Have These Recent Cool Temperatures Impacted My 2013 Soybean Crop...In Short Not Yet

Delayed soybean plantings coupled with unseasonably cool temperatures in late July and early August have many growers and crop consultants concerned over the stage and state of the WI soybean crop. Though NASS reports March 1 to August 3 GDU accumulation (base 50 F) to be normal, developmentally the early planted WI soybean crop is tracking ~7 days behind normal. At Arlington WI our early planted High Yield Study is just entering the R5 crop growth stage (seed is 1/8 inch long in the pod at one of the four uppermost nodes on the main stem with a fully developed trifoliolate leaf node).

Image 1. Fourth node pod and bean of R5 plant. Image courtesy of D. Marburger.

Though cool temperatures can reduce photosynthesis and crop growth rate (Table 1), they also extend the number of days in a specific growth stage which allows total dry matter (TDM) to equilibrate thus limiting potential yield loss in early reproductive stages (Board and Kahlon; Seddigh and Jolliff, 1984 a,b).  However if cool conditions (< 50 F) due continue through seedfill or an early frost appears then significant yield loss can occur due to reduced seed size and/or number (Board and Kahlon) (Table 1). In short we are ok to date we just need average temps moving forward and no early frost to finish this crop off.

Table 1. Summary of cold stress effects on soybean physiology, growth, and yield componets. Taken from: Board and Kahlon.

Literature cited:
J.E. Board and C.S. Kahlon. Soybean Yield Formation. What Control it and How it Can be Improved. In Soybean Physiology and Biochemistry. http://www.lsuagcenter.com/NR/rdonlyres/84746337-8BFE-4903-BEB8-420D0D2B7271/82639/InTechSoybean_yield_formation_what_controls_it_and.pdf

Seddigh, M. and Jolliff, G.D. (1984a). Night temperature effects on morphology, phenology, yield and yield components of indeterminate field-grown soybean. Agron J. 76: 824-828. 

Seddigh, M. and Jolliff, G.D. (1984b). Effects of night temperature on dry matter partitioning and seed growth of indeterminate field-grown soybean. Crop Sci. 24: 704-710. 

USDA NASS. Wisconsin Crop Progress. Vol 13. Number 18.