Issue No. 9, Article 7/May 23, 2008
Corn with Issues: Keep, Replant, or Change Crops?
I traveled from Champaign to near Mt. Vernon on May 19, stopping to assess heading date of the wheat varieties at Brownstown, to see fields and crops, and to attend a wheat field event south of Centralia. The wheat crop looks great, but heading was just underway at Brownstown. This is the latest heading I have ever seen there, and the crop was not much more advanced at Mt. Vernon. If the rule of thumb holds that it takes 6 weeks from heading to harvest, harvest and double-cropping even south of I-64 might not start before the last week of June this year.
Of considerably more concern is the fact that very little of the planted corn crop south of I-70 has emerged, even though it was planted early enough to have emerged by now. I stopped by one field north of Centralia after noticing a few plants that had emerged in a high corner of the field. There were basically no other plants emerged in the entire field. Soil in the planted rows had run together during heavy rainfall. The few seeds I dug up had germinated and were not soft, but they had hardly any trace of a root or shoot, and they were definitely dead. Rainfall in this area has exceeded 5 inches since May 1, and the seeds and seedlings appeared to have died from lack of oxygen.
I did not see a single cornfield between Effingham and Mt. Vernon with a good stand, and in most planted fields there were no visible emerged plants at all. It has been wet in that area since early in May, so most planted fields were planted early enough that they should have emerged by now. If they have not, there is very little chance that they will, and nearly all of these fields will need to be replanted. Even in fields with some plants emerged, there are likely to be areas where water stood or where soils were saturated long enough to kill seeds or seedlings.
Up until the end of May, it is likely that even uniform stands of less than 15,000 per acre will benefit from replanting. Even early-planted and emerged corn has not yet grown very much--our April 8 planting at Urbana is at about V4 (4 leaf collars visible), but the plants are very short (less than 6 inches tall) and small for this stage, reflecting the generally poor (cool, wet, and cloudy) conditions during most of the last 6 weeks. About 350 growing degree-days have accumulated since planting, and that should be about the number needed to reach late V3, so development is keeping pace with temperature. This slow growth is not a good thing, but compared to a more favorable spring, such as the one we had a year ago, it means that replanted corn will not start out as far behind early-planted corn as it often would. So in relative terms, replanting is "favored" this year.
Our most recent data (see "Thinking About Corn Planting Date and Population" in issue 2 of the Bulletin, April 4, 2008) suggest that we have lost about 15% of our yield potential by May 20 and that we are losing about 1%--1.5 to 2 bushels--per day of delay during the last 10 days of May. Given the slow growth so far, these losses from planting delays might be a little less in 2008, at least on a relative (percentage) basis. This is due more, however, to the likelihood that yield potential has been decreased some by the slow growth so far than to any expectation that we can expect less absolute loss from late planting this year. In other words, even early-planted corn will by June 1 appear to have been planted later than it was due to limited growth. GDD accumulations continue to run considerably behind normal, with totals so far in May running 30% to 40% below average. As an example, we received over 500 GDD in May 2007, but the total through May 20, 2008, has been only 171, or 80 less than average.
For those who will need to replant corn, one question might be whether to keep a low stand and replant next to the old rows in order to take advantage of the ability of established plants to contribute to yield. This involves some guesswork, but given that existing plants will often have compromised root systems due to saturated conditions, and that they will likely have grown little by the time replanting is done, the plants from the first planting might not contribute much. They will compete some with the replanted plants, and if they do not produce enough more per plant to make up for this, the overall effect will be loss of yield. Existing plants are often damaged by the replanting operation, which will lower both the ability to produce and the ability to compete. In short, while there is clearly a risk that late-planted/replanted corn may encounter dry conditions later in the season and not yield very well, keeping a low percentage of older plants alive through the replanting process probably does not add very much potential, and in many cases it will not pay. The first priority needs to be getting good establishment of the replanted seeds, not trying to preserve plants from the first planting.
As I noted last week, there is no need to change hybrid, or to change maturity if hybrid has to change, when replanting through late May. Planting without tillage will help conserve soil moisture and can often result in more favorable seedbed conditions than using full-width tillage. If only a few plants are emerged from the first planting, using row cleaners might well destroy these or damage them to the point where they don't compete much with the new planting.
There is no need to change planted population when replanting or planting late the first time. After June 1, it may help to lower planted population slightly if planting into good soil conditions. This is mostly because we can expect emergence percentage to be greater with warmer soils. There is also an increased risk that late-planted corn will encounter water stress later in the season, but it is not advisable to prepare for this by lowering the population, both because of the possibility that the season will be favorable and the crop responsive to higher populations and also because lowering the population by several thousand per acre will likely not help much if serious water shortage develops. Late-planted plants often grow taller than early-planted ones (which is more likely this year due to slow growth of early-planted corn), but they may have less leaf area per plant, so populations cannot be lowered much without decreasing canopy cover.
Is it time to think about switching from corn to soybean if planting or replanting is delayed to the end of May or into early June? Making such a decision is complicated, but in general, poor conditions later in the season tend to affect both corn and soybean to a similar extent, with the exception that late-planted soybean can be revived considerably by late-August rains, while late-planted corn often will not be. Over the last 10 days of May, we are likely to be losing about 1%, or half a bushel per acre, per day of delay. In terms of gross income, that is less than the 1.5 to 2 bushels per day that we are losing from the delay in planting corn. The base for corn has been eroded more than the base for soybean, because we are farther past the optimum planting date for corn. On the other hand, prices are still sending a strong signal to plant corn.
One important factor affecting the decision of changing from corn to soybean is whether nitrogen has been applied for corn. Fertilizer N is likely to have little effect, if any, on the yield of soybean. Soil nitrate inhibits nodule formation, and it can keep nodules from forming altogether. But 150 to 180 lb of N is roughly the amount that soybeans usually fix from the atmosphere, so lack of nodules in N-fertilized fields may have no effect on yield. The problem is that this represents a cost of $75 to more than $100 per acre, and none of this investment will be recouped if soybean is grown instead of corn. Even with reduced yields from late planting, we expect such an investment to return at least two times that amount in corn. So switching from corn to soybean should probably be considered only in fields where N has not been applied, at least into the first week of June.--Emerson Nafziger