Predicting Prostrate Knotweed (Polygonum aviculare L.) and Wild Barley (Hordeum spontaneum [C. Koch]) Seedling Emergence Using Thermal Model in Karaj

Increasing public awareness and concern about the impacts of herbicides on the environment, development of herbicide-resistant weeds, and high economic cost of herbicides have increased the need to reduce the application of herbicides in agriculture. The prediction of weed emergence timing would help to reduce herbicides through the optimization of the timing of weed control. Seedling emergence is probably the most important phenological stage that influences the success of an annual weed. Wild barley (Hordeum spontaneum [C. Koch]) grows in diverse habitats in the eastern Mediterranean and in south-western Asia and is widely distributed in winter fields of Iran. Based on the recent weed surveys, it is now present in more than 16 provinces in Iran and it is increasing in in winter wheat fields. Prostrate knotweed (Polygonum aviculare L.) is one of the most widespread weeds in nearly all the temperate regions of the world. It is an annual weed that colonize open, human-made habitats and able to adapt to different ecological conditions.

In order to predict the emergence of wild barley and prostrate knotweed using the temperature model, an experiment was conducted at the research field of college of agriculture and natural resources of the University of Tehran located in Karaj during 2012-13. The first objective was to determine whether accumulated heat degree-days after wheat planting can adequately predict shoot of wild barley and prostrate knotweed emergence. The other objective of this work is that how the presence or absence of wheat affects the emergence patterns and total emergence of aforementioned weeds under growing conditions in Karaj. The experimental area was infested with natural weed populations of two species. No herbicides were used during the course of the experiment to allow the greatest number of weeds to emerge. Ten quadrats 1.0 by 1.0 m were used. Four quadrats placed at the center of the field, four and two quadrats placed at outer and inner margins of field, respectively. Half of the quadrats were assigned for sampling of seed bank and the half of them for emergence recording. Three soil samples were taken from depth of 5 cm. Soil samples were initially poured sieve (9-mesh) and the residues were placed in oven at 65ºC for 24 hours. After that, they were placed in cloth bags under running water. Weed seedling emergence was recorded every week. Weed seedlings were counted by species and cut at the soil surface to minimize soil disturbance. Weather data were obtained from weather stations located next to the experimental field. Heat sums (temperature units or cumulative growing degree days) were calculated based on the base temperature (Tbase) of 5ºC for both species. The nonlinear regression model (logistic 3 parameters) emergence versus cumulative growing degree days was used.

Results showed that the emergence of prostrate knotweed occurred earlier than wild barley in the outer margin and the center of the field at fourteen weeks after planting. Also based on the results, emergence of wild barley in outer margin of the field was happened at lower mean GDD and seedlings were appeared over fourteen weeks in 35.8 GDD. Cumulative emergence of wild barely was increased after received 50 GDD in the absence of wheat (outer margin of field) but decreased at inner margin and the center of wheat field. The presence of wheat affected the emergence of wild barely. Wheat canopy was probably developed to affect light levels or soil temperature needed for weed germination and, consequently, seedling emergence. Required GDD for 50% seedling emergence of wild barely was higher than prostrate knotweed at outer and inner margin of field and center of the field. Therefore, it is expected that wild barely was emerged later than prostrate knotweed.

Integrated weed management systems require a comprehensive knowledge of weed biology. Timing of the emergence often determines whether a plant competes successfully with its neighbors, is consumed by herbivores, infected with diseases, and whether its flowers reproduce, and mature properly by the end of the growing season. The simplicity and accuracy of this model would make it an excellent tool to predict wild barley and prostrate knotweed seedling emergence in field situations, facilitating the determination of the timing of scouting in integrated weed management systems. More prostrate knotweed seedling emerged at the margin of field before 100 GDD, thus, control methods such as herbicide must done as soon as possible at early of growing season in the field infested with this species.