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Lodging is estimated to cause up to £300/ha in lost profit. In this, the first of three Academy modules focusing on key arable issues, Dr Pete Berry from ADAS outlines the causes of lodging

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What is lodging?
There are two forms of lodging that cause cereal crops, such as wheat and barley, either to permanently lean or, in the worst cases, lay horizontally on the ground. The better known is stem lodging where the stem or shoot buckles between the internodes, i.e above ground level. The second type, root lodging, occurs because of a failure of the anchorage system. In both cases lodging usually only occurs after the ear has emerged. Research by ADAS with the Universities of Nottingham and Birmingham over the past 15 years have shown both types are problems in UK cereal crops – a fact well recognised by the farming industry.

Impact of lodging
Severe lodging years occur in the UK on average every 3-4 years. Aerial surveys suggest in those years over half the fields have over 10% of their area lodged. Overall 15-20% of the area lodges in a severe year.

Lodging reduces grain yield and quality while at the same time increasing combining time and drying costs. The earlier lodging occurs the more impact on yield. Losses from crops lodged at ear emergence have been shown to be 75%. Average yield losses from a lodged patch are estimated at 25%, or put another way, 2t/ha from a crop with a 8t/ha yield potential.

Lodged crops take longer to dry, and increase the risk of a low Hagberg falling number which can drop by as much as 25 per day. Milling premiums are at risk if Hagberg’s fall below a minimum standard, usually 250. Overall costs of a lodged crop of milling wheat are estimated at £300/ha (see Table 1). Factors affecting lodging Lodging is a complicated phenomenon influenced by many factors including wind, rain, topography, soil type, soil nitrogen residues, husbandry and disease.

Wind acts on the shoots to create a leverage force, the size of which is determined by wind speed, area of the ear and height of the crop. When this force exceeds the strength of the stem base then stem lodging occurs (Figure 1). The strength of the stem base is determined by the diameter, wall width and the strength of the stem wall material.

When the leverage exerted by all of the shoots belonging to a plant exceeds the strength of its anchorage system then root lodging occurs (Figure 2). The strength of the anchorage system is determined by the spread of the root plate and the strength of the surrounding soil.

Soil strength is critical for root lodging and it is affected most by rainfall, but also by soil type and cultivations. Just 7mm of rain is sufficient to wet up the soil to 40mm – the average depth of the anchorage roots – and this weakens the soil strength several-fold.

Varietal choice has a major influence on lodging risk. The HGCA Recommended Lists give a lodging resistance score for each variety which ranges from 1 (very susceptible) to 9 (very resistant). In practice, a score below 3 is too susceptible to be grown commercially.

Varieties are often more prone to one form of lodging than the other. However, the current varietal lodging resistance scores represent an average risk to stem and root lodging.

Changing husbandry practices, such as drilling date and nitrogen rates, can increase or decrease lodging risk (Table 2). Different factors affect the risk of stem or root lodging by different amounts depending on their effects on shoot leverage, stem and anchorage strength.   A straightforward method for calculating the effect of husbandry factors on varietal lodging resistance score is described in the HGCA Guide Avoiding Lodging in Winter Wheat – Practical Guidelines published in 2005 (www.hgca.com).

 

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Complicating factors
The risk of stem lodging is increased by the stem-based disease eyespot. Recent work by ADAS funded by BASF showed a moderate infection of eyespot (lesions girdling more than half the circumference of the stem) at GS83-87 reduced stem strength by 17% and reduced the stem lodging resistance score by one point.

A severe infection of eyespot (lesions girdling more than half the circumference of the stem causing stem softening) reduced stem strength by 46% and reduced the stem lodging resistance score by three points. Take-all is unlikely to increase lodging risk because the reduction in yield and shoot leverage outweighs any reduction in anchorage strength. 

How to avoid lodging
The most effective strategy is to avoid sowing lodging susceptible varieties early or at high seed rates. This is doubly important if soil nitrogen residues are high. Of course this is not always possible and a method of calculating the “effective” varietal lodging resistance score after accounting for sowing date, plant establishment and soil nitrogen residues is also described in the HGCA guide. It then describes the best course of tactical action for avoiding lodging depending on the effective lodging resistance score. For low to moderate risk situations rolling the crop and a single plant growth regulator (PGR) applied in the spring between late tillering and early stem extension is likely to be sufficient. For higher risk situations a second PGR applied before flag leaf emergence and delaying N applications will be required. Effects of successive PGRs are additive, so a two-spray programme could increase the lodging resistance score by as much as three points. In very high risk situations, farmers should consider reducing N fertiliser, since the costs of lodging are likely to be much greater than the yield penalty of reducing N by around 50 kg N/ha. pete.berry@adas.co.uk

 

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