Pre and post dispersal environmental factors affecting germination and emergence of Digitaria sanguinalis (L.) Scop. in soybean crops

FERNANDO OREJA; ELBA DE LA FUENTE; DIEGO BATLLA

Hungria

Simposio; 15th European Weed Research Society Symposium; 2010

European Weed Research Society

Digitaria sanguinalis (L.) Scop. is a troublesome annual weed of soybean in the Rolling pampas, Argentina. The extended period of seedling establishment may explain the success of this weed, whose establishment depends on seed dormancy and germination, and on seedling emergence. These processes are regulated in one way by pre-dispersal environmental competitive factors (light, water and nutrients availability) and non-competitive factors   (temperature, light quality and humidity) occurring during the growth and the development of the seed in the mother plant.  And in another way seed dormancy, germination and seedling emergence are regulated by post-dispersal soil environmental factors (temperature, moisture)  moulded by the different kinds of stubbles. Quantifying the environmental factors that regulate seedling establishment would be useful to develop efficient weed management and to reduce the use of herbicides. The objectives of this work were: 1) to study the effect of soybean crop structure (inter-row spacing and maturity group) on the crop-weed pre-dispersal environment (light and temperature) and on weed plant structure and seed germination; 2) to analyze the effect of post-dispersal soil temperature and water conditions on seed dormancy and germination. Experiments were carried out in the Facultad de Agronomía de la Universidad de Buenos Aires. Chamber experiments had a completely randomized factorial design with five replications. Factors were: 1) seed post-maturation temperature (5ºC, 20ºC and 30ºC) within three seed post-maturation times (during 14d, 21d and 28 days) and 2) alternating temperatures (8/16hs: 10/20ºC, 15/25ºC and 20/30ºC). A field split plot experiment was carried out where the main plot was crop-weed competition (with/without soybean) and sub-plots were inter-row spacing (0.15 - 0.45 m), soybean maturity group (MG III - IV) and stubble type (soybean, corn and control). Periodically, radiation, far-red: red relationship, temperature and moisture under the canopy were measured. At the end of the experiment plant height, number of tillers, reproductive tillers and plant biomass were determined. The dormancy level of the harvested seeds was studied in chamber germination tests using alternated temperature (20/30ºC, 8/16h) and light. In all the treatments soybean crop reduced light availability for the weed plants, reduced the far-red:red ratio and the temperature under the canopy. At the beginning of the crop cycle, the inter-row spacing of 0.15m reduced light availability more than 0.45m, and at the end of the crop cycle MG IV reduced light availability more than MG III..The MG IV reduced the far-red:red ratio more than MG III at seed filling of the crop.  The crop increased weed plant height, reduced the number of tillers (89%), reproductive tillers (93%) and plant biomass (94%) and affected germination of viable seeds. Seeds dispersed later in the cropping season were less dormant (15%) than seeds dispersed earlier (55%). Seeds of plants growing together with soybean were more dormant than those from plants growing alone. Low (5ºC) and moderate (20ºC) temperatures were the best conditions to break seed dormancy. The highest germination was observed using alternating temperatures of 20º/30ºC (8/16h). Soybean crop structure affected the thermal and light environment under the canopy where the weed was growing, these changes where accompanied with changes in the plant structure and seed dormancy. These results could help to develop a demographic model for this weed in this crop, which according to the different crop structure, could be useful to design management strategies to reduce the number of seeds in the soil seed-bank or the incidence of the weed in the soybean yield loss.