This section will provide extended information on numerial assessments of where, how, and what to sample in the field.
Seedling counts
Use a stick one meter long with a mark at 0.5m to assess plant density. Put the stick between two rows in one of your preselected areas and count the seedlings on both sides of the stick. Make two counts in each of your five different areas and record the 20 numbers on the field sheets. Then work out the average of your 20 counts for the entire field. Seedling counts are done most quickly when there are only one or two leaves per plant (Z1.2 or 1.3). The presence of tillers later makes counting more difficult. If the crop is broadcast measure a square with 0.5 meter sides using your stick and count all seedlings inside the square.
Spike counts
Spike numbers should be assessed when the flag leaf (final leaf) has emerged on main shoots (Z3.9) even though the spikes aren't out at that stage. (This is the latest you can add nitrogen to improve yield.) Make two counts in each of your five selected areas just as you did the seedling counts. Do not include any late regrowth tillers. Count all shoots that have nodes at this stage (the term 'shoots' always means both tiller shoots and main shoots) and that will give a good approximation of the final spike number. Only water stress at later stages could reduce this number.
Ground cover
Estimate this by looking at the crop 2 meters in front of you through a circle formed with the thumb and index finger and held about 10cm from the eye. Guess the ground area covered by leaves and shadows. Decide first if it is more or less than 50%. Make your final estimate to the nearest 10%. Repeat for different areas and looking in different directions. Do not estimate when the sun is low and casting long shadows. Try for around midday. These values are important to gauge if and when full ground cover is reached. Ground cover and yield are generally closely related in irrigated wheat.
Green leaves
Determine the average number of green leaves per spike-bearing shoot at each visit to the crop after Z3.9. Check four shoots in each sampling area. Count leaves from the flag leaf (top) down and include fractions of green leaves if other parts are yellow.
Wilting score
(This method follows that used by R.A. Fischer.) Assess your wilting score between 11 am and 3 pm. Pick off the lowest green leaf on a shoot and remove any dead material at the end of the leaf.
Follow these three steps:
- Hold the leaf between the fingers with 5 cm protruding.
- Bend the leaf towards the ground until it is vertical.
- Let the leaf spring back and watch its movements.
The wilting score is determined by how far the leaf springs back. If the leaf springs back above the horizontal level, its wilting score is 0: the leaf is fully turgid. Scores 1 to 3 represent decreasing levels of spring-back. The score of 4 is given when the leaf has no spring, but remains hanging vertical. If any leaves have become tightly rolled to half their usual width, this is score 5, indicative of a severely stressed plant. This will probably be the top leaf, as others will have died from drought.
Always check the score before irrigation. A score of 0 means irrigation is unnecessary that day. Score 1 (on the day of irrigation) indicates that watering is just in time. A score of 2 or more indicates that the soil is being allowed to dry too far and growth and yield are suffering. You need to reduce the time between irrigations (see Moisture stress).