Issue No. 12, Article 6/June 13, 2008
Picking Up the Pieces after a Rough Start to the Season
Dry weather has returned to Illinois this week, and except in fields or parts of fields where water is still standing, we should be able to finish planting or replanting, and to do maintenance--herbicide or fertilizer application--on recently-planted fields or fields where it has been wet for a long time.
How this late start will affect yields in 2008 will remain unknown, with helpful clues coming only after we get through pollination and into the grainfilling period in corn, and until we get well into podfilling in soybean. It is safe to say that the general condition of both crops is not very good in many parts of the state, and even if the crop condition ratings return to better numbers by late July, compromised root systems will mean more than usual vulnerability of crops--especially corn--to periods of dry weather.
Is there anything we can do to improve root systems in a corn crop? Most fields this year were planted into soil conditions that were less than ideal, due both to “puddle” surface soil and thick crusting from heavy rainfall, and to the fact that most tillage and planting operations were done before the soils were as dry as would have been ideal. This might well have been an appropriate compromise, since those who waited often waited for weeks as the rains continued, then planted into conditions that were little or no better, if they planted at all.
Cultivation is often the first idea that people have as a way to aerate the soil and to help roots recover. At best, cultivation might help break up a hard crust, and so introduce some air (oxygen) into the soil. However, roots are most dense, and so in most need of aeration, near the plant, and cultivation close enough to disturb the soil in this zone will often break roots, and so do more harm than good. Cultivation can also mulch the surface of the soil, and thereby actually slow the movement of water to the soil surface and reduce the rate of evaporation. Water is moved from the root zone most effectively by the roots themselves, which extract water to replace water as it is lost from the leaves. Leaves that are yellow and those on plants that are sitting in water are not photosynthesizing very well, so are not using water very fast. Such plants will be slow to recover root function due to slow drying of the soil.
Why is the corn crop so yellow, even where nitrogen was applied before or after planting? This problem is most noticeable in corn that follows corn, but it is apparent in many of the wettest fields and parts of fields, and so is directly related to the crop’s standing in saturated, low-oxygen soils. One reason for this is that root systems have suffered damage and are physiologically unable to take up nutrients very well. Roots that are in water suffer from low oxygen and also from a buildup of carbon dioxide, which is toxic. They also are not fed very well by sugars from the top of the plant, so the energy needed to take up nutrients like N may be in short supply.
The other main reason that plants standing in water have poor color is that nitrogen may be depleted near the roots, thus causing deficiency of N. Water movement usually helps bring N to roots, and as noted above, water movement to roots is slow when plants are struggling. Excessive rainfall has also helped move N to below the active root zone, and by now the soil temperatures are such that some loss of N through the denitrification process has taken place. Denitrification is the conversion of nitrate to N gas, which is lost to the air, and it takes place when oxygen levels are low, as they are in flooded soils.
Besides lack of oxygen and lack of N, some root systems have likely been damaged by diseases that attack under wet conditions. Corn plants are relatively hardy when they have enough resources, but flooding and low oxygen make plants more vulnerable to root rotting organisms, and in some cases part or most of the root system may be lost to disease. Aerating through drying might help roots to start to regrow, but if roots need to regenerate instead of just revive, recovery might be slow.
Is there anything to do to help plants green up? Until the soils dry out to allow roots to function better, there may be little we can do to help. One of the complicating factors is that the weather turned from cool to warm suddenly in early June, and this tends to favor growth of the tops of the plant at the expense of root growth. Larger plants with more leaf area have higher demand for water and nutrients, and this can make it more difficult for roots to recover to meet the needs of the plant.
In light of the problems roots are having finding and taking up N, many have wondered if finding "creative" ways to get nitrogen into struggling plants might help speed up recovery. If, as we discussed above, the main problem is getting oxygen to roots so they can recover, then just getting N into the plant is unlikely to do much good, especially if that stimulates top growth even more and puts more demand on roots. More leaf area and greener leaf area will, once the root system recovers, increase growth rates and use of water, so will help to dry out soils. But the key is to have root systems recover, and just finding ways to make the leaves greener might make us feel better, but is unlikely to help speed this process very much. The same can be said for micronutrients and other products that have been promoted as helping make plants green up faster.
It takes patience, but if soils are starting to dry out, about the best we can do is to let the root systems come back to full function, without doing things to “help.” The lowest areas of fields, where water stood for several days to weeks, will likely need to be replanted. If the plants aren’t obviously dead, with death of leaf area, then you can cut plants open to see if the growing point is a healthy, white color or if it is showing discoloration or other signs of deterioration. Failure of new, green leaf area to appear within the whorl of the plant is often a signal that the plant won’t be coming back.
Does it make sense to plant or replant corn this late? Depending on insurance coverage or herbicides applied, it might be the only option. If fields still have enough water so that it will be the end of June before they can be planted to corn, then use earlier hybrids than normal in most of Illinois. In many cases, having the option to harvest late-planted corn as silage will be advantageous. But as we discussed last week, fair yields are still possible for corn planted this late in the southern part of the state. In northern Illinois, the number of growing degree days expected from late June to the date with a 50% chance of frost is inadequate to bring normally-grown corn hybrids to maturity, and so moving to an earlier-maturing hybrid is advised. Fortunately, most corn that needs to be replanted in the northern half of the state is in low-lying parts of fields, and the area is less extensive than in parts of southeastern and southwestern Illinois. Use early hybrids in these areas, and hope for the best. If it gets wet again, then the crop will likely die again. If it gets dry, then the diminished root system will likely mean drought stress “early and often”. It’s not impossible, but getting good yields from corn planted after 20 to 25% of the seasonal GDDs have already accumulated is not a very sure thing. --Emerson Nafziger