|Looks fertile to me...|
I hoped briefly that the discrepancy was a case of comparing apples and oranges: milling, the process of removing husk and bran layers from rice after harvest so it becomes edible, can be very wasteful. In Vietnam, the ratio of milled to rough rice is about 65%. But farmers and statisticians, it seems, all talk about rough, straight-from-the-paddy, rice when they compare yields.
IRRI , the authoritative rice institute based in the Philippines, is quite specific in its documentation of rice yields not just in Vietnam, but in the Mekong Delta in particular - and per season at that: "The rice yield is highest in the spring season (5.3 t/ha) " . The write-up does say that flooding and salt intrusion have been major constraints. The article seems to date from 2000 or 2001 (don't you hate undated content on the web?!). Since then, of course, water control infrastructure has made a big difference. A great big difference for the farmers we stopped randomly near completed sluice gates.
The farmers we talked to tended to have small plots of land (often less than a hectare). A colleague suggested that small exaggerations may have been multiplied in converting their yield per cong (one-tenth of a hectare) into yield per hectare. These farmers may also have been more successful than the average rice farmer because of rice varieties, extension services and the location of their plots. We spoke to a dozen farmers in three different provinces but they all had in common land right along the irrigation and drainage canals with good soil. Regional yield averages can mask striking differences between farms depending on the type of soil and whether they are irrigated or rain-fed.
Since my last post, I've put together a slideshow  with images by Chau Doan , a Hanoi-based photographer, describing the impact of a Bank-financed water resources management project in the Mekong Delta. Of course, I'm not trying to suggest 12 images are worth a good Excel table...but take a look anyway. The sluice gates are often quite beautiful.