No. 19 Article 4/July 31, 2009

Soybean Growth and Development: Late-July Update

Two weeks ago, I indicated that soybeans planted in early June were growing and developing as if planted in mid-May due to the mild temperature patterns of June and early July. Last week, Dr. Chad Lee at the University of Kentucky also wrote about small soybeans resulting from late planting and cool temperatures. There is no doubt that a little more heat would help our midwestern crops produce better yields and take more advantage of the adequate soil moisture.

Dr. Lee noted two main points that I want to reiterate. First, the most important growth analysis by yield factor in soybean is that full canopy closure is attained by flower development (blooming). Some yield is surely lost if sunlight is reaching the ground during the reproductive growth stages. Second, foliar applications of fertilizer and/or fungicides are no substitute for sunlight and temperature, so they are not likely to buy instantly larger beans and closed canopies. I'm not saying they shouldn't be used, but they are no replacement for sunlight and heat.

USDA NASS Weather and Crops reports have indicated that soybean reproduction (flower development) was 11% on July 12, 24% on July 19, and 46% on July 26. These figures contrast with 22%, 37%, and 51% for the same weeks in 2008 and 51%, 69%, and 81% for the previous five-year averages for those weeks. According to the Illinois State Climatologist Office, July 2009 has been the coolest July on record in Illinois. As of July 27, the statewide average temperature for the month has been 70.2°F, which is 5.7 degrees below normal.

What does this mean for the outlook on soybean yields? It is too early to tell for certain, though as already indicated we know a little more heat would be good. Last year, the state soybean yield average was about 4 bushels above an "expected" average when yields were regressed on the dates that 50% of the state soybean acres were completely planted. A graph of this was published as Figure 3 in issue 6 of the Bulletin (May 1). The 1996 state soybean yield average was 40.5 bushels per acre and was about "average" for yields regressed on 50% completed statewide planting dates. So while the cool temperatures and slow reproductive soybean development are apparent, yields at or above expected levels when accounting for delayed planting still seem attainable. Another way of putting it is that delayed planting date is still a good indicator of yield expectation. Since we continue to have mostly adequate or above-adequate soil moistures in most of the state, let's keep hoping for a little more heat through August to maximize crop development. The cool weather has at least made for pleasant outdoor activities and field day educational events.--Vince M. Davis

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