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Issue No. 4, Article 6/April 20, 2007

Epilogue on the April Freeze: Tough Decisions Remain

We continue to get a lot of questions about the effects of the cold temperatures of April 5 to 9 on wheat and alfalfa. The answers for alfalfa are probably more certain than those about wheat, due to the uniformity of alfalfa damage. That is, nearly all of the top growth, or at least the leaves and upper stem, froze on alfalfa, while the lower stem generally survived. Despite the fact that alfalfa height at the time of the freeze ranged from less than 6 inches in northern areas to 20 inches or more in the southern part of the state, the entire crop was in vegetative growth, and regrowth is also vegetative, meaning that the amount of growth has less effect on regrowth potential.

On the question of whether dead alfalfa tops should be cut and harvested to "help" regrowth, the answer is no. There is very little to harvest in the top growth in the northern half of Illinois, and by the time the crop is cut (we'll call it first cutting, even though it might resemble second cutting), the dead residue from the first cutting will be mostly gone. Even if some gets included in the first harvest, its effect on quality will not be so negative. It is possible that alfalfa seeded last summer might have limited root reserves that make it difficult to grow back. If new leaves and shoots appear in the next few weeks, regrowth should be okay, even though it might be slow.

Questions about wheat are more difficult to answer, and they are not getting much easier over time. Here are some questions and answers regarding the wheat crop:

Might here be damage to the head even where it was less than 6 inches above the ground and less than an inch long during the freeze and it still looks healthy? We continue to think that such heads are intact, though we realize that the normal protection from surrounding leaf tissue might have been compromised by the breezy conditions during the freeze. Heads at that developmental stage have not completed forming the cells that will become the pollen and egg cells, so unless there are physiological disruptions from the cold temperatures, we think heads should be normal on such plants. At the same time, we recognize that this was truly an unusual event, and we cannot rule out some unhappy surprises that may appear only after heads emerge. Cold weather when wheat is in the boot stage is usually blamed for heads emerging twisted or even bleached and sterile. We hope that heads as small as many were at the time of the freeze will not show such symptoms, but it might help to split some stalks occasionally over the next few weeks to see if heads are developing normally. In one photo that I was sent, the head looked normal but there was a darkened area at the base of the head that looked like it had moved up from the stem. Freeze injury of the lower stem likely resulted in formation of this material, and it moved in the xylem up to the head. It is not at all certain that the plant will be able to fill its head normally after this happens.

Can plants grow out of lower stem injury caused by freezing? There have been a number of reports that larger (jointed) plants that showed freeze damage on lower stems seem to be holding their own, with some new leaf growth and an improved appearance. But many fields like this may not continue to thrive. Though some stem function returned after the freeze, the plant is not able to completely repair damage to its "plumbing," and stems are likely to stay more fragile and subject to breakage where they were frozen. It has been relatively cool since the freeze, and heads are still small, so things have worked okay so far. Once the heads start to develop and temperatures increase, demand for water and the need for physical support might well cause stems to break, or leaves might start to wilt because stems can't conduct water fast enough.

What about leaf damage? In the fields with larger plants, much of the exposed leaf area was damaged or destroyed by freezing. If plants were at Feekes stage 8 (emergence of the flag leaf), then about two-thirds of the total leaf area was exposed. Stage 9 is complete emergence of the flag leaf, at which time all of the leaf area is exposed. While tillers may come on in some damaged fields, the amount of leaf area left to fill grain will become an issue. As with damage to the stem, the larger the plants at the time of the freeze, the more damage there was to leaves. Many leaves that showed only minor darkening during and after the freeze may have recovered some, but unless leaves have a deep green color, they do not have full photosynthetic capacity. If only some of the leaf area is left and it does not have a good, green color, the ability to produce grain is decreased. Wheat tends to have a little more leaf area than it needs to produce high yields, so the amount of leaf loss may not be directly proportional to yield, but we can't lose much leaf area without reducing yield.

What about planting another crop into damaged wheat? Recent questions about this have included ones on allelopathy and other problems that might carry over from wheat to corn. Allelopathy is the production of something (a chemical or group of chemicals) that form as residue breaks down and can damage the next crop. There is some potential of this as wheat residue breaks down, though we usually do not know what the chemical(s) are that cause it. The rate of formation of such breakdown products is usually fast early in the breakdown process, then less over time. It also takes moisture and biological activity, so it will be slow if conditions are cool and dry. Incorporating residue with tillage might increase rates of formation, but if these materials form away from the seed and seedling, they will not do much damage. Thus it might help some to delay planting into residue so that it can dry down, and residue that is not incorporated might stay drier and away from soil microbes and might break down more slowly. But the surest way to avoid allelopathy is to keep residue away from the row. Most do this by using planters equipped with trash movers to clear off the row before planting. We usually worry more about corn than soybean, but research has shown that allelopathic chemicals are often toxic to many plants. On the positive side, allelopathy seldom kills plants, and many crops will grow out of early damage caused by allelochemicals.

How do we treat nitrogen applied to wheat if we're planting another crop? Some work at Kansas State indicated that about half of the N applied to wheat as an early topdress should be available to the corn crop. If N was applied late, more of it might be available. But the N taken up by wheat will become available only slowly to the corn crop, and some of it might be lost in different ways, so we can't count on recovering all of it. I would suggest that half the total N applied for the wheat crop be considered available to the corn crop that replaces damaged wheat, if the wheat was clearly past jointing when it was damaged. If smaller wheat was damaged, especially if N was applied late, then up to 3/4 of the N should be available to the following corn crop. If soybean replaces destroyed wheat, the N applied to wheat should have little effect on the soybean crop, but of course this means no return to the N investment.

How long can we wait to decide to keep a wheat crop or not? Knowing how well the wheat crop is recovering from freeze damage is taking longer than we expected, due in part to the very slow return to warm temperatures. If you are not seeing some good stem growth--if plants have not increased in height by more than an inch or two--by this weekend, then the crop may have suffered more damage than it seemed at first. An increase in height should be accompanied by emergence of healthy, green leaf tissue at the top of the plant. Healthy wheat gets marginally harder to kill the older it gets, but this might be countered by warmer temperatures and by the fact that in damaged wheat, leaf area that appears after damage might help take up herbicide. This presents a dilemma: the longer we wait, the more chance the wheat crop has to show its potential to recover and produce good yields, but waiting to destroy the crop means more residue to contend with and later planting of the replacement crop. When the crop that will replace wheat has a high price like it does this year, planting delays and planting difficulties due to more residue carry higher penalty costs. This suggests making the decision on replacing damaged wheat earlier rather than later.

Will there be enough seed and fertilizer to replace wheat with corn? We think that there is enough corn seed in Illinois to plant an additional 2% or 3% of corn acres that replacing a sizeable chunk of wheat acres would mean. Choice of hybrids and even seed companies was already restricted due to the large increase in corn acres, so adding additional acres will only restrict such choices a little more. Nitrogen fertilizer is somewhat similar in that we think there will be enough to go around. But with so much yet to be applied this spring, spot shortages could develop. With the N that was applied to wheat, the corn crop that replaces it will not need N at or before planting, so delaying N application to sidedress time might make sense.--Emerson Nafziger

Author:
Emerson Nafziger

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