POLICY  •  NEWS  •  MARKETS

AG POLICY & MARKETS DAILY

FRIDAY, AUGUST 21, 2026   |   SPECIAL REPORT & ANALYSIS

MARKET PERSPECTIVE  |  USDA CROP YIELDS

NASS Field Checks Could Put More Pressure on USDA Corn Yield

Late-August sampling will test whether wet-weather stress cut kernel weight

Analysis  ·  August 21, 2026

The next important test for USDA’s 2026 corn and soybean yield forecasts begins next week, when National Agricultural Statistics Service (NASS) enumerators start physically measuring crops in selected fields across the major producing states. The timing is especially significant this year because excessive rainfall and prolonged soil saturation across parts of the eastern Corn Belt may have damaged yield potential in ways that are difficult to capture through crop-condition ratings or windshield observations.

What the field checks actually measure

NASS enumerators will begin visiting corn and soybean Objective Yield plots Aug. 25, making plant and fruit counts, measuring ears and pods and assessing crop maturity. As sampled fields reach maturity, enumerators will hand-harvest portions of the plots and send ears and pods to laboratories for grain-weight and moisture analysis. Those physical field observations will become an important new input into USDA’s Sept. 11 Crop Production estimates.

Sample plots are visited near the end of each month during the growing season. At those visits, enumerators count plants and fruit, measure ears and pods and determine maturity. The destructive sampling comes later: NASS says the final crop cutting is made at maturity or immediately before harvest, when sample ears or pods are sent to a laboratory for fruit weight, threshed-grain weight and moisture analysis. That means the first visits beginning Aug. 25 will primarily involve field counts, measurements and maturity assessments; actual laboratory samples depend on how mature an individual field is.

A separate Agricultural Yield Survey — the producer survey — runs alongside the field work. For the September survey, the reference date is Sept. 1. NASS says mail data collection can begin no earlier than Aug. 25, telephone collection no earlier than Aug. 28, and collection generally concludes around Sept. 5, depending on the Crop Production release date. The resulting September Crop Production report is scheduled for Friday, Sept. 11, at noon ET.

Figure 1. Two separate surveys feed the Sept. 11 report: enumerators measuring sample plots in the field, and producers answering the September questionnaire. Source: USDA NASS.

That represents an important change from USDA’s Aug. 12 estimates. The August corn and soybean forecasts were based primarily on farmer-reported yield expectations along with administrative acreage information and satellite data; they did not yet include this season’s physical Objective Yield measurements. USDA currently forecasts the national corn yield at 180.7 bu. per acre, down 2.3 bushels from its previous projection, while soybean yield is forecast at 52.7 bu. per acre.

Table 1. What changes between the August and September crop reports — and the one input that arrives only where fields have reached maturity. Source: USDA NASS.

Where the next downside risk sits

Corn kernel weight may be where the next downside risk emerges. In saturated fields, nitrogen losses through denitrification and leaching can leave plants short of nitrogen precisely when they need to maintain photosynthesis through grain fill. Corn may have established a respectable number of ears and kernels earlier in the season, but severe nutrient stress during the dough and dent stages can reduce final kernel weight as the plant deposits less starch in the endosperm. Purdue notes that nutrient deficiencies and other stresses that reduce photosynthetic leaf area during grain fill can lower kernel weight and cause premature maturity.

Figure 2. Illustrative arithmetic: what a lighter-finishing kernel would do to the national corn yield if ear and kernel counts hold. Baseline is USDA’s Aug. 12 forecast of 180.7 bu. per acre. Not a forecast.

That is why the market should pay increasing attention not simply to ear counts but to ear weight. A field can look impressive from the road and even produce a respectable kernel count while ultimately yielding fewer bushels because those kernels finish lighter than normal. Purdue specifically warns that preharvest yield estimates can overstate production in seasons with poor grain fill because the calculations often assume a more normal kernel size and weight than ultimately develops.

Test weight is the symptom, not the yield

There also could be a quality signal. Saturated, nitrogen-deficient corn that loses green leaf area prematurely may produce corn with test weights 1 to 3 pounds per bushel below normal in some fields because starch accumulation was curtailed. But test weight needs to be interpreted carefully. A 54-pound test-weight crop versus 56 pounds does not automatically translate into a proportional yield reduction. Test weight measures bulk density — the amount of grain packed into a fixed volume — while yield is determined by harvested dry-matter weight per acre. Purdue and Nebraska research both caution that the relationship between test weight and yield is not especially strong.

Table 2. A 54-lb. test weight versus 56 lb. is not a proportional yield cut — Purdue and Nebraska research finds the link is not especially strong. Sources: Purdue University; University of Nebraska–Lincoln.

Still, widespread reports of lighter kernels would matter. If NASS plots begin showing both reduced kernel weight and lower grain weight per sampled area, USDA’s models will have objective evidence that the crop failed to finish as well as previously assumed. In other words, lower test weight would be the symptom; reduced kernel dry matter would be the yield issue.

Soybeans: a different stress pathway

Soybeans face a somewhat different risk. They are not dependent on applied nitrogen in the same way as corn, but prolonged saturated soils can suppress nodulation, damage root systems and increase Phytophthora and other root diseases. Flooding during reproductive stages also can reduce pod retention, seeds per pod and final seed size. University of Minnesota research indicates soybean yield losses from flooding can become particularly severe during the R3-to-R5 reproductive stages.

Table 3. How prolonged saturation reaches yield. Sources: Purdue; Minnesota.

Bottom line

The September USDA report therefore carries considerably more information risk than the August report. The Aug. 12 numbers established USDA’s starting point, but the late-August Objective Yield work will begin answering the question markets are now debating: Did the crop simply endure a wet, uneven summer, or did prolonged saturation materially reduce the weight of the grain that ultimately makes it into the bin?

For corn especially, the difference may not show up in the number of ears. It may show up inside the kernels.

AG POLICY & MARKETS DAILY   |   MARKET PERSPECTIVE  |  USDA CROP YIELDS — FRIDAY, AUGUST 21, 2026