P & K content of straw & chemical use |
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Potash offtake by cereals and therefore their fertiliser requirements are very considerably affected by straw removal. (P content of straw is generally low so that straw removal makes little difference to phosphate needs). To make calculation of fertiliser needs simpler, typical offtake standards are used, but in practice there is enormous variation in the potassium (K) content of straw.
As cereal plants mature towards harvest, potassium retention in the straw is markedly affected by weather and the speed of senescence. The developing use of strobilurin fungicides in recent years has been associated with "stay-green" effects on cereals at harvest.
A small survey of crops was therefore undertaken last harvest to examine whether fungicide treatment had an effect on nutrient offtake.
P & K content
Winter wheat
Disease pressure varied at different sites with grain yield responses to fungicides reflecting this variation. Strobilurin programmes gave higher grain yields than "conventional" (triazole based) programmes with visual greener foliage retention.
Nutrient content in straw ranged from 0.03-0.05% P and 0.7 - 0.9% K in the dry matter with no meaningful difference between fungicide treatments.
Winter barley
Strobilurin treatments prduced grain yield responses between 0.5 and 1 t/ha depending upon variety compared to "conventional" programmes which gave up to 0.5t/ha response. There was little variation in the P or K content in straw with values around 0.02% for P and 1.5% K in DM. - again there was no significant effect of fungicide programme on P or K content.
Desiccation
Two sites were also examined to investigate possible effects of Glyphosate desiccation on P & K content. Crop desiccation had no effect on nutrient content.
Straw yield
Whilst this survey showed that fungicide use had no effect on nutrient content, there was a marked increase in straw yield related to fungicide treatment, particularly with strobilurin programmes.
| Fungicide programme | ||||
| Nil | Triazole | Strobilurin | ||
| Winter wheat | ||||
| Site A | Straw yields t/ha | 3.7 | 4.1 | 4.5 |
| +11% | +23% | |||
| Straw as % of grain yield | 65% | 56% | 74% | |
| Site B | Straw yields t/ha | 5.2 | 7.1 | 7.8 |
| +37% | +48% | |||
| Straw as % of grain yield | 54% | 71% | 75% | |
| Winter barley | ||||
| Straw yields t/ha | 3.9 | 4.3 | 4.9 | |
| +11% | +25% | |||
| Straw as % of grain yield | 54% | 71% | 75% | |
Standard fertiliser recommendations assume typical P & K straw content and a straw:grain yield ratio of 65%. i.e. 0.65t of straw for every tonne of grain yield. For winter cereals when straw is removed, an extra 6.2kg of potash for each tonne grain yield is required compared to grain only. E.g. 10t/ha winter cereal crop requires an extra 62 kg/ha if straw is removed. The results from this survey suggest that strobilurin treatments increase straw yield relative to grain yield so that the extra potash offtake is 7.2 kg per tonne of grain yield.
Where fungicide use increases straw yield and straw:grain ratios as above, "standard" allowances for potash may need to be reviewed.
Acknowledgement: Many thanks to Arable Research Centres, Morley Research and Syngenta UK for their help in undertaking this work.
For further information please contact:
PDA info@pda.org.uk
Potash Development Association
update: Dec 2002
