Library

Notes: A comprehensive research project was set up to characterize by different approaches some Italian populations of Lolium spp. susceptible and resistant to diclofop-methyl. The present study was aimed at the taxonomic identification of these Lolium populations on the basis of the electrophoretic analysis of seed reserve proteins. Electrophoresis confirmed the great range of variation detected by preliminary morphological analysis. Approximately half the individuals in each population could be classified as belonging to Lolium multiflorum or Lolium rigidum, the most common Lolium species present in the area. However, electrophoretic patterns revealed a significant number (40–60%) of hybrid individuals in all populations. The origin of these off-types was postulated as resulting from both intrageneric hybridization among different Lolium species and intergeneric hybridization between Lolium and Festuca genera. No significant differences were detected in the taxonomy of herbicide-susceptible and -resistant populations.

Notes: The mechanisms of herbicide resistance were investigated in two diclofop-methyl-resistant Lolium spp. populations from central Italy, Roma ’94 and Tuscania ’97. These two populations were compared with two susceptible Italian populations (Vetralla ’94, Tarquinia ’97) and a resistant and a susceptible population from Australia, SLR31 and VLR1. The activity of acetyl Co-A carboxylase (ACCase) extracted from susceptible (S) or resistant (R) individuals from the Italian populations was inhibited by both aryloxyphenoxypropanoate (diclofop acid and fluazifop acid) and cyclohexanedione (sethoxydim) herbicides. Diclofop-methyl was rapidly de-esterified to diclofop acid at a similar rate in both R and S populations. In all populations, diclofop acid was subsequently degraded to other metabolites. The rate of degradation of diclofop acid was not significantly faster in R than in S populations; however, diclofop acid was degraded more completely in Roma ’94 and Tuscania ’97 compared with the S populations. Application of the mixed-function oxidase inhibitor 1-aminobenzotriazole (ABT) significantly enhanced diclofop-methyl toxicity towards both R populations, but not in S populations. However, enhanced herbicide metabolism does not completely account for the measured resistance level. A mechanism other than an altered ACCase and enhanced herbicide metabolism appears to be responsible for resistance to diclofop-methyl in Roma ’94 and Tuscania ’97.

Notes: Four tests were carried out in 1980 and 1981 to determine: (a) the economic threshold density of Abutilon theophrasti Medicus (velvetleaf) in maize, and (b) seed production with varying densities of infestation, both in the presence and in the absence of maize. The infestation was artificially created, and the density of the weed ranged from 0 to 80 plants m−2. The economic threshold, calculated using the Cousens (1987) model, varied between 0⋅3 plants m−2 and 2⋅4 plants m−2, depending on the variables considered. The presence of maize reduced the seed-rain of A. theophrasti by 50%. This seed-rain reached its maximum level at 20–30 plants m−2 in maize, and at 30–35 plants m−2 in weed monoculture. However, with only 4–5 plants m−2 in competition with maize, A. theophrasti produced 8–10 thousand seeds m−2. The usefulness of threshold density in weed management is debatable when one considers the ecological characteristics of the A. theophrasti seed, and the great capacity of seed production of this weed.

