Lodging of Cereal Crops

Introduction

Two types of lodging occur in cereals. These are: root lodging and stem breakage. Root lodging occurs early in the season and it is the most common type of lodging. Stem breakage, on the other hand occurs later in the season as the stalk becomes more brittle due to crop maturation or due to sawfly infestation.

Causes of Lodging

Lodging in cereal crops is influenced by morphological (structural) plant traits as well as environmental conditions. Lodging in cereals is often a result of the combined effects of inadequate standing power of the crop and adverse weather conditions, such as rain, wind, and/or hail. Lodging is also variety (cultivar) dependent. For example, a tall, weak-stemmed wheat cultivar has a greater tendency to lodge than a semi- dwarf cultivar with stiffer straw . Under conditions of high moisture and nitrogen fertility, semi-dwarf varieties are less prone to lodging than standard ones. Furthermore, short thick-strawed cultivars resist lodging better than tall cultivars. Plants that are initially resistant to lodging may stand erect during favourable conditions but these plants may fall down when exceptionally bad weather, such as heavy rain or wind, prevails. A crop that lodges early will recover through the formation of "elbow joints" at the lowest stem nodes. The cells on the lower side of the node elongate and force the stem erect. As plants mature the stem cells mature and are no longer capable of elongation to enable plant recovery.

High nitrogen fertilization, caused plants to be more susceptible to lodging. This is due to lusher growth which also provided an excellent environment for the spread of diseases such as leaf, stem and stripe rust. In addition, increased plant densities, heavier seeding rates, shading, and high moisture content especially under cloudy and humid conditions, have been found to increase the tendency of cereals to lodge. These conditions can result in lodging problems even with semi-dwarf cereal varieties.

Balanced fertility helps to decrease lodging. Straw from potassium deficient plants appear to be more brittle (hence greater tendency to lodging) than those fully supplied with potassium.

Although many reports have indicated the causes of lodging, there are disagreements about how these take place in cereal plants. Different theories and explanations were given by various researchers as to how and why cereals lodge. For example, the tendency of cereals to lodge is reported to be associated with elongation, yellowing, and thinning of the lower internodes of the stem. This can be caused by high rates of seeding or high fertility, especially abundant nitrogen, resulting in long and weak internodes with little resistance to bending. As a result, lodging of the cereal crop would easily occur.

In addition the tendency of a crop to lodge depends on the resistance especially of the lower internodes. This is because the lower internodes have to resist the greatest movement of force. The weight of the higher internodes of the stems plus leaves and heads in relation to the stem (culm) will affect the resistance of a crop to lodging. The heavier the higher parts of the stem are and the greater the distance from their centre of gravity to the base of the stem, the greater is the movement of the forces acting upon the lower internodes and the roots. Supporting this argument, it was found that the breaking strength of the lowest internode and shoot per root ratio were the most suitable indices of lodging.

The tendency of a crop to lodge is dependent on its straw length. The ability of a crop to withstand lodging also depends on the length of the stems particularly the length of the peduncle (the distance from the last node to the base of the head). Some of the factors that will increase the length of the stem include: the genetic potential of the cultivar; high fertility level, especially nitrogen and low solar radiation during the formative growth (that is, under cloudy weather); and stem diameter and stem-wall thickness (particularly the basal internodes).

Lodging can also result from failure of the root system, occurrence of fungal diseases and/or weakness of the stem. Lodging may occur as a result of irreversible bending and breaking of the lower internodes or by uprooting (that is, bending or breaking of the roots). Several researchers reported the importance of a well-developed root system as providing resistance to lodging . Structural failure in small grain cereals occurs by buckling rather than by loss of anchorage.

Plant morphological (structural) characteristics such as plant height, wall thickness and cell wall lignification can affect the ability of the plant to resist a lateral force. Tall plants have a higher tendency to lodge than short plants. A small change in plant height can have a strong influence on lodging.

