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• Spacing needs to be adjusted to field conditions to find what is the optimum for the specific farmer, recognizing that these conditions can also change over time. As soil quality improves due to better management practices, wider spacing gives higher yield per unit area.
• Water management can be done several ways to achieve the same objective: having moist but well-aerated soil during the vegetative growth phase. It should be understood that rice roots need oxygen like other living things. When grown under flooded conditions, part of the roots' cortex disintegrates to form air pockets that get some oxygen to root cells and under hypoxic conditions rice roots die back. Good water management that avoids continuous soil saturation keeps roots alive and intact.

  In some countries, even without SRI practices, it is now recommended to practice alternate wetting and drying, rather than continuous flooding, during the grain filling period. So possibly the presently recommended SRI practice of keeping 1-3 cm of water on the field after panicle initiation can be improved upon. Farmers are advised to experiment with reduced water application to determine how best to provide "a minimum of water" with an understanding that roots die back in soil that is continuously saturated (hypoxic).

• The best age to transplant seedlings will depend on a number of factors that affect the length of the plant's phyllochron -- temperature, day length, soil moisture, soil and plant nutrient status, spacing, etc. Phyllochrons are periodic intervals of plant growth in which one or more phytomers (units of root, tiller and leaf) are produced from the plant's meristem. The length of a phyllochron can vary from 4 or 5 days to 8 or more, and the number of phytomers emerging during a phyllochron can range from 0 or 1 to 20 or even more.

  What is important is to transplant seedlings before the start of their fourth phyllochron of growth. This is physically indicated by the seedlings still having only two small leaves. The fourth phyllochon usually starts about 15 days after emergence in the nursery seedbed, plus or minus about 3 days. We usually recommend using seedlings 8-12 days old, but 14-day-old seedlings may be quite satisfactory and easier to handle because they are larger. In cold climates, 18-20-day-old seedlings may still be in their third phyllochron of growth. Some farmers who have experimented with this variable prefer to use very young seedlings, only 5-6 days old. There may be some small yield advantage in this, but transplanting such tiny seedlings requires more skill and effort.

• Fertilization is also a matter of choice, with best results depending on soil quality and structure. In general, compost gives the best results, but the benefits of this often take a few years to be realized. Some farmers who have limitations on their time or biomass availability will decide to use chemical fertilizers. Under some conditions, a combination of compost and fertilizer will give best results. So experimentation -- and possibly patience -- will be needed to arrive at the best solution to nutrient management. Many Madagascar farmers have concluded that they get the best results from compost by putting it on the preceding (inter-season) crop of potatoes, beans or other vegetables and then benefiting from the residual fertility of the compost for their rice crop.
• Plants per hill is factor that warrants experimentation, with the "best" practice depending on soil and other variables. SRI was developed using single plants because, when combined with the other practices, this gave more vigorous root growth. However, experimentation by Bruno Andrianaivo, senior rice specialist with the Madagascar government's agency for agricultural and rural development research (FoFiFa), has found that at least under some conditions, planting two plants per hill gives better yield than one or three. With appropriate soil conditions, two plants may give more tillering and grain filling than one without significant root competition and inhibition as occurs with three or more.

  In Sichan, China, A.B.Liu has developed a "triangular" system in which he plants three plants per hill, but with wide spacing (7cm) between them in a triangle shape, and alternately in evey other hill (a staggered pattern). There are only half as many hills per m2 with 3 plants per hill, so a 50% increase in plant number is achieved while still having significant space between plants. (The grid is 15X30 cm, but only every other hill is planted.) This also achieves "the edge effect" or "the border effect" throughout the whole field. With this spacing strategy, Mr. Liu achieved a yield of 16 t/ha using a hybrid rice variety, which was cerified by the Provincial Department of Agriculture, winning him the 2002 prize for highest yield in Sichuan Province. Farmers are encouraged to experiment with such innovation in plant spacing; fewer plants really can give higher yield per m2.

• Weeding is necessary with SRI and can actively enhance yield through soil aeration which stimulates soil biological activity. The issue is always, how much weeding is optimal? Also, there are at least three main kinds of weeding: (1) manual hand weeding, (2) chemical control of weeds using herbicides, and (c) manual mechanical weeding using the rotating hoe.

  Tefy Saina advises using the latter method "early and often." This means starting 10-12 days after transplanting with a first weeding, and then every 10-12 days until the canopy has closed in enough so that further weeding is difficult and plants shade out further weed growth.

  Several analyses have shown that additional weedings beyond the first two can add 0.5 to 2.5 t/ha to final yield. Presumably this is not due to elimination of weeds so much as to active aeration of the soil and promotion of microbial activity. These relationships remain to be studied scientifically. The recommendation to do three or four weedings or even five before canopy closure is based on empirical observations. Two weedings is the minimum for good results. Beyond this, third, fourth or fifth weedings are optional investments that should prove to be very cost-effective.