Effect of Land Preparation Methods on Growth, Seed Yields of Jasmine 105 Paddy Rice (Oryza sativa L.) and Growth of Weeds, Grown in Northeast Thailand

DISCUSSION

In growing rice crop under rain-fed conditions, growers of lowland paddy in Northeast Thailand normally cultivate their rice plants in the rainy season, i.e., starting from May to November. Within the months of the rainy season, monsoon normally comes and then cool weather with short day-length follows in October. Most growers in Northeast Thailand normally plough their paddy fields once and left them out for a few weeks to allow all buried grasses or plant materials after ploughing to decompose and then they plough the paddy fields again once follow by broadcasting of rice seeds and a raking follows. They usually broadcast rice seeds to their paddy fields in May or June to obtain a dense amount of young seedlings and when the seedlings reach an age of 25-30 days after emergence then growers pull out their seedlings for transplanting into their paddy fields. The paddy field being used for transplanting of rice seedlings must be ploughed again followed by raking once. The method in transplanting rice seedlings has been practiced for many decades and it is known as a conventional practice in rice culture in Northeast Thailand. This method in rice culture normally provides a considerable amount of seed yields at the final harvest if a severe drought condition was not occurred during the growing period (Tongkamdee, 2006). The high seed yield attained with transplanting of rice seedlings was also found with the work of Singh et al. (2001) where he compared seed yields among other treatments used.

In Thailand, now a days the practice in transplanting rice seedlings has been declined due to lack of labour inputs as a result of industrialization of many products such as clothing, food processing, electronic components and many others where a number of youngsters stay away from their paddy fields but migrated to many factories of industrial zone instead. This phenomenon has caused a huge problem for villagers who grow rice although some sorts of mechanization have been employed during the past decades but it is rather expensive to invest and many growers could not afford it but hiring a tractor unit to work on their paddy fields where the hiring cost for paddy preparation could be relatively high with a small margin of profit at the final harvest. Thus large numbers of growers have shifted themselves from the conventional method of transplanting rice seedlings to broadcasting of rice seeds to their paddy fields instead. In practicing this way, nowadays growers realized that their annual harvests of rice seed yields became relatively small compared with that of the transplanting method due to many reasons such as (1) broadcasting method creates a tremendous amount of weeds, particularly when erratic rainfall occurred, i.e., a few centimeters of water above ground level was not available for rice plants since no rainwater filled up the paddy fields. This condition normally favours the germination of weed seeds and it is generally found with many higher elevated paddy areas in Northeast Thailand, (2) poor growth of rice plants with, more or less, no tillers could be produced due to small spaces available for rice plants to develop tillers, (3) it may be possible that a large amount of roots of rice plants could not be developed due to perhaps a high competition for nutrients among rice plants and weeds and also the competition for radiant energy or even CO₂ uptake.

It was found with the results of this work that there were no significant differences on plant heights due to the three methods of paddy land preparation, i.e., commencing from the first up to the fourth sampling periods except at the end of the experimental period (at day 156 after sowing) when the final sampling of all treatments were harvested, i.e., T₃ gave a mean value of plant height significantly higher than control (T₁) but similar to T₂. The results suggested that the use of rotavator blades for digging or ploughing the paddy land after the broadcasting of rice seeds of T₁ may not be a suitable method in growing rice due to perhaps some amounts of seeds did not bury into the soil or some could have been buried at a considerable depth thus poor germination could have been occurred and perhaps birds and insects could have taken away some amounts of the seeds those left out on the soil surface. It was found that a moderate amount of annual rainfalls was attained during the experimental period. Therefore, the method used for T₁ may not be an appropriate method for growers to practice. The method used with T₂ could probably be better although T₂ gave seed yield not significantly different from T₃ but highly significant over T₁. There should be some advantages derive from T₂ under a condition that if it is so happened that in a year when erratic rainfall pattern occurs when rainwater may not plentiful then high percentages of germination of seeds of T₂ could have been achieved due to a moderate amount of soil moisture content in soil aided germination of seeds, particularly those buried in soil at a considerable depth. However, a reverse condition may be resulted if rainwater could have been plentiful in that year then a condition on water-logging could have been occurred, hence the germination of seeds may not be possible thus the practice of T₂ may not be of advantage (Srisa-ard, 1982).

