No. 11 Article 5/June 17, 2010

Corn Crop Canopy: Are We There Yet?

Continued heavy rains in parts of Illinois have resulted in flooding and some loss of stands in low-lying areas. Many are concerned about loss of nitrogen, and some have taken remedial action, dropping nitrogen into the crop. In fields where rainfall has not been excessive, the continuing warm weather has provided the expected boost to growth rates. Growing degree-day accumulations since May are above average in most of Illinois now, keeping us well on track for an early start to pollination.

By now (June 11), corn planted at Urbana on April 5 is at V11 and about 45 inches tall; on April 21, V8 and about 28 inches; on May 10, V6 and 18 inches; and on May 28, V3 and about 7 inches. Once past stage V8 or so, corn needs only about 50 GDD to add a leaf stage. With warm temperatures, the crop can add two to three leaves and 12 inches or more in height in a week.

With warm temperatures and adequate water during early vegetative growth this year, leaves appear to be a little larger than normal, and this has caused the rows to "fill" earlier than we sometimes see. It's considered a positive thing for the crop canopy to develop quickly, so that by the longest day (June 21) the crop is capable of intercepting much of the sunlight that falls each day.

The way we typically decide that the canopy has filled is to see little or no gap between rows as we drive past the field. The V11 corn shown in the first photo below would qualify as having filled the rows. As most people know who have walked a lot in corn fields, the look from the end of the field at 55 miles an hour can be deceiving. The second photo was taken facing down into the canopy shown in the first shot. It shows clearly that, while there is a lot of leaf area, it is not enough to intercept all of the sunlight. In this case, probably 20% or so of the sunlight falls on the ground, and it is lost in terms of photosynthesis. This figure will drop below 5% by pollination and should be only 2% to 3% in a fully formed canopy soon after pollination.

Corn at stage V11 and about 42 inches tall. The crop was planted on April 5 and photographed on June 10.

Photo facing down into the canopy shown in the previous photo. Note the sunlight reaching the ground.

Even though sunlight interception by a rapidly growing crop in midvegetative stages is not complete, it's hard to imagine that such a crop has no advantage over a later-planted, smaller crop. The third photo shows a V6 crop photographed on the same day as the V11 crop in the first photo. The V6 crop is intercepting less than 25% of the sunlight, while the V11 crop is intercepting three to four times as much.

Corn at stage V6 and about 16 inches tall. The crop was planted on May 10 and photographed on June 10.

Even though the smaller crop will begin to grow rapidly and may reach V11 (the same stage as the larger crop now) within 15 to 18 days, it will lose out on a great deal of sunlight in the meantime. It's likely that the smaller crop will intercept no more than half the sunlight that the larger crop will intercept during the first three weeks of June. But if previous patterns hold, the smaller plants (planted May 10) should yield 90% or so as much as the early-planted (April 5) crop. And with late planting and cool temperatures in May last year, most of the June sunlight hit the soil, not the crop. Yields were still good.

The lesson in this is that while early canopy development is a desirable thing, we can get very good yields without it, provided that the canopy eventually does develop fully and that we get enough sunlight during and after pollination, when it counts most toward yield. By the same token, various forms of injury during vegetative growth, such as mild nutrient deficiency, some water stress, and modest leaf damage from hail, often have little effect on yield potential unless the causes of the injury remain uncorrected.--Emerson Nafziger

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