Weed seed predation in organic and conventional fields

Enhanced biological control of weed seeds may improve sustainability of agricultural production, Biological control due to seed predation may be higher in organic fields because organic production generally supports more seed predators. To investigate such a difference, weed seed predation was studied in autumn in eight organic and eight conventional mixed cropping fields in New Zealand. Predation rates were estimated by observing removal rates of seeds of the annual weeds fathen or common lambsquarter (Chenopodium album) and Persian speedwell (Veronica persica). The seed losses were recorded at distances of 0, 3,9 and 50 in from the field edge. The weed seeds were placed at the above distances on small dishes of finely sieved soil and sunk into the ground. Removal rates were recorded after 48 h. Seed predators were identified using video recording at an organically grown husbandry unit at Lincoln University and in two of the fields used for estimating seed predation. Recording of predators had therefore limited overlap with seed predation assays but was expected to give important information on key seed predators in the region. The mean seed removal rate was 17% in organic fields compared with 10% in conventional fields although variation was high. Significantly different removal rates between the two field types were found only 9 in from the field edge with approximately four times higher losses of fathen seeds in organic fields. There was also a strong tendency towards higher seed losses at organic field edges. Overall, there was no consistent effect of distance from the field edge. Vegetation had a significant influence on the predation rates, with maximum rates at a medium-dense plant cover. Based on the video images, birds were the most important seed predators. The higher weed seed predation rate in the organic fields indicates that there may be an economic advantage associated with the well-established trend that bird populations are generally higher in organic agricultural situations. (c) 2008 Elsevier Inc. All rights reserved.