Effects of Lodging

Severe lodging is very costly due to its effects on grain formation and associated harvesting problems and losses. It takes about twice the time to harvest a lodged crop than a standing one. Secondary growth in combination with a flattened crop makes harvesting difficult and can subsequently lead to poor grain quality and high yield losses.

Lodging alters plant growth and development. It affects flowering, reduces photosynthetic capabilities of the plant, hence affecting carbohydrate assimilation. Severe lodging interferes with the transport of nutrients and moisture from the soil, and thus with food storage in the developing kernels. Lodging always results in some yield loss, and if permanent lodging occurs shortly after heading, the yield reduction can be as high as 40 per cent. Yield losses from lodging become smaller as the crop matures, but losses will continue until the kernels are completely filled. Incomplete filling results in small kernels, lowered carbohydrate content, and lower test weight.

Lodging often contributes to uneven maturity, high moisture constant and loss of grain quality due to sprouting and possible moulding. Excessive moisture has often delayed harvest and may necessitate grain drying. Lodging can cause severe pickup problems and slow harvest due to green, immature kernels and slowing of the combine speed. All the above can result in increased harvesting costs.

Lodging is reported to be the most limiting factor in attaining high yields from increased nitrogen fertilization, especially during humid conditions. The effects of lodging on yield losses depend on the growth stage of the plant, the weather conditions prevailing after lodging has taken place, and the severity of lodging. When a crop lodges before flowering, the culms (stem) may regain their upright position if favourable weather conditions prevail. Under adverse weather conditions, however, the crop will easily be lodged, resulting in deformed head and shrivelled kernels. If a crop lodges sometime after flowering, the heads will not regain their upright position. At this stage, kernel numbers are not affected, but the grain weight may be severely reduced. The extent of weight reduction depends on prevailing weather conditions after the crop had lodged.

Yield loss comes from poor grain filling, head loss and bird damage. Yield losses are greatest when a crop lodges during the ten days following head emergence. Yield losses at this stage will range between 15% and 40%. Lodging that occurs after the plant matures will not affect the yield but it may reduce the amount of harvestable grain. For instance, when lodging occurs after the plant matures, neck breakage and the loss of the whole head can result; these often lead to severe harvest losses. In theses cases, farmers who straight combine their grain will likely incur higher losses than those who swath them.

Control of Lodging

Cultivar selection
The first step to help prevent lodging is to select a variety that has short, strong straw. Lodging resistance ratings are given in the annual Provincial Variety Description publication and at this internet site under Variety /Agronomic performance. Semi-dwarfs are your strongest strawed varieties but may be short some other trait(s) need on your farm.

Lodging has been reported to be severe where plant densities are high. It is therefore recommended that plant densities should be reduced since these will promote stronger straw and more tillers. Farmers should firm-up the seedbed for good seed/soil contact allowing for optimum seeding depth, plant spacing and plant population. The optimal seeding depth is generally 2.5 to 5 cm( 1 to 2 inches), if severe drought conditions are not present, with plants 2.5 cm apart and row widths of 12 to 18 cm, or random spread giving a seeding rate of 15 to 30 plants per square foot depending on the rainfall usually received in the area being shown. Winter cereals (winter wheat, fall rye, winter tritcale) should not be sown deeper than 2.5 cm. When sown deeper, winter hardiness drops off rapidly (winter wheat is the most sensitive to seeding depth).

Crop rotation
Crop rotation is necessary for the prevention of diseases such as common root-rot, scald , net blotch, root rot and take-all. When a cereal crop is grown on a broad leaf crop stubble, such as canola or flax, the less severe is the disease pressure. Crop rotation practices can be particularly important for irrigation farmers. In the absence of summerfallowing, a crop rotation scheme is useful for the maintenance of soil fertility, disease and weed control. In addition, careful rotations can aid in lowering protein levels in soft white spring wheat and malt barley.

Cultivar selection
The first step to help prevent lodging is to select a variety that has short, strong straw. Lodging resistance ratings are given in the annual Provincial Variety Description publication and at this internet site under Variety /Agronomic performance. Semi-dwarfs are your strongest strawed varieties but may be short some other trait(s) need on your farm.