What's the difference between dry seeded rice (DSR) and aerobic rice?
The answer to this question depends a lot on what is meant by both terms—dry seeded rice (DSR) and aerobic rice.
It is difficult to give a simple answer to the question. The bottom line is that we need to carefully define what we are talking about when we use each term:
DSR means different things to different people. In thinking about the answer to the question, we need to be conscious of three totally different crop management aspects:
Strictly speaking, dry seeding is a planting method, nothing more. How water is managed is a totally separate matter.
Before dry seeding, the soil may have undergone dry land preparation. This is done either through conventional tillage , or through zero till dry seeded rice (ZT-DSR).
As the term suggests, in dry seeding, the soil is not puddled or flooded at the time of seeding.
In irrigated areas in India, the term DSR is commonly used to define more than just the planting method. It is often taken to imply that the crop is NOT grown under flooded conditions i.e., the crop receives irregular but frequent irrigation, such as in wheat. Thus, for a number of people, the term DSR is about both the planting method and water management. This causes the confusion.
In fact, water management for DSR can vary greatly (just as it can for puddled transplanted rice and wet seeded rice), from continuous flooding for most of the growing season to frequent alternate wetting and drying (AWD), to less frequent AWD, to rainfed.
In the irrigated areas of India where DSR is being introduced, the goal is to achieve similar yield to that of puddled transplanted rice, while minimizing the irrigation input to the rice.
Therefore, the dry seeded rice is not continuously flooded, but is irrigated frequently to avoid yield loss. This means keeping soil water content of the root zone (~0−20 cm) between saturation and field capacity much of the time.
Aerobic rice is grown under conditions where the root zone is maintained in a non-flooded or aerobic condition for most of the time. This means that the soil's water content in a significant part of the root zone is allowed to dry down to less than field capacity.
This comes with a yield penalty due to water loss stress. To try and reduce the penalty, specialare being developed with greater drought tolerance, but at the same time with the ability to respond to inputs, especially nutrients.
The System of Aerobic Rice involves growing these specially developed cultivars under aerobic conditions. At present, the yield potential of aerobic cultivars (under well-water conditions) is less than the yield potential of high yielding cultivars bred for puddled transplanted systems grown.
Aerobic rice is usually dry seeded into dry or moist tilled soil like in DSR, although it doesn’t have to be. Aerobic conditions can also be introduced into fields after puddling and transplanting.
Shades of grey and clear definitions
Whether dry seeded rice is the same as aerobic rice or not depends on how the water is managed—if a significant proportion of the root zone soil dries below saturation for a significant period of time. But what proportion of the root zone are we talking about, and for what period of time?
Thus it is not helpful to debate about whether the production system is aerobic or not, whether or not dry seeded rice and aerobic rice are the same thing or not, nor what the differences are.
The important thing is to clearly define the system you are talking about—which means defining three things—the planting or crop establishment method, the land preparation or tillage management, and the water management.
The term DSR is already being widely used in irrigated areas across India, by farmers as well as extensionists and scientists, to imply a combination of crop establishment method and alternate wetting and drying water management. This meaning of DSR in India, and probably South Asia as a whole, is here to stay.