Issue No. 13, Article 5/June 23, 2006
What We Can Learn from Plant Stands
With corn off to a good start and soybeans a little delayed but now coming along, many people who scout crops are wondering what to look for in fields that seem almost perfect from the road. I'll try to put this into perspective with the following comments.
Stand counts in corn are of considerable importance, since they directly affect yield. This should be one of the first things we measure. I've described the measuring wheel method, which is a way to count plants in a bigger sample of row for more accuracy. But when stands tend to be good, like they are this year, the traditional method of counting plants in 1/1,000 of an acre is good enough in most cases. Instead of using a string or tape, measure off the distance (17 ft. 5 in. in 30-inch rows, or 523 divided by the row spacing in inches) in 1/1,000 of an acre and practice until you can walk it accurately. If several different counts are within two or three plants of each other, the stand is fairly uniform and not too many counts are needed.
In soybean, stand counts can be taken in a fourth as much row length, or 131 divided by the row spacing in inches, and multiplied by 4,000 to give plants per acre. It's often useful to calculate the percentage of dropped seeds that established plants to help set seeding rates in the future. Most cornfields should have stands of 27,000 to 33,000, and soybean needs about 100,000 plants per acre to ensure full yields, especially this year when the start of the crop was delayed by planting (or replanting) date.
Plant Spacing Uniformity
Uniformity of plant spacing in corn has been much studied and talked about. Most research has shown that if plant populations are as high as desired, less-than-perfect distribution of plants down the row, when using modern planters with proper adjustment, is unlikely to have much effect on yield. Still, lack of "precision" of spacing is easy to see, and so is a talking point, used to tell people to slow down planting, to better adjust and maintain planters, and so on. I certainly have no problem with doing everything possible to improve planter performance, but the price of trying to reach perfect uniformity might in some cases exceed the return.
Skips are a problem in that they lower plant population, but because not every seed emerges, some skips are inevitable, so absolutely perfect spacing uniformity is impossible to establish. Doubles may waste a little seed but usually have little effect on yield, unless they result in populations much higher than desired. Having that happen with the monitoring systems on planters today requires considerable carelessness or planter malfunction. For those who see even uniform plant spacing as an opportunity for improvement, taking a lot of plant-to-plant measurements down the row and calculating standard deviation from these measurements (e-mail me if you want to find how to do this on a spreadsheet) will give an indication of the margin for improvement. Standard deviations around 2 (inches) are typical for very uniform stands. Standard deviations of 4 inches or more indicate a problem with planting or emergence, and they usually are associated with stands that are lower than normal.
Probably because soybean plant stands tend to be considerably above the minimum needed for highest yield, spacing uniformity in soybean has not been studied much. Many have reported on the ability of soybean plants to grow to fill gaps up to 6 or 8 inches down the row, and we know from planting rate studies that stands as low as 60,000 plants per acre often produce maximum yields. From this information, it appears that soybean plant spacing would have to be extremely variable, with skips four to five times the average spacing in length, to affect yields. Even the worst drills in the state probably do a better job of planting than that.
Plant Size Uniformity
Plant size uniformity is more important for corn than plant spacing uniformity, but it is difficult to assess, at least in terms of numbers. Work that we and others have done suggests that corn plants that are one or two leaf (V) stages behind their neighbors before V6 or so will probably suffer some yield loss, and this loss will not be fully compensated by the neighboring plants. In corn that is V9 to V10 or larger, the same growing-degree-days (GDD) lag that had a plant two leaf stages behind at V6 now will have that plant three or four leaf stages behind, and such plants often "fade in the shade" as they get outcompeted for light. This developmental delay is often made worse by the fact that something other than just late emergence--for example, insect injury--accounted for, or contributed to, the delay. When underground competition is added, delayed and/or damaged plants have much less ability to catch up.
While there's usually nothing to do to correct such size differences once we find them, it helps to evaluate stand uniformity. A simple way to do this is to record, in 30 plants down the row, how many are delayed behind the larger plants and how much, in leaf stage and/or height, they are delayed. We don't have great guidelines to convert such measurements to yield loss, in part because causes of nonuniformity may or may not be still active later in the season. We found in some early work that planting delays of about 10 days (120 to 150 GDDs) in some plants caused yield decreases of about 7% to 8%, but replanting for uniformity would have reduced yield just as much. If half or more of plants were delayed by about 21 days (300 to 350 GDDs), yield losses exceeded 20% and replanting might have paid even with the late timing. When we cut the mesocotyl on small plants, detaching the seed and seminal root system from the plant, such plants yielded more than 30% less than undamaged plants, even though their development was delayed by only a leaf stage or two.
Because of the abundance of plants in most fields and the ability of the soybean plant to respond to changes in competition from its neighbors, we generally consider plant size uniformity to be less important in soybean than in corn. Soybean plants tend to "self-thin" to some extent, with stand counts decreasing from emergence to maturity. This happens more when initial counts are high, above 150,000 or so per acre. But the plant-to-plant size variability in soybean is usually high, even when populations are not excessive. This nonuniformity tends to worsen during the season, and it is common to find some plants without any pods at harvest. At best, such plants represent a waste of seed. Of course, we would expect higher yields if all plants, rather than only some, were robust and productive, but there is no clear evidence that the presence of nonproductive plants lowers overall yield, at least when moisture is adequate such that water use by nonproductive plants does not lower overall yield. The ability of soybean to tolerate nonproductive plants might be related to the fact that soybean often has a leaf area index of 6 or more, while corn's index is usually around 4. This means that soybean produces leaf area in excess of its need, so the loss of some plants with their leaves may not be a problem. As indicated in last week's article (issue 12, June 16, 2006), leaf area and light interception drive yield of both crops, and it's critical that there be enough healthy leaves to intercept nearly all of the sunlight during the time seeds are filling.
In both corn and soybean, the health of the leaf area and of the stems and roots that support the leaf area is critical for yield. When scouting stands, note the color of leaves and any nutrient deficiency, insect, or disease damage. The health of stems and roots is not directly visible in most cases, but you can see it indirectly by noting leaf size and moisture status. If a plant has smaller than normal roots, for example, leaf area is often smaller if the damage occurred early, and leaves seem to be under water stress (grayish color, curling in corn, drooping in soybean, wilting in both crops if stress is serious enough) if the damage was relatively recent.
As you scout, try to see in your mind's eye, based on your previous experience and crop development guides, what the crop will look like a week and a month later. I know this is hard to predict when we don't know what the weather will be, but it is possible to get a sense of how the crop will fare during the critical weeks coming up, based on how it's doing now. This might be no more complicated than recording present condition and adding a place on a scouting sheet for "expected condition at [a later time]." We can never know exactly what's coming, but much of the future prospects for a crop, or at least best-case yield levels, can be approximated based on crop condition in mid- to late vegetative stages. Plant and canopy problems caused by insects, diseases, or nutrient deficiencies may be correctable. Many of the crop developmental problems discussed in this article are not correctable, at least during the current season. Proper scouting can distinguish between the two types of problems.--Emerson Nafziger