With Spring drilling fast approaching and seed orders coming in thick and fast I thought it might be pertinent to discuss a simple but important question.
What seed rate to use?
Of course this depends completely on the context for instance, soil type, fertility, climate zone and sowing scheme and sowing date, as well as additional biotic stress including disease and weed pressure.
First and foremost , whilst it is difficult for an organic farmer to find technical advice and guidance for a specific plant population to target, this should be the foundation of any seed rate calculation, working from the number of desired plants per m2 to the kg/ha value that is eventually used to calibrate the drill to. The thousand grain weight (TGW) is used in the calculation as well as adjustments being made for germination % and potential field losses. If TGW is not incorporated in the seed rate, plant populations could vary wildly form season to season and between seed lots, or new and home saved seed. It might be one contributory factor as to why organic yields can be so variable.
The AHDB has a very simple tool for converting seed rates but is too simplistic as it fails to include the adjustment factors.
KWS have a seed rate calculator that does include germination %
However, my preferred calculation also takes into account the additional correction factor of suspected field losses and could be thought of a small insurance around 5-10%, unless for any particular reason, particularly poor conditions around drilling, it might be higher.
A useful tool for calculating seed rate and quantity, with all the correction factors is from Cope Seeds.
The calculation for seed rates determined from target plant population is thus;
This comes from the PGRO website Choice and use of seed | PGRO
If for a Spring Bean a target population of 40 plants/m2 is desired, with the seed having a thousand grain weight of 600 and a germination of 97% and suspected field losses of 5% then this would give a seed rate of 260kg/ha
For spring drilling the risks associated with higher seed rates that exists for Autumn drilling (higher weed pressure, higher disease pressure, greater lodging risk) are less pronounced. The shorter growing season, less time for yield component building and compensation and the fact soils are colder and often wetter than in the Autumn, mean target plant populations should generally be higher than for winter crops. Adjustments should be made for the environmental conditions for example, for higher fertility situations and for earlier drilling, seed rates should generally be lower. I would still argue, without the ability to fertilise the crop in situ and with the shorter foundation phase, that, slightly higher seed rates should be used, as an insurance, and to offer higher ground cover to compete with weeds.
Not to sound like a broken record but without the research into optimal organic seed rates, the answers must always lie in experience, your own context, and in your own farm research, varying seed rates to see within a given situation which seems to work best. A management system that uses wider rows will necessarily need to reduce seed rates as the number of rows per m2 is reduced, maintain seed rates at the level of narrow row systems would result in too much intra row competition.
So far I have been thinking more in terms of yield, but seed rates, and hence final plant populations will also affect quality which is another important thing to consider.
Seed rates that are recommended in the farm management handbook are;
Wheat 500-550 seeds/m2
Oats 650-700 seeds/m2
Beans 45 seeds/m2
The wheat and beans targets seem appropriate whilst the barley target may be a little low and the oat target seems far too high.
Please be aware we still have spring seed on offer, with stringent disease testing and the quality and vigour likely to be superior to home-saved seed.