Notes: Des simulations d'échantillonnage ont été realisées sur des données provenant d'une expérimentation commune du groupe de travail EWRS ‘Potentiels semenciers des sols agricoles’. Cinq parcelles agricoles ont été choisies et leurs stocks semenciers analyses. Une étude par simulation de la relation liant la précision relative et le nombre de prélèvements regroupés en composites a montre (1) que l'hypothfèse de normalité des distributions des moyennes spécifiques n'est réalisée que pour environ 30% des séries lorsque le nombre de prélèvements est égal a 30 et pour 75% des séries lorsque ce nombre est de 150; (2) que la précision relative est fonction d'une part de la taille du composite (nombre de prelevements) et d'autre part du nombre moyen de semences par prélévement; (3) I'existence d'une relation de type exponentiel liant la précision relative et le nombre moyen de semences par prélevement sur les différentes parcelles expérimentales. EWRS collaborative study of seed bank estimation, II. Estimation of the relative accuracy of the mean using the compound sample procedures Sampling simulations were made using the data from a joint experiment of the EWRS working group ‘Seed banks of agricultural land’. Five agricultural plots were selected and the seed banks analysed. A simulation study of the relationship between the relative accuracy and the size of the compound sample (number of soil cores) showed (1) that the hypothesis of normality of the distribution of the species means was true for only about 30% of the sets when the compound sample size was 30 and for about 75% of the sets when the size was 150, (2) that the relative accuracy was dependent partly on the size of the compound sample and partly on the mean number of seeds per soil core, and (3) that there was an exponential relationship between relative accuracy and the mean number of seeds per soil core for the various experimental plots. EWRS-Ringsversuch zur Bestimmung der Samenbank im Boden. II. Schatzung der re-lativen Treffsicherheit des Mittelwerts von Sammelproben Mit Datenmaterial aus einem Ringversuch der EWRS-Arbeitsgruppe ‘Samenbank land-wirtschaftlich genutzter Boden’ wurden Pro-bennahme-Simulationen vorgenommen. Dazu wurden 5 Felder ausgewahlt und deren Samenvorrat analysiert. Eine Simulations-Studie über die Beziehung zwischen der relativen Treffsicherheit und der Sammel-probengröße (Anzahl der Bodenproben) ergab zum einen, daß die Hypothese der Normalverteilung der Mittelwerte von Arten nur in etwa 30% der Datensätze bei einer Sammelproben-größe von 30 und in etwa 75% bei einer Sammelprobengröße von 150 zutraf. Zum anderen hing die relative Treffsicherheit teils von der Größe der Sammelproben, teils von der mittleren Samenzahl pro Bodenprobe ab. Drittens bestand für die einzelnen Versuchsflächen eine exponentielle Beziehung zwischen der relativen Treffsicherheit und der mittleren Samenzahl pro Bodenprobe.

Notes: Thresholds for winter wheat weed control have been determined for different herbicides or mixtures effective against individual weed species. For Avena sterilis L. subsp. ludoviciana (Durieu) Nyman the economic threshold was between 7 and 12 plants m−2, for Alopecurits myosuroides Hudson and Lolium multiflorum Lam. it varied between 25 and 35 plants m−2, while for Bromus sterilis L. the values were just under 40 plants m−2. For Galium aparine L. the threshold was as low as 2 plants m−2, while for Vicia sativa L. it was between 5 and 10 plants m−2. Within the same species the different values of economic threshold result from the different costs and efficacy of herbicides. Nitrogen fertilization was important: in its absence, the competitiveness of B. sterilis and G. aparine was negligible, and it was not possible to calculate an economic threshold for some of the treatments which could not be justified economically. Estimation de seuils économiqu.es de désherbage chez le blé d'hiver Les seuils économiques de désherbage du blé d'hiver ont été determinés pour différents herbicides, seuls ou en mélanges, pour plusieurs espèces demauvaises herbes. POUT Avena sterilis L. subsp. ludoviciana (Durieu) Nyman, le seuil économique était compris entre 7 et 12 plantes m−2. PourAlopecurus myosuroides Hudson et Lolium multiflorum Lam., il variait entre 25 et 35 plantes m−2 et était juste au-dessous de 40 plantes m−2 pour Bromus sterilis L. Pour Galium aparine L., le seuil était2 plantes m−2 et était compris entre 5 et 10 plantes m−2 pour Vicia sativa L. Les différences de seuil pour une même espèce sont dues aux différences de coût et d'ef-ficacité des herbicides. Le rôle de la fertilisation azotVe Vtait important: en son absence, le pou-voir compVtitif de B. sterilis et de G. aparineétait négligeable, et il n'était pas possible de calculer de seuil économique pour certains traitements, qui ne pouvaient se justifier économiquement. Bestimmung von wirtschaftlichen Schadensschwel-lenfiirdie Unkrautbekdmpfung in Winterweizen Für verschiedenen Herbizide und -mischungen wurden entsprechend ihrer Wirksamkeit gegenüber bestimmten Unkrautarten Schadensschwellen zur Unkrautbekämpfung inWinterweizen bestimmt. Für Avena sterilis L. spp. ludoviciana (Durieu) Nyman lag die Schadensschwelle zwischen 7 und 12 Pflanzen m−2, fur Alopecurus myosuroides Hudson und Lolium multiflorum Lam. variierte sie zwischen 25 und 35 Pflanzen m−2, während die Werte für Bromus sterilis L. bei knapp 40 Pflanzen m−2 lagen. Bei Galium aparine L. war die Schadensschwelle ganz niedrig, nämlich 2 Pflanzen m−2, während sie für Vicia sativa L. zwischen 5 und 10 Pflanzen m2 betrug. Fiir ein und die-selbe Art ist die okonomische Schadensschwelle je nach den Kosten und der Wirksamkeit der Herbizide unterschiedlich. Die Stickstoffdün-gung ist zu beachten: wenn sie fehlt, ist die Konkurrenz durch Bromus sterilis und Galium aparine verachlässigbar, so daß keine gerechtfertigten ökonomischen Schadensschwellen berechnet werden konnten.

