Effects of different methods of cultivating degrading pastures on stem emergence and seedling of Rumex obtusifolius L. in winter: Ecological weed control in the renovation of degrading pastures

This study was designed to examine effects of different methods of cultivating degrading pastures on the stem emergence and seedling of Rumex obtusifolius L. (abbreviated to Ro) in winter. The objective was to examine a mechanism for the growth and germination of Ro for its ecological control as weed in the renovation of degrading pastures. The experiment was carried out in degrading pasture sods that were established more than 20 years ago and have been covered with Ro in the field at Kuju Agricultural Research Center of Kyushu University located at an altitude of 900 m in the middle of Kyushu highlands area. The treatment No. 1 (control) was the experiment for cultivating only once in the first ten days of October. In the treatments No.2 to No.6 there were additions of cultivating in the same period of the following months; November (No.2), November and December (No.3), etc., every month until next March (No.6), In each treatment, the sods were cultivated 500 cm in length, 180 cm in width and 20 cm in depth by a rotary machine. The sods were cultivated 5 times at the cultivation in October, and 3 times in each of the following months. In these experiments the number of stems and that of seedlings of Ro per m2 were measured for 6 months (before winter: October 26, 2001; winter: December 3, 2001, January 9 and February 9, 2002; spring: March 1 and April 3, 2002) prior to each cultivation of the sods. The temperature and precipitation in the field were also measured during this experimental period. The number of stems of Ro increased in the order of October (25.7) January (32.0) March (62.0) April (68.0) in the case of control. The treatment No.2 showed a drastic decrease in the order of October (20.3) January (4.7) April (4.3). There were also large decreases in the case of No.3 - No.6, especially Nos.5 and 6 showed a decrease to 0.3 or less in next April. When compared with control, the number of stems in No.2 - No.6 in January was less than 15% (P 0.05) with an extraordinary decrease (less than 0.02%, P 0.01) for No.3 in next April. The number of seedlings of Ro increased even in winter in the order of October (7.7) January (9.4) April (22.0) in the case of control. In the treatments No.2 - No.6 there was a reduction to 41% or less (p 0.0l) of control in December, and at last they showed 0.0 after January. In the case of control, the number of stems and that of seedlings of the plant increased in winter. It is conceivable that the tap root of Ro in winter could have the ability of stem emergence because the average of minimum temperature of 5 cm below the soil surface did not show a minus degree and the average of maximum temperature was over 10 deg C. However, the number of stems and that of seedlings of Ro in the treatments No.2 - No.6 gradually decreased in February. It is also conceivable that those plants withered and died of the physical damage such as the root cutting caused by cultivating the soil to the depth of about 20 cm using the rotary machine from late autumn to winter repeatedly, In addition there was an effect of weather conditions such as drying and freezing as a result of minus degrees of the minimum temperature of the soil suface in winter. The results from this study suggested that the method of cultivating degrading pastures in winter controlled stem emergence and seedlings of Ro, especially affected seedlings physically or environmentally. The present method would be extremely effective to improve the renovation technology of grasslands in terms of ecological control of Rumex obtusifolius as weed in the middle of Kyushu highlands area.