Section 3: Assessing and measuring your crop

Why bother with measurement?

Often you do not need to make any measurements of number, length, weight and distribution because you can quickly gauge the extent and importance of problems by eye. But there are sometimes benefits of measurement.

• Measurement makes you look at the crop or field in detail and actually helps you to see things that otherwise you would miss at a quick glance.

• Assessing a crop with many thousands of components is very difficult and difficult to do objectively. Measurement forces objectivity. Without structured measurement, we tend to look at large or brightly coloured things and give them greater significance than small things.

• Measurement leads to numerical descriptions of the crop. It helps you determine not only that you have a problem but also that you have a problem that is big enough to fix or small enough to ignore. Is the problem worse or less than last year? Are you really a better farmer than your neighbour? You don’t assess your yield by eye; you weigh it.

Using your senses to assess problems in your crop

Examine the crop and soil with all your senses and try to perceive patterns.

• Feel your plants. Rapidly growing leaves feel cool and soft. Leaves that are stressed feel too warm and can be floppy (p 15) and possibly harsh to touch (p 66). Feel your soil. Sandy soil is rough; clay is smooth (p 15).

• Smell. Smell the soil. Waterlogged soil smells sour. Smell the crop; the disease bunt smells like bad fish.

• Taste. Taste your irrigation water, and a water extract of the soil. Saline water and saline soils really do taste salty (p 34).

• Look. Most of all use your eyes. The following checklist depends mainly on eyeballing the crop.

First level of assessment: your eyeball check list

1. Look at the crop as a whole. Is it uniform?

Colour and colour distribution

Is the crop a good colour; are there patches of off-colour?

Size and shape

Is the whole crop as tall and thick as you would hope?
Are there patches where the crop is short or too tall?
Patches where it is too thick or too spindly?

2. Look at the crop as single plants. Compare good and poor plants.

If you have decided there are patches of crop that are different, pull up a plant from inside a patch and compare it with one from outside the patch, one plant in each hand. Smell the roots; do they smell different, sour? Feel the leaves; are they soft but turgid, are they floppy?

Colour and colour distribution

Are the good and poor plants different colours?
Is the colour difference all over or
Is colour just different on some parts?

Is it the old leaves that are different?
Is it the young leaves that are different?
Is it the stems, or is it the roots that are different?

Size and shape

Are your plants different in size?

Is it because one has more shoots than the other?
Is it because the leaves are longer?
Is it because the plant is taller?

Always try to split up the problem into smaller things you can describe.

3. Reassess what you have seen.

Are the differences in your comparisons, different in other plants? Go back and take a few more samples and make sure. Are their symptoms more or less? Look back to the crop as a whole. Are these symptoms actually more widespread than the patches you first noticed but not quite so obvious as in the initial samples?

4. Use your information to identify the problem.

By now you should have enough information to make a good first guess of the cause of the problem using the descriptions in Section 5 starting page 25. If you later want to assess the severity of the problem on a numerical scale, use the methods following.

Where, how and what to sample in the field? A numerical assessment

Much of this section is adapted from the Australian guide, SIRAGCROP, Field observations and crop standards for wheat by Maarten Stapper and David Murray.

Use field sheets for recording observations of your crops

Take a copy of the field sheets (p 17) with you to the field to help determine growth stages of the crop and so you can write down your observations and counts in a structured way. Use the same field sheet each time you visit that field. At each visit, make notes of unusual conditions of the crop such as herbicide damage, frosted leaf tips, frost damage on spikes and any leaf curling. An accurate record of crop management as the season progresses (sowing, fertilizers, weed and disease control, irrigations, harvest) will help you to identify which factors limit yield most.

Where to sample

Your observations should be representative of the whole field. Select five separate representative areas and mark them. Avoid headlands, crop edges, and areas near trees. Visit the same areas each time you return to the field.

Seedling counts

Use a stick one metre long with a mark at 0.5 m 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 whole 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 metre sides using your stick and count all seedlings inside the square.

After the flag leaf emerges, assess the whole crop

HM Rawson

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 final spike number. Only water stress at later stages could reduce that number.

Ground cover

Estimate this by looking at the crop 2 metres in front of you through a circle formed with the thumb and index finger and held about 10 cm 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 (details on p 44).

Estimate ground cover by looking at a small circle of crop defined by your thumb and forefinger. Crop cover here is about 60%

HM Rawson

Green leaves

Determine the average number of green leaves per spike-bearing shoot at each visit to the crop after Z3.9. Check 4 shoots in each sampling area. Count leaves from the flag leaf (top) down and include fractions of green leaves if other parts are yellow.

Leaf diseases

Look closely for diseases in each sampling area on each visit and as you walk between sampling areas. First identify the diseases. To estimate the percentage of the crop that is infected, walk diagonally across the field and collect 50 shoots as you go. If the field is 50 metres across, pick a sample at every stride. From the collection, count the total number of green leaves, and the number of green leaves with a particular infection. Calculate the percentage of infected green leaves in the 50 shoot sample for each disease present as:

Whether or not to spray will depend on the susceptibility to the disease of the variety and the growth stage. Consult the section on diseases in the booklet (p 73) and the latest recommendations on control for your district.

Lodging score

This has two components

1. the percentage of the whole field that is affected by lodging
2. the degree of lodging from slight to moderate to severe within those areas.

Slight lodging is when the plants are just leaning over. With moderate lodging the plant height is reduced to half the plant length. Severe lodging is when the crop is flat on the ground. Check the information on lodging on page 49.

Use this wilting score to indicate when you should irrigate

After RA Fisher

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 the three steps in the illustration. When the leaf springs above the horizontal to score 0 the leaf is fully turgid. The score of 4 in which the leaf has no spring, but remains hanging vertical, is the worst score illustrated. 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 each 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 page 64 for suggestions on irrigation timing).

Evaluation of soil texture by feel

Method and drawings after Ilaco (1985)

What is your soil type?

Drip water onto about one tablespoon of fine soil held in your hand. Squeeze and roll the soil until it just starts to stick to your hand. The extent to which it can then be shaped as in the drawing, gives a rough idea of its texture class.

(A) Sand. Soil remains loose and single-grained and can be heaped, but not formed.

(B) Sand loam. Can be shaped into a ball that easily falls apart. With more silt (C) it can be rolled into a short thick cylinder and is called a silt loam.

(D) Loam. About equal sand, silt and clay and can be rolled into a thick thread about 15 cm long that breaks when bent.

(E) Clay loam. Soil can be rolled as above but can also be bent carefully to a U shape without breaking.

(F) Light clay. Soil feels smooth and can be bent into a circle with some cracks.

(G) Clay. Handles like plasticine and can be bent into a circle without cracks.

How to estimate how much soil moisture is available to your crop

Once you know the textural class for your soil you can use the next table to estimate how much soil moisture is potentially available to your crop.

Wheat roots may eventually grow to 1.2 m, but if their depth is less than 1 m the amount of moisture available to them is reduced proportionately (so if roots have grown to 0.5 m, halve the red values in the table). Check your rooting depth by the method described below.

Table used to estimate the amount of water in your soil by feel
Red values are the amount of water potentially available to roots per metre of soil depth

Determining rooting depth and any barrier to root penetration

This method follows Lafitte (1994). The root system of a mature wheat plant can reach a depth of 1 to 1.2 m, but if some barrier exists in the soil above 1.2 m, the roots will stop at the barrier and be unable to tap deeper water and nutrients. Potential yields will be reduced. If there seems to be a problem check for barriers in two or three places in a field though one place may be adequate if the crop looks uniform and is on a level area.

Pick an area where the plants look uniform, and start digging with a spade, preferably when the soil is near field capacity (p 16), as digging in dry soil can be difficult. Dig until you strike any barrier. This can be a plough pan, a hard pan, a rock layer, an impermeable clay layer, acid subsoil, a water table, or a salt band. If you are uncertain that what you find is a barrier, examine the roots of the crop to see if they penetrate the area. If not, record this depth. Dig to about 40 cm, as plough pans usually form above this level and use a soil corer from there to 1m. Break the cores at several places to see if roots are still present at different depths.

Field sheets. A guide to recording your observations

You will find the field sheets in a pocket at the end of this booklet. There are two sets of pages; the first includes example data from an actual crop and pictures of the Zadoks growth stages while the second set is blank for you to add your own data. Make as many copies as you need and then replace the originals for future use. In case someone borrows the originals, A5 (small) copies are included here.

Take a copy of the field sheets with you to the field to help determine growth stages of the crop and so you can write down your observations and counts in a structured way. Use the same field sheet each time you visit that field. At each visit, make notes of unusual conditions of the crop such as herbicide damage, frosted leaf tips, frost damage on spikes and any leaf curling. An accurate record of crop management as the season progresses (sowing, fertilizers, weed and disease control, irrigations, harvest) will help you to identify which factors limit yield most.

At each visit, write down the date in the first column opposite the plant growth stage that you have identified. Notes in the second column and pictures on the sample sheets will help you to decide on the plant growth stage. The third column is for your estimate of ground cover (p 14) as it increases from seedling emergence towards flowering (p 44) and green leaf number reductions thereafter (p 14 and p 48). Pages 44 and 48 tell you what numbers to expect. The fourth column is for a summary of your management decisions as the crop advances and for dates of your actions. The rows that cut across the full table width are for your calculations of the state of the crop. Example target numbers for a high yielding crop are provided for comparison. If your calculated numbers disagree substantially with the targets, things that you might like to check in your crop are also listed in those rows.

Field sheets adapted from Maarten Stapper and David Murray (SIRAGCROP)

Field sheets adapted from Maarten Stapper and David Murray (SIRAGCROP)