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Investigating PGRs

I’m going to focus my information on plant growth regulator (PGR) options, and mostly on gibberellin biosynthesis inhibitors. Gibberellin is a type of plant hormone that makes plants taller and skinnier. Gibberellin biosynthesis inhibitors prevent this taller growth. The only gibberellin biosynthesis inhibitor registered in Canada has the active ingredient chlormequat chloride (CCC) with the tradename Manipulator, and at this point, only on wheat. It was registered for use by PMRA in 2015, but it wasn’t until April 2018 that the EPA in the U.S. approved a maximum residue level (MRL) for CCC. Manipulator is registered in all of Europe, Australia and most wheat-producing countries.

Another gibberellin biosynthesis inhibitor is trinexepac-ethyl (TXP) under the trade name Moddus from Syngenta. It’s registered for use in many countries, and there are MRLs in place in the European Union, the U.S., and Japan. It was submitted to PMRA for registration in 2017, but it is not yet registered in Canada. A registration decision may be provided later in 2020 for hopeful product use in 2021.

The label of Manipulator states that you can apply Manipulator anywhere between growth stage 12 and 39. So, that’s the two-leaf to the flag leaf stage, but if you want to be really effective with Manipulator, you want a single application between growth stages 30 to 32. Growth stage 30 is the beginning of stem elongation. To determine these growth stages, you need to pull out plants, and split the base of the stem open. What you’re looking for is one centimetre (cm) of growth between the basal node and the first node. That’s the sweet spot for spraying Manipulator or Moddus. The end of the sweet spot is growth stage 32, defined when node two is at least two cm above node one.

Another PGR with a very different mode of action, Ethrel, is registered in Canada. It increases ethylene production and staging is incredibly particular or crop losses can occur. Bayer makes you sign a waiver before you use it. Application timing is when most of the tillers are between early flag leaf emergence to swollen-boot stage (Zadoks stage 37 to 45).

Growth stage 37 is when the flag leaf is fully emerged but really tightly rolled. Growth stage 39 is when the flag leaf has unfolded, and is when you want to target application. At growth stage 45, which is late boot, you can see the swollen sheath where the head is pushing out of side of the sheath. Growth stage 49 has the first awns visible. The label on Ethrel says, “Do not apply Ethrel after 10 per cent of the awns have emerged.” If you count 10 plants and two of them have awns emerged, this window is closed. The label of Ethrel also states that “Correct timing is critical for successful results and to ensure crop safety.” If this product is new to you, be scared of it.

PGR considerations
Research has documented some of the risks of PGRs. PGRs will not eliminate lodging in highly susceptible crops but may delay the onset of lodging. If lodging was originally going to happen at the end of July, you might delay it until mid-August, which could be beneficial.

In the absence of lodging, research has found variable effects on yield – sometimes a yield increase, sometimes a yield decrease, and sometimes no impact.

PGRs can also affect other hormones in the plant, and result in developmental and physiological changes. Green colour of the foliage can be intensified, and this can result in slower maturity.

The optimum temperature for gibberellin biosynthesis inhibitors is around 5 C, but it’s very possible to have a 3 C morning in early June when you’re out there spraying. That is not the optimum temperature for PGR application.

Height reductions may also be only short-term. Two weeks after a PGR application, we can see beautiful height reductions, but by the end of the season, it can be surprising that it is the same height. Sometimes the crop is taller and sometimes it’s only a centimetre shorter. We’ve seen a lot of inconsistency.

PGRs can also result in the production of late, unproductive tillers. Those may not make it to maturity and are going to cause harvest headaches. Reduced grain weight has also been very frequently documented.

PGRs should also be used with caution under environmental stresses. The label for Ethrel states, “Do not use under drought, excessive moisture, or excessive heat.” The Manipulator label says, “Don’t use under waterlogging, drought, or nutrient deficiencies. In hot, dry weather a better result may be obtained from application in the early morning or evening.”

We’ve done a number of field trials, and have seen yield reductions, mainly when soil moisture content is 10 per cent or less, or when the humidity is quite low at less than 50 per cent. I think in these situations where you have hot, dry conditions at the beginning of June, Mother Nature is going to be your PGR because if you have drought, you don’t need a PGR.

AAC Brandon (81 cm, G) at Barrhead, Alta., on Aug 28, 2017 treated with dual PGR – Manipulator + trinexapac-ethyl (left); no PGR (centre left), trinexapac-ethyl (centre right) and Manipulator (right).

PGR performance in wheat
I really like this quote by Wilhelm Rademacher in regards to his research on PGRs that, “The risk of lodging is strongly affected by variety and husbandry factors including sowing date, seed rate, drilling depth and rate of nitrogen application.”

A fairly old study in Indiana in 1988 looked at PGR application on four different cultivars. They found that chlormequat chloride (the active ingredient in Manipulator) increased yield in one of the four cultivars, while Ethrel application produced similar or a slight decrease in yield. This suggested to me that we needed to dive deeper into looking at how different cultivars respond to different plant growth regulators in Western Canada.

In our research in Alberta, we looked at a few different varieties. We found that the varieties reacted differently to PGRs. For example, when we applied Manipulator to AAC Brandon (good lodging rating) it did a great job of helping it stand up beautifully. But when we applied Moddus, AAC Brandon did not stand quite as well. It was better than no PGR, but not perfect.

AAC Elie (good lodging rating) lodged under higher fertility and good moisture conditions. Manipulator definitely helped reduce lodging, but it was still starting to go down. But with a Moddus application, AAC Elie stood poker-straight.

With Stettler, even though it has only a “fair” lodging rating, it stood beautifully with or without PGRs. The message is that nothing is textbook with lodging and PGRs. We can’t necessarily have a nice set of rules and just apply them directly.

