Issue No. 10, Article 9/June 1, 2007
Early June Stands and Crop Prospects
Rainfall over much of Illinois in the past week has brought much-needed relief from dry surface soils that have caused problems with plant, including root, establishment in many parts of Illinois in May. As usual, some parts of the state did not receive as such rainfall as was needed to replenish surface soil water, but the root growth problems I discussed last week have been greatly decreased.
In some areas, planting conditions were not ideal, and emergence of both corn and soybean was not very uniform. Rainfall in these areas will generally bring the rest of the crop up, if the seed has not been compromised by germination's having started and then stopped as soils dried. We expect such problems to be much more likely in soybean than in corn. If you see some plants coming up within a week after rain but others are not emerging, dig up seed immediately to see if it is dead.
If soybean stands are much less than 90,000 to 100,000 per acre (21 to 23 plants in 10 square feet), or if there are large areas with few plants, replanting might be necessary. In research we have done, filling in soybean stands by planting without destroying plants from original stands has worked as well as, and in some cases better than, first destroying the older plants using tillage or herbicide. Replanting should be in 15-inch rows or drilled to help make up for smaller plants that typically grow from late plantings. It is often difficult to set "supplemental" seeding rates when replanting across uneven stands. If most of the stand is thin and there are relatively few areas without plants, you might subtract the existing number of plants from 150,000 and plant the difference. For example, if there are 60,000 plants, then replant using 90,000 seeds. Areas without any plants can be double-planted at the same planter setting to provide adequate seed.
Where corn came up at two distinctly different times due to replenishment of marginal soil moisture by later rainfall, the result will be a "two-tiered" stand, with late plants as many as two or three leaf stages behind the early-emerging plants. This will result in complex competition among corn plants, and it is difficult to predict what the effect on yield might be. We do expect some yield loss from this unevenness, but it is unlikely to be great enough to justify replanting this late, especially given the costs and uncertainties in trying to establish a late-planted crop. The greater the proportion of plants that adjoin plants of a different size, the greater the yield loss will be. So unevenness that is mixed down the row is more damaging than unevenness that runs in streaks, with stretches of older plants alternating with stretches of younger plants down the row.
On the positive side, most stands are good to excellent, and crop color has improved since rain fell. The earlier-planted fields look as good as many fields looked at this time in 2004, our "benchmark" year for high yields. This year, as in 2004, May was warmer than average, and crops spent little time in cool, wet conditions early in vegetative growth. At Urbana, growing degree day (GDD) accumulation in May 2007 will total about 570, compared to 409 in 2006 and 519 in 2004. We think that warm May weather tends to give corn a yield advantage, though most evidence that supports this is anecdotal.
The corn that we planted earliest here at Urbana (April 2) has been growing with little interruption and is now at stage V8, with plants about 24 inches tall. Corn planted on April 20 is one leaf stage behind and perhaps 2 inches shorter. By the V8 stage, corn is entering the rapid stem elongation stage, and it will add 12 to 18 inches of height in each of the next four weeks, as long as temperature and moisture conditions are favorable. Leaves in smaller plants are pushed out of the whorl mostly by the expansion of leaf size, which is relatively slow. Once the stem starts to elongate rapidly, leaves are pushed out by both the expansion of leaf size and the increase in stem length. This means a reduction in the growing degree days required between leaf stages; GDD per leaf stage decreases from about 80 in plants smaller than V6 to about 50 in plants larger than V10. At the same time, average temperatures are increasing, with weekly accumulations of GDD in June averaging about 150 compared to about 90 in May. So new leaf appearance will move from about one leaf per week in early May to as many as three leaves per week as the plant nears tasseling.
Before the recent rains, some commented that the crop was entering the stage "when ear size is determined," with the worry that stress during this time could seriously limit yield. Because V8 plants have at an upper node the small ear shoot that will develop into the main ear on the plant, many consider it reasonable that poor growing conditions at this stage will limit the number of potential kernels that can develop on this "pre-ear." In practice, however, potential ear size is relatively well protected, and high yields are possible even if plants show extensive leaf curling and other signs of drought stress at around V8. There is extensive leaf tissue surrounding the stem where the upper ear initial is attached, and this tissue tends to buffer the effects of insufficient water in the plant. We have also seen that in years like 2005, some fields that experienced serious stress during much of June ended up yielding quite well after rain fell in late July. I thus consider it unlikely that there has been any loss of yield potential caused by stress encountered by the Illinois corn crop so far in 2007.
Soybeans are now at VC (cotyledonary leaves expanded) in many fields, and some are starting to expand their first trifoliolate (3-leaflet) leaf, meaning that they are approaching stage V1. There has been some confusion recently about when a leaf is large enough to count in the V-stage system. I find it best to count the uppermost trifoliolate leaf as a leaf once it has expanded to be about as large as the leaf below it on the stem, and when the trifoliolate leaf above it has unrolled to the point where the edges of the leaflets are no longer touching. During early vegetative growth, each new soybean leaf tends to be larger than the leaf below it, so this guide works well. Dry weather will reduce leaf size, but soybeans tend to produce more leaf area than they need for full light interception (by stage R3 to R4) and yield, so smaller leaves at some nodes is not a big problem, as long as the canopy ends up being complete. Warm temperatures in June will help the crop develop adequate leaf area, as long as the water supply stays adequate. Excessive water is usually more harmful to soybean plants in June than are moderately dry conditions. This is because maintaining root growth and health is very important to maintaining the ability of the plant to function well during seed filling.--Emerson Nafziger