Understanding Starch Digestibility in Corn Grain
Posted: December 19, 2011
Starch digestibility affects rumen health and milk production and directly impacts a dairy’s bottom line. Dr. Bill Mahanna, Nutritional Sciences and Sales Support Manager at Pioneer, offered participants at the 2011 Penn State Dairy Cattle Nutrition Workshop a look at corn kernel physiology, starch digestibility research, and some practical tips for increasing and managing corn grain in rations for dairy cows.
Digestion of starch in corn grain by dairy cattle is primarily limited by large particle size and the pericarp, which was designed to protect the endosperm. The three key factors that influence starch digestion in order of importance are 1) fermentation or extensive processing (such as steam flaking), 2) particle size, and 3) kernel characteristics. Dry corn is less digestible than corn that has been fermented (high moisture corn or corn silage), and digestibility of corn increases with the length of the fermentation period and with the amount of moisture present at ensiling. Steam-flaked corn is also more digestible than dry corn. Fermentation or steam flaking have greater impact on digestibility than particle size or kernel characteristics. The second most important factor affecting starch digestibility is grain particle size. Finely ground corn has higher digestibility than cracked corn, and the consistency of particle size in dry corn and corn silage should be closely monitored. The final factor in starch digestibility is kernel characteristics including the amount of hard or vitreous starch and the prolamin (zein) content. Fine grinding increases starch digestibility more for vitreous grain than for floury grain. In addition, although research investigating the extremes in vitreousness and prolamin content has provided valuable insight into the process of starch digestion, in most field situations commercial corn varieties are more moderate in starch density and prolamin content.
Mahanna offered the following practical approaches to increasing starch digestibility. For high moisture corn: ensile at more than 26% kernel moisture, roll or grind into storage, and keep in mind that ruminal starch digestibility increases approximately 2% units per month up to about 12 months of storage. For steam-rolled or steam-flaked corn: aim for a flake weight less than 30 pounds per bushel and keep in mind that vitreous hybrids tend to produce larger, more stable flakes. For dry corn: grind finely and avoid large particles; vitreousness is not a significant factor when corn is ground to 800 to 1000 microns. Also keep in mind that grinding floury corn may produce more fines and that the zein content of most commercial hybrids falls in a narrow range. For corn silage: use a kernel processor and monitor frequently during harvest as well as submitting samples for laboratory analysis after harvest. In addition, chopping slightly shorter (at 17 mm) can improve roller mill effectiveness. Keep in mind that starch digestibility increases with storage time and fermentation greatly diminishes any effects of vitreousness.
In summary, Mahanna suggested that the focus on most dairies should be measuring and managing the variability in kernel moisture/maturity, starch content, particle size and distribution, and changes in digestibility associated with fermentation and time in storage for both corn grain and corn silage.
- Summarized by Coleen Jones, research associate, Penn State Department of Dairy and Animal Science