It was found that narrow leaf type of weeds of T₃, in most sampling periods, were relatively smaller than the rest. However, with broad leaf type of weeds of T₃, in most cases, was significantly higher than control (T₁), particularly at days 30 and 45 after emergence but later both became similar. The results indicated that the use of rotavator blades in rotavating or ploughing the paddy soil of T₁ may have caused some relative effects on weed seeds of broad leaf type perhaps by buried seeds into the soil at a considerable depth where the seeds were not able to germinate hence the amount of collected weeds were relatively smaller for T₁ than T₃. Another reason for this could have been due to previous availability of weed plants in the chosen plots, which were perhaps lesser for T₁ than T₃. A better result on weed problems among the treatments with respect to seed yields was found with T₃ where growers recognised the preparation of land of this method as a replacement of the conventional method. It seems more likely that growers must have had experiences or tested this method for several seasons long before they decided to choose this method as the most appropriate method for their land preparation.

When it comes to yield components, the results showed that unfilled seeds were similar for T₁ and T₃ but with filled seeds, it was found that T₃ gave the highest where T₁ and T₂ became similar. This could possibly be attributable to perhaps the small amount of weeds of both types (narrow and board leaves) and the rapid growth of both roots and leaves of the rice plants of T₃ as a result of the lesser amounts of weeds than the other two treatments favoured the rapid filling up of seeds. Romyen et al. (1986) stated that broadcasting of rice seeds created weed problems enormously and rice seed yields decreased significantly. The lesser amounts of weeds with T₃ than the rest could have been due to the raking practices when most grown weeds were taken away by hand at the time of raking. Tindall et al. (2005) reported that high amount of weeds significantly affected rice seed weight than the effect due to the damages caused by rice stink bug where this type of insects affected quality of rice grains rather than seed yield. A large amount of weeds in the paddy field area could always affect growth and yields of rice plants as reported by Srisa-ard (1982) where he attained only a maximum seed yield of 967 kg ha^-1 with direct seeding. Similarly, Watanabe et al. (1997) and Azmi and Abdullah (1998) carried out experiments on paddy rice they stated that weeds of many kinds severely and significantly reduced rice yields at harvests.

Some considerable amount of paddy rice seed yield (2077.50 kg ha^-1) derived from direct seeding was attained with the experiment reported by Karnjana (2001) where he added cattle manure to the paddy field at a rate of 6,250 kg ha^-1 together with a complete chemical fertiliser at 30-30-15 (N-P₂O₅-K₂O) kg ha^-1. He stated that most paddy field requires some large amounts of animal manure, particularly cattle manure. The highest seed yield (kg ha^-1) found with this work was relatively lesser than the experiment reported by Tongkamdee (2006) where he obtained an average seed yield of rice in northeastern region of Thailand up to 1,875.00 kg ha^-1 with the use of transplanting of rice seedlings. With the present work, the highest seed yield was attained with T₃ (1,136.25 kg ha^-1). However, this amount of seed yields may be recognised as a high level of seed yield. This could possibly be attributable to the low amount of chemical fertiliser applied to the paddy soil (62.50 kg ha^-1 N, P₂O₅, K₂O) and above all this soil series is one of many poor soil series found in Thailand. Thus poor soil fertility apart from the effect due to the high amount of weeds tremendously affected rice seed yield. The seed yield of T₃ may be improved largely if some amounts of chemical fertiliser had been added again to the rice plants, at least of the same amount as applied with initial application, particularly during the vegetative stage or perhaps the application should be done at approximately a few weeks prior to booting stage. Thus land preparation method of T₃ could be most appropriate if soil fertility could be relatively high. The method used with T₂ could be another choice for growers to choose when erratic rainfall occurs but it could be rather risky and it is difficult to forecast the conditions of the weather. For the results of this study, it may be possible to state that the practice of T₃ could be somewhat of a retrospective explanation of what and why growers practice this method rather than other methods. It may be of important value to find some further means and ways to eliminate weeds in paddy fields before broadcasting of rice seeds such as the use of ploughing twice at 3 week intervals and the sowing of dense legume crops right after each harvest of rice yield. The practice in crop rotation or the rotating crop patterns may be off help, particularly the use of legume crops of a high leaf density to cover the ground areas such as soybean, cowpea, mung bean, African sesbania and others where shading of leaves of legume crops could avoid the germination of weed seeds and at the same time the plant materials of these legume crops could help in improving soil fertility, particularly soil nitrogen. The growth of legume crops could largely increase soil nitrogen has been reported by Russell (1988), Garrity and Flinn (1988), Lizhi (1988), Miller and Donahue (1990), Rekhi and Bajwa (1993), Suksri (1999) and Srisa-ard (2007).