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Issue No. 10, Article 7/May 28, 2004

Crop Growth Charges Ahead

Although recent weather has not been kind to crops everywhere in Illinois, virtually all of the corn crop and more than half of the soybean crop are up and growing fast, in response to the warm temperatures. A swing around south-central Illinois this week showed that the corn planted from early to mid-April is in the 6- to 8-leaf stage. Except where excessive rainfall has caused standing water, the color and uniformity of the crop are excellent.

Is there any particular advantage to having a corn crop develop this fast, this early in the season? In most years, the answer is yes. We think it's a very good thing to have a fully canopied crop as early in the season as possible, especially during the long days of mid- to late June. The definition of a "full canopy" is not exact, but we might think of it as a crop with enough leaf area to intercept more than 90% of the sunlight that hits the crop during a day. Based on some measurements we have made, corn reaches 90% interception by about the V12 to V13 stage in 30-inch rows, and slightly earlier in narrower rows.

Corn planted on April 15 at Urbana is now in the V6 stage and 12 to 14 inches tall. With average GDD accumulation from now, we expect corn planted in mid-April in central Illinois to reach V8 by the end of May and V12 by June 12 or so. Corn at V12 is about 4 feet tall. Can corn reach that height in the next 2-1/2 weeks? It certainly can; once stem growth takes off at about stage V6, plant height (a combination of stem and new leaf growth) can increase by 3 to 4 inches per day, with most of the increase coming at night. Thus, adding 15 inches of height in a week is no problem at all, as long as temperature accumulations remain average or above.

The GDDs required to add a new leaf actually decrease after V10 or so, to about 50. This is because the stem growth is so rapid that it takes fewer days to push out new leaves. At 50 GDDs per leaf, we can add easily three new leaf collars in a warm week in June. If the crop is at V12 by June 15, it is likely to show tassels and perhaps even silk by the end of June. This is more likely in early hybrids, since most of their earlyness comes from reduced GDDs to silking rather than after silking.

There is almost no downside to having silks appear by late June or early July, especially when there are a lot of fields showing this, so there is less attraction of pests such as corn borer to more advanced fields. In general, drier June weather typically is good for the crop, due to a combination of high sunlight and perhaps some encouragement to deeper rooting. Then in July, we want average or above-average rainfall, followed by a cooler-than-normal August, with lots of sunshine and just enough rainfall to provide what the soil can't. A crop this far ahead will need little of September to complete its grain filling (grain fills for 55 to 60 days past silking), though cooler temperatures in August and some extra days of fill in September would be a great finish to a great start.

In areas where hail has hit corn in the past week, it is important to look at the crop right away to see what stage it was in and to start to assess regrowth potential. If you have hail insurance, involve the adjusters right away. Corn with less than six leaves can usually recover with little if any yield loss, in part because it doesn't have stem tissue above ground to take direct damage. Severe damage, such as plants completely destroyed down to the soil surface, may result in plants unable to grow back. With warm temperatures, you should be able to see within 2 days whether plants will regrow. We have occasionally seen plants that should grow back "by the book" but fail to thrive after damage. In such cases, insurance adjusters might delay counting stands until a clearer picture emerges.

The end, if not the start, to soybean planting has been delayed in many areas, though some 3/4 of the crop is planted by now. The rapid emergence of the crops under warm soil conditions has been positive, though it does mean earlier death of seeds or seedlings if it rains after planting and soils stay saturated for more than a couple of days. In areas that have received hail, you might want to check to see what part of the small plants was injured; heavy hail can break off both cotyledons and the growing point, leaving no opportunity for regrowth. After a few days, it should be possible to count stands of regrowing plants, though badly damaged plants might never recover their full potential. We don't have precise guidelines, but a reasonable approach might be to count plants that show little regrowth as either not plants or as partial plants. If you can replant into warm soils in May, though, there isn't much reason to hope for recovery that might not happen. Many people simply "repair-plant" injured stands, using lower seeding rates to assure there will be enough healthy plants to make up a full stand.

Some people use the hula hoop method to count stands of drilled soybeans, though counting sections of rows works just as well and may be easier for some. Remember that there are 17,424 feet of 30-inch rows per acre, and calculate stands accordingly: 1/1000 acre is 17.424 feet of row in 30-inch rows, 34.848 feet in 15-inch rows, and 69.696 feet in 7.5-inch rows. A practical way to use this method is to measure off 4 feet 4 inches (one-fourth of 17.424 feet) of row; count the number of plants in one row for 30-inch rows, in two rows for 15-inch rows, and in four rows for 7.5-inch rows; then multiply by 4 to get thousands of plants per acre. One advantage over the hula hoop method is the larger area counted, which means less variability.

One observation of our wheat tour through south-central Illinois was that the crop is considerably ahead of normal development. In some fields, in fact, grain is in the soft-dough stage, and there is less than full canopy left to fill the crop. Heading and flowering were a few days ahead of normal, but the rapid pace of development, caused by the warm weather, is not a positive for the crop. Temperatures above 80°F are not ideal for wheat, and especially on days when it's cloudy, grain fill is likely to be slowed. The bottom line: The crop looks relatively uniform and relatively free of serious disease problems, but it lacks the outstanding look that we saw at this time in 2003, and it's unlikely to yield as well as last year's crop. Cooler temperatures would add bushels, but we do not have very many more days of filling left, and it is likely that grain size will not be above average. On the positive side, the crop should come off very early compared to last year, which should allow early planting of double-cropped soybean. As we have learned, though, early harvest of wheat, like early harvest of most crops, usually comes at a cost of lower yields.--Emerson Nafziger

Author:
Emerson Nafziger

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