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The Wisconsin soybean crop is slowly starting to mature, however many growers and crop consultants are still concerned about the risk of frost damage to late planted fields.  In soybean an extended period (several hours) of temperatures 28 degrees F or lower is required to completely kill a soybean plant, though temperatures 32 degrees or less can still damage top growth. Those growers considering the state of their soybean crop and wondering the odds of making it to maturity before significant yield loss occurs must first correctly identify the soybean growth stage. 

Once the crop growth stage has been determined we can estimate the number of days it will take for your field to reach R7 or physiological maturity.  Across our Arlington and Hancock field sites it has taken 5-8 days to go from R3 to R4, 7-8 days to go from R4 to R5, 10-14 days from R5 to R6 and 14 days from R6 to R7. Note: we have seen crop development expedited the past few weeks due to heat and drought conditions. Next using the three figures below that show the 10th percentile, median, and 90th percentile date when you can expect a freeze event you can estimate the risk of a frost based on your crop growth stage.

For example: If you lived in SW Marathon county there is a 10% chance that a freeze event would have occurred prior to September 11-20, a 50/50 chance that a freeze event would occur prior to September 21-30, and a 90% chance a freeze event would have occurred prior to October 1-10. So if your soybean crop just entered the R5 crop growth stage today 9/7/13 there is a greater than a 50/50 chance that crop won’t make grain based on historical weather data.

Lastly if you are concerned about a freeze event please refer to Table 1 below that provides yield loss estimates of freeze damage by crop crop stage. This may help you decide whether you should risk taking the late planted soybean field as a grain crop or would that field be more valuable as a forage or green manure?

Table 1. Soybean Response to Freeze Damage
Growth Stage	Yield Reduction
R4 – Full pod	70%-80%
R5 – Beginning seed	50%-70%
R6 – Full seed	15%-30%
R7 – Beginning maturity	0%-5%
R8 – Full maturity	0%

Source: Saliba et. at. Kansas State University, 1982

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.

Delayed planting coupled with wet early season growing conditions across much of Wisconsin has dampened grower enthusiasm to enter the 2013 WI Soybean Yield Contest. Excellent growing conditions over the last several weeks has renewed grower interest. To facilitate this interest we are extending the entry due date to September 1st to enter the contest.  Below please find links for entry forms and rules. Good Luck!!!

The WI soybean crop ranges anywhere from just planted (JP) to beginning flower (R1). As we enter the soybean reproductive growth phase there are a few things to keep in mind. The first is that soybean will produce flowers for ~3 to five weeks, depending upon planting date and environment. During that time soybean will abort anywhere from 20 to 80% of the flowers that they produce. Generally it is the first and last flush of flowers produced that are most likely to be aborted.

Mike Bertram a WSA board member reminded me to add the category almost planted (AP).  That’s the treated seed that’s in the drill that will hopefully gets planted if the field dries out this week.  It might end up as forage.

R1 soybean growth stage

Next, the timing window for glyphosate applications in our early planted soybean is quickly closing. Glyphosate labels indicate that applications can be made through R2 or full flower. The R3 growth stage begins when one of the four top nodes with a fully developed leaf has a 3/16 inch long pod. Applications made after the R3 stage begins are off-label applications. On average it takes ~ 4 days to move from R1 (beginning flower) to R2 (full flower) and ~10 days from R2 to the start of R3 (beginning pod).

Last but not least, wheel track damage made from ground applications may start to reduce yield. Sprayer wheel traffic from first flower (R1) through harvest can damage soybean plants and reduce yield (Hanna et al. 2008). Our research suggests that an adequate soybean stand (more than 100,000 plants per acre) planted in late April though mid-May can compensate for wheel tracks made when a field is sprayed at R1. Yield loss can occur, however, when wheel tracks are made at R1 or later in thin soybean stands (less than 100,000 plants per acre) or late planted soybeans. Regardless of stand, plants could not compensate for wheel tracks made at R3 (early pod development) or R5 (early seed development). The average yield loss per acre is based on sprayer boom width (distance between wheel track passes). In our trials yield losses averaged 2.5, 1.9, and 1.3% when sprayer boom widths measured 60, 90, and 120 foot, respectively. Multiple trips along the same wheel tracks did not increase yield loss over the first trip.

Spraying soybean at the R1 crop growth stage

Wheel track damage to drilled soybean at R1

Hanna, S., Conley, S. P., Shaner, G., and Santini, J.  2008.  Fungicide application timing and row spacing effect on soybean canopy penetration and grain yield.  Agronomy Journal: 100:1488-1492. 

Scouting our field trials today we noticed an area presenting phytophthora symptoms. Further scouting showed plants were actually dying due to excessive flood injury and soil saturation. These fields have been subjected to nearly 5 inches of rain this week. This specific pocket has been anaerobic for 48 to 76 hours. Couple that with the heat, humidity, and larger crop growth stage (R1) plants are going downhill quickly. For more information on flood damage to soybean please visit www.coolbean.info.