Notes: Weed density was sampled in a number of quadrats in five crops and at eight locations in Italy. For every location and species of crop and weed, a mean/variance pair was calculated. The relationship between these two parameters was significantly influenced by crop and, to a lesser extent, by site, but not by species of weed. The goodness of fit of the negative binomial distribution to the samples was assessed. Its k exponent was estimated on the basis of the observed variance/mean regression. A good fit was found for weed populations with fewer than 10 plants m−2. The applicability of a sequential sampling procedure based on the negative binomial distribution to a weed control strategy that uses a threshold density is discussed. Denombrement de la fréquence des adventices et applicabilité a une méthode d'échantillonnage séquentiel pour un desherbage integré La densité en adventices a étééchantillonnée dans plusieurs cadres dans 5 cultures et à 8 endroits en Italic. Pour chaque lieu et espèce de culture et de mauvaise herbe un couple moyenne/variance a été calculé. La relation entre ces deux paramètres était très dépendante de la culture et à un degré moindre du lieu, mais n'était pas influencé par l'espèce d'adventice. La validité de l'application de la distribution binomiale négative aux échantillons a été estimée. Son coefficient k était estimé sur la base de la regression observée sur variance/ moyenne. Une bonne adéquation a été mise en évidence pour des populations inférieures à 10 plantes m−2 L'applicabilité d'un protocole d'échantillonnage séquentiel basé sur la distribution binomiale négative, à une stratégic de désherbage utilisant un seuil de densité est discutée. Frequenzverteilung in Unkraut-Vegetationsaufnahmen und Aufnahmemethode im Rahmen der integrierten Unkrautbekämpfung Auf der Basis der Dichte von Unkräutern in 5 Kulturen an 8 Orten in Italien wurden Mittelwert und Varianz berechnet. Das Verhältnis dieser beiden Parameter wurde durch die Kulturpflanzenart signifikant, weniger durch den Ort und gar nicht durch die Unkrautart beeinflußt. Die Güte der Anpassung an dienegative Binomialverteilung der Erhebungsdaten wurde bestimmt. Ihr k-Exponent wurde nach der beobachteten Varianz/Mittelwerts-Regression bestimmt. Für Populationsdichten von 〈 10 Pflanzen m−2 ergab sich eine gute Anpassung. Die sich aus der negativen Binomialverteilung ergebende Bekämpfungsstrategie aufgrund von Schadensschwellen wird diskutiert.

Notes: Populations of Lolium spp. collected in central Italy were screened for resistance to acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides and compared with known susceptible and resistant Lolium rigidum (Gaud.) populations from Australia. Populations Roma’94 and Tuscania’97 were up to 8- and 7.5-fold more resistant to diclofop-methyl, respectively, than susceptible populations in pot experiments. However, populations Tarquinia’97 and Vetralla’94 were not resistant. Diclofop-methyl resistance levels in the Italian populations were lower than in the Australian populations SLR31 and WLR96 (16.5 and 〉 64 times more resistant than S respectively). In an agar germination test, Tuscania’97 showed low levels of cross-resistance to fluazifop-p-butyl, whereas no cross-resistance was found in the Roma’94 population.

Notes: A prerequisite for a precise estimate of the abundance of weed seeds in sou is prior knowledge of sampling variability, Based on an analysis of EWRS working group trials, it is shown that the sampling variance (s2) of seed counts increases with the sampling mean (S2) and can be predicted with the logarithmic form of Taylor's power law: log10(sz)=a+b log10(x̄). This relationship is constant over time for means greater than 0.1 seeds per core within each of the five different sites studied (temporal variability) but differs slightly among sites (geographical variability). An attempt is made to use a general s2 : x̄ relationship to predict the number of samples as a function of precision and density.