soil or mixed evenly between the soil surface and each of these depths. Total emergence ofStellaria media(L.) Vill.,Matricariaspp.,Veronica persicaPoiret,Veronica arvensisL. andPolygonum aviculareL., declined with increasing depth of burial. A similar pattern was observed forChenopodium albumL. with the exception of emergence from the surface layer. The reduction in emergence with increasing soil depth was greater for seeds in the narrow‐banded treatments than for those in broad layers. A model was developed using the data from the narrow‐banded treatments to predict emergence from distributions over broad layers. Predictions from the model closely agree with the data from the present broad‐layer treatments and those from a similar experiment made in the 1960s. By including the effects of depth of seed burial on seedling emergence, this model could be combined with models that determine the effects of cultivation on seed distribution, and therefore improve precision of predictions of seedling emergence from the see

ISSN:0043-1737    

DOI:10.1111/j.1365-3180.1996.tb01667.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryWeeds in the secondary vegetation of the fallow in slash/mulch or shifting cultivation systems have been identified as important components of the system. Weeds found in the traditional slash/mulch orfrijol tapadobean (Phaseolus vulgarisL.) production system in southern Costa Rica were identified and given preliminary rankings based on fanner surveys. The variance of these rankings was used as a measure of farmer uncertainty. Weed rankings were further characterized by key informant interviews. This study also investigated the ecological implications of the vegetative dynamics in the system. Biomass estimates of individual weed species, nutrient dynamics and crop yield response are considered in conjunction with the farmers' rankings of weeds. A negative correlation was found between weed rankings and bean yield, and between weed rankings and C:N ratios.

ISSN:0043-1737    

DOI:10.1111/j.1365-3180.1996.tb01668.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryThe effect of nitrogen (ammonium, urea and nitrate), applied during a conditioning or germination phase, on germination ofOrobanche crenataForsk, seeds was investigatedin vitro.Ammonium sulphate (8 mM) applied during conditioning in combination with the nitrification inhibitor nitrapyrin, reduced germination of seeds from 46% to 26%. A lower concentration of 4 mM ammonium sulphate or supplying nitrogen as 8 mM urea or 16 mM nitrate did not inhibit germination. Applied after conditioning during the germination phase, 4 mM ammonium sulphate strongly inhibited germination, especially in combination with a nitrification inhibitor: germination was reduced to less than 5%. Urea (8 mM) reduced germination to only a limited extent (from 58% to 40%). When a urease inhibitor (carbon disulfide) was additionally applied, germination percentage was not affected. Nitrate had no effect. It is hypothesized that the inhibition by ammonium ofO. crenatagermination is connected with a reduced ability to detoxify ammonium, owing to the reduced activities of the enzymes GS and GOGAT. The inhibitory effect of urea may be indirect, occurring through conversion to ammonium. The effect of a short‐term exposure to ammonium was largely reversible and no increase in mortality could be detected. The presence of 6 mM phosphate, Hepes or Mops buffer was found to intervene with the inhibition by ammoniu

ISSN:0043-1737    

DOI:10.1111/j.1365-3180.1996.tb01669.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryThe spatial variability of weeds within fields was studied for six sets of count data. Heterogeneity for a given mean population density was measured using the variance of the counts between sample units at different locations; relatively large values of sample variance imply aggregation. The dependence of variance on mean was measured using the relationship known as Taylor's power law, ubiquitous in animal ecology but seldom used for plant populations. This was fitted to an extensive set of plant counts and 69 estimates of its parametersb, an index of aggregation, and log10a were computed. Estimates were corrected for bias when the number of samples was small. Overall,bvaried between 1.32 and 2.61, and log10a varied between ‐0.85 and 1.58. agreeing well with previous estimates for both plant and animal populations. Parameter estimates varied with sample size and spatial sample scale, but unpredictably. Parameter values when species counts were combined were compared with individual species analyses. Knowledge of the likely range of these parameters for weed populations provides an important basis for future modelling of the relationship between weed density and crop yield los

ISSN:0043-1737    

DOI:10.1111/j.1365-3180.1996.tb01670.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryAdsorption 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 (Kd) 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

ISSN:0043-1737    

DOI:10.1111/j.1365-3180.1996.tb01671.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryBud‐sprouting dynamics, sprouting base temperature and reserve mobilization of rhizomes and stolons ofCynodon dactylon(L.) Pers. were characterized in a laboratory experiment under dark conditions. In addition, the dynamics of sprout emergence were determined during 2 years under field conditions. In the laboratory experiment, buds from the vegetative structures did not sprout at temperatures below 7^C. The rate of sprouting increased with temperature within the range 11–33^C, and a base temperature of 7.71^C was determined for bud sprouting of rhizome and stolons of the weed. An exponential model is proposed relating the percentage of sprout emergence in the field to accumulated thermal units, calculated as the sum of the daily mean air temperature above the base temperature of bud sprouting. This model satisfactorily predicted the dynamics ofC. dactylonsprout emergence in sunflower (Helianthus annuusL.) crop fields. The efficiency of carbohydrate reserve use during sprout growth was highly dependent on temperature and type of vegetative structure The efficiency was maximum at 20^C and was higher for rhizomes than for stolons. The main factors influencing bud sprouting and early weed establishment are discus

ISSN:0043-1737    

DOI:10.1111/j.1365-3180.1996.tb01672.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryAlcoholysis (methanol or ethanol) and hydrolysis (pH ≤ 8) of the herbicide bensulfuron‐methyl at 30 or 50^C involve only the breakdown of the urea part of the molecule. A high yield of the pyrimidinamine is always obtained, along with the corresponding carbamate (alcoholysis) or benzylsulfonamide (hydrolysis). The latter compound was easily cyclized (pH ≥ 6). In alkaline solution, the carbomethoxy substituent of the aromatic ring was preferentially hydrolysed. In all cases, the alcoholysis and hydrolysis rates could be described well with first‐order kinetics. Alcoholysis rate constants of bensulfuron‐methyl and bensulfuron ranged from 0.08 to 0.15 d−1at 30^C. Hydrolysis rate constants of bensulfuron‐methyl, bensulfuron and benzylsulfonamide varied strongly with pH. The hydrolysis rate constant of bensulfuron‐methyl was minimal around pH 8. The hydrolysis rate constant of bensulfuron decreased with increasing pH, whereas that of benzylsulfonamide increased wit

ISSN:0043-1737    

DOI:10.1111/j.1365-3180.1996.tb01673.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY