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Friday, July 30, 2010

How Is Your Soybean Yield Being Estimated?


The sweep across the Cornbelt continues for hundreds of statisticians who are trying to estimate the size of the 2010 corn and soybean crops. Their effort will come to life on August 12 when the USDA releases the August 1 Crop Report, which is the first objective yield survey for the year. Yesterday the focus was on measurement procedures in the corn crop. Today we take a look at how soybean yields are estimated.

We have all counted ears in 1/1000 of an acre, factored in rows and kernels and arrived at an estimated corn yield. We have counted pods and nodes, factored in 2 and 4 bean pods and estimated a soybean yield. But the gut-based guesses of most farmers are not good enough for the National Agricultural Statistics Service, which has honed its crop measurement procedures over the years. Its yield forecasting procedures are outlined with justification for the methods that are used.

NASS enumerators are now working in eight major soybean producing states of Arkansas, Illinois, Indiana, Iowa, Minnesota, Missouri, Nebraska, and Ohio, where over 1,300 plots have been selected and will be scrutinized before successive surveys are made in September, October, and November. Just before the field is harvested, the plot will be harvested and samples sent to a laboratory for weight and moisture testing. After the plot is harvested, the enumerators will return to the spot to measure field loss, so an accurate measurement of the crop harvested can be calculated.

A sample consists of two independently located units (or plots), each of which consists of two parallel 3.5 foot sections of row partitioned into a 3-foot section and a 6-inch section. Field enumerators use a random number of rows along the edge of the field and a random number of paces into the field to locate each unit. Plant counts are made in the full unit while detailed fruit counts are limited to a small 6-inch section at the end of each row, which usually consists of 1 - 4 plants. All 3.5 feet of each row is picked and weighed at harvest to establish gross yield. The yield is measured as bushels of beans per acre at 12.5 percent moisture.

Enumerators will also measure the distance between the two rows, as well as the distance among five adjacent rows. To estimate the number of pods per plant, they will county the plants in each section of row, the number of main stem nodes in the 6-inch section, the number of lateral branches in the 6 inch section, the number of dried flowers and pods in the 6 inch section, and the number of pods with beans in the 6 inch section. To measure the weight of the beans, the enumerator will weigh the beans from the 6 inch section and the moisture content of the harvested beans will be taken.

Gross yield for a unit is forecasted by multiplying the forecasts of the number of plants per 18 square feet, the number of pods with beans per plant, and the historical average bean weight per pod, and converting this forecast to bushels per acre. All of that data is inserted into a series of calculus equations to arrive at a yield for the field, the state and the nation. If the enumerators land in the best area of a field, the yield will be large; but those have to be averaged with the estimates taken in the middle of ponds where there may not be any soybeans.

Soybean yield estimates begin in August, although they become more accurate as beans mature. Statisticians will collect data from a plot, consisting of two 3-foot sections, plus an additional 6-inch section. Plants will be counted in the longer rows and beans will be counted in the smaller sections then the numbers will be expanded over the plot. Plots are selected by random numbers of rows into a field and paces down the row, with enumerators landing in areas of good and bad soybeans, which are all averaged out for the national yield estimate.

Posted by Stu Ellis on 07/30 at 01:39 AM | Permalink

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