Indeed, the lack of growth was a far more regular topic of conversation. But things change, and hopefully we will feel the need for plant growth regulation (PGR) shortly.

One might suggest that the application of growth regulators has become too automatic, as it is often done with due consideration of what it is trying to do.

Sometimes thick crops are being treated in a way that will thicken them further and thus lead to other problems, one of which is increased lodging risk in winter and spring barley, for example.

However, this article primarily focuses on winter crops and on wheat and barley crops in particular.

Growth regulation needs to be divided into three separate functions based on application timing:

Manipulation;

Stem stiffening;

Straw shortening.

Growth manipulation is done generally during tillering, where products are applied to help break the apical dominance in the main tiller(s) and hence facilitate more tillering.

Apical dominance is a feature of many plants, whereby the main stem expresses its dominance by minimising tiller/branch/shoot production, and, if this dominance is broken for a period, then more tillers can be produced. This is most easily seen when the ‘leader’ on a tree is damaged.

Tiller manipulation

Manipulation is the process by which crop growth is modified for the purpose of altering crop canopy or plant rooting structure.

This is normally used to increase tiller production on plants during autumn growth but it can also be done in springtime. It is also used to try and hold up fertile tiller number in crops to increase ear count.

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Growth manipulation is normally done during tillering because that’s the only stage where useful additional tillers can be produced.

Once the plant goes into stem extension, no new tillers will be produced, but ones already formed could still appear.

For tiller manipulation to happen, there must be growth after a treatment is applied to produce new tiller buds. If there is no growth then it is possible that the chemical could be gone from the plant before it can do its job. But most years, there is growth at the November or late February timings and, in these situations, enhancing uptake of products like CCC can be an advantage.

There is little doubt that these treatments can influence crop appearance and structure. What is less certain though is whether or not a yield benefit will result.

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But it is now probably too late to achieve this objective in most winter wheat and barley crops this year.

The process of rolling crops post-emergence also tends to cause some damage to the strongest tiller(s) and thus break apical dominance. This is why rolling is associated with increased tillering.

The main tool used for tiller manipulation has been CCC in its different forms. For many, this was CCC plus an adjuvant like Arma or Kantor to help get penetration, but you still need growth to get the desired effect.

In latter years, more growers moved over to ‘K2’ because it tends to have a longer duration of persistence and activity, thus helping secure the desired effect.

Tiller number can also be manipulated (increased) by application at the end of tillering and the start of stem extension.

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Treatment with CCC at this stage (GS30/31) will again weaken apical dominance and this allows for the survival of somewhat weaker tillers which might otherwise be aborted.

Stem extension is the period of maximum growth in crops and so this is the time when each plant assesses its ability to fill all the grain sites present. If a plant senses that it does not have the available resources to fill all the current grain sites, then it tends to abort some of the weaker tillers.

By reducing apical dominance, there can be a more uniform spread of yield devoted to additional tillers, thus increasing yield potential in barley in particular.

Tiller manipulation should not necessarily be regarded as a routine practice. You need to know what the crop is doing and what you want to achieve from a treatment.

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It is common to have more than enough tillers in a crop to begin with and forcing additional tillers can merely add to crop density, disease pressure, higher lodging risk and decreased grain quality.

Enhanced rooting

A second and direct consequence of increased tillering in wheat is that each tiller develops its own independent root system which remains even if the tiller dies.

But this is primarily a wheat phenomenon and it does not happen in barley to the same degree. As well as having more roots, these also tend to grow more laterally (more left and right rather than down), at least initially, and this can play an important role in anchoring the plant in the ground.

This is an increasingly important phenomenon as stem strength is generally good but plants carry more tillers with bigger ears and so stems are inclined to lean in a wet summer if roots break free in the soil.

As well as being associated with increased tillering, increased root growth is also claimed from PGRs like Meteor and Moddus.

These products impact on normal root growth patterns to help increase rooting, as well as providing standard stem strengthening and stem shortening effects. So they both manipulate and regulate/shorten.

BASF recommend Meteor on winter wheat at 2.5l/ha between GS25 and GS30, which is about now on many crops. This can also be applied as a split application with 1.75l/ha at the start of stem extension followed by 0.75l/ha before GS32. But Meteor is only for use on winter wheat.

