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Issue No. 19, Article 3/July 29, 2005

Looking at Corn and Soybean as Seed Filling Gets Under Way

The crop condition reports show little improvement in corn and soybean ratings, but that is to be expected; these ratings seldom improve when reports come in toward the end of a week with 100° temperatures. Rainfall in many places during the past week and a return to cooler temperatures might brighten the general mood a bit, so ratings might recover some, whether or not actual crop prospects improve. At least the decline in both crops has been halted for now, in time to do both crops some good in many fields but probably not in all.

Lower temperatures will slow water loss rates some, and the cooler nights are a definite improvement, especially for corn, where high night temperatures decrease the amount of sugars available for growth. While most fields that pollinated starting in early July likely have kernel numbers fixed, the improvement in conditions should help prevent kernel abortion in many fields north of I-80. We have seen an unusual amount of unevenness in pollination this year, with the tail end of silking maybe 2 weeks later than normal, often on scattered plants. This may go back to differences in time of emergence or in plant damage early, with increased competition on individual plants causing a delay in development as conditions turned dry. This could be a small positive in some cases if it helped more kernels to form in the field. However, plants that get behind stay behind, and even if their larger, neighboring plants didn't pollinate very successfully, they will still be competitive during grain fill.

Most corn plants are in the milk stage (R3), moving into the dough stage, when starch starts to form from sugars in the kernel. A few advanced fields are starting to dent, while some later-pollinating fields are still in the blister stage (R2). This means that corn in most fields is using all of its resources to fill grain, with photosynthesis producing sugars, which move to the stalk and then rapidly into the kernels. Except for blips due to conditions that reduce photosynthetic rates (cloudy, too dry), the rate of dry matter increase is steady during this period, which lasts for about 35 to 40 days. The grain-filling rate slows down a week or 10 days before black layer, which marks the end of the process. On the best day possible--good water supply, healthy canopy, bright sunshine, low/high temperatures of about 63 to 88--the crop can add about 10 bushels, or close to 500 pounds of dry matter per acre. Few days are so ideal, and average filling rates during this period are more typically in the range of 4 to 5 bushels per day.

While we can sometimes help the crop-filling rate by protecting the canopy from insects and diseases, kernel number and canopy cover are set in most fields. If we continue to get good weather, kernel number could well limit yields in some fields. In the more typical case where kernels don't reach their maximum size in most fields, we don't think that kernel number limits yield. We don't really know in most cases what maximum kernel size is, but we think it's determined during the week or so after fertilization, when kernel cells divide to reach their maximum number and potential size. If you are curious about what maximum kernel size might be, remove every other plant (which greatly reduces competition and so increases filling rates), leaving eight plants in each of four rows; take four ears out of the center of this "miniplot," shelling and taking kernel weights in this and an adjoining area with normal plant numbers.

Conditions that lead to poor kernel set often do not get corrected completely, so kernel number is likely not to reach a maximum, even in fields with low kernel numbers. Keep this in mind as you take kernel counts; filling conditions, canopy cover, and health are still likely to limit yields in most fields. To assess canopy cover, check when the sun is high how much light is reaching the ground. In fields with the poorest canopy development this year, I expect that 20% to 25% of the sunlight will bypass the leaves and hit the ground, lost forever to the crop. If the canopy loses color or lacks enough water to function well, even light that is intercepted may not be very effective.

The soybean story is looking somewhat like the corn story this year, even though the rain has done the soybean crop slightly more good than it has the corn crop. Many soybeans in the southern half of the state are at or near full height and so are near the end of the time when they can add nodes, flowers, and pods. I have gotten more reports of what seem to be low pod numbers, often only one or two pods per node, and maybe none on the lower part of the plant. Remains of flowers are still attached to many nodes, and one question was whether these can still develop into pods. We doubt that can happen, but even if the tiny pod is still attached and rainfall can now make it start to grow, the competition among pods at individual nodes tends to be intense, and it's doubtful that a small pod can compete successfully with a large, established one. Each leaf tends to feed the pods attached at the same node, though there might be movement from one node to another if there are no pods at a node.

Although making stand counts and pod counts and estimating the number of seeds per node is the standard way to estimate soybean crop prospects, it takes a lot of time and often gives unsatisfactory results. It is necessary to get into fields to see how podding and pod filling are progressing, however. I suggest a simpler but more subjective approach: take a stick and move the canopy sideways, looking at general pod density and the length of the stems along which pods have formed. Are there one, two, or three pods per node? Along how many inches of stem have pods formed? Are there gaps or nodes that lack pods at the bottom or top of the stem? Look at the ground surface as well, to see if pods have dropped off. Note how much pod filling has taken place in the pods. Besides low pod numbers in some fields, I have seen some unusual variability in pod size, probably due to uneven survival of pods as the weather changed.

As we learned from 2003 and 2004, the extent to which seed filling has started by the end of July can give a hint about the crop's vulnerability to stretches of dry weather during August. The crop this year is at about the same stage as it was in 2004, which is an advantage. Still, even though we think root systems are generally healthy, the return of dry weather and depletion of soil moisture will likely cause some pods to drop and will reduce seed numbers in pods that are still flat where seeds have not yet begun to fill. Both will be negative for yield prospects, but as we discussed for corn, seed number probably doesn't limit yield as much as seed size in most fields. This means that having and maintaining a healthy canopy will be critical for soybean over the next 5 to 6 weeks--Emerson Nafziger

Emerson Nafziger

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