Feb07

IRRI agronomy challenge: don't spray unless it's necessary

Author // Dr. Achim Dobermann Categories // Achim Dobermann's blog

February 2: As a purist I can’t be satisfied by how uneven our crop looks. The small patches with few or no plants we see scattered throughout the field are a stark reminder of what went wrong early on. But it’s tillering time and we still have hopes that some may fill more. That’s one of the fantastic features of rice – it can tiller profusely when it has a lot of space to fill. Leigh says the best thing to do is to walk away for a couple of weeks and not worry about it. I won’t go that far.

Finbarr Horgan explains how best to manage pests - without pesticides.

Complete video playlist in this series

We were out there this morning with Finbarr Horgan, our entomologist, to look at actual and potential insect problems we may face. For most people insects tend to have a bad reputation and when they see them in the field a natural reaction is that they must be doing something bad to the young, juicy rice plants. You don’t see a lot when you’re standing outside the field, but once you go in and stare down a little closer you’ll find a whole biological microcosm in and around the shallow water layer and rice plants. Watch the video clip to see some of the critters we found this morning. Amazing stuff, particularly if you have somebody around who can tell you what all these things are and what they do.

Many rice farmers in Asia still have a tendency to spray insecticides whenever they see some damage on the rice leaves. Particularly at early growth stages, during the first 5 to 6 weeks after planting or sowing, this is almost always the wrong thing to do. Yes, we also found a little bit of leaf damage by insects such as the whorl maggot (a fly) or leaffolders, but it rarely causes a major yield loss - and we saw lots of “good” insects, those who are natural enemies of the pests we may be worrying about. Spraying now, particularly with a broad-spectrum insecticide, would have the opposite effect because it would also kill most of the beneficial insects we need to keep for biological control through the normal food webs. So, we’re happy to make that decision: no need to spray at this stage. That’s also what our guidelines for Integrated Pest Management (IPM) say.

Achim holds up leaves of a rice plant that could be afflicted with leaf folder.

Some leaves show symptoms of whorl maggot damage, but this is nothing to be worried about.

What has cropped up as a new problem are perennial grasses that have spread from the bunds into the field. In fact, much further than we had thought possible. We had no choice but to hire eight laborers for a couple of hours to do handweeding right away. Not good for the balance sheet, but it may also mean that we may not need to spray a post-emergence herbicide.

February 6: With profuse tillering ongoing, it’s time to apply the first topdressed nitrogen, as recommended in our site-specific fertilizer prescription. I came out yesterday morning to drain off the surface water. Why? We’ll be using urea and applying it on a wet soil surface followed by re-irrigation will greatly enhance the efficiency of the fertilizer.

Urea dissolves quickly and the water will move it into the root zone, where the rice plants can take it up quickly, as much as 5kg nitrogen per ha per day at this stage. What’s not taken up gets absorbed as ammonium on clay particles in the soil or consumed by microbes – or is lost. The latter is what we want to avoid.

If we would drop a large amount of urea into the standing floodwater layer the risk is great that a lot of it gets converted to ammonia gas and thus lost to the air rather than going into the plant, where we want it to go. Scientists spent a lot of time in the 1980s to figure out the biological and chemical processes behind all that. It all has to do with a large increase of the pH in the floodwater during daytime, when algae are biological most active.

Our rate was 2 bags of urea per ha, or half a bag for our field (46 kg N/ha). We roughly quartered that amount into buckets and spread it. How evenly we’ll see in just a few days. Nowadays urea is of much better quality than it used to be. It’s more granular and less prone to becoming sticky or caked up than the prilled urea that was common 30 years ago. If you spread it by hand you appreciate that difference immediately. Water has gone back on at about 5 cm depth. From now we’ll stick to the alternate wetting and drying (AWD) water management we had planned to do.

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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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