Recommended quote format: Irwin, S. And also D. Good. "Understanding “Implied Ear Weight” in USDA’s respectable Corn productivity Forecast." farmdoc daily (6):156, room of farming and customer Economics, university of sdrta.net atUrbana-Champaign, august 18, 2016. Permalink

The National agricultural Statistics business (NASS) that the USDA is responsible for making U.S. Chop production forecasts and estimates. NASS released the very first survey-based estimate of the size of the 2016 U.S. Corn crop on august 12. New forecasts will certainly be issued on September 12, October 12, and also November 9 and the last estimate of yield and production will be exit in the second week that January 2017. The august 2016 corn yield projection was 175.1 bushels per acre, which would certainly be a brand-new record if realized. The yield forecast was also a major surprise to the market, through average industry expectations closer come 170-171 bushels (farmdoc daily, respectable 12, 2016). ~ the report to be released there was significant discussion that the “implied ear weight” that NASS calculates because that each monthly forecast and final estimate. Specifics the include ear weight for the august 2016 yield forecast was the biggest of the critical 13 years, a period that includes several very an excellent growing seasons such as 2004, 2009, and also 2014. The really high include ear load raises the inquiry whether NASS may have overestimated yield potential by utilizing an suspect ear weight that is as well high. However, the must also be stated there appears to it is in a extensive misunderstanding of just how the implied ear load is in reality derived and also therefore exactly how it must be interpreted. Several of that misunderstanding may stem from a lack of knowledge around the NASS methodology because that generating monthly productivity forecasts, but NASS additionally provides very couple of details top top the computation of comprise ear weights. The function of this post is to administer a better understanding the the NASS include ear weight calculation and also analyze accessible data come determine how likely that is that the august 2016 ear weight calculation will decline substantially.

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NASS Corn yield Forecasting Methodology

We begin with a review of the NASS corn productivity forecasting methodology. Additional details deserve to be uncovered in this NASS publication (Vogel and Bange, 1999; NASS, 2012), research reports (Irwin and Good, 2006; Irwin and Good, 2011; Irwin, Sanders and also Good, 2014), and also farmdoc daily posts (August 28, 2013; august 29, 2014; September 4, 2014). From ours experience, lot of the misunderstanding the USDA yield estimates for corn (and other crops) revolves about the truth that the USDA uses two species of surveys to collect data for the monthly NASS manufacturing forecasts in August through November. These are described as the farming Yield survey (or the farmer-reported survey) and also the objective Yield survey (or the ar measurement survey). Data for the final approximates released in January are built up in the December farming Survey in which respondent report actual acres harvested and also the actual productivity or production.

For the august 2016 forecast, the agricultural Yield inspection (AYS) contained 22,144 operations because that all crops and was performed in 32 states for corn. The sample of farm yard operations surveyed was drawn from those that responded come the inspection of planted acreage in June. The sampling style used to pick the to work to be surveyed employs multiple control items, such as number and form of products planted and desired sample dimension for every commodity, to determine the probability of selecting a certain operation. The exact same operations will be interviewed every month native September with November. Most of the inspection data are gathered in electronic type using computer-assisted phone call interviewing. Each state is supposed to attain a minimum solution rate that 80 percent.

In the august survey, respondents were asked to recognize the number of acres the corn to it is in harvested and also to administer a projection of the final yield. Harvested acreage responses will certainly be retained from month-to-month and also the concern will no be request in succeeding surveys. The AYS, however, does contain a distressed acres sub-survey that targets details crops in states that have actually experienced too much weather problems in order come measure alters when excessive weather does occur. Respondents will certainly be asked to update yield prediction in succeeding surveys. Keep in mind that respondents room not asked to calculation plant populace or mean ear weights, only yield every acre.

The monthly AYS data space reviewed because that consistency through previous surveys because that the individual respondents and also an across-record evaluation is carried out to identify any extreme worths that need to be re-checked. A review program i beg your pardon accounts for sampling weights and also includes one adjustment for non-respondents is provided to create an point out of expected average yield for each state surveyed. The yield indications indigenous the inspection reflect the referee of respondents (farmers) and also historical relationships suggest that respondents tend to be conservative in estimating final yields (under-estimate productivity potential) particularly under drought conditions. This tendency is quantified and factored into the main yield forecasts.

The objective Yield survey (OYS) is design to generate yield forecasts based upon actual tree counts and measurements, eliminating some of the biases associated with the farmer report yields. Some have referred to this inspection as “ground truthing” the yield estimates reported by farm operators in the AYS. The sample of fields for the OYS survey is selected from ranches that report corn planted or to it is in planted in the June survey of acreage. Samples room selected in the 10 principal corn developing states. Documents from the June survey are sorted by state, district, county, segment, tract, crop, and also field. A random sample of areas is drawn with the probability of selection of any details field gift proportional come the dimension of the tract. For the respectable 2016 OYS, a full of 4,544 plots to be sampled for corn, soybeans, cotton, and winter wheat.

Two counting areas, or plots, space randomly selected in every field. Objective dimensions (such as counts that plants and ears) are created each plot every month during the survey cycle. Once mature, the plots space harvested and also yield is calculated based upon actual manufacturing minus an pin money for harvest loss. During the august survey, the operator is asked come verify, field-by-field, the acreage report in June.

For corn, every of the two independently located sample plots in a field is composed of 2 parallel 15 foot part of row. Every plot is selected by using a random number of rows along the sheet of the field and a random number of paces right into the field. Enumerators count all fruit and also fruiting positions in corn and, if ears have formed, a sample of ear is measured for length and also circumference. Just prior to the ar is harvested, both plots room hand harvested and weighed by the enumerator. Four ears are sent to the NASS lab for shelling and measurement the moisture. These data are used to compute gross productivity at 15.5 percent moisture. Harvest lose is measure in separate locations near the productivity plots.

Data collected from each corn plot throughout the forecast cycle are used to measure size of the plot area and to measure or projection the number of ears and also grain weight. These data incorporate (as available) heat width, variety of stalks every row, number of stalks v ears or ear shoots every row, number of ears with kernels, kernel heat length, ear diameter, ear load in dent stage, weight of shelled grain, humidity content, total ear load of harvest unit, lab weight of sample ears, load of grain from sample ears, and also moisture content of shelled grain from sample of mature ears.

At each visit, the enumerator creates a corn maturity group for the plot, varying from 1 (no ear shoots) to 7 (mature). Prior to the blister phase (see this publication for summary of corn development stages), the number of ears is forecast based upon the variety of stalks, ear shoots, or ears and also both the load per ear and harvest loss space forecast based on the 5-year average. Native the blister through the dough stage, the load per ear is forecast based upon kernel heat length and also harvest lose is forecast based upon the past 5-year average. Ear load is measure in the dent and/or tires stage. Harvest lose is measured adhering to harvest.

Prior to maturity and harvest, the OYS corn yield is forecast based on the projection of the number of ears, the projection of the weight per ear, and the estimate of harvest loss. Prediction are based on conditions as of the inspection date and projected assuming normal weather problems for the remainder of the cultivation season. The OYS forecast of gross corn yield climate is based upon the adhering to formula:

Gross Yield= ÷ 56

Number the ears and ear weight space either projection or actual and also 56 is a conversion from weight to bushels.

The state average gross corn yield because that the OYS is the basic average of the gross returns for all the sample fields. In addition, a state yield estimate is also made by an initial averaging the projection or actual yield factors (such together stalk counts, ear counts, and ear weight) and also then forecasting the state average yield straight from this averages. This projection is based upon a regression evaluation of the historical relationship (15 years) in between the productivity factors and the state typical yield. Historical relationships show that OYS productivity indications tend to it is in liberal in estimating last yields (over-estimate yield potential). This propensity is quantified and factored right into the official yield forecasts.

