These are the important aspects in weed control in natural rice farming. This is the summary taken from 'One straw revolution' and some from others experience and some taken from my own experience. I haven't mastered these techniques yet, but think these are the things to be taken care of.
1. No-tilling - Tilling brings out the buried seeds back to surface and this has better germination capacity. So without tilling, seed resources on the field will be exhausted and hence weed population will decrease eventually. But there are other sources of seeds like rain water, wind etc. Flooding of rain water from other fields has to be prevented for better weed control.
2. Mulching - Mulching with remains of previous crop reduces weed, since mulching does not provide sunlight and hence the weed population decreases. Considerable amount of mulching will be required for better weed control. So when you are staring, mulch with as much as you can. Twigs, branches etc..provide good mulch and weed control.
3. Leguminous cover crop before - Have a leguminous cover crop in the field before planting grain and it is said that grass varieties does good in leguminous cover crop. If the grain is sown in leguminous cover crop, and then cover crop is cut and mulched, good weed control can be assured.
4. Leguminous cover crop along with rice - If there is another cover crop which grows along with grain, it is the best. Like Fukuoka san used clover along with rice. Clover does not interfere with rice, and if it takes control flooding the field weakens the clover and once the rice takes over, clover will grow beneath the rice. Once the clover is weakened and rice comes out, he later Fukuoka drains water so that clover again establishes underneath. This kind of water control is required to control the cover crop. Clover does not grow well in all the climates, so for each region a suitable cover crop has to be found out.
5. Rice that can compete with weeds - Rice also was a wild growing variety in the past, but after cultivating in controlled environment with all the extra care, they are no longer wild. But while selecting the rice, if we can use the variety which is not hybrid, rather local variety they will compete with weeds more effectively. See the video - http://www.youtube.com/watch?v=LdYAS_OPhJA where Thiru. Karikalan explains his rice cultivation using local variety called 'Kattuyanam'. This takes 6 months to mature and grows 7 feet tall. It is pest resistant and gives good yields without applying any manure. Till 1 feet there will be weeds, afterwards the rice still grows tall and weeds loose the competition. This variety is supposed to be drought tollerant as well as flood tolerant and has medicinal values.
Fukuoka san used a variety which he had developed called Happy Hill. According to his words in Natural farming book.
"I have created, together with the insects in my fields, a new strain of rice I call
"Happy Hill." This is a hardy strain with the blood of wild variants in it, yet it is also one of the highest yielding strains of rice in the world. If a single head of Happy Hill were sent across the sea to a country where food is scarce and there sown over a ten- square-yard area, a single grain would yield 5,000 grains in one year's time.". Not able to find the source of its seeds, so if anyone knows please report back.
http://ro119.com/archive/osaka.cool.ne.jp/shojaku/ - According to the following source..these are the variants of the Happy Hill rice..
Fukuoka invented and registered three varieties of rice.
Fukuoka 1 gou(Fukuoka No.1)-fast growing,
Fukuoka 2 gou(No.2)-late growing,
Fukuoka mochi 3 gou(glutinous No.3).
There are not much information available on this, but it is said to have wild strains in it.
6. Timing of crops - Depending on the season, the crop should be sown, so that it emerges first before the weeds. For e.g in kerala, weeds will start emerging heavily as the rain starts. Before the weeds start germination, rice should be sown and established. Once the crop is established, later weeds won't come that easily. A few sentences from 'One Straw Revolution' - Coping with Weeds -
If seeds are sown while the preceeding crop is still ripening in the field, those seeds germinate ahead of weeds. Winter weeds sprout only after the rice has been harvested, but by that time winter grain already has a head start. Summer weeds sprouts right after the harvest of barley and rye, but the rice is already growing strongly. Timing the seedling in such a way that there is no interval between succeeding crops gives the grain great advantage over the weeds.
7. Continous coverage - Once the field is left uncultivated after a season, weeds take over. It is better that field is continously used for one crop or other. When there is no crop, better to use a cover crop so that there is no chance for weeds to emerge. Again, depending on climates, the cover crop for that region has to be selected.
8. Crowding out weeds - The crop should be planted dense so that it can crowd out the weeds. In my first rice planting experiment, I maintained 1 feet distance between seedlings and weeds came up strongly in the vacant spaces. This kind of distance may be OK for SRI rice, since there field is completely ploughed and seasoned so that weeds are not given a chance at all. But in natural farming, we have to crowd out weeds using crops.
9. Allow weeds to germinate in summer - In one of the book Fukuoka san mentioned to water the area in the summer so that weeds germinate and then stop the watering, and weed seeds gets destroyed.
10. Weeder Ducks. Fukuoka used ducks to control weeds in his rice paddy in the initial days.
http://www5b.biglobe.ne.jp/~Aigamo21/Furuno%20book/Book1.html The following link also talks about this. Ducks don't eat rice, but young weeds, may be this is applicable in transplanted fields where the rice plants are more mature. Links -
http://www.detourjapan.com/furuno.html,
http://permaculturenews.org/2009/03/07/the-one-duck-revolution/
11. Consideration for weeds - As some body told, weeds are sown by god and crops by man. Control weeds only when it tries to control your crop, otherwise allow it to grow, change your attitude towards weeds.