10. IRRI Agronomy Challenge 2: Measuring the Components that Determine Grain Yield (1 April 2013)

See Achim's blog at: http://irri.org/index.php?option=com_... Also see the hybrid rice harvest at    • 11. IRRI Agronomy Challenge 2: Hybrid...  

Achim Dobermann, deputy director general for research at the International Rice Research Institute (IRRI; http://irri.org), and Leigh Vial, head of IRRI's experiment station, begin IRRI Agronomy Challenge II. It is the continuation of a special project in which they demonstrate how to grow a productive rice crop in a 25 x 100-meter field on IRRI's research farm.

In this installment, Achim and Crisanta Bueno, senior associate scientist at IRRI, discuss how to measure the various components that determine rice grain yield.

Commentary from Achim:

Measuring the components of yield provides some indications for what may have caused a high or low yield. With help from our crop physiology group we collected 10-hill samples at physiological maturity stage, about a week before the combine harvest. That is the time when grain filling has been mostly completed. In the video Leny explains how it was done through pinpointing 10 random sampling locations for each variety half and then collecting 10 hills along a single row at each location. Those samples were then processed in the lab to determine the three components that make up the final yield:

yield = no. of panicles per m2 x no. of filled grains per panicle x average weight of a grain

The latter is usually expressed as "1000-grain weight", to make it easier. One can measure many other interesting things on such 10-hill samples. You can find the entire data set here [D7-2013DS Yield components.xls]. Let's just look at a basic summary of the 10-hill sampling: See: http://irri.org/index.php?option=com_...

So what does this tell us? First, you will notice that the yield measured from the 10-hill samples collected at 10 locations in each field is greater than the total grain yield measured with the combine. That is because the latter includes the whole plot and thus all gaps and areas with poorer growth, whereas the 10-hill samples may not be fully representative of that. It's a common problem and a good reminder that one cannot measure the real crop yield per area using such small samples. Small sampling areas are good for analyzing yield components, but not for measuring productivity as a whole.

The mean yield collected from those 10-hill samples was also higher in the inbred than in the hybrid, but, due to the larger variability associated with such small samples, it was not statistically different. But there were significant differences in a number of yield components. Generally speaking, the hybrid had less filled grains per panicle, more unfilled grains, and lower more unproductive tillers than the inbred. It seems pretty clear that we simply didn't have enough biomass to produce a large sink size for a higher yield.

Machine planting at 30-cm spacing resulted in a low hill density of only 16-17 hills per m2 (as compared to 25 hills if you plant at 20 x 20 cm). Moreover, the lower yield of the hybrid was probably also because it had one week shorter growth duration (less time to fill grains) and it had a large proportion of unfilled and immature grains due to the rat damage we reported on earlier, which didn't affect the inbred.