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Corn Still a Worry, Soybean's Turn Next

May 24, 2002
Travels this week took me to St. Louis and back, with a good look at the crop--or, rather lack of a crop--through a swath of south-central Illinois, both on and off the interstate. I saw only one field (in Shelby County) where planting was being done. Corn stands looked okay on the higher areas of fields, but almost every field with corn planted (and almost all planted corn was up since there has been little planting for 2 weeks or more) had areas drowned out. Corn all had poor color, due mostly to cool temperatures. That should improve slowly as temperatures return to more normal levels. So far in May, we've had barely more growing degree-days than we had in the month of April, so it's no mystery why the corn has not been growing very quickly. If we're lucky and the weather continues to be good during the next week, a lot of the remaining corn will be planted, though planting around wet areas in fields will be very common. According to the official estimates, there was no planting progress at all for the week ending May 19.

If planting can get under way at a good pace in most of the unplanted areas within the next week, we think that most producers can probably still plant the hybrid they had planned to plant a month ago. Exceptions might be hybrids relatively late for the area and perhaps hybrids that are known not to do very well under stress. Most people who still have water standing in low areas of fields are going to want to get enough seed of early hybrids to replant (or plant for the first time) the low spots. Most of these will not be "drivable" until early June and, by then, only if it doesn't rain and refill them.

There are also reports this week of frost injury to corn in the northern half of the state. As usual, such injury was scattered, but some fields had most of the leaf area of emerged corn plants killed. Leaf color was not very good and leaf tissue was not very healthy, so loss of existing leaf area should not greatly affect yield prospects in most fields. Plants with one or two emerged leaves can be killed by frost, but because the growing point, which is about 3/4 to 1 inch deep in the soil, needs to be frozen for that to occur, we don't expect that to have occurred. Still, it pays to watch corn for a day or two after lows in the lower 30s, especially in lighter soils (where cold air penetrates much more easily) and in lower-lying parts of fields, where cold air settles at night. Frost injury of leaf area will delay development of the plants, but because growth is slow in undamaged fields and parts of fields, this delay will not be as great as it might have been.

With corn planting at least starting to move again, thoughts will swing toward soybean planting. Only 10% of the crop was planted as of May 19, and for many producers soybean planting will not start until corn planting is complete. That's appropriate, as corn is losing yield much faster with planting delays now than is soybean. We do not have a lot of solid data on the effects of late planting on soybean yield, but our data from last year showed that yield decreases, while quite variable, averaged about 3/10 of a bushel per day of delay during the last half of May in the northern half of the state; soybean planted at the end of May would be expected to yield 3 to 5 bushels per acre less than that planted in early May. We expect this loss rate to be a little less in the southern half of the state, but with average weather during the season, we expect yields to start to decline as planting is delayed past May 25 or so. We did not plant into June at most locations in this study, but in the southern part of Illinois, we would expect losses of about a quarter of a bushel per day of delay or so during the last week of May and the first half of June. Effects of planting delay in soybean are as variable as the weather in July and August, though, so one can be pessimistic or optimistic about what to expect this year.

While it is tempting to try to "finesse" soybean management to compensate for late planting, we do not see evidence to support making many changes. The real problem that late-planted soybeans often encounter is lack of sufficient growth to make a complete canopy by the time the crop flowers or even for the entire season. Such lack of full canopy almost always results in yield decreases. This principle should guide management decisions for late-planted soybeans:

· Changing to an earlier-maturing variety should not be done because earlier-maturing soybean varieties usually grow shorter than fuller-season ones. Flowering is triggered by warm temperatures and decreasing day length, and early varieties will flower earlier and with smaller plants than later-maturing ones. This might get them to mature quicker, but it will also mean they are smaller plants and, often, plants with less ability to survive stretches of dry weather later in the season.

· There is little reason to increase seeding rate if planting is delayed. When emergence percentage is normal, seeding rates used by most producers are already substantially higher than those needed to maximize canopy formation and yield. Delays in planting, even if they result in smaller plants, usually do not call for higher seeding rates unless planting is delayed past mid-June. And later planting into warmer soil usually results in higher emergence percentage, providing that seed was stored well and didn't lose germinability during the delay.

· The yield advantage of narrow (20 inches or less) rows over wider rows tends to increase as planting is delayed. If the option exists, planting at less than 30 inches might decrease the size of the yield penalty from delayed planting. That is the case, however, only if the planting equipment for narrow rows is good; I would not want to drag an old drill out of the shed just to get narrow rows if the drill weren't capable of doing a good job of seed distribution and placement.

· With warmer soils and more mineralized nitrogen, we would typically expect less response to inoculation when soybean is planted late. Tests of inoculant have been inconsistent in Illinois, in any case. In general, given the lower yield potential, late planting is usually not the best time to make additional expenditures on inputs without a proven record of return.

A final note: late planting in both corn and soybean, especially when done in soils that are marginally too wet, usually means that the crop develops with smaller-than-usual root size and rooting depth. Warm temperatures during vegetative growth also tend to favor top growth at the expense of root growth. Except for waiting until soils are in better shape to plant, there is little we can do about this, though it will pay to think about this when supplying nutrients (and water if we have any control over that) to the crop. Restricted root growth is a major reason why stress tolerance and yielding ability tend to be lower in later-planted crops.--Emerson Nafziger

Author: Emerson Nafziger

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