IRRI agronomy challenge: another tricky choice... water management

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19 January: On the surface it seems like a no-brainer: unlike any other major food crop rice just loves to grow in fields with standing water. There are many good reasons for that. The water layer ensures that there is no water stress during critical growth stages, it enhances soil nutrient availability, it provides a home for a whole range of biological activities, it suppresses weed growth and it acts as a nice temperature buffer. Yet, it also takes a lot of water to grow a kilogram of rice. Here is a scary number: irrigated rice consumes about 25% of the total world’s freshwater withdrawals each year.

Liz Humphreys talks about water management with Achim Dobermann.

Achim Dobermann shows Golden Apple Snail and Leigh Vial talks about counting plants.
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So, our goal is to grow the rice crop in our field with safe Alternate Wetting and Drying (AWD), a best management practice for producing a good yield with 15-30% less irrigation water than normally used. Sometimes it is also called intermittent or controlled irrigation.

To get going, we installed a simple plastic pipe on one side of the field. It’s perforated, i.e., it has many holes through which the water can enter into the tube, allowing us to watch how deep the water level has fallen below the surface. The rule is simple: whenever the water is below 15 cm from the surface, we can see the bottom soil in the tube and we need to irrigate again. Straightforward, but based on a lot of research and we should be ok with that choice.

Achim Dobermann installs water tubes to help determine when to irrigate.

Achim Dobermann installs a water tube to help determine when to irrigate. 

We did the first irrigation right after the first fertilizer application on January 19, hoping to wash the dissolving fertilizer into the root zone for greater efficiency. But we now have to weigh our water management options. Without a shallow water layer, grassy weeds such as Echinochloa (Barnyard grass) may come up fast in some higher areas in the field.

On the other hand, we worry a lot about the Golden Apple Snail. It cuts off the young seedlings and moves faster through the field if there is standing water. Our field has plenty of these snails, either dug into the soil or in places where there is some standing water. We step on them all the time when walking through the field, which is not exactly pleasant. But we’re also not keen to apply a very toxic chemical to control the snails. It’s a gamble – should we manage the water for better controlling the Echinochloa, or should we try to control the snails? We can’t easily achieve both to full satisfaction.

We opted for irrigating more frequently during the week following the first irrigation, trying to keep most of the field under shallow flooding, but also hoping to not make it too easy for the snails to move around.

Leigh Vial counts rice plants in the field.

Leigh Vial counts rice plants in the field. 

26 January (from Leigh Vial)
: We did plant counts today, and just in time as tillering has started. I fabricated a 0.1m2 ring, which I threw randomly around as I walked through the field. We averaged 12.8 plants inside our ring, or 128 plants per square meter. The count ranged from 1 plant to 32 plants and 6 of the 23 counts were less than 10...perfectionists will hate this! This is a touch less than the average of 150 plants we aimed for, but given the low weed burden, I have few concerns. I have seen many excellent yields from well less than 100 plants per square meter.

We have transplanted about a 1 meter strip down the side where our birds ate so well, and manually removed some perennial grass encroaching from the bunds. Shall we call it precision management? The field is being irrigated twice a week for now to keep close to water coverage...we will wait for something like canopy closure (another 2-3 weeks?) before changing to AWD to preserve some water.


About the Author

Dr. Achim Dobermann

Dr. Achim Dobermann

Achim is a soil scientist and agronomist with 25 years experience working in Asia, North America and Europe. He is recognized internationally as an authority on science and technology for food security and sustainable management of the world's major cereal cropping systems. He has authored or co-authored over 250 scientific papers and two books on nutrients in rice and has received numerous awards from various academic, government and industry organizations. He is a Fellow of the American Society of Agronomy and a Fellow of the Soil Science Society of America.

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