Summary A total of 74 independently run bioassays with soil incorporated metsulfuron‐methyl from 12 different laboratories was analysed by a logistic dose‐response curve to assess the precision of regression parameters and relate ED50 to soil properties. The potency in terms of ED50 of metsulfuron‐methyl in Brassica rapa L., which was used by all laboratories, varied between 0.05 and 3.9 g a.i. ha‐1. ED50 was negatively correlated with pH and positively correlated with organic matter. The majority of laboratories had ED50 within the interval 0.1‐1.0 g a.i. ha‐1. At one laboratory using three test species, the most sensitive species was Beta vulgaris L. followed by Brassica rapa L. and Lepidium sativum L. The coefficients of variation were smallest for the ED50 and ED90 response levels and largest for the ED10. The slope of the response curves had considerably lower coefficients of variation than the EDs. The results are discussed in relation to a previous collaborative bioassay study. Finally it is suggested that standardization of bioassays with herbicides could be achieved in the same way as standardization of chemical analyses.

Summary: This study examined the effects of competition on the growth of Avena fatua, winter wheat and winter barley. Plants were sampled at frequent intervals from replacement series experiments at two contrasting sites in the U.K. A. fatua was much slower to establish than the two cereals, but thereafter exhibited a faster rate of growth. In monoculture, it took a considerable time for A. fatua to reach a size equal to that of the cereals, but by the end of the experiments it was the largest of the three species. The change‐over from cereal dominance in mixtures to A. fatua tiominance was rapid, and in three cases coincided with cereal flag leaf emergence. In the fourth case, it appeared to coincide with the art of canopy height extension. At one site the rder of competitiveness at anthesis was A. fatua= barley>wheat, and at the other site the order was A. fatua>wheat>barley. In order to explain and predict differences between years and sites, more studies are required on morphological development in relation to abiotic variables.

Imbibed seed of 10 common arable weeds were placed in trays in initially moist soil and, after imbibing for 2 h, heated in ovenshncubators set to 31°C 42"C, 56°C 75°C or 100°C for 0.5, 1, 2, 4, 8 or 16 days or at 102"C, 155"C, 204°C or 262°C for 0.5, 1, 2, 5, 7.5 or 10 min. After heating, seeds were incubated for 28 days at 10/20"C or 20130°C on a 12 h darwlight regime, depending on species, and germination recorded. At the lower temperatures, germination of all species was prevented by temperatures of 75°C or higher for periods of 0.5 days or more. Germination was lower after treatment at 56°C than at 31°C or 42°C for all species except Rumex obtusifolius. The maximum temperature required to prevent germination varied among species and was of greater importance than the duration of heating. Germination was variable with duration of heating. At the higher temperatures, there was very little germination of any species after heating at 204°C for 7.5 min or 262°C for 5 min or more. Seeds were greatly buffered from the air temperature by 3 mm of soil, throughout the shorter duration of heating. The average temperature of the soil, over the 10 min heating required to prevent over 90% germination, varied among species and ranged from 48°C for Avena fatua to 65°C for R. obtusifolius. This work implies that composting systems maintained at 65°C are unlikely to provide an efficient method of weed control. Recommendations for improvement of the laboratory technique are suggested.

Aryloxyphenoxypropionate (APP) and cyclohexanedione (CHD) herbicides are used extensively in the UK to control grass weeds, including Avena spp. (wild‐oats). Reports of resistance to APP and CHD herbicides are a particular concern for the agricultural community. In this study, the responses of four UK Avena populations were characterized towards the APP herbicides fenoxaprop‐P‐ethyl and fluazifop‐P‐butyl, and towards the CHD herbicides cycloxydim and tralkoxydim. An A. sterilis ssp. ludoviciana population (T/41) was found to be highly resistant to fenoxaprop‐P‐ethyl and fluazifop‐P‐butyl, but did not show cross‐resistance to cycloxydim and tralkoxydim. In contrast, one A. sterilis ssp. ludoviciana (T/11) and one A. fatua population (Dorset) showed partial resistance to both APP herbicides and also showed cross‐resistance to the CHD herbicide tralkoxydim, but not to cycloxydim. Before this study, the biochemical mechanisms that confer resistance to the APP and CHD herbicides in UK Avena populations were unknown. Results from the present study show that an enhanced rate of metabolism of fenoxaprop‐P‐ethyl was found to confer resistance in the two partially resistant Avena populations (T/11 and Dorset), and the presence of an insensitive form of the target enzyme, ACCase, was responsible for target site resistance to fenoxaprop‐P‐ethyl and fluazifop‐P‐butyl in the highly resistant population T/41. Cross‐resistance to the CHD herbicide tralkoxydim in the T/11 and Dorset populations was not conferred by insensitive ACCase, and was most probably caused by enhanced metabolism. This is the first report that resistance to fenoxaprop‐P‐ethyl can be conferred by enhanced metabolism in Avena spp.

Summary: Résumé: Zusammenfassung N‐(phosphonomethyl)glycine (glyphosate) at 0·5‐1·0 kg a.i./ha controlled Agropyron repens grown in specially established 1‐m2 field plots in three out of four experiments. The addition of butyl acid phosphate or ammonium sulphate to the spray liquid improved the level of control given by glyphosate alone, where there was scope for such improvement: the addition of ammonium nitrate had no effect. When ammonium sulphate was added to either dalapon or asulam there was also no increased control of A. repens. However, when added to activated aminotriazole at 2·0 kg a.i./ha, ammonium sulphate significantly increased the level of control of A. repens in one experiment. Addition de sets d'ammonium ou d'un ester phosphorique à des herbicides en vue de la destruction de l'Agropyron repens (L) Beauv. La N‐ (phosphonométhyl) glycine (glyphosate) à al dose de 0.5–1.0 kg m.a./ha a été efficace contre l'Agropyron repens poussant au champ dans des parcelles spécialement établies de 1‐m2 dans trois essais sur quatre. L'addition de phosphate acide de butyle ou de sulfate d'ammonium au liquide pulvérisé a augments le niveau d'efflcacité obtenu avec le glyphosate seul, là oú il y avait possibilité de réaliser cette amélioration; l'addition de nitrate d'ammonium fut sans effet. Lorsque le sufate d'ammonium fut ajouté soit au dalapon, soit à l'asulame, il n'y eut pas non plus augmentation de l'efficacité contre A. repens. Toutefois, lorsque fut ajouté de l'aminotriazole activéà raison de 2 kg m.a./ha, le sulfate d'ammonium dans un essai accrut significativement le niveau d'efficacité contre A. repens. Der Zusatz von Ammoniumsalzen oder eines Phosphatesters zu Herbiziden zur Bekämpfung von Agropyron repens (L) Beauv. In drei von vier Versuchen wurde Agropyron repens in 1 m2 grossen Feldparzellen durch 0.5–1.0 kg A.S./ha von N‐(Phosphonomethyl)glycin (Glyphosate) bekämpft. Die Zugabe von Phosphorsäure‐dibutylester oder von Ammoniumsulfat zur Spritzbrühe erhöhte die Wirkung, verglichen mit Glyphosate alleine. Der Zusatz von Ammoniumnitrat brachte keine Verbesserung. Auch wenn Ammoniumsulfat zu Dalapon oder zu Asulam zugesetzt wurde, wurde die Bekämpfung von A. repens nicht verbessert. Wenn aber zu ‘aktiviertem’ Aminotrizol (Aminotriazol+Ammonium‐rhodanid) bei 2,0 kg A.S./ha Ammoniumsulfat zugegeben wurde, wurde in einem Versuch die Bekämpfung von A. repens deutlich verbessert.