Even the greatest of optimists would have to conclude that there’s a lot of corn headed for a confrontation with the first fall frost.  Over the next couple of months, much will be said and written about the effects of a fall frost on immature corn and soybeans.  First and foremost, producers need to be realistic in their expectations of whether corn has a snowball’s chance in you know where of making grain.  Once the crop gets beyond the point where silage is an option, there is no turning back.  The logical place to start is to define what it is that constitutes a killing frost for corn.  After all, not all frosts are created equal and certainly there will be a wide variation in corn maturity when that first frost does occur. 

        Corn is killed when temperatures are near 32 degrees for a few hours, and when temperatures are near 28 degrees for a few minutes.  Less damaging frost occurs when temperatures are around 32 degrees and conditions are optimum for rapid heat loss from the leaves to the atmosphere, i.e. clear skies, low humidity, and/or no wind.  The stem on a corn plant is a temporary storage organ for material that eventually moves into the kernels.  Grain yield will continue to increase about 7 to 20% after a light frost that only kills the leaves as long as the stem is not killed (see Table 1).  

Harvesting Immature Corn for Silage 

        Virtually all of the post-monsoon planted corn will be harvested for silage.  What can we expect from the crop in terms of yield and quality?  Let’s begin to take a look... 

Moisture

        When dealing with frosted, immature corn, it's often difficult to gauge whole-plant moisture content.  Frosted leaves can offer the appearance of a drier plant than what may actually be the case.  Leaves comprise only about 10-15% of plant dry matter.  However, the same rules apply with immature corn that apply with mature corn.  Whole plant moisture is a critical factor that drives quality and storability.  A three-year UW study at Marshfield indicates that immature whole-plant corn will be excessively high in moisture for direct ensiling without some dry-down (Table 2).  As a "rule of thumb", corn will lose 1/2% per day in moisture from milk stage to black layer.  There is no evidence of increased whole-plant drying rates following frost and prior to maturity.  However, waiting for multiple frosts will result in lower forage quality.  The optimum time to harvest then becomes a trade-off between moisture and quality.  Immature corn often needs to be ensiled at a higher than desirable moisture content to maintain quality and uniform particle size.  

Forage Quality and Yield

        The nutritional value of slightly immature corn made for silage is not dramatically different than for mature corn (Table 3).  Yield increases while quality decreases from just before silking until corn reaches the dough stage.  The concept to remember is that soluble carbohydrates are primarily in the stalk in immature corn rather than having been converted to starch in the kernel for "normal" corn.  Either way, the animal will utilize the product.  Farmer experience and research data both confirm that crude protein content will be higher in immature corn silage.  Obviously, dry matter yields will be lower with the immature corn and final animal performance will weigh heavily on proper fermentation and freedom from molds in the silage.