Population growth rate of Setaria viridis in the absence of herbicide: resulting yield loss in foxtail millet Setaria italica

P>Setaria viridis (green foxtail) is a common weed and is the putative ancestor of foxtail millet (Setaria italica). Population densities of S. viridis and its effect on foxtail millet yield were studied for 4 years in two locations, with three cultivars, under monoculture and no use of selective herbicide. Each year the density of S. viridis plants increased in all fields at a population growth rate lambda = 1.13. The stability of the lambda estimate in monoculture could serve as a reference value to predict the fate of a weed population in multi-year demographic models, using corrective factors to account for specific effects of given crop rotation and cropping practices. The low lambda value was due to a very low reproduction rate, around 6 10-4 adults per seed, which suggests high dormancy and seed predation. The yield of millet decreased linearly as the infestation increased, with the same slope whatever the cultivars. The yield loss was 0.58 tonne ha-1 for each additional plant m-2, indicating high sensitivity to competition from the wild relative. The close relationship between the weed and the crop, which could lead to similar biological responses to environmental conditions, could explain these results.