Title: Rice and Advice, Ep. 04: Rice Leaf Discoloration, Midseason N, and Boot N (7/2/21)

Arkansas Row Crops Radio providing up to date information and timely recommendations on row crop production in Arkansas.

Welcome to Arkansas Row Crops Radio. I'm Jarrod Hardke, rice extension agronomist for the University of Arkansas System Division of Agriculture. Today on Rice and Advice, I'd like to visit a little bit about some of the things we're seeing in the field and upcoming timings and issues that we need to try to stay on top of, or even stay in front of. 

One of the main topics of really this past week and a number of phone calls has really been about leaf tip discoloration on a lot of rice out there in the state at this point. Particularly that which has moved onto reproduction growth stages. Writing an article on that this week in the Arkansas Rice Update that will be posted on-line on the Arkansas Row Crops blog, ArkansasCrops.com, if you want to check that out and not only read some of the same comments to be made here but also see some of the pictures associated with differences and some of the leaf tip effects that I'm going to discuss. Certainly feel free to go have a look at that article and again that will be in the Arkansas Rice Update 7/2/21. So again, deficiency disorders or high yield disease? So a majority of what we're seeing out there with this leaf tip discoloration, is what has been referred to us in the past starting years ago. I didn't come up with the name but I've ridden with it since because it's continued to be appropriate. This high yield disease condition that usually what you're looking at is the brand new growth, the newest leaves coming out are nice and pretty green and clean, and it's really the second or third leaf down, again a fully expanded leaf collar there that that leaf tip has kind of a burnt look toward the end. Even kind of extending down the center vein a little bit or even down the edges of the leaf and again some say it looks a little bit like chemical burn or is it some kind of deficiency look, what's going on? What usually seems to be the case, is again, the rice is in reproductive growth stage. It's using a lot of nutrients and under the heat and sunny days that we've been having, the rice is undergoing very, very rapid growth. You can also add in that it's been so wet this year that the rice crop is a little shallow rooted. Really hadn't had a lot of drying time between getting planted and trying to get to flood and then we're flooded from then on. So what you're really looking at is a nutrient draw from those leaf tips. Again usually associated with our most highly mobile nutrients such as nitrogen and potassium, which are probably why you're getting that odd look. It's not sure one way or the other. The biggest thing to take home about it is, it's not something to be concerned about when you see this. The new growth should be in good shape and it's those last two leaves usually this rice we're talking about. This leaf that's showing the effect will end up being the third leaf down on the plant. So once you get to late boot, have that flag leaf, which is the last leaf, have the flag leaf come out, it's actually flag leaf minus two. This will be that leaf that's showing the effect. That's been very common over the years. What you do want to watch out for in this scenario is, is that the only leaf showing some type of effect? Because if it's the high yield disease situation then again the new growth is good. Really it's primarily this one leaf on a given plant or tiller that's showing this tip effect and then those leaves down below it also look pretty normal. That's a pretty good indicator that it's the high yield disease condition. However if you see something like this effect and it's occurring on a lot more leaves on the plant, then certainly we have more questions. And I would also strongly advise, and I typically advise this anyway, anytime you see anything that looks out of the ordinary with the plant, we need to pull some plants up and look at the roots and see if there's anything else going on. In some instances of what can be confused with the high yield disease leaf tip effect, it's actually a more expanding deficiency issue throughout the plant. Still may not be specific of a particular nutrient because when we pull the plants up, you may find you have hydrogen sulfide toxicity, which means you have blackened roots, so the roots are being compromised and not able to take up and deliver nutrients into the plants, so then suddenly you're getting a much larger drawl from a lot of leaves and the plant's trying to overcome and make up for that lack of root uptake. So again, if you see this occurring on a lot more leaves on a given plant, even if there's not hydrogen sulfide toxicity found, we certainly have more questions and we need to investigate further about what's going on with those plants or any possible further deficiency issue that could be there. The other thing that's certainly possible is if you're getting certainly a much more burnt appearance to the leaf tips where it's beyond just a little paling or lightening, but getting some more reddish orange color to it, things like that or even some brown spots showing up. Now we may be talking about actual potassium deficiency. Again, not high yield disease, not something associated with hydrogen sulfide, but actually a true potash deficiency. And in that scenario again, because potassium is a highly mobile nutrient, the newest growth on the plant will again look very good, but once you get to a few leaves down, you'll start to see deficiency symptoms on the leaves, and again that will continue down the plant. One quick check on that, particularly in leveed rice fields is to go to the rice and the bar ditches or to deeper water areas in the field that are displaying some of the symptomology and in deeper water areas, potassium deficiency is going to be more pronounced. We do have a lot of time within a given growing season to correct in-season potassium deficiency. So we don't want to move too quickly unless we don't find it until right at heading or near heading, so if we're in the early to middle mid-season stages where we still have a couple of weeks or more to get the heading, we certainly want to take wide leaf samples. And that would be the upper most fully expanded leaf with a collar. Take those samples and send them off for diagnostic testing and see what our actual potassium concentration is to confirm that it is for sure potassium deficiency and that that's the only deficiency that we have in play at the time. We always want to make sure that we don't also have something else going on that we're going to miss that we could possibly still be able to correct while we're going across the field trying to correct this one. And really from recent testing it's shown that from those wide leaf samples if we have potassium concentration of 1.5 percent or greater, we have adequate potassium in the plant. If it's lower than that then we may have a deficiency occurring. Certainly the lower it goes the more severe that may be, so we'll have to have a look at that. Most of our past research has shown that should we identify a potassium deficiency in-season, we can correct and recover most, sometimes almost all, but most of our full yield potential all the way out until kind of just before late boot with an application of one hundred pounds of potash per acre, Sixty pounds K2O per acre. What we have still that amount of time to be able to correct the problem. So those are really the three main kind of leaf tip effects currently going on. Obviously not a lot of reports of nitrogen deficiency out there but we certainly want to be on the lookout for that general paling and yellowing of the lower part of the plant. Again new growth still may look good but be on the lookout for the nitrogen in part of it as well. 

