USDA corrects September corn yield estimates after weighting error

The U.S. Department of Agriculture’s National Agricultural Statistics Service (NASS) said on 23 October that it had found a processing error in September corn data and corrected the affected figures in its reports and Quick Stats. An additional explanation on 24 October described the cause in more detail: incorrect acreage weights had been used to combine irrigated and non-irrigated objective-yield observations for Kansas and Nebraska.

The correction affected September plant populations and ear counts in both states, September ten-state averages for those measures, and corn yield estimates for Kansas, Nebraska and the United States. NASS’s 24 October notice gave corrected September yield estimates of 119 bushels per acre for Kansas, 3 bushels below the prior figure; 182 for Nebraska, up 5; and 178.8 for the United States, up 0.3.

The agency separately reviewed and validated its October data and estimating procedures and said the October yield and production forecasts published on 9 October were unchanged. That distinguishes the September correction from the newer October forecasts. The notice does not restate the October forecast figures; it says they remained the same after review.

NASS said it would introduce additional pre-publication checks to verify acreage weights and validate objective-yield calculations. It is also evaluating further ways to strengthen quality-control procedures. The agency linked the review to concerns raised after the October crop report and described the review as comprehensive across its September and October corn-estimation procedures.

NASS explains that corn yield estimates draw on two complementary survey approaches. Its Agricultural Yield Survey asks growers for a monthly assessment of yield prospects, while the Objective Yield Survey uses counts and measurements from random plots in sampled fields. The agency’s methods manual says the objective data reflect growing conditions as of the start of each month and are combined with broader survey information to produce forecasts. September plant and ear measures are therefore inputs to a layered estimating process, rather than counts of the final national harvest.

The corrections were entered into NASS Crop Production reports and Quick Stats. Users should therefore rely on the corrected September estimates rather than the original September figures. By contrast, NASS said its October forecasts had not changed. The agency thanked farmers and data users who raised concerns and said it remained committed to promptly communicating and correcting errors in published statistics.