Brassica carinata A. Braun is a non‐edible, oilseed winter crop for biofuel production that can diversify crop rotations and improve integrated weed management. The research objective was to evaluate the influence of B. carinata on weed population dynamics in the southeastern United States cropping systems. B. carinata was grown after cotton (Gossypium hirsutum L.), peanut (Arachis hypogea L.), or a clean summer fallow, and winter weed pressure was modified with or without S‐metolachlor. The emergence of Lamium amplexicaule L. increased at least 40% after cotton or peanut in 2018–2019 and 50% after cotton in 2019–2020 compared to summer fallow. Stellaria media (L.) Vill. emergence was increased over threefold after peanut or cotton in 2019–2020. Senna obtusifolia (L.) H.S. Irwin & Barneby emergence after cotton was increased at least 43% during both experimental years, whereas Amaranthus hybridus L. emergence was increased over 50% after peanut in 2019–2020 compared to summer fallow. Even without S‐metolachlor, B. carinata reduced A. hybridus (>27%) and S. obtusifolia (>25%) emergence compared to weedy winter fallow. After B. carinata harvest, A. hybridus emergence was reduced >40% with or without S‐metolachlor compared to winter fallow in 2018–2019. Overall, B. carinata biomass was highest after peanut for both seasons. B. carinata seed yield did not differ among crop history treatments in the first season, whereas in 2019–2020, the yield was higher after peanut (2417 kg ha−1) or fallow (2520 kg ha−1) compared to cotton (1710 kg ha−1). Weed management treatments were not different for B. carinata biomass or yield in any year. The results indicate that B. carinata can enhance integrated weed management strategies at the rotational level for summer crops by reducing seed banks of summer weed species, in addition to its potential as a winter biofuel crop for the southeastern United States.

Herbicide resistance is a widespread issue that impacts management of conventional farms, but also has ramifications for the weed community assembly; it is therefore important to see how these species factor into the weed community assembly of farms throughout the countryside. This research analysed species richness and community diversity in 98 field relevés from 48 organic and 50 conventional farms evenly distributed between two major production regions of the Czech Republic and then evaluated the incidence of species which have been reported resistant in the Czech Republic and its neighbouring countries. Farms were selected independently of any acknowledged resistant species. Out of 164 species found in this survey, only eight species have had herbicide‐resistant biotypes reported in the Czech Republic, while a total of 19 species had herbicide‐resistant biotypes reported in neighbouring countries. Species with recorded resistance to PSII inhibitors in the Czech Republic tended to be found together and were mostly associated with the beet production region, characterised by low altitude: Amaranthus retroflexus, Chenopodium album, Echinochloa crus‐galli and Solanum nigrum. Species with reported resistance to ALS and ACCase‐inhibiting herbicides were not clearly associated with a particular region or farming type. Of the species which have had reported herbicide resistance in the neighbouring countries, several were found in conventional fields within the growing season and we recommend immediate screening for herbicide resistance in these species and more diligent action in management according to anti‐resistance strategies: Bromus sterilis resistant to ACCase‐inhibiting herbicides, A. retroflexus resistant to ALS‐inhibiting herbicides or Avena fatua resistant to ACCase and ALS‐inhibiting herbicides. This work is unique in that it is evaluating weed species diversity in organic and conventional farms and using the context to frame the prevalence of high‐risk herbicide‐resistant species; thereby putting the potential incidence of herbicide resistance into perspective at the landscape level.

The two currently independent countries -the Czech Republic and Slovak Republic -have a common historical development. During the period of shared national identity from 1918-1992, agriculture completely changed and the agricultural policy of one state largely unified conditions throughout the whole country (Krejčí and Machonin 1996). In the second half of the century, there was violent collectivisation, large-scale socialist management and further intensification of agriculture especially resulting in the existence of large farms with large areas of land, narrow crop rotations, high doses of mineral fertilisers and the broad application of herbicides (Grešlová-Kušková 2013, Pinke 2020). The introduction of a new political market economy regime after 1989 combined with private property restitution, privatisation of state property and transformation of agricultural cooperatives (Kanianska et al. 2014).Of course, these changes also affected weed communities and diversity. Intensive farming in large