Finally, after 40 some odd days, our crop appears to be growing. Scattered rainfall earlier this week (June 29th and June 30th) helped but most showers were hit-or-miss. Today (July 1st), however, the rainfall across the area has been widespread. The crop has been waiting on this rain and I expect it will begin growing very rapidly by the end of this weekend. With rapid growth and an increase in number and size of squares, expect plant growth regulator (PGR) and insecticide applications targeting plant bugs to be required in the immediate future. In this blog, I cover a few points on PGR application timing and products, as well as provide some insight from Dr. Scott Stewart on our best options for plant bug control prior to bloom and immediately after bloom.
Beginning plant growth regulator applications during 2021
Making a PGR application to this crop will be difficult, given we’ve watched it struggle for so long. Still, with our forecast, it is likely the crop will be growing very rapidly by the end of the weekend. In recent years, it has become increasingly common to make very aggressive PGR applications by first week of bloom. In our area, I believe this trend has developed due to the consistent and substantial amount of rainfall the area has received over our summer months (with the exception of 2020) and an increase in planting of certain varieties which require very aggressive management (most notably, DeltaPine 1646 B2XF).
Consider these four factors when picking your initial rate for 2021:
- Varieties vary substantially in their response to PGRs, and this variance is greater today than when I started years ago. Although most varieties we’ve planted this year fall within the moderately responsive to responsive category, we have a few that are on the far ends of the spectrum. DeltaPine 1646 B2XF is a very aggressively growing variety that requires aggressive applications. On the opposite end of the spectrum is NexGen 4936 B3XF- that variety is very responsive and requires much lower rates to slow growth. If you are uncertain how a variety will respond, start with lower applications and follow-up with additional applications based on plant growth measurements. Feel free to reach out to me or your seedsman for more insight into specific variety responses to PGRs.
- Internode length and Height to node ratio (HNR)
- I am typically a big supporter of using the internode between the fourth and fifth nodes down from the top of the plant to gauge current growth potential and guide rate. If the distance between fourth and fifth nodes is:
- Less than 2”: Growth not adequate. Application is not warranted.
- Between 2” to 3”: Growth is adequate; a low application may be warranted on ‘strong’ ground, but applications in this range are not universally warranted.
- Exceeds 3”: Growth is excessive. The potential for rank growth is high and an application should be strongly considered
- For the initial application this year, given our rough start, I would also consider the height to node (HNR) ratio. On stressed plants, an aggressive application will likely be detrimental. Incorporating this measure into your rate selection will help give insight into plant health and help you pick an appropriate rate. See table below for HNRs for cotton at varying growth stages under stressed, normal and vegetative growing conditions.
Height to node ratios for cotton PGR decisions. (Jost et al. 2005)
|Large Square – First Flower||1.2-1.7||<1.2||>1.9|
|Early Bloom + 2 weeks||2.0-2.2||<1.8||>2.5|
Jost, P., S. M. Brown, S. Culpepper, G. Harris, B Kermerait, P. Roberts, D. Shurley, and J. Williams. 2005. 2005 Georgia Cotton production guide p 37-39.
- This is a big one that bit us last year; several (myself included) made aggressive applications based on a forecast that included rains. Those rains failed to develop and as a result those applications ended up to be more aggressive than necessary. If the forecast holds rain, an increased rate may be necessary to slow growth into a reasonable range. However, if you don’t see a substantial chance for rain in the forecast, it might be wise to either hold or reduce the rate you apply.
- Similarly, we must take into account field history. Cotton grown on deep, silt loam bottoms tends to grow cotton with more horsepower. A rolling hill that could be characterized as shallow depth-to-fragipan will not have the same growth potential. Rates on those two farms should vary, with the bottom receiving slightly more than the hill, even at this timing.
Product availability has changed in recent years. We typically talk about rates of 4.2% mepiquat chloride and the rate discussion that follows will be based on the 4.2% mepiquat chloride formulations. If you are running mepiquat pentaborate, a cyclanilide containing product, or mepiquat chloride at another concentration (23.5%, for example) you will need to adjust your rate down.
My rate extremes currently fall between 6 and 12 ounces. I took a brief tour through the area today and saw only one field that I thought might tolerate a 16 ounce application now; most fields appeared to be just moving into a window in which a squaring application would be appropriate. On a very responsive variety growing off on a rolling hill well into square, I’d consider a 6 ounce application. On a very aggressive variety growing in a bottom, I’m in the 12 ounce range. Keep in mind that we can make follow-up applications to increase the concentration of mepiquat in the plant and subsequently restrict growth if we undershoot rate. We cannot, however, correct an overly aggressive application.
Counts are increasing, with applications being made in some parts of the state. I’ve included two comments below from Dr. Scott Stewart’s post from last year titled, ‘Early Season Management of Plant Bugs in Cotton.’ Please review this article at your convenience.
Again, quoting Dr. Scott Stewart:
- The very first fields that begin to square often receive the brunt of the plant bugs until more fields start squaring. Watch them closely.
- The goal is to maintain at least 80% square retention through early bloom. There are a number of methods to count square retention. It is often suggested to focus just on first positions, but the most important thing is to count the number of missing squares in ‘X’ number of fruiting sites. You’ll want to look at multiple plants in several, representative places of the field. Usually, checking retention for 100 total fruiting sites is enough, especially if square retention is well above or below the 80% mark, but you may want to look closer when you are riding the edge.
- ‘During the first two weeks of squaring, I think full rates of imidacloprid, Centric at 1.75-2.0 oz per acre, or combinations of these two product will protect against square loss, despite some “slippage” of efficacy. Transform at 1.5 oz/acre is another option and Vydate at 10-12 oz/acre is worth consideration, but honestly, I’d try to reserve those products until at least the third week of squaring and beyond. Once we hit the third week of squaring, mixing in Diamond at 4-6 oz/acre is common to delay and reduce the build-up of nymphs.