Late Planted Drought Stricken Soybean as a Forage?...Check the Label First

Severe alfalfa winterkill coupled with late soybean plantings followed by dry conditions have growers considering chopping their soybean as a forage. Before you even consider this option make sure you check the label of the pesticides applied to the crop before you grease the chopper.

• Let's start with the herbicides first. In short, outside of glyphosate (14 to 25 day, depending upon timing/use) and a handful of pre's and posts (please refer to Table 3-3 in A3646, Pest Management is WI Field Crops) most soybean herbicides are listed as "not permitted" for forage use. 
• Next, many common insecticides used for soybean aphid management implicitly state "Do NOT graze or feed treated forage or straw to livestock" (please refer to Table 3-8 in A3646, Pest Management is WI Field Crops) . 
• Lastly, fungicide labels are as equally exclusive with pre-harvest intervals ranging from 14 days to "Do NOT graze or feed soybean forage or hay" (please refer to Table 3-12 in A3646, Pest Management is WI Field Crops) .

If you somehow pass the gauntlet of "Do not" or "Not Permitted" and the forage value is greater than the grain value then the highest protein and yields are obtained from soybean harvested at the R6 to R7 growth stage. Harvesting soybeans for forage between the R1 and R5 stage will result in a very high quality silage, but dry matter yields will be reduced significantly. Forage quality will be reduced from R5 soybean forward if a conditioning process is used during harvest as conditioning will cause significant seed shattering. For additional information please refer to Soybeans for Hay or Silage.

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.