Now that the calendar has turned to September, U.S. combines will begin rolling to collect the world’s largest corn and soybean crops, but meaningful harvest results are unlikely to surface until November.

On Monday, the U.S. Department of Agriculture’s statistics agency will update its monthly crop production report. This includes potential adjustments to the initial U.S. corn and soybean yield estimates released last month.

During the growing season, USDA’s National Agricultural Statistics Service updates corn and soybean yield projections just four times, monthly from August through November, before the final number is published in January. Therefore the agriculture market tends to hang on each monthly update for news on how big the crops will be.

The downside of relying too heavily on the September yields is that only a small fraction of corn and soybean fields are cut by the time the estimates are made. Since October is the biggest harvest month for both crops, NASS forms the September estimate in a similar way to the August one.

In the week ending Sept. 4, more than 80 percent of the corn crop had been harvested in Georgia, Louisiana and South Carolina, and over 50 percent had been cut in Arkansas, Mississippi, North Carolina and Texas.

These seven states account for 7 percent of national corn production. By comparison, No. 2 producer Illinois had only 1 percent of its corn harvested by the same date.

Only four states were reporting soybean harvest progress as of Tuesday: Arkansas (6 percent), Louisiana (25 percent), Mississippi (12 percent), and Texas (39 percent). These states combine for 9 percent of national soybean production.


Using the past five years as a guide, about half of the U.S. corn crop will be harvested in October, and the other half is split fairly evenly between September and November.

The recent five-year average shows that the corn harvest is around 23 percent complete by the end of September and around 73 percent complete by the end of October. Harvest is generally completed by the Thanksgiving holiday in the fourth week of November.

Of the major corn producing states, Illinois is the only one that generally harvests faster than the national average due to location. But because the bulk of the high-yielding crop is chopped toward the latter half of the harvest campaign, we sometimes have to wait until November or beyond for a truly representative yield number.

This coming Monday, NASS will publish the percentage of corn harvested in its crop progress report for the first time this season. The initial overall percentage may be slightly higher than usual since the early-harvesting Southern states are currently ahead of schedule.

But for the U.S. crop as a whole, recent maturity ratings do not suggest that this year’s corn harvest will necessarily move along faster than normal. Harvest progress corresponds well with maturity, and as of Sunday, the 2016 crop was 18 percent mature – 1 point higher than last year and 2 points below the five-year average.

October is an even bigger month to cut soybeans, as nearly two-thirds of the U.S. crop is typically harvested then. About 20 percent is completed in September and the last fields are harvested in November.

In contrast to corn, many of the major production states harvest soybeans earlier than the national average. Most of these states have a big corn crop to harvest as well, but the window in which to harvest soybeans is very small, and most farmers harvest soybeans first.

This is because mature soybeans are far more vulnerable to losses than mature corn. At this stage, the pods tend to lose moisture quickly. If harvest occurs slightly too late, pods that are too dry can shatter at the combine head and cause losses to the farmer. The pods are also at risk of shattering in the event of severe weather such as hail or high winds.

When soybeans reach maturity, the green plants turn from yellow to brown and lose all their leaves. As of Sunday, 12 percent of the U.S. soybean crop was dropping leaves. This is equal to the five-year average and 3 points behind last year at the same time.

This year’s soybean harvest progress will likely be published a week from this coming Monday. The four states currently reporting progress are generally on pace with average.

History confirms that September yield estimates are not always permanent. In the last four years, NASS’s September soybean yield projection has deviated much further from the final January figure than corn.

Additionally, September soybean yield has been too low for five years straight, while only three of the past five September corn yields were lower than the final yields.