Current PGR and Irrigation Considerations (Collins & Edmisten)
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Current Crop Status:
This crop is all over the board. The current crop status can vary widely from field to field and in close proximity to one another. Recent rains have resulted in much needed growth in some areas, while others still appear thirsty. As more and more fields begin blooming, we are seeing some fields that are too short to be blooming (as predicted from the cool early June and some due to dry weather), some that are right at the target height to be entering the bloom period, and a few fields that are borderline too tall for their current growth stage. This is largely attributed to rainfall frequency and amounts, but some of it can be related to soil texture as well.
We’re thankful for the recent rains that have spurred growth. However, our soil moisture status can change quickly in the temperatures that we are experiencing. Soil moisture will rapidly deplete when temperatures are in the low to mid 90’s, if not replenished by rainfall, with sandier soils depleting noticeably faster than heavier soils. In these temperatures, it is important to remember that we are never more than 4-5 days (occasionally shorter) away from a serious drought in sandier soils, and perhaps a couple of days longer in heavier soils. Just today, several fields with sandy soils were showing some of the earlier signs of wilting, and these fields recieved ample rainfall at some point over the weekend. When daily highs fall back into the upper 80’s, our soil moisture status can be prolonged noticeably.
We wanted to remind growers of our general guidelines for irrigating cotton, shown below:
These weekly rates should be divided into two or more irrigation events in order to account for and adjust for rainfall, and to allow uptake by the soil while avoiding runoff or seepage. For example, if your weekly water requirement is 1.5 inches, then it should be applied in two 0.75″ irrigation events, or three 0.5″ irrigation events, spread evenly over that week. If rain occurs during that week, the rainfall amount can be credited to the weekly water requirement (within reason) in the next irrigation event, which saves you money.
In a hypothetical scenario, we wanted to provide a few examples. Lets say that your weekly water requirement is 1.5 inches, so you would want to schedule irrigation events for Monday, Wednesday or Thursday, and Saturday at roughly 0.5″ per irrigation event (3 events), or on Tuesday and Friday at 0.75″ per irrigation event (2 events). Keep in mind that the crop will not repond to much more than 0.75″ to 1″ at a time, nor will most soils absorb more than that at one time. Therefore, sandier soils may require 3 events whereas heavier soils may require only 2 events. It is advised that you observe the maximum amount that your soils will absorb before runoff occurs. It is generally better to have more irrigation events with lower rates (no less than 0.25″) versus higher rates applied in fewer events. This enables adequate soil uptake, ensures continuous soil moisture or at least prevents intermittent depletion, and allows for maximum crop response.
Now let’s say you recieve rainfall during that week. Credit should be applied to any meaningful rainfall (0.2″ or more) up to a maximum of 1″ for 3-4 days, depending on when that rainfall occurs. Therefore, if you were planning on 3 irrigation events at 0.5″ per irrigation on Monday, Wednesday/Thursday, and Saturday, and you recieved a 3″ rain on Monday night (after you irrigated), then you would forego the Wedneday/Thursday irrigation event, and resume your irrigation at 0.5″ on Saturday. Why would we do that if that 3″ rain exceeding our weekly requirement? Well, most of that water was either lost due to runoff, or seepage below the rooting profile, nor could the crop take up that much water within that time frame. In hot temperatures, any excess moisture is likely gone by the end of that week. Therefore we only apply credit for up to 1″ of rainfall to the next irrigation since it was scheduled within 3-4 days of the rain event, but we would resume normal irrigation beyond that 3-4 day timeframe.
Adjustments to this model, especially for soil type, can be made, however, DO NOT wait until wilting occurs before irrigating or resuming irrigation. When wilting is observed, some yield has likely been lost already.
We’ve had a number of calls over the past week or so regarding the need for PGRs. This year is more challenging than most in that we can not provide any “cookie-cutter” approaches that apply to all, or even most situations. In several fields, PGRs are warranted, mainly due to frequent rains and the resulting strong growth. In other fields, it depends. In some cases, PGRs are clearly not needed. This goes back to our previous comment on the wide range of current crop status.
As mentioned in earlier articles, this crop is late and will therefore need to managed for earliness. HOWEVER, that does NOT mean that we need to be arbitrarily aggressive with PGRs just because the crop is late. If at all possible, we are encouraging growers to make these decisions AT First Bloom, UNLESS a field is clearly exhibiting incredibly vigorous growth but is not yet blooming. There are several fields that are beginning to bloom and are far too short to do so (15 to 18″ tall and blooming). If soil moisture is not replenished frequently, a crop in this situation can expect to reach cutout very rapidly. Frequent rains are the only thing that can keep it going and progressively setting fruit. Other fields range between 20 to 25 inches tall when they start to bloom, and this is fairly on-target. In these situations, we encourage growers to look for all the signs of vigorous growth (internode distances between 4-5th leaf from the terminal, presence of large squares in the terminal, NAWF, etc) and most importantly, current soil moisture status. If any signs of stress, such as wilting, are observed, PGRs are likely not necessary. In other cases, we are seeing cotton that is 26″ to 28″ tall and not yet blooming. If soil moisture is plentiful, PGRs are warranted in these scenarios.
With temperatures like what we are experiencing, it is important to scout, and to do so frequently. This will allow you to catch changes in soil moisture status and changes in growth. Also, and probably more importantly, it will enable you to stay on top in plant bugs, or other insect pest that may arise.