We've talked some in the past few weeks, shifting gears a little bit to mid-season nitrogen. Just a quick reminder on that, that what we're after on applying mid-season nitrogen, of course we're talking about varieties, pure line varieties, where we want to make mid-season nitrogen applications, we want to be at least three weeks after the pre-flood nitrogen was incorporated by the flood, not necessarily three weeks after it went out but after it was actually incorporated where the plant could start taking it up. At least three weeks and then be into reproductive growth, so green ring or beyond. So meet at least those two minimums. The real preference is to wait four weeks. That's when the timing is lined up much better to get maximum uptake and maximum yield response from a mid-season nitrogen application. So if you were looking for more of a growth stage, that's the optimum time in most every scenario, applying before half inch does not yield the greatest benefit. So applying at green ring usually doesn't give us the greatest benefit from mid-season nitrogen and sometimes it doesn't at all because unfortunately more often than not in recent years is we've been delayed getting our pre-flood nitrogen out. We're too close to that pre-flood nitrogen incorporation when we hit the green ring for the plant to be ready to take it up and we just don't get the full benefit from it. Sometimes not much benefit at all, again until just a little bit later once the plant's ready for it. So four weeks after pre-flood nitrogen incorporated and preferably half inch joint movement or beyond for that mid-season nitrogen application and that still gives you a couple of weeks of a window to get a really good response. The mid-season nitrogen response starts to decline as you get into late boot, flag leaves all the way out, so we want to be just in front of that, otherwise we can still get a great and full response to that application. 

Speaking of those later nitrogen applications, and moving over to talking about hybrids, where we recommend applying boot nitrogen or more specifically, late boot nitrogen. Again, flag leaves all the way out where you can see the collar on the flag leaf. You're waiting on no more leaves, you're just waiting on heading at that point. We don't want to make that late boot application until we're fully into late boot. Flag leaves all the way out, you've got a week or so window in there to make that application, it's perfectly fine to go all the way right up to the point you start seeing the first few heads. That's kind of an ok indicator that the majority of the field is fully into late boot at the time you make that application. At that point the plant is done growing vertically and you're not going to increase plant hide or make it more rank down low with more tillers and that's going to give us the best benefit there. The late boot nitrogen application in most of our research hasn't shown one single large increase to any one thing, but very consistently at every site. We've done it with a number of different pre-flooding rates that it's followed. Still that late boot seems to give us a few bushels of yield, a small bump in head rice yield and a small bump in total rice yield and ends up leaving us with a twenty, thirty plus dollar net return over the cost of that application. And a quick reminder again that on the boot nitrogen application we only recommend going with sixty-five pounds of urea per acre, just thirty units of nitrogen. Some will say, well I'm paying for a minimum of a hundred pounds to be flown whether I put out a hundred pounds or not, so I'm just going to go ahead and have them apply a hundred pounds. Well, that's certainly one way to go but you're actually spending more money on more fertilizer just to justify the fact that you're paying that minimum, especially with urea prices where they are. Again, we don't need more than that thirty units. Let's save the cost of that other thirty-five pounds of urea you're talking about adding to it. Let's save that money and keep it in our pockets. It's not giving us additional benefit at that time. One other thing that can happen is some lightening of the plant around this time and make someone a go a little earlier than late boot. Put it out kind of at more mid-season time. Again we see no reason with the majority of hybrids to do that. Certainly if it takes on a real pale look and it looks like it's really going the other direction and running out of nitrogen, fine, but a little bit of lightening up is not uncommon and quite often rice looking slightly low on nitrogen, means we really hit the number very well and optimally fertilized that rice without putting out too much access. So that's actually kind of a goal is to actually have it look, make us feel like maybe we left it just short. That means we nailed it right on. So that's actually a pretty good place to be and will help us reduce our lodging potential, which the boot nitrogen application in addition to the other positive mention helps to do as well on our more lodging prone hybrids when lodging does occur. Generally the boot application reduces lodging by half over what it would have been without it, so certainly not all hybrids it's going to be a big benefit for or do a lot because some don't lodge very much but you have all the other benefits that go along with it is well beyond just the lodging part of it. 

So I hope everybody learned a little bit of something. I covered a wide range of topics that we're experiencing and kind of going through out there in the rice world today. Certainly if you have any questions feel free to reach out to myself or any of our other specialists as well as reaching out to your county agent for any questions. They are there to help you, very nearby as well. So, look forward to seeing you out in the field. Thanks for joining us for this episode for Rice and Advice on Arkansas Row Crops Radio. Have a rice day!

Arkansas Row Crops Radio is the production of the University of Arkansas System Division of Agriculture. For more information please contact your local county extension agent or visit uaex.uada.edu

End notes: Arkansas Row Crops Radio is a production of the University of Arkansas System Division of Agriculture. For more information, please contact your local county extension agent or visit uaex.uada.edu.