What will happen to late-sown wheats if spring 2020 turns dry?

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Peter Kettlewell, and Martin Hare, of  Harper Adams University look at the threat of a dry spring following such a wet autumn and winter in this months edition of Agronomist and Arable Farmer

Over England and Wales as a whole, the 2019 autumn rainfall was well above average, but still below that of the wettest autumn on record in 2000. The total for September, October and November 2019 rainfall was 412 mm compared with 502 mm in 2000. But this national value hides remarkable regional differences. Parts of Cambridgeshire, Essex and Suffolk had about average rainfall, whilst Yorkshire and Lincolnshire suffered with nearly twice average rainfall.

Many fields in the wetter counties have been sown late with wheat in wet soils or remain unsown. On lighter soils, drier weather in February may allow winter wheat seed to be sown very late, but only for varieties with an end February latest safe sowing date, such as Skyfall or Leeds – check the AHDB Recommended List for the latest safe sowing dates for other varieties. If the rain continues, and especially on heavier soils, spring cropping will be the only solution.

Problems from dry weather will be far from grower’s thinking at the moment, but we all know our weather is very fickle and it’s not unknown for a wet autumn and winter to be followed by a dry spring. This happened after the autumn 2000 record. The winter continued wet, but the rain diminished in May which had only 42 mm rain compared with a long-term (1766-2019) average of 64 mm. What will be the consequences for the late-sown wheats if this pattern happens in 2020?

Late-sown crops don’t have time to put roots down to the 1.5 to 2.0 m depth that wheat sown early into sufficient depth of well-structured soil can achieve. This limitation of insufficient time will have been exacerbated in less well-drained fields. Roots need oxygen to grow, which is in poor supply in waterlogged soil. The lack of oxygen also leads to growth-inhibiting acids and the plant hormone ethylene being produced from rotting trash, which can both further reduce root growth.

If spring and early summer bring adequate rainfall, then many late-sown crops may yet turn out well. But in a dry spring these shallow-rooted crops will be unable to exploit subsoil water reserves and will suffer at the crucial flag leaf to booting stage. Once a wheat crop is past establishment, then the greatest damage to yield from water shortage occurs at these stages. Current theory is that lack of water in the plant at these stages reduces the proportion of living pollen grains, so fewer grains form and yield goes down.

If we get a dry spring, what can be done? A few farmers irrigate wheat, but the capital investment is only justified in the UK by growing high value crops such as potatoes. The wheat generally only gets a turn if there is plenty of water available and the high value crops are not yet at their critical stage for irrigation. At present, serious water limitations are too infrequent to justify the capital investment needed for the wheat crop alone.

There is, however, another management option for wheat in a dry spring which doesn’t need any capital investment – spraying a waterproofer. Maybe you think of a waterproofer as something you mix with cement or spray on a jacket or boots to help keep water out. But the same principle applies to keeping water in the plant.

At Harper Adams we have been studying drought, irrigation and waterproofers on wheat in field experiments over the last 24 years. We’ve found that commercially-available plant waterproofers can give reliable yield improvements as long as two conditions are met: the spray needs to go on around flag leaf to booting, and the soil moisture deficit needs to be at least half the available water – in essence the same requirements as for irrigating wheat. We haven’t done many trials with very late-sown or spring- wheats, but the threshold deficit might possibly be even lower for these shallow-rooted crops.

The timing for a waterproofer is around the T2 fungicide spray timing – so that may make it possible to add the waterproofer in a tank mix. Some plant waterproofers are also sold as adjuvants so that they may be compatible with many fungicides used at T2.

As always, there will continue to be regional weather differences and although the weather forecasts are much more accurate than in the past, seasonal forecasts are too unreliable yet to be sure how wheats will fare in 2020.



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