Fundamentally Speaking
Joel Karlin DTN Contributing Analyst

Thursday 11/15/12

Is There An Inverse Relation Between Soybean Pod Numbers & Pod Weights?

Do soybeans compensate for lower pod numbers with higher pod weights?

That is an interesting question in light of this year’s performance where the USDA in a graphic following up on last week’s November crop production report estimated the derived soybean pod weight around 0.306 grams per pod in what appears to be the highest pod weight in at least ten years and perhaps ever.

Meanwhile the USDA also estimated the number of soybean pods per an 18 square foot area at 1,450 pods, well below the ten-year average we estimate to be around 1,800 pods per an 18 square foot area.

There has been some talk that with fewer pods per area, each pod was able to maximize their nutrient and water intake resulting in much greater pod weights than seen before.

The accompanying graphic suggests this may be the case as it plots the number of soybean pods in an 18 square foot area that is reported by the USDA vs. the derived pod weight in grams per pod for the state of Illinois.

Note that all years use final data except for 2012 where we use the November USDA figures of an Illinois soybean pod count of 1,382 pods and their yield of 43 bushels per acre.

Based on this yield, the derived Illinois soybean weight is 0.35 grams per pod, the highest we have in our database going back to 1990.

Other years that have relatively low pod counts such as 1990-1992 had above average pod weights though only 1992 really stands out.

Conversely, years with high pod counts such as 2010 and 2000 have relatively low derived pod weights with 2000 being the lowest.

A couple of caveats here which include the fact combined August and September rainfall does not appear to be a factor in determining soybean pod weights as we might have imagined.

Secondly the R squared for the scatterplot relation between Illinois soybean derived pod weights and pod counts is 42.4% and that is far higher than the R squared for the other six objective states with IN at 0.192, IA at 0.132, MN at 0.179, MO at .115, NE at 0.084, and OH at 0.232.

(KA)

Posted at 12:43PM CST 11/15/12
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