Issue No. 8, Article 11/May 18, 2007
Wheat Watch Continues
Despite some favorably dry weather, the Illinois wheat crop continues to struggle in its attempt to recover from the low temperatures of April. While there are some good fields of wheat in Illinois, others that we expected to fall into the "good" category by now are recovering poorly.
We continue to get reports of short plants, uneven plant height, and general failure of the crop to make the rapid, upright growth expected at the time of heading. Some of this may be because tillers have taken over after the main plant suffered damage from the low temperature. But if there is little dead tissue visible, it's also possible that stem damage to the main stem slowed its growth so that it resembles a tiller as it develops. It makes little difference at this point, since both main stems and tillers have about the same ability to produce heads and fill grain. The overriding concern is that, whether they're tillers or not, many heads are small, and small heads do not portend good yields.
Adding to the general concern about the crop is the poor leaf color in some fields. Some appear to be nitrogen-deficient and have never recovered a healthy leaf color since the freeze. Dry surface soils in some areas may be contributing to this problem. In the past few days we have also been hearing reports of yellowing of flag leaf edges, which may be a late infection of barley yellow dwarf virus (BYDV). It was expected that the low temperatures would kill the aphids that carry BYDV from plant to plant, but some likely survived, then spread rapidly in the warm temperatures.
What is expected from here on? The first step may be to try to estimate possible yield before making a final decision to keep a crop. As I wrote earlier, 20 normal-sized kernels per square foot is about a bushel per acre. Including some tillers with few kernels, the average number of kernels per head is often about 20, so one head per square foot might translate into about one bushel per acre. This year, with small heads emerging in many fields and some question about whether kernels will fill to normal size, that estimate may be too optimistic.
It's a tedious process, but you account for reduced kernels per head by counting "mesh" (spikelets) per head, and assuming two kernels per mesh. This may be more accurate where there are a lot of small heads, but we still can't guess at final kernel size with any confidence. Kevin Black of Growmark suggests calculating the number of kernels per square foot and multiplying this by 0.045 to predict bushels. That formula predicts 16,000 seeds per pound, while we use 15,000. Both of those numbers could be a little optimistic this year.
Robert Bellm, Extension crop systems educator at Edwardsville, reported that the unusual variability has made it difficult to find representative parts of fields in which to make yield estimates. For those with experience in looking at wheat, just walking through a field to note head size, head number, and general plant health compared to normal years might be more helpful than doing a lot of counting.
In the northern part of the state, where there is little chance for a double crop, it may still be wise to consider converting damaged wheat fields to a different crop. From reports of the crop in the Kankakee-Pontiac area, it is clear that the low temperatures did more damage to the smaller plants than we had expected. If straw is a primary product, remember that short plants will produce less harvestable straw. In southern Illinois, double-crop potential may justify keeping a wheat crop even if the yield potential is only 35 or 40 bushels.
As a general rule of thumb, we expect harvest about six weeks after heading. Heading this year ranges from somewhat late in southwestern Illinois to moderately early; most varieties in the trial here at Urbana headed out late last week, so 3 to 4 days earlier than normal. It is unlikely that harvest in the main wheat-growing areas will be early, unless it gets hot and dry, in which case yields (and test weights) will suffer even more.--Emerson Nafziger