Issue No. 14, Article 12/June 24, 2005
This past week, I traveled across Iowa and South Dakota. Crops in Iowa looked good, except that soybeans were mostly planted late. Southeastern South Dakota was a disaster zone, with more than half the fields not planted and water still standing in many fields. Thus, we have the usual pattern of too much rainfall in some places and not enough in others. Few things are as frustrating to those on the two extremes of this pattern. I know it's common to have this pattern, but that doesn't make it easier to experience.
As clouds and moisture continue to bypass Illinois, often "dashing" on the high pressure to our southeast, we keep wondering when the pattern will break. Unlike some places to the west of us, history tells us that the pattern will change and rainfall will return. But signs are scant that this will happen soon. Until it does, we can only hang on and hope that the crop is keeping its yield potential high. Is this a realistic hope?
While research tells us that short periods of dryness are most detrimental at pollination time in corn, such research uses methods of quickly withdrawing water and then supplying it quickly at the end of the defined period. That doesn't very well duplicate what we're experiencing now in our driest areas, where stress on the crop has been increasing for weeks. Our question really is this: how quickly is the crop losing yield potential as it continues without renewal of soil water?
We don't have a good answer for this, but the crop is probably "hanging on" better than we think it is. I say that based on similar patterns in previous years, in which dry conditions were relieved and high yields followed. The crop is still growing in most fields, which indicates that it is tapping soil moisture. Soil moisture supplies are still present, though in lighter soils they might be starting to deplete. Still, there's no question that the sooner we get relief, the more certain the recovery to produce high yields.
By the numbers, plant-available soil water storage in the top 5 feet of soil ranges from about 10 to 15 inches in most of our soil types, though the extent to which the roots can tap this varies based on factors like soil compaction, claypans, and how well the roots got started. Daily water use rates approach 1/4 inch under as canopies become complete, but when the crop goes under stress and leaves roll, this indicates that stomata are closing. This decreases water loss rates. If we estimate that the crop has used half of the available soil water, has access to the rest, and is using 1/5 inch per day, then we still should have 20 days or so of water, at least adequate to keep the plant functioning.
The numbers, however, are probably a little optimistic, and it's highly likely that 3 more weeks without rain will reduce yields in most fields. This is because pollination events will occur within 3 weeks, and developmental events during pollination are more sensitive to water stress than is vegetative development. We discussed this last week and will revisit it next week unless it rains 2 inches over the entire state and it's no longer a topic. Let's all hope that happens.
Soybean growth is being inhibited more than corn growth under current dry conditions, mostly because the root system is less extensive. Under these warm conditions, we can expect flowering to begin in some fields in the next week. If it stays this dry, many early flowers will probably not set pods. The only positive in the case of soybeans is that they can flower and set pods over a period of 3 weeks or more and so will retain their ability to respond positively to rainfall for the next month or more. Rainfall will also spur stem growth, but the crop planted in wide rows may not be able to grow enough to produce a full canopy. When it's dry like this, size and length of stems, petioles, and leaves are reduced, and canopies often remain "narrow" as a result.--Emerson Nafziger