No. 19 Article 7/July 30, 2004

Guessing at Yields

Although people like any cool weather we can get this time of year, lows in the 50s and highs in the 70s are not exactly what the crop needs to thrive here at the end of July. Corn responds by slowing its development rate, and photosynthesis (and the water loss that accompanies it) slows as well. If good weather continues through August, the only noticeable effect of cool temperatures now will be a delay in harvest. The crop is still ahead of normal development, so such a delay will not be a problem unless early harvest could bring a premium. The chance to use days in early September to fill, rather than to dry, grain would likely boost yields.

Soybean plants respond less favorably to low temperatures than corn plants do. As a result, especially of low night temperatures (less than 50°F in some places on Tuesday morning, July 27), soybean flowers and pods could abort in larger than normal numbers. But if temperatures return to normal by the end of this week, as the forecast indicates, the temporary drop in temperatures should not have a serious detrimental effect. Soybean development is, on average, a little ahead of normal, with much of the crop now at R4, the stage when pods are present up and down the stem. Some are at R5, the start of seed filling. Look for flowering to end soon, and then the critical pod-filling stage begins. Seeds will take on almost all of their weight during the month of August.

With a large corn crop filling in the field and a soybean crop that at least is promising at this point, many people are starting to think about estimating crop yields. With corn, estimating is easier to do now than it normally is at the end of July, because pollination is long finished in most fields and the number of kernels is fixed or close to fixed. Once that happens, all we need to do is estimate the number of kernels there are per acre, then guess at how large the kernels will get.

I've had several questions about the "corn yield slide rule calculator" in the past week and found out that this venerable instrument, published originally by the Department of Agricultural and Biological Engineering here at the University of Illinois, is no longer available. Why there is such nostalgia is not clearnot too many people use slide rules to multiply and divide these days, and the yield calculating slide rule is just as easy to replace with a hand calculator as a regular slide rule was. The right-to-left scale on the slide rule calculator was easy to misread, and had we had electronics at the time, it probably would never have existed.

Here, then, are the two simple steps to estimating corn yield before harvest:

  1. Estimate number of kernels per acre. Actually, we estimate the number of kernels in 1/1000 of an acre, which is 17 feet 5 inches in 30-inch rows. Just count the ears that have kernels, then pull or husk three ears (or four or five if ear size is really variable), and count kernels per ear by multiplying the number of rows by the number of kernels per row. An example: you count 28 ears in 1/1000 of an acre--ear 1 has 16 rows with 35 kernels = 560 kernels; ear 2 has 16 rows with 34 kernels = 544 kernels; and ear 3 has 18 rows with 32 kernels = 576 kernels. The average number of kernels per ear is (560 + 544 + 576)/3 = 544. The number of kernels in this 1/1000 of an acre is 544 x 28 = 13,056.
  2. Guess how large kernels will get. The slide rule calculator mentioned above is "hard wired" at 90,000 kernels per bushel. That may be a reasonable average, but depending on filling conditions we might be able to guess that kernels will end up larger or smaller than that, and thereby improve the estimate. If the canopy is in good shape and there is soil moisture and kernels are starting to dent, it makes sense to raise this guess, say to 80,000 kernels per bushel. If the leaf area is drying up and kernels will clearly not get very large, then raising the number per bushel to 100,000 or even more might make sense. In any case, just divide the number of kernels in 1/1000 of an acre by the number of thousands of kernels (this cancels out that we counted kernels in 1/1000 of an acre) you expect to have in a harvested bushel. In our example, dividing 13,056 by 90 predicts a yield of 145, while using 80,000 kernels per bushel predicts 163 bushels per acre.

If trying to do calculations in the field isn't your thing, just write down the numbers, then use the calculator in the On-Line Illinois Agronomy Handbook, or work with a hand calculator after you come out of the field.

It's early to count the number of soybean seeds filling or even pods in some fields, but soybean yields can be estimated the same way as described for corn. If the stand is uniform, you can get a reasonable estimate of plant number in a smaller area--maybe a fourth as much row length (4 feet 4 inches in 30-inch rows), or two or four rows that same length in 15- and 7.5-inch rows, respectively, to give the same area. Count pods and seeds per pod from 8 or 10 plants to get an average, then multiply by 4 to get seeds per 1/1000 of an acre, just like we do with corn.

One problem in soybean is that variability among plants is usually large: one plant may have hardly any pods, while the plant next to it might have 40 or 50 pods. The more time and patience you have to count pods on more plants, the better your estimate will be. It helps if you simply don't count plants with few pods when you count stands.

Once seed number is determined, yield estimates are made by dividing by the expected number of seeds per bushel at harvest. This number is often more variable than the number in corn, but you might start with 180,000 (3,000 seeds per pound) as a conservative number, and decrease that if it appears that seeds may be larger. As an example: 35 plants in 1/4000 of an acre, with 24 pods per plant and 2.8 seeds per pod is 9,408 seeds per 1/1000 of an acre. At 180 (thousand) seeds per bushel, that gives a yield estimate of 52.3 bushels per acre.

As of now, there seems to be little reason to think that corn kernels will not reach at least average size. If we get rainfall and the weather remains mild, kernels may well get larger than normal, and yields will follow right along. It's too early to say whether this is likely to happen in soybean, as well, but we can always hope.--Emerson Nafziger

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