The survey and also forecasting measures described above produce a number of indictors of the net yield the corn from the AYS and OYS. In August this indicators encompass the average ar level returns from the OYS, median state level counts indigenous the OYS, and the mean yield report by farm operator in the AYS. After ~ harvest begins, returns reported by farmers are additionally included as an indicator of last yield. Every of the signs results in a point yield forecast for which projection errors are computed based on the historic relationships in between forecasts and also actual yield. The variety of returns is evaluated relative to all of the pieces of available data to assist in the choice of the main yield forecast. This process is completed individually in every state and also at the national level. A formal farming Statistics board (ASB) consisting of 7 to 10 statisticians is convened to review local yield indicators and determine an official yield forecast.

Implied Ear Weight

As shown above, the yield of corn is basically a duty of the variety of ears every acre and the weight of each ear (see this recent write-up by Emerson Nafziger the the university of sdrta.net for added details). So, to understand the factors driving a yield forecast it is valuable to understand the underlying ear count per acre and also average ear weight. Because that each monthly yield forecast and also final estimate, NASS calculates these statistics but they space not published in the monthly crop Production reports. Instead, the average ears per acre and ear weight are shown graphically in the executive summary released in tandem through the reports. Figure 1 mirrors the calculated ears per acre and ear load for the respectable 2016 productivity forecast along with the final calculation for the vault 12 years. 2 things space notable about the observation for respectable 2016. First, the comprise ear weight, at about 0.363 pounds is high in compare to current years (2014, 2015) and also earlier years through very an excellent growing seasons (2004, 2009). The relatively high include ear weight raises the inquiry whether NASS may have overestimated productivity potential by making use of an assumed ear weight the is as well high. Second, ear per acre, around 28,100, is more than 1,000 ears reduced than in 2014 and 2015. This can be because of lower plant populations, emergence problems, or pollination problems, or some combination of all three.

To assess whether the august 2016 ear weight is “too high” it is necessary to understand how NASS computes the implied ear weight. As displayed in figure 1, the calculation is as follows,

Implied Ear load = (published yield estimate x 56)/number of ear per acre.

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Some misunderstanding about this calculation probably stems native the absence of descriptive detail detailed by NASS. First, the calculation is minimal to the 10 states included in the objective yield survey, which means that data for states external of the objective yield region do not impact the implied ear load estimate. Second, when the published yield can be calculated because that those 10 objective yield states, NASS does no report that average directly. Third, the ear count per acre and also calculated implied ear weight space not reported directly, yet must be read from the plot in number 1. Because that example, the weighted average yield estimate for the 10 states included in the OYS in respectable this year is calculated together 182 bushels, or 10,192 pounds. The average variety of ears every acre in those 10 states shows up to be estimated at 28,100. The calculated implied ear weight, then, is around 0.363 pounds.

Probably the biggest source of misunderstanding about the include ear load calculation is the failure to acknowledge that productivity forecasts are obtained from two independent surveys. The implied ear weight, then, is no an assumed ear weight or a measured ear weight, even though some elements of ear load measurement or ear weight forecasts are used in the OYS. In the OYS procedure, NASS might use some mix of presume or measured ear weight, relying on maturity that the crop. As defined above, before the blister stage, 5-year average ear weights are assumed in OYS productivity estimates and also after blister stage, kernel counts are provided to projection ear weight. So, NASS does use some element of forecasting ear weights once making OYS yield approximates prior come actual harvest. However, over there is no analogue available for the AYS yield estimates, where farm operator are just asked come report harvest acreage and also yield. This means that any kind of ear weight presumptions or calculations supplied by respondent to the AYS space not known. The an essential then to a suitable understanding the the NASS calculation of “implied” ear load is to establish it is based on the average population count from the OYS and also the 10- state average yield forecast based on both the OYS and the AYS. In other words, the include ear load is a “hybrid” measure up reflecting information from both the OYS and also AYS surveys. The ear counting per acre is drawn from the OYS but the typical yield is based upon both the OYS and AYS surveys. The is why NASS call the measure “implied” rather than “assumed” or “measured.” there is only one way that the derived ear load measure report by NASS can equal the assumed 5-year mean ear weights for the OYS–the average ear load and populace used by farm operators in responding come the AYS would have to be specifically the exact same as the 5-year median weight and population assumed by NASS for the OYS. Obviously, this is very unlikely to it is in the case, and hence, one have to not equate acquired ear weights report by NASS as the same thing as the 5-year median ear weight supplied by NASS in one component of the productivity estimation process.