In the Alberta research, we looked at 11 different CWRS wheat variety responses to Manipulator over one to two years. Three varieties, AAC Brandon, AAC Elie and Thorsby tended to lodge frequently, but also responded to PGRs. AAC Brandon lodged 83 per cent of the time, and Manipulator made it stand better 80 per cent of the time. AAC Elie lodged 100 per cent of the time, and 100 per cent of the time Manipulator made it stand better. Thorsby lodged 100 per cent of the time, and 100 per cent of the time a PGR did improve standability.

Conversely, AAC Redwater lodged 67 per cent of the time but Manipulator did not significantly improve standability. Based on this data set, I wouldn’t necessarily recommend applying a PGR on AAC Redwater.

We looked at six different CPS varieties. AAC Crossfield, AAC Foray VB and SY Rowyn lodged fairly frequently and PGRs helped with standability. With AAC Crossfield though, the PGR only helped out 50 per cent of the time. AAC Penhold though did not lodge at all. It’s a variety that has some really good genetic standability, so we didn’t need a PGR on that variety, based on our research trials.

In terms of the take-home message for the interaction between cultivars and PGR response, for CWRS we’re fairly confident that AAC Brandon, AAC Elie, and Thorsby would be a good choice for varieties to use Manipulator on. These are the most responsive varieties but you won’t see a benefit 100 per cent of the time.

In the CPS class, AAC Foray and SY Rowyn would be the varieties where we most consistently get a response of improved standability, and AAC Crossfield, in the Alberta environment at least, we get fairly severe lodging but only a 50-50 chance that a PGR would actually help. I do want to put the caveat out there that not all cultivars have been tested, and different cultivars will respond differently in different environments in Western Canada. So, it is this complex interaction of genetics versus PGR performance.

PGRs on barley
Barley certainly is a crop that has standability issues, but at present, no PGRs are registered on barley. [Editor’s note: Manipulator was approved for use on barley in May 2020.] Ethrel was registered but no longer is.

Our research on CDC Meredith barley showed a little bit of height reduction with Manipulator, but not a large reduction. Moddus application produced a beautiful height reduction, and it carried on quite nicely throughout the growing season. Manipulator doesn’t seem to be the right product on barley. Moddus looks good, but a higher rate than what is recommended in wheat is required on barley.

Cultivar-specific responses also complicate PGR use in barley. A 1977 study in Ontario looked at the response of 53 different barley cultivars to chlormequat chloride (the active ingredient in Manipulator). Thirty-three had a height reduction, five had no height reduction, and 15 varieties were taller. What works on one cultivar isn’t necessarily going to apply to others.

Research by Laurel Thompson at Lakeland College looked at how CDC Austenson feed barley, and CDC Copeland, AAC Synergy, and CDC Bow malt barleys responded to Moddus application. With CDC Austenson (good lodging resistance) at Barrhead in 2019, lodging was just as bad with or without a PGR. This is a great example where a PGR will not prevent lodging in all conditions when you have that perfect storm for lodging with high fertility, a cultivar that tends to fall over, and favourable growing conditions.

On barley, the take-home messages are that to date, no PGRs are registered and a higher rate of Moddus will be required. I want to highlight that when you have a variety like Amisk barley with really good standability, you don’t need a PGR. The magic bullet to prevent lodging is to use cultivars that have great genetic standability rather than depending upon a PGR.

Trinexapac-ethyl (left), Manipulator (centre) and no PGR (right), on CDC Meredith barley at Barrhead, July 5, 2016 – 22 days after PGR application. PGR was applied June 13 at BBCH 31-32.

Oat and PGRs
We’ve done a little bit of work on oats, although PGRs are not currently registered. [Editor’s note: Manipulator was approved for use on oats in May 2020.] With Manipulator, we saw an eight per cent height reduction, a 13 per cent height reduction with Moddus, and when we tank mix the two actives we get a 25 per cent height reduction. We did a little bit of work on different oat cultivars: AC Morgan, CDC Norseman, ORe5342M, OT3085, Summit, and Triactor. Five cultivars had height reductions, four cultivars had improved standability, four had reduced bushel weight, and three had improved yield. Again, this cross-species trend of “not all cultivars respond the same” is an important message.

Field pea and PGRs
On field pea, we looked at Ethrel, Manipulator, and Moddus. There were very small, very inconsistent results. We also found that under dry conditions and peas under stress, our pea yields were cut in half.

Future research
A very common trend in Europe is to tank mix Manipulator and Moddus for improved performance. Manipulator works earlier in the gibberellin biosynthesis pathway and Moddus later. A tank mix can improve performance and consistency, however this option is not registered in Canada. In our research on wheat, the frequency of height reduction with Manipulator alone was 67 per cent, Moddus alone was 22 per cent, and tank-mixed together was 78 per cent of the time.

We also tended to get larger height reductions with the tank mix. Again, there were differences in standability response between cultivars where Manipulator worked quite well by itself on AAC Viewfield, AAC Wheatland VB, CDC Landmark VB, and Stettler, and the tank-mix (which is not yet registered) worked better on AAC Brandon, CDC Plentiful, AAC Foray VB, and AAC Goodwin.

In summary, we need to remember that PGRs will not eliminate lodging in highly susceptible crops. For wheat, Manipulator seems like the best options and is the only product currently registered. It works fairly well on AAC Brandon, AAC Elie, Thorsby, AAC Foray VB, and SY Rowyn. In other crops, no PGR is currently registered. Our barley research found Moddus is the PGR of choice, but we need to use a higher rate than the wheat rate. On oats, Moddus or Manipulator may be good choices. On peas, just don’t spray PGRs on them because there’s more risk than gain.