Harvesting Corn Silage: Kernel Processing and Theoretical Length of Cut

Harvesting Corn Silage: Kernel Processing and Theoretical Length of Cut

Adapted by Noelia Silva del Río, UCCE Vet Med Dairy Specialist, VMTRC Tulare

Kernel Processing improves whole plant feeding value by breaking all the corn kernels and reducing the presence of large cob pieces.  This has a positive impact on handling and packing, reduces feed sorting, and increases rumen availability and total tract digestibility of starch, fiber utilization and feed intake.  However, excessive processing may negatively impact rumen health by decreasing effective fiber and favoring rapid fermentation and ruminal acidosis. On the other hand, if kernel processing is inadequate, sorting will increase, kernels will be lost in feces, and the silage will be more difficult to pack.  In California, most corn harvested for silage is processed.  But, 5- 15% of dairy producers do not choose to kernel process at harvest in order to reduce harvesting costs (Collar and Silva-del-Río, 2010). 

Several factors affect kernel fragmentation during processing: processor roll clearance, length-of-cut, machine throughput and corn maturity at harvest.  Adequate processing will result in most kernels cracked (at least 95%) with 70% of the kernel particles smaller than ¼ of a kernel.  The extent of kernel fragmentation should be evaluated at the silage structure during harvest.  This can be easily done using a bucket of water.  The kernels sink and can be separated from the fodder for evaluation.

Some commercial labs offer corn silage processing evaluation using a score system developed by Dr. Mertens (USDA Forage Center).  The forage sample is dried and shaken vigorously through a series of sieves.  Particles are separated into three categories: coarse, medium, and fine.

• Coarse fraction: particles larger than 4.75 mm. Fiber in this fraction will stimulate chewing, but starch will be poorly digested and may escape the rumen. 
• Fine fraction: particles smaller than 1.18 mm. Fiber from this fraction does not contribute to chewing activity or physical effectiveness.  Thus, NDF from this fraction can be subtracted from peNDF.  

Kernel processing is excellent when the proportion of starch that passes through the coarse sieve is more than 70%, average when it is 50-70%, and inadequate when it is less than 50%.  Data from a commercial lab (Cumberland) indicates that 42% of the corn silage samples evaluated (n=1131) were inadequately processed.

Theoretical Length of Cut (TLC) of corn silage should be set so particle length is long enough to supply effective fiber for optimal rumen function and adequately short to favor packing and fermentation.  Based on the DM of the harvested crop, TLC should be adjusted.  If the corn crop is too dry, it should be chopped finely to improve packing.  However, if ensiled corn is the only roughage source at the dairy, it is recommended to chop long to ensure enough effective fiber in the ration. 

Dr. Mike Hutjens, the Dairy Extension Specialist from the University of Illinois, has developed some guidelines on the theoretical length of cut and the roll clearance based on whole plant maturity (http://www.livestocktrail.uiuc.edu/dairynet/paperDisplay.cfm?ContentID=615):

• DM < 33%: TLC should be 0.75 – 0.90 in. and the rollers open.
• DM 33-38%: TLC should be 0.75-0.90 in. and rollers with 0.12 clearance inches. 
• DM 38%: TLC should be 0.5 in. and the rollers should be closed. 

Measuring forage particle length using the NASCO Penn State Particle Separator continues to be a popular way to objectively evaluate on the farm if forages have optimal particle length.  The recommendations are different for processed and unprocessed corn silage (see table). 

Furthermore, if corn silage represents the major forage source in the ration, the target is to have more material in the middle two sieves and less in the top sieve and bottom pan.  Penn State Separator guidelines can be found at:  http://www.das.psu.edu/research-extension/dairy/nutrition/pdf/evaluating-particle-size-of-forages.pdf. 

Results of a recent custom harvester survey (Collar and Silva-del-Río, 2010), indicates that the settings of the roll clearance opening ranged from 0.5 to 3.0 mm (0.02 to 0.12 inches) and TLC from 10 to 21 mm (0.4 to 0.8 inches).  Research needs to be conducted to evaluate if the particle length and kernel processing of corn silage in California falls within the desired target.