Primary symptoms

A wheat crop requires around 100 spikes/m2 for each t of grain (as on average each spike produces 1g of grain). Therefore, the crop requires good water and nutrient management to develop the needed number of tillers from the initial number of plants.

A good wheat crop will cover 90% of the ground by booting.

Summary

A good crop of 4 t/ha has to produce more than 400 spikes/m2 as on average each spike produces 1 g of grain. With normal sowing rates of 100 kg/ha (10 g/m2), which is 200 viable seeds/m2, these spikes could all be from plants with a main shoot and one tiller. But main shoots have few leaves, so do not capture enough solar radiation early enough to produce a good crop. Using its tillers a plant can produce many leaves quickly. These in turn capture the radiation needed for rapid crop growth.

Wheat plants can produce a tiller in the axil of every leaf and a tiller bud is ready to grow into a tiller shortly after its surrounding leaf is fully expanded. The first tiller usually appears in the axil of leaf 1 when there are 2.5 leaves on the main shoot.

Any tillers can be missing from the normal sequence. If water or nitrogen are limiting at the time the tiller bud is ready to grow, or if light is very low, that tiller bud will not grow and will not be used by the plant. Missing tillers, by their position, tell when the problem occurred.

Once the developing spike on the main shoot of the plant reaches Z3.0, few further tillers will start; maximum shoot number per m2 has been reached. If conditions have been poor until then, fewer shoots will have been produced.

Growing conditions after Z3.0 determine how many of the shoots or tillers survive to produce spikes. Even if you do everything right, some tillers will die, but with poor management many tillers will die without bearing a spike. Greater death means less interception of solar radiation during the important period leading up to anthesis. As importantly, it means less spikes and grains.

Is the crop producing enough tillers and spikes?

• Pull up some plants at several stages during tillering. How many leaves are there on the main shoots and how many tillers are there? Check the previous figure to see how your crop rates. For example, if your plants have 3.5 main shoot leaves, and no tillers, your crop is poor. If your plants have 3 tillers, it is a good crop. Do your records on the field sheets indicate that tiller numbers are increasing? The numbers of tillers per main shoot leaf shows whether the crop has problems that should be fixed.
• When the flag leaf has emerged, check spike numbers as described on p 13. Do you have the 400 spikes/m2 required for a 4 t/ha crop (80-100 /m row if using a 7" or 18 cm row spacing)?
• As your crop grows, estimate the percentage of ground covered by the crop (method) or measure it with a light bar as radiation interception. Both numbers are similar and essentially interchangeable. Compare ground cover (i.e. radiation interception) for your crop with that on the accompanying graph for the same Zadoks stage. If you are hoping for a yield of more than 4 t/ha your crop should cover about 80% of the ground by stem elongation (Z3.0 or jointing). For even higher yields you should be able to see almost no soil surface through the crop by flag leaf appearance (Z3.7). That crop is in the right hand pane of the photograph. All the light is being captured and used by the crop in producing growth. The three crops in the graph were grown at the same place but were different because they had different amounts of nitrogen applied at sowing (0, 80 and 240 kg/ha). Nitrogen increased radiation interception by making more leaf area, and yield responded in proportion.

Causes of low tiller and spike populations

• Seeds were planted very deep. If the first one or two tillers are absent and the sub crown stem section is very long, this is the likely cause.
• Basal fertilizers were inadequate to stimulate tillering. Check how much nitrogen was used and when it was applied. Pull up some plants and assess whether some tillers in the tillering sequence are absent in spite of correct sowing depth. An absent position will indicate the problem occurred when that tiller was ready to grow.
• Death of tillers has resulted in low spike numbers. Was nitrogen provided at node 1 (Z3.0)? Count spike numbers and compare with earlier tiller counts. Did many tillers abort?
• Irrigations were inadequate during tillering? Did the wilting score fall to or below 1 at the time of irrigations? Again check the timing of the problem by noting which tillers are absent or weak.
• Waterlogging occurred in association with hot weather during early tillering. Two or more days waterlogging will kill tillers if temperatures exceed 30° C.
• Solar radiation was low and it was hot. Were there several days of very overcast or foggy weather during tillering?
• Stem borers, root rots that reduce green area have suppressed tillering.
• A variety with genetically suppressed tillering may have been used.
• The seed rate was too high.
• Planting was late during rapidly rising temperatures with long days. This caused the crop to rush through to heading quickly (see the developmental stages of wheat) leaving no time for tillering.

What you can do to increase tiller and spike number?

• Planting depth: Plant as shallowly as is practicable (see planting depth and method) using the recommended seed rate for the area.
• Nutrition: Ensure that nitrogen and other fertilizer recommendations for the area are followed to optimise tiller production and then keep them alive to produce spikes (see graph which illustrates the effect of 3 levels of nitrogen applied at sowing on ground cover and yield). Consider split applications of nitrogen and placement (banding) rather than broadcasting.
• Irrigation timing: If irrigation water is very limited, at least apply water at crown root initiation (Z1.2 – Z1.3), around 21 to 30 days after sowing in warm areas.
• Adequate moisture: Preferably apply irrigations at the frequencies recommended locally and whenever the wilting score of 1 is reached.
• Waterlogging: Reduce water logging following recommendations (see waterlogging).
• Pests and diseases: Keep populations of pests and disease under control.
• Variety: Choose the right variety, which does not develop too rapidly towards heading.