How do farming practices influence the richness and floristic composition of weeds in some Mexican maize fields?

The milpa is a historically successful Mesoamerican agroecosystem, consisting of a mixture of cultivated plants with associated vegetation that is partially managed. Conventional farming practices, like weeding and fertilization, have modified some ecological parameters of weeds, affecting their ecological, evolutionary, cultural, and productive roles. This study explores these changes from various perspectives: field experiments in fields with different management histories, contrasting treatments within these fields, farmer interviews and field walks, in San Bartolo del Llano, Ixtlahuaca, Mexico. A field experiment in milpas with different histories of farming practices (traditional vs conventional) was conducted to understand (a) the effect of the history of farming practices on richness, composition, and biomass of aboveground weed vegetation and the seed bank, (b) the effect of contrasting weeding treatments within these fields on the biomass of weeds and maize, and (c) farmers’ traditional knowledge and use of these plants. Farming practices did not affect species richness (43 species) of the aboveground weed vegetation, but a history of herbicide use increased monocot biomass and reduced useful weeds. More introduced monocots were recorded in conventionally managed fields; manual weeding led to more species, including more monocots and dicots, a higher biomass of weeds, and more native and exotic species. The composition differed between plots with different farming practices and treatments. The seed bank contained more species (54) than the aboveground vegetation. Maize ears’ and total biomass were somewhat higher in conventionally treated fields, but no differences were found between treatments. We found a considerable species turnover, with some species disappearing or becoming rare, particularly useful ones, and others being introduced‚ particularly grasses. Traditional management can help farmers keep useful weeds and higher biodiversity, thus contributing to overall productivity. Though herbicides reduce labor requirements, improved management should combine the benefits of traditional farming with some modernization.