Corn silage that has been passed through a roller mill is referred to as processed corn silage, and the roller mills are sometimes called "kernel-crackers." While use of processed corn silage is relatively recent in North America, it has been used extensively in Northern Europe for more than a decade. Trends in North America have fueled interest in roller milling of corn silage. Corn breeders have given emphasis to more rapid dry down of the grain, resulting in a dryer and harder kernel at corn silage harvest. Corn geneticists have also been responding to the interests of grain buyers who want a hard, strong kernel that resists breakage during handling. Some of the qualities desired in corn grain from a production or marketing point of view may reduce the digestibility of kernels, particularly the intact kernels often found in corn silage.
Another development stimulating interest in processing of corn silage is the increased size of dairy and beef operations. Wider use of custom harvesters, larger farms, and availability of processing equipment have played a role in the growing popularity of processed corn silage in North America. The need for high producing dairy cows to have sufficient long fiber in their diet has resulted in more producers chopping corn silage at a longer theoretical cut. This results in larger cob pieces and more whole kernels in the silage, as well as more corn in the manure. A longer cut, combined with processing, results in longer fiber particles while ensuring complete kernel breakage. Cob pieces are also broken, resulting in little or no sorting of cobs in the feed bunk.
Factors Influencing the Value of Processed Corn Silage
Maturity of Corn Silage at Time of Harvest
Grain development is incomplete when corn silage is harvested too early, thus reducing the nutritive value of the silage. Harvesting at black layer results in overly dry kernels, and can result in reduced starch digestion. Digestibility of the vegetative portion of the corn plant also decreases with advancing maturity. Processing of corn silage can reduce the loss in starch digestibility due to over-maturity, but will not reverse the loss in digestibility of leaf and stalk tissue. It is still important to harvest corn silage before the black layer stage. However, processing may lessen the adverse effect of overly mature corn silage through its action on the kernel portion of the silage. Optimum maturity for harvest is 1/2 to 2/3 milk line, either with or without processing.
Machine Setting
The normal theoretical length of chop (without processing) is 1/4 to 1/2 inch. This needs to be lengthened if the silage will go through a roller mill, because in passing through the rollers, forage particles will undergo reduction in size. Presently, cutting at 3/4 inch theoretical length of cut is recommended when processing corn silage. Typically, particle size of processed corn silage chopped at 3/4 inch theoretical length of cut will have about the same particle size as unprocessed silage cut at 3/8 inch. Cutting at lengths longer than one inch is not recommended at the moment. Cutting at 3/4 inch takes less power than cutting at 3/8 inch, and tends to compensate for the extra power required for operation of the roller mill.
In addition to obtaining appropriate length of cut, proper setting of the roller mill is important. The gap between the rolls is adjustable, and can range from less than one mm (thickness of a dime) to 15 mm or more. (Note: One mm equals about 40 thousands of an inch). To obtain complete breakage of kernels, a one to two mm gap will be required. Too often, equipment operators open the gap to increase harvest capacity. This can result in incomplete kernel processing. The goal is to have complete kernel breakage. Table 1 illustrates the effect of roll spacing on kernel breakage.
Processing Before or After Ensiling
Corn silage can be passed through a roller mill either before or after ensiling. The most common approach is to have a roller mill as an attachment to a forage harvester. A stationary roller mill/blower combination also works well when tower silos are filled. An alternative approach is to roll the corn silage as it comes out of the silo. Very little research data is available regarding processing of corn silage before or after ensiling. In a 1997 study by Harrison et al., milk production was 1.5 lb more per cow with silage rolled prior to ensiling compared to silage rolled on the way out of the silo. Rolling drier corn silage may have more of an effect than rolling corn silage with adequate moisture (65-70%). One might expect better consolidation or packing of processed silage, because stalks and cob pieces are more flexible or pliable after passing through the rollers.
Processing Effects on Digestibility
Processing can increase dry matter digestibility of corn silage. It appears that much of this increase is due to increased kernel or starch digestibility. Harrison et al. (1997) reported less corn grain in feces when processed corn silage was fed compared to control silage (0.86% versus 3.15% of manure dry matter, respectively). Bal et al. (1998) noted an increase in starch digestibility from an average of 83.8% to 87.9% in lactating cows when processed corn silage replaced unprocessed corn silage. Small increases in fiber digestibility have been reported with processing, suggesting the vegetative part of the plant may also be slightly improved in terms of digestibility. However, the modest improvements in digestibility that can be expected with processing will be due mostly to improved starch digestibility.
Milk Production Response
The average milk production response due to processing corn silage in 15 feeding trials (Satter et al., 1999) was 1.0 lb more milk per cow per day. Four of these trials were conducted with rather low producing cows. Using just the 11 trials with the better producing cows shows an average response of only 0.6 lb milk per cow per day due to processing. Milk fat averaged 0.07% units higher for the processed silage for all 15 trials, and 0.1% point higher for the 11 trials.
This small difference in fat test may reflect more complete consumption of cob (fiber) with the processed silage. Although observed production responses have been small, the trials were conducted using good quality corn silage. Overly mature or dry corn silages were generally absent from this group of studies. One might expect more benefit from processing when applied to corn silage having more mature or harder kernels. The responses from these studies should, therefore, be considered as minimal responses. Most of the studies with processed corn silage have been short term where changes in body weight or body condition could not be reliably measured. If starch digestion is increased with processing, then either an increase in feed efficiency or in body condition would be expected.
Economics of Processing
For purposes of estimating the potential return from processing, assume feeding of processed corn silage will increase milk production by 0.6 lb per cow per day. In a 305-day lactation, this would yield about 183 lb more milk per cow. If milk is worth $12/cwt, gross returns would be increased by $22 per lactation. Also, assume there is a small increase in feed efficiency, as well as a small increase in feed intake, to support the 0.6 lb increase in milk production. Accordingly, an increase in net return of $18 per cow per lactation will be assumed, with the $4 difference assigned to cover the increased feed cost associated with the increase in milk production. Keep in mind, many factors affect the response to processed corn silage. The production response to processing dry corn silage is expected to be greater than processing responses for silage at typical moisture levels. Also, a better response is expