Library Categories


Corn yield 2012: too early to tell

Upon hearing that his obituary had been published, Mark Twain stated: “The reports of my death are greatly exaggerated.”

Jumping to conclusions without all the necessary information is, I’m afraid, part of our human nature. I’ll admit it is part of my nature anyway!

Can you imagine reading your own obituary? That could be enlightening, embarrassing, a cause for perhaps unfounded pride, or possibly well-deserved humility! We may feel some of those emotions this fall as harvest proceeds and certainly in January when USDA’s National Agricultural Statistics Service publishes the state’s final yield figures.

If the 2012 corn crop could talk, it might say something much like what Mark Twain said in 1897 in his famous quote, 13 years before his death. As I write this in July, drought scorches the Corn Belt. Many people, including myself, attempted to understand the impacts of the abnormally dry conditions on growth, development and yields. The bottom line is that until harvest is complete, and the numbers recorded, processed and interpreted, we won’t know how things actually will come out. In the meantime that doesn’t prevent us from speculating, and most of us do that without hesitation.

The first 2012 NASS corn yield forecast was scheduled to be published on Aug. 10. Remember, these forecasts arise from meticulous late-July counts and measurements by the team at NASS. In late July, enumerators use stand and ear counts to estimate ears per acre.

Statisticians generate yield estimates based on how these counts compare to those of the last five years.

The big unknowns in late July are final kernel counts per ear and final kernel weights. The assumption is that weather during the remainder of the grain filling season is normal when compared to the previous five years. Thus, kernel numbers and kernel weights are “normal,” too. Of course, no one knows if that is true or not until after the fact.

Later vs. earlier estimates

Each successive monthly forecast through November is a “new” forecast — not simply an update. The NASS group uses new and expanded observations from a large number of sample locations to formulate expected yields based on field observations for every subsequent crop report. By the time the September yield forecast is published (based on late-August samples), kernel numbers will likely be set, and thus, yield forecasts will be more comparable to the final yields for 2012. Of course, it is possible the weather could be better than average for crop yield and actually increase the yield forecast in subsequent months. This is usually associated with temperatures that extend the filling period beyond normal. Don’t forget, when acres are dropped from the harvested area — due to drought, flooding, etc. —“yield” increases, because “yield” means “yield per harvested acre.”

As you know, groups other than NASS attempt to get an early assessment of yield, too. Their procedures undoubtedly differ from those of NASS. But in any case, the biggest issue with yield forecasts before harvest is obtaining an accurate estimate of kernel numbers per acre (plants per acre times ears per plant times kernels per ear) and final kernel weights. It becomes a numbers game: The more samples taken, the more accurate the estimate, and the closer to harvest the estimates are made, the closer the estimate to final yield.

Look at corn yield trends

Nevertheless, considerable uncertainty remains regarding the outcome of the 2012 corn crop and will remain until the combines roll. Given that, there is some certainty in the world — we know historical corn yields (see chart). With that information, perhaps we can fine-tune our speculation about this year’s prospects.

For example, if corn yields maintain the 30-year trend line increase of 2.6 bushels per acre per year, Iowa corn yields will average 180 bushels per acre, and the U.S., 160 bushels per acre (U.S. yield trend line increases 2 bushels per acre per year). If Iowa corn yields end up at 162 bushels per acre, 10 bushels below last year, that is 10% below trend line yields of 180 bushels per acre. As a reference point, the July World Agricultural Supply and Demand Estimates gave a U.S. 2012 corn yield estimate of 146 bushels per acre, almost 9% below the U.S. trend line.

Track record tells the tale

The other thing to keep in mind regarding the August NASS yield forecast is it doesn’t perfectly reflect final yield estimates for the year. The second chart shows the track record for the August forecasts. With a few exceptions, August yield forecasts are within 10% of the final yields for each year.

Most of you will remember plausible reasons for the exceptions shown in the chart: 1983 drought, 1993 floods, 2004 long seed-fill period, etc. As mentioned above, as we go into the final stages of kernel fill, the monthly yield forecasts will be progressively closer to the final annual yield estimates for the state.

Meanwhile, with high night temperatures and dry conditions in 2012 during the early seed set and seed-fill period, we can expect fewer kernels per ear and lighter kernel weights than we’ve seen for several years. If corn could talk, let’s hope that as it reminisces in some year far in the future, it remembers the year when, “The reports of my death were greatly exaggerated.”

Elmore is the Iowa State University Extension corn agronomist.

30-year corn yield trend line


TREND: Iowa corn yields have increased an average of 2.6 bushels per acre per year over the past 30 years. For the United States, it’s 2 bushels. Iowa’s yield is the solid line; U.S. yield is the hashed line. Clearly, the trend won’t be reached this year. Source: USDA NASS

August forecast vs. final yield


TRACK RECORD: Here’s how the August corn yield forecast from the USDA National Agricultural Statistics Service compares to final yields for each year. With a few exceptions, August forecasts are within 10% of the final yields in most years. Source: USDA NASS

This article published in the August, 2012 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.