Determination of critical period of weeds control in French bean (Phaseolus vulgaris L.)

Pulses are the second important for human diet. Pulses planting in agriculture systems had multiple outputs. In addition to their food importance for human and livestock, these plants play an important role in soil fertility. Growth and yield of French bean are substantially reduced by weed competition for nutrients, water and light. Application of pre emergence herbicides is quite common for weed control and it is often associated with post-emergence herbicide treatments. Alternatively, French bean growers rely on machine hoeing techniques, especially in organic farming systems. These techniques are often expensive, time consuming but they are not often successful or cost effective. The Critical Period of Weed Competition (CPWC) is a key consideration for IWMS programs and for the development of alternative weed management strategies. By definition, the critical period of crop growth cycle is length of time during which weeds must be controlled to prevent the unacceptable yield losses. The results showed that the bean is vulnerable to weeds, and weed control in bean production is the main problems in many countries, including Iran. The objectives of this study were to determine the CPWC in French bean, to gather specific information on the competition effects of weeds to this crop, and to understand the time during which yield-reducing competition occurs. Field experiments were conducted in 2009-2010 at Azad University Shoushtar Branch Agricultural Research Station (32o 15′ N, 48o 28′ E). The soil was as: 42% clay, 51% silt and 7% sand, pH=7.8, total organic matter 0.4%, and a height of 84 meters above sea level. The soil was prepared according to the local practice for French bean production. Primary tillage consisted of spring chisel plugging and it was followed by two harrowing. The trials were preceded by wheat (Triticum aestivum L.). French bean was sown at 40 seeds m-2 with the rows spaced 0.9 m apart and at a depth of 4 cm. A randomized complete block design with 3 replicates was used for all trials. Individual plots consisted of 5 rows of French bean plants, each 4 m long. In order to determine the critical period of weed removal, the duration of tolerated competition (DTC) and weed- free period (WFP) were calculated. In order to determine the DTC, plots were left weedy for 12,24, 36, 48, 60, 72 and 84 days after emergence (DAE) corresponding with 2-6, 6-10, 10-14, 14-18, 18-22 leaves, 50% flowering and 50% pod production respectively and weed free for the rest of the growing period. To determine the WFP, plots were kept weed-free for 12, 24, 36, 48, 60, 72 and 84 DAE and weedy for the rest of the growing period. The treatments were compared with two control plots kept weed-free and weed-infested throughout the crop cycle, respectively. We used MSTAT-C software to variance analyze of data. In addition, we used Duncan test to compare mean. In this research, among wide leaf weeds Physali divaricat and Amaranthus viridis has the highest density. Narrow leaf weeds included Cyperus rotundus and Echinochloa crus-galli that Cyperus rotundus had the highest number. With prolongation of interference period of weeds the number of weeds primarily showed an increasing trend, and then decreased. In interference treatments the population of broad and narrow leaves reached the highest value i.e. 45 and 49 plant m-2 respectively 48 days after germination of French bean (14-18 leaves stage). After that their population decreased. Finally, their density respectively reached to 26 and 28 plant m-2 at harvest stage (Tables 1 and 2). The impact of interference and interference free treatments on dry weight of broad and narrow leaf weeds was significant (Table 3). With increase of competition period interference length treatments, dry weight of weeds increased such that the highest value of aggregated dry matter in broad leaves in throughout season interference treatment was 426.7 gm-2, and in narrow leaves was related to interference treatment up to 72 days after germination of French bean (50% flowering) that reached 78.3 gm-2. Dry weight of broad leaves in interference treatments was higher than narrow leaves (Table 8). With the closure of crop canopy a significant reduction occurred in density and biomass of narrow-leaf weeds. The results showed that the impact of weed interference and weed free period treatments on the performance of green pod and biological yield of French bean is significant at 1% probability level (Table 4). The comparison of green pod yield means of French bean showed that interference treatments significantly decreased the yield so that the lowest yield of green pod was seen in interference treatment up to 84 days. Its value was 349.8 gm-2 that were located in the same statistical group with complete interference. In weed free period treatments, the highest value of yield was related to control treatment up to 84 days after germination that was 1959.4 gm-2, and was located in the same statistical group with complete weed free treatment. Decreasing trend of pod yield can be attributed to weeds shade, flowers fall due to the presence of competition and more allocation of photosynthesis materials to growth. Therefore, French bean yield was decreased by increasing interference periods of weeds (Table 8). According to our research, the critical period for weeds control in French bean was determined 58 and 72 days after germination for acceptable reduction yield of 5% and 10%, respectively. The results revealed the sensitivity of French bean in competition with weeds. Cultivation of beans requires correct implementation of weeds control operation particularly before planting and germination.