Standard procedure for determining yield components at harvest

Final grain and straw yields are measured at harvest time. A set number of hills (called hill samples) are collected at physiological maturity (PM) stage. This “hill sample” is used to obtain the harvest index (based on grain and straw yield in this sample), components of yield, green leaf area, and N concentration.  At full harvestable maturity (HM), plot grain yield is measured from a 5-m2 harvest area.  Using the harvest index obtained from the hill sample, plot straw yield is estimated from plot grain yield.

Procedure

1. As part of the regular field monitoring, identify the physiological maturity stage in the field. PM is visually identified when grain on the lower portion of secondary and tertiary panicles reaches the hard dough stage and begins to lose its green color. This typically occurs several days before harvestable maturity. Samples for determining yield components and total nutrient uptake are taken at PM stage to avoid loss of plant tissues to rapid weathering of straw that occurs in tropical climates. Therefore, make sure that sampling at PM does not get delayed.

2. In each plot, use the sampling layout in Appendix 1 to collect the hill sample from the sampling zone close to the harvest area. Select areas with a homogeneous crop stand that are representative of the whole sampling plot. Do not select sampling locations close to quadrants harvested during destructive plant sampling at earlier growth stages (border effects!). Collect components of yield samples as follows:

• • Obtain an estimate of the hill density (number of hills per square meter) of the sampling plot.

  • The number of hills (TNH) to be collected is roughly equal to one-half the hill density.

  • Collect 12 hills (3 adjacent hills from each of the four sides) of the grain yield harvest area.

  • Cut all plants at the surface. Make sure that no leaves or stems (including dead tillers) are lost. Try to take these samples without smearing soil on the leaves or stems.

3. Avoid loss of grain by carefully placing the sample collected from step 2 upside down in large paper bags or cloth bags. The bag should be pre-labeled with year, crop season, growth stage, treatment, and date of sampling.

4. To avoid deterioration, process the sample shortly after sampling.

5. If there is any soil on the stems, carefully rinse adhering soil off with clean tap water.

6. Count the number of all panicles in the sample (PANh) and calculate panicles/m2 (PAN) as

PAN = (PANh × HD)/TNH

    where TNH is the total number of hills taken and HD is the average hill density equal to Nht (defined later), divided by the sampling area.

7. Strip all spikelets from the panicles.  Strip both filled and unfilled spikelets and place these in a paper bag that has been properly labeled. Place the remaining straw in another properly labeled paper bag.

8. Processing of the grain sample: 

• • Oven-dry the whole sample at 70°C for 3 days to reduce moisture content.  After this initial drying, record the weight (g) of each sample (SpWh).

  • Immediately (do not let the grain rehydrate!) remove a 30–40 g subsample and record subsample weight (SpWss).

  • Separate and count the total number of filled (FSpNoss) and unfilled spikelets (UFSpNoss) in the subsample.

  • Place filled and unfilled spikelets in separate bags with proper labels and again oven-dry as before to attain complete oven-dryness.

  • Weigh and record oven-dry weight of filled (FSpODWss) and unfilled spikelets (UFSpODWss) in each subsample.  Avoid rehydration before weighing!

  • Save these subsamples for grinding and nutrient analyses.

• Calculate total spikelets per panicle (SpPan), filled spikelets per panicle (FSpPan), filled spikelet percentage (FSpPct), and 1,000-filled-grain weight based on components of yield (TGODWCOYOD) using

9. Processing of the straw sample: 

• • Weigh and record the total fresh straw weight (StFWh) after removing all spikelets.

  • Take a representative straw subsample of 200–250 g immediately after weighing the total fresh weight to avoid moisture loss. Record fresh weight of the subsample (StFWss).

  • Oven-dry the straw subsamples at 70°C to constant weight.  Avoid overpacking samples in the oven because good air circulation is needed for rapid and even drying.

  • Record the final oven-dry weight of the subsample (StODWss). Calculate the oven-dry weight of the hill sample (StODW) using

StODW12 = (StODWss/StFWss) × StFWh

• • Save this subsample for grinding and nutrient analysis.

10. Calculate the grain:straw ratio (GSR) as

GSR = FSpODWh/StODWh

11. Measure grain yield at harvestable maturity from the 5-m2 harvest area:

• • Harvest all hills in the prescribed grain-yield harvest area (see Appendix 1) of 6 m2.

  • If there are damaged hills (insects, diseases), estimate and describe the pest damage, but do not replace the damaged hills with undamaged hills from outside the harvest area.

  • Count the number of damaged hills (Nhd), undamaged hills (Nhu), and missing hills (Nhm) in the harvest area and record the total number of hills in the grain yield harvest area (Nht) as

Nht = Nhu + Nhd + Nhm

• • Cut all panicles and place them in cloth bags or plastic sacks labeled with site, date, treatment, replication, and plot number.

  • Thresh and clean the spikelets from each plot.

  • Dry these samples to reduce moisture content to 10–16%.

  • Remove unfilled spikelets.

  • Measure the weight of the plot grain yield in grams (PlotGy) and immediately measure the grain moisture content (MCPlotGY) with a moisture meter.

  • Correct the plot grain yield to 14% moisture content using

PlotGY14 = PlotGY x [(100 - MCPlotGY)/86]

• • Calculate grain yield adjusted to 14% and 3% moisture content (in kg/ha) from plot grain yield and the harvested area

  • (HAGY; m2):

GY14 = (PlotGY14/1,000) × (10,000/HAGY)

GY3 = GY14 × 0.887

 12. Calculate the straw yield (in kg/ha):

StYOD = GY3/GSR

13. Measure thousand-grain oven-dry weight in a grain subsample (TGODWGY3): 

• • Take a representative 100-g subsample of grain (GYss1) and oven-dry it to constant weight at 70°C.

  • Remove a 30- to 35-g subsample. Dry, weigh, and record the oven-dry weight (GYODWss2).

  • Count the number of grains in the subsample (GYNOss2).

  • Calculate TGODWGY3:

TGODWGY3 = (GYODWss2/GYNOss2) × 1,000

• • Compare TGODWGY3 with TGODWCOYOD measured at physiological maturity in the 0.5-m2 or 4-hill sample.

NOTES:

• • All calculations will be done in an automated spreadsheet template containing all equations needed.

Estimate grain yield from yield components and compare with actual grain yield measured in the 5-m2 harvest area. Typically, GY estimated from yield components is about 10–15% greater than grain yield from the large grain yield harvest area, but both should be highly correlated.