The first ever dedicated growth regulator product for winter oilseed rape was recently launched on the Irish market.
The product, called Caryx, has been introduced by BASF as an anti-lodging agent with an associated impact on stem and root growth.
Up to now, growth regulation in oilseed rape was a side effect of triazole fungicide use. The effect depended on the product used and its application rate – higher rates had a greater effect.
But it was not a label claim and could be inconsistent.
Caryx has a label guarantee of growth regulation. BASF states “it works for you when you want it to”, but this is a qualified statement.
Caryx is a mix of two actives with growth regulation capability – metconazole (30g/l) (in Caramba or Juventus) and mepiquat chloride (210g/l) (also in Terpal and Canopy).
It can only be used on winter oilseed rape, at two application timings.
On forward crops, it can be applied in the autumn from the three-leaf stage to the end of autumn growth (GS13-19). In spring, it can be applied from the start of stem extension to the yellow-bud stage (GS31-59).
It can be applied in autumn or spring, or both, but it cannot be split at either timing. The maximum individual dose rate is 1.4l/ha.
Caryx claims to shorten the crop and manipulate its canopy structure, thus reducing the risk of crop lodging. But it needs to be applied when conditions are broadly suitable for growth.
For the moment, straw from treated crops should not be used for composting.
How it works
The growth regulatory characteristics of the two actives are different and seem complementary.
Table 1 indicates that mepiquat chloride is relatively slow off the blocks, lasts in the plant for some time and is active at low temperatures.
Metconazole needs higher temperatures, works rapidly, and has a relatively short period of activity.
While mepiquat is active at lower temperatures, it is recommended that Caryx only be applied when prolonged active growth is likely.
It works on the plant hormone balance by inhibiting gibberellins and stimulating cytokinins and so needs growth to work.
It decreases apical dominance in the plant to reducte plant height. It reduces the size of the terminal raceme and promotes stronger branching on the plant. It also strengthens the stem by reducing stem elongation.
The hormonal effect also helps to increase the growth of tap and adventitious (side) roots. In addition, it increases chlorophyll density, which helps give higher photosynthetic activity and longer green leaf duration.
Together, all these effects help deliver more seed sites, longer seed fill and higher yield.
The height of the crop is reduced and the altered main raceme results in greater light penetration into the lower canopy, where increased secondary branching is taking place.
Ultimately, treatment decreases lodging while increasing root growth and rooting depth.
The alteration to crop structure will be influenced by the timing of the spring treatment.
While all timings will delay the onset of flowering by up to seven days, a mid-March green-bud treatment will tend to give a shorter plant compared to a mid-April yellow bud stage treatment.
At this latter timing, some extension has taken place and a few flowers may already be visible. The earlier timing may also give a more bushy plant.
Because Caryx limits plant growth, application should not be made to crops that have a poorly developed canopy.
This is defined as a green area index (GAI) of less than 0.8. Applying Caryx to crops with poor canopy development could restrict future growth and the plant’s ability to fully compensate.
While there are different ways that one can assess a crop’s GAI, a useful guide is that GAI = 1 is approximately 50% ground cover.
This can be a very useful guide. A GAI = 2 is approximately 75% ground cover.
While lodging prevention is the main objective, application of Caryx (1.0l/ha) is said to increase seed yield by 0.21/ha on average in the absence of lodging.
This is because of the range of beneficial influences mentioned previously.
Where lodging occurs, the yield benefit of the same rate of Caryx increases to 0.43t/ha.
The level of lodging prevention achieved will equate to the rate used.