Introduction
Soybean harvest has progressed rapidly across the state, and should be near completion in west-central IA by the end of the week.  Degree-day accumulations ended the year at 130 (DD) below the 13-year average.  So far, weather conditions have been favorable for in-field dry-down of corn. Use the following information to calculate grain shrink when artificially drying corn.  Corn harvest should pick up pace this week.  The first potentially killing frost occurred on October 2^nd.  Caution should be used when grazing sorghum sudangrass when frost occurs.

Row-Crop and Forage Information
Growing Degree-day  West-Central IA ended the season with a 130 degree-day deficit.  But at one time was nearly 100 DD above and 250 degree-days below normal.  The warm September made up for 130 degree-days. Final Degree-day numbers (average for NW Iowa) can be accessed at this web site:  http://www.extension.iastate.edu/carroll/crops/degree-days-2004.htm

Dry down rates of corn.  Estimated corn dry-down rates for last and this week are given in Table 1.

Table 1.  Estimated Corn "In-Field" Dry-Down
	Current Moisture Between
	30% - 25%	25% - 20%
	Moisture lost (%)
Last Week	2.5	1.6
This Week	2.4	1.6

Calculating Shrink
Weight loss due to drying is referred to as “shrink”.  Shrink is comprised of weight loss due to removal of water and miscellaneous handling losses.  Shrink factors used by grain handlers typically account for both water shrink and handling loss.  Weight loss due to water shrink is by far the largest weight loss factor.  There are several mathematical equations and methods to determine “water” shrink, but to simplify the process, the water shrink factors have already been calculated and are presented in table 2.  The “water” shrink factor to use depends on the desired final moisture content of the grain.  A good estimate of handling loss is 0.5%, which is added to the water shrink to obtain “total shrink”. 

Water Shrink = (percentage points removed) * (water shrink factor)

Total Shrink = (Water Shrink + 0.5%)

Table 2.  Water shrink factors for drying shelled corn to various moisture levels.
Final grain moisture content	Water shrink factor
--%--	--% shrink per point--
15.5	1.183
15.0	1.176
14.0	1.163
13.0	1.149
12.0	1.136

Example:  You plan to dry shelled corn from 25% to 15.5% moisture (a removal of 9.5 percentage points), the water shrink would be [9.5 * 1.183 (from table 1)], or 11.24% of the original grain weight.  Total shrink would be [11.24% + 0.5% (handling loss)], or 11.74%.  So if we started out with 1000 lbs of 25% moisture corn, we would end up with 882.6 lbs of grain dried to 15.5% moisture; [1000 lbs * 0.1174 (total shrink loss) = 117.4 lbs of lost weight; then 1000 lbs – 117.4 lbs (lost weight) = 882.6 lbs of dry grain]
Source:  Calculating grain weight shrinkage in corn due to mechanical drying, D.R. Hicks and H.A. Cloud, Univ. of MN.  NCH 61.  http://www.extension.iastate.edu/Publications/NCH61.pdf

What is a killing frost?  The first killing frost for Northern Iowa occurred on October 2.  Most row crops at this time have reached or nearly reached physiological maturity, so a killing frost should not have any impact on crop yield potential.  But what constitutes a killing frost?  For corn, soybean and sudangrass, temperatures at or below 28 degree Fahrenheit for several hours would be a killing frost.  Alfalfa is generally more tolerant, being able to withstand temperatures to 25 degrees Fahrenheit without severe foliar damage. 

Is frosted alfalfa toxic?  (Information provided by Stephen Barnhart, ISU Extension Forage Specialist)  Frost injured alfalfa, clovers, and the commonly used perennial cool-season forage grasses Do NOT have the potential to form hydrodynamic acid, are NOT considered toxic and can be safely grazed or harvested for hay or silage following a frost.  There is probably a slightly higher bloat risk for grazed alfalfa and white clover the first few days after a frost.  Follow normal bloat preventing grazing management when grazing alfalfa and clover.

Managing frosted sorghum sudangrass and sudangrass.  (Provided by Stephen Barnhart, ISU Extension Forage Specialist) The potential for prussic acid poisoning and management suggestions are related both to the size of the plant when frosted and the extent of frost damage. Prussic acid, more correctly called hydrocyannic acid (a cyanide based compound) is formed in sudangrass or sorghum sudangrass hybrids which are severely stressed or frost damaged.  The hydrocyannic acid develops within a few hours after the frost and usually dissipates within a few days. 

The safest management is to remove cattle and sheep from frosted fields for several days.  Livestock can be returned to frost injured sudangrass that is 18" or taller and sorghum sudangrass 30" or taller after about 3 or 4 days.  If the grass was shorter than these heights when frost injured, withhold cattle and sheep for 10 days to 2 weeks following the frost to avoid problems. Then watch for new shoot regrowth, (tillers or “suckers”) on partially frost killed plants! Direct grazing of these fresh new shoots can be toxic too. Where new shoots appear following frost, avoid grazing until 2 weeks after the "killing" frost that kills the new shoots.

Frost damaged sudangrass or sorghum sudangrass hybrids can be cut and stored as silage.  Sudangrass and sorghum-sudangrass hybrids are difficult to dry thoroughly enough for safe storage as dry hay.  As with wilting and ensiling, most if not all of the hydrocyannic acid is dissipated in the drying process.

Pest Management
Please find the “Treated vs. Non-Treated” on-line and printable form at these URL’s:

On-line form:  http://www.extension.iastate.edu/carroll/crops/aphid_treatment_form.htm 

Printable Form: http://www.extension.iastate.edu/carroll/crops/Soybean%20Aphid%20Insecticide%20Treatment%20Survey.pdf

Refer back to the “Aphid Page” for survey results (http://www.extension.iastate.edu/carroll/crops/soybean-aphid.htm).