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Issue No. 3, Article 8/April 13, 2007

Wheat After the Freeze

Temperatures dropped into the lower 20s or upper teens throughout nearly all of Illinois over this past weekend. As I predicted last week, the most advanced wheat crop was hurt the most by low temperatures. Wheat in the northern part of the state may have suffered some leaf injury, but it will likely recover well, except in fields that were not in very good shape before the freeze.

In the southern half of the state, there is real concern about whether the crop will grow out of the injury done by freezing temperatures. In some cases, such as those fields giving off a silage-like smell and with darkened leaf tissue, the crop is basically dead, though some tissue at the base of the plant is still alive. Some of this living tissue is likely to be small tillers that had stopped developing at the base of the plant. These might start to grow as the competition from the larger stems decreases, but they would be starting very late, and so would be flowering and trying to fill grain very late into the season. This makes it unlikely that late tillers will produce high yields, and harvest will certainly be late.

In fields that were in Feekes growth stage 8 (flag leaf emergence) or 9 (flag leaf completely out, or early boot), the growing point (developing head) was 6 or more inches above the soil surface. Many plants this size may not have had the majority of their leaf area killed by the freeze, but stems may have frozen below the growing point, and many such plants have "flopped," ending up nearly flat on the ground due to weakened stem tissue. Dead stem tissue beneath the head means that the head is basically cut off from nutrients and water. Such plants may show green color for some time, but there's little chance that they will recover to produce good yields.

Plants less advanced at the time of the freeze show a range of symptoms, from minor leaf burn (death of leaf tissue) on the upper leaves and leaf margins, to considerable loss of leaf area. Loss of leaf area is seldom a positive thing, but if the plants were only starting to joint (grow upright), only a third or so of their total (eventual) leaf area was exposed, so they should recover their green color as new leaves emerge and expand. Loss of lower leaves might have little effect on yield. The important factor in these fields is that the growing point was nearer to the ground and better protected from the low temperatures.

If none of these descriptions fits for a particular field, then there may be no choice but to wait for a week or so to see what parts of the plants recover and start to grow back. In most cases the extent of recovery will be rather obvious: if heads are healthy and stems start to elongate, with a majority of leaf area still intact, then the crop should recover. If any of these is missing, then recovery is unlikely. In any case, however, this period of low temperatures will mean a delay in development of the crop, including later heading and harvest. The crop in the most advanced fields was on pace to head early, so the delay may not be serious. But any delay means, on average, warmer temperatures during development. This could be a problem, especially if it rains during the flowering period.

Destroying badly damaged wheat to plant another crop can be a challenge. Removing the crop as forage is possible if the crop has not been treated with a herbicide whose label prohibits such use. Planting into a damaged crop will get easier as the crop residue dries, but it can be difficult to kill plants if they have green lower stems and leaves but little green leaf area in the upper plant. Such plants do not absorb herbicide very well, though Gramoxone might kill exposed tissue. Mechanical residue removal will allow soils to dry faster and can make no-till planting easier, though it is moderately costly. In some cases, mechanical tillage might be the best way to remove residue and to make a good seedbed, especially in fields with low erosion potential. This is more likely to be the case when planting corn than when planting soybean.--Emerson Nafziger

Emerson Nafziger

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