About half of corn silage's energy value comes from its starch content, provided by the grain fraction. Total tract digestibility of this starch can range from about 80 percent to nearly 100 percent in lactating dairy cows.
The major factors associated with this digestibility variation are kernel particle size, length of time that the corn silage remains in the silo prior to feeding, and kernel maturity, moisture content or hardness at the time of harvest.
The focus on improving starch digestibility has intensified with the upward movement in corn grain and corn silage prices. Furthermore, many dairies are short on corn silage inventories and will not have the luxury of feeding long-ensiled corn silage later this year.
Room for processing improvement
A kernel processing score was developed at the U.S. Dairy Forage Research Center which entails sending a corn silage sample to a laboratory where it is dried and sifted through variable sized wire mesh sieves. A starch analyses is then performed, and the proportion of the starch in the sample that is retained or has passed through a 4.75 millimeter sieve is determined. The starch that passes through this sieve is more highly digestible in the cow. The researchers provided the guidelines shown in the table above.
To evaluate how well we are doing as an industry with the kernel processing of corn silage, we have compiled data from both field research trials and commercial testing laboratory surveys (below).
Collectively, these datasets show that kernel processing can be improved, as a low proportion of samples fall into the excellent processing score classification. A controlled digestibility trial with lactating dairy cows showed a 6 percentage unit boost in total tract starch digestibility as the kernel processing score rose from adequate to excellent.
Our calculations indicate that this difference in starch digestibility could be worth 2 pounds of milk per cow per day or a 2-pound per-cow per-day reduction in the feeding rate of shelled corn, both of which have economic consequences.
Check the kernel processing score of your corn silage, and see what steps can be taken to improve it during the next harvest season. Focus on forage harvester theoretical length of cut (TLOC), roll gap settings and roll maintenance. Long TLOC, especially if combined with a wide roll gap setting, will reduce your kernel processing score. Past research with processed corn silage indicated that 19 mm (3/4 inch) TLOC and 1 millimeter roll gap settings were best after taking into account silage packing and fermentation, kernel processing, digestibility and lactation performance by dairy cows.
A longer TLOC with shredlage
Due to the feeding of high corn silage rations and high prices for hay and straw as sources of physically-effective fiber, there has been quite a bit of interest in lengthening the TLOC beyond 19 mm when harvesting corn silage. This, however, can create some real challenges with regard to proper kernel processing with conventional rolls.
Attracting recent interest has been corn silage harvested with a self-propelled forage harvester fitted with aftermarket cross-grooved processing rolls. The self-propelled forage harvester is set for a longer TLOC than commonly used, 22 to 30 mm. This silage has been termed corn shredlage.
Compared to conventionally processed corn silage (CPCS) harvested at 19 mm TLOC, the most obvious difference for corn shredlage, harvested at a longer TLOC, is a greater proportion of coarse stover particles. When fed in rations for lactating dairy cows, this can elevate the physically-effective fiber content of the ration which is important for proper rumen function, cow health and milkfat content.
An important aspect of the corn shredlage concept is that excellent kernel processing may be achieved, even with a longer TLOC, allowing for high starch digestibility. Furthermore, the cross-grooved rolls used for producing corn shredlage may cause greater damage to the coarse stover particles and allow for greater digestibility of the fiber, but the controlled research on the fiber digestibility aspect is not clear cut at this point.
We conducted a feeding trial with lactating cows in our university dairy herd to compare corn shredlage (30 mm TLOC) to CPCS (19 mm TLOC). The proportion of coarse stover particles was greater for shredlage than CPCS for samples collected during feed-out from the silo bags throughout the feeding trial (32 percent versus 6 percent as-fed particles retained on the coarse or 19 mm screen of the Penn State Separator Box).
Kernel processing scores on feed-out samples averaged 75 percent for shredlage and 60 percent for CPCS. The shredlage and CPCS were similar in dry matter (34 percent) and starch (36 percent) content, pH (3.6), and silo bag packing density (17 pounds DM per cubic foot).
Midlactation cows were used in a 10-week continuous-lactation experiment with respective treatment TMRs containing 50 percent (DM basis) from either shredlage or CPCS. Both TMR treatments contained 10 percent alfalfa silage and 40 percent (DM basis) of the same concentrate mix.
For the TMR fed throughout the trial, the proportion of coarse particles was greater for shredlage than CPCS (16 percent versus 4 percent as-fed particles retained). Our measurements of weigh backs did not reveal any sorting.
Averaged over the treatment period, DM intake and 3.5 percent fat-corrected milk (FCM) yield tended to be greater by 1.4 and 2.3 pounds per day per cow, respectively, for shredlage than CPCS. The average daily per cow FCM yield was 100.1 pounds for shredlage compared to 97.8 pounds for CPCS. Milkfat, protein and milk urea nitrogen contents were unaffected by treatment. Total tract dietary starch and neutral detergent fiber digestibilities were greater for cows fed shredlage.
Additional data will be available
More research is needed regarding fiber digestibility and the physically effective fiber in corn shredlage compared to other forage sources.
Harvest of corn shredlage may improve starch digestibility more when silage is harvested drier than normal and for hybrids with harder kernel texture, but research is needed. Also, controlled data on packing densities in bunker silos for corn shredlage is lacking. But, more information will become available as shredlage silos are fed out this year.