Search

Description of the subject. Weed pressure is a main biotic constraint in tropical agriculture. Cover crop mixtures have increased in popularity to limit weed growth through competition for resources, but the relationship between cover crop diversity and weed suppression is still under debate. Objectives. This study aimed to assess the impact of increasing cover crop diversity (one to four species) on weed control during two growing seasons (tropical summer and winter) in Reunion Island. Method. Weed control was expressed regarding ground cover by weeds and weed aboveground dry mass in the mixtures during four months of growth and its response to cover crop traits was tested using structural equation models. Results. While cover crops reduced weed ground cover and dry mass by 60% and 68% on average in summer and winter, respectively, a higher number of cover crop species within a mixture did not increase mean weed control. Nonetheless, weed control was influenced by the mixture composition and improved when including Guizotia abyssinica. Additionally, cover crop traits explaining weed control differed between growing seasons. In summer, weed control was mainly explained by the final cover crop aboveground biomass and leaf area (depletion strategy). In contrast, weed control was mainly explained by the cover crop rate of increase in ground cover (obstruction strategy) in winter. Conclusions. Using traits to characterize cover crop mixture enables us to identify mixtures of species and traits adapted to different growing conditions. Our study suggests that particular attention on species identity rather than diversity should be paid in mixture to improve weed control in tropical conditions.

In Réunion, sugarcane represents more than half of the useful agricultural area and is the main export product of the Réunion economy. The humid tropical climatic conditions promote the development of a high diversity of weeds in sugarcane fields (from 6 to 48 species at the field level but more than 200 species for the whole sugarcane area), which can cause yield losses from 30 to 80% if weed control is not carried out in time. For farmers, optimizing the efficiency of weeding requires a good knowledge of the weed flora that will develop on their fields. However, knowing this weed flora in advance is particularly difficult. The presence and abundance of species depend strongly on environmental factors, geographic location, seedbank and season. In our study, we formalize the weed-prediction problem as a multi-label supervised learning problem. We propose a comparative study of different multi-label learning algorithms to predict presence/absence and abundance of weeds in a sugarcane field. The experimental dataset used for this study is based on 20 years of historical data from phyto-ecological surveys conducted on sugarcane fields in Réunion. The results show that the ML-ARAM and ML-kNN algorithms obtain the best performances. However, the correspondence between the predictions and the ecological profiles and the confrontation of the results to experts' opinions is not always excellent. New perspectives are proposed to improve the quality of the predictions.

International audience

International audience; The EPPO code used for plants is a five-letter code more or less based on the species name. It is short, simple, unambiguous and does not change even when botanical nomenclature evolves. In tropical weed research work, EPPO codes are used in several ways: For weed community studies, EPPO codes are used for species records in the field, for data storage, aggregation and management, and for statistical analysis, mainly for tables and graphs to facilitate understanding of the results. For botanical specimens stored in the reference herbarium, the EPPO code is mentioned on the label, while for botanical photos it is associated with the date and time as combe-20210312-983421.jpg to name the photo file and facilitate its storage without risk of duplication. Finally, all species pages of the Wiktrop collaborative portal (http://wiktrop.portal.org), which is dedicated to tropical and Mediterranean weed species, have a "Brief" chapter mentioning the EPPO code of the species, which facilitates the search for a species by this code. This EPPO code is very useful for our scientific work

International audience; Le contexte tropical Objectif : Utiliser des approches relatives à la science des données pour analyser les contraintes d'enherbement dans les cultures tropicales à une échelle globale. Une masse de données hétérogènes complexes à assembler, stocker, partager et analyser

Une enquête nationale ayant recueilli 1320 réponses (839 analysées dans cet article), dont 56% d'agriculteurs, 26% de conseillers, 14% de techniciens/expérimentateurs et 4% de chercheurs, a permis de sonder leur perception des adventices. Les effets positifs des adventices ont moins d’importance aux yeux des sondés que leurs effets négatifs. Pour les effets négatifs, la perte de rendement (nuisibilité primaire directe) et l’augmentation du stock semencier (nuisibilité secondaire) sont les plus importants. Entre 60% et 80% des personnes enquêtées jugent importants les effets négatifs sur la conduite des travaux agricoles et la perte de qualité la récolte. Les effets positifs « sources de pollen pour les insectes » et « refuge pour les auxiliaires des cultures » sont ceux auxquels les répondants ont accordé le plus d’importance. Les adventices les plus citées comme problématiques sont le vulpin des champs (cité par 47,5%), le ray-grass (45,3%), le chardon des champs (42,6%), la folle avoine (17,5%) et le gaillet gratteron (16,2%). Vingt-deux relevés adventices virtuels différents étaient présentés aux personnes sondées, qui devaient indiquer leur perception du risque malherbologique et leur volonté de désherber une telle communauté. Une différence existe entre la perception du risque malherbologique et de la décision de désherbage pour les agriculteurs, les expérimentateurs, les conseillers agricoles, mais pas pour les chercheurs. Il existe une différence de perception au sein de chaque type d’acteurs, aussi grande qu'entre les acteurs. Différents profils, associés à leurs déterminants, ont pu être identifiés. Perception of weeds by farmers, crop advisors and researchers in France. A national survey (1320 responses, 839 analyzed in this article, 56% of farmers, 26% of consultants, 14% of technicians and 4% of researchers) sound out their perception of weeds. The positive effects of weeds are of less importance to the respondents than their negative effects. For negative effects, yield loss (direct harmfulness) and increase in seedbank (indirect harmfulness) are the most cited. Between 60% and 80% of the respondents consider that the negative effects of increasing costs of agricultural work and the loss of harvest quality are important. The positive effects "pollen source for insects" and "refuge for auxiliaries" are the most cited positive effect. The most cited problematic weeds were blackgrass (cited by 47.5%), ryegrass (45.3%), and Canada thistle (42.6%), wild oat (17.5%) and cleavers (16.2%). Twenty-two different virtual weed surveys varying according to different criteria were presented to the people surveyed, who had to indicate their perception of weed risk and their willingness to weed such a community. There is a difference between the perception of the weed risk and their decision to weed for farmers, experimenters, advisors, but not for researchers. There is a difference of perception within each type of actors, as big as between the actors. Different profiles, associated with their determinants, could be identified.

The Mentor, Volume 5

The Mentor, Volume 3