Seed rates

Last week we introduced the topic of canopy management. This week we look at seed rates, the starting point of the change to produce a canopy to optimise yield. A word to the wise would be: "Try a small acreage first".

Generally, sowings after October show reduced yields. However, experiments with different sowing dates show mixed effects on yield. Sowings in early September can give very high yields, but the results are less consistent and such crops are much more prone to problems of delayed establishment (due to dry seedbeds), lodging and take-all, than sowings in October.

Establishment: In East Yorkshire, seedbeds are often too dry to support full germination in September. Seedlings are very vulnerable to drying during emergence. Plants whose emergence is delayed by dryness show reduced performance throughout their life.

Tillering: The earlier that a plant emerges, the more leaves it produces. Increases in leaf number give exponential increases in tillering capacity. Sowings earlier than mid-October can produce more than 20 shoots per plant, so seed rates can be very low compared to later drillings, which may only be able to produce five shoots per plant.

The full tillering capacity of wheat is only realised if plants are widely spaced and have plentiful nutrition. The main determinant of tiller production appears to be the plant's ability to capture Nitrogen (N). Tillering of early sowings will be curtailed until spring if soil N residues are low, or the plant population (per square metre) is dense.

Lodging: Early-sown crops form longer stems than late-sown crops, so they are taller and more vulnerable to lodging. Measures must be taken to reduce the lodging risks of early -sown crops. PGRs do little to strengthen stem bases.

The best way to counter lodging risk of early drillings is to establish sparse plant populations, through low seed rates. Conventional seed rates do not exploit the remarkable capacity of wheat to tiller, and to increase grain sites per ear, decisions on seed rate per square metre must be made on the basis of the expected percentage establishment and target plant population.

Percentage establishment is a result of two factors, germination (pc) of the seed used, but mainly by the seedbed conditions on, and soon after, the day of sowing.

Target plant populations should be set to balance the risks of (i) inadequate tillering and ear expansion if plants are sparse, and (ii) reduced yield and quality due to the increased lodging if plants are too dense.

Comparing low plant populations with conventional, they:

1. Have higher percentage establishment.

2. Tiller for longer (less inter-plant competition), so tiller more, even on soils with low soil N reserves.

3. Retain tillers better (i.e. show greater economy of tillering).

4. Produce more grains per ear, and slightly larger grains.

5. Are less competitive with weeds.

6. Are less prone to lodging.

7. Are less damaged by late frosts, since they have more "late" tillers.

8. Show no delay in ripening (even down to 10-15 plants per square metre).

There have been no significant differences in these effects between varieties.

Thousand Grain Weight (TGW) can vary from 35-60 grams.

A seed lot with TGW of 35 g drilled at 180 kg/ha will give 514 seeds per square metre.

A seed lot with TGW of 60 g drilled at 180 kg/ha will give 300 seeds per square metre.

It is therefore important to know TGW and drill to a seed number rather than seed weight.

Allowance for percentage establishment of seeds can be fine tuned by farm experience, but on average about 70pc of seeds establish plants that survive through to the spring.

These are my rule of thumb numbers for optimum plant establishment under average conditions. If you move outside this, you must think and adjust accordingly.

If seed lot has low germination (below 80pc) or conditions are not favourable, expected establishment should be reduced to 60pc or 50pc.

Conversely, if conditions are good and germination percentage is known to be above 90pc, expected establishment can be increased to 80pc or 90pc.