Moddus is also recommended at GS30/31 on wheat and barley (and oats), in combination with CCC, at 0.2l/ha plus 1.0l/ha respectively.

This can be followed, in the case of winter wheat and barley, up to when the flag leaf is fully emerged. Higher rates can be used, but it is generally advised to keep rates down at around 0.2l/ha.

The combination of Moddus and CCC is regarded as providing a good combination of strengthening and shortening.

There are now a number of Moddus equivalents in the market, including Optimus and Freeze, but be careful with rates because concentrations differ.

Canopy can also be used for this purpose and it has a clearance from GS30 to GS40. It helps rooting, provides some shortening and is generally used in sequence with other products.

The maximum total dose is 1.5l/ha and individual doses tend to be between 0.3l/ha and 1.0l/ha but most GS30-GS32 applications on wheat and barley will tend to be between 0.6l/ha and 0.8l/ha.

While a number of products can be used to enhance plant rooting (and we can add some fungicides to this list), this can only be of benefit if the soil around these roots is firm enough to hold these roots. So, sometimes the roller is still the best single action to use to help stabilise plant roots to prevent late summer leaning.

Stem shortening

For many, straight CCC will still be the major growth regulation product used, especially on wheat, and this needs to be targeted between GS30 and GS31.

Any products applied at this early stem extension timing provide both stem stiffening and a degree of plant height reduction.

However, later applications tend to have a greater overall effect on plant height because the internodes are longer at this later timing and the shortening effect can be more pronounced.

It remains very important to apply a treatment to strengthen the base of the plant where lodging risk is high, because weak stem bases continue to have a lodging risk even if plant height is reduced.

This is an additional concern this year where late spring growth increases the risk that stem extension will be rapid and that stems will be weaker as a consequence.

But while this is a risk, it is not a certainty.

Options for late season shortening include Moddus/CCC at later stem extension stages or the application of products like Cerone or Terpal somewhere between GS32 and GS37. The latter two can be applied a bit later but it is important to prevent as much of the height gain as possible, if and where late straw shorteners are deemed necessary.

Later applications can increase the risk of ears not fully emerging from the leaf sheaths and thus increase the risk of disease transfer to the ear.

Where Moddus/CCC is being used on winter barley, a second application should be applied by flag leaf emerging at rates similar to those mentioned previously.

The application rates for Terpal and Cerone are influenced by whether or not a previous growth regulator was applied.

Where this was the case, it is generally recommended to use only half the full rate, which means 1.0l/ha for winter wheat and winter barley with Terpal (with non-ionic wetter). For Cerone, the rate would be 0.5l/ha on winter wheat and barley. Neither of these two products are recommended for oats.

The standing package

Preventing lodging is not just a matter of applying chemicals. Lodging prevention begins at variety selection. After that, sowing date and seed rate interact to impact on lodging risk. This is mainly a result of the amount of tillering that can take place, so use lower seed rates with earlier drilling.

Then comes rolling. This is not always possible because conditions post-planting may not allow. But having the soil firmly pressed around where the roots are growing is important to help provide good anchorage in a wet summer. It is easier for roots to lose their grip in loose wet soil than in tight soil.

Nitrogen is an important factor. The more N that is available to the crop early in the season (either applied or via soil fertility), the more foliage that may be produced, resulting in denser crops and increased lodging risk.

Nitrogen at stem extension tends to have less lodging risk, providing optimum rates are used. Excess total nitrogen is obviously another precursor to lodging so get N application accurate.

The final tool is PGR. If the others are wrong, they increase the pressure on PGRs and the size of the job they have to do. As we move towards integrated crop management, it is increasingly important to use, and to show you use, alternative options to just depending on chemicals.

Excessive shortening should be avoided, as it may generate other problems, like increased ear blight risk or direct disease transfer from leaf to ear.

KEY POINTS

Know what you want to achieve before routinely applying plant growth regulators.

Lodging prevention begins with choice of variety, combined with other sensible husbandry decisions.

Different growth regulators can be applied to manipulate, strengthen or shorten the crop.

Avoid excessive shorting to help prevent other potential problems.