Early season NASS calculations that ear weight deserve to differ native final approximates of ear load for a number of reasons. First, the last estimate of the number of ears per acre may differ from early on season estimates, although the approximates of plant populations reported because that the 10 objective yield states carry out not change much indigenous month-to-month. Those plant population estimates room not report in “real” time, but are summarized in the crop Production Annual summary report. Second, beforehand season forecasts of ear load in the OYS based upon 5-year averages may not correctly reflect ear weights in the current year. Third, early on season productivity expectations report by producer in the AYS might differ from their last estimates. Together the crop reaches maturity so that ear weight have the right to be measure in the OYS and also as harvest progresses so the farmer productivity expectations become much more accurate, the calculation of median ear weight also becomes more accurate.

Evidence top top the really variation of NASS implied ear load estimates throughout report release months is gift in number 2. The ear load data shown in the chart are approximate values based on the charts uncovered in executive recaps for the August-November reports over 2011-2016. The respectable 2011 estimate is lacking because the ear weight chart was no presented in the executive summary for that report and the October 2013 estimate is missing because a crop Production report was no issued due to the federal federal government shutdown. Additionally note that final ear weight estimates are no released in the crop Production yearly Summary, while, curiously, last ear counts per acre are released. Number 2 shows there is some variation across the report months because that the reasons outlined above, v an average adjust from respectable to November (September come November in 2011) the +0.004 pounds every ear, so over there is a slight propensity for the ear weight approximates to boost over time. The average absolute readjust is 0.006 pounds every ear, or 2.1 percent. The largest August-November change, +0.12 pounds every ear, developed in 2012; possibly not surprising offered the especially nature that the drought conditions that year.

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In sum, the obtainable data on NASS ear weight estimates indicates castle change fairly little as soon as the initial estimate is made in August. This way one have to be quite mindful in concluding that the august 2016 ear weight estimate is an outlier that will be readjusted downwards considerably in future chop Production reports. Utilizing the data native 2011-2015, we have the right to project the the November 2016 estimate is most likely to it is in in the range of 0.363 +/- 0.006 pounds per ear, or 0.357 to 0.369 pounds. So, if historic patterns hold, the last (November) approximated average ear weight is likely to exceed the previous high of about 0.352 in 2004 and also remain well over the averages because that 2014 and also 2015.

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Implications

This review of the USDA’s corn yield and production forecasting steps is a reminder that the size and robustness that the survey and also forecasting methodology. The methodology is time-tested and also has no parallel in the private sector. Among the objections of the respectable corn yield estimate this year was that assumptions about ear weight included in the forecast process may have been also aggressive, resulting in a projection that overstates actual productivity potential. The “too high” ear weight argument may reflect part misunderstanding of the productivity forecasting methodology provided by NASS, the USDA agency responsible because that making the forecasts. In particular, the include ear weight reported by NASS is a “hybrid” measure reflecting details from both the target yield survey (OYS) and farming yield survey (AYS). Any type of ear weight presumptions or calculations used by farm operator respondent to the AYS are not known and this provides it impossible to “back out” the 5-year typical ear weight presumption used by NASS because that the OYS side of the productivity estimation process. Regardless, the available data top top NASS ear weight approximates indicates castle change reasonably little as soon as the initial estimate is make in August. This method one must be quite mindful in concluding the the august 2016 ear weight estimate is one outlier that will be adjusted downwards dramatically in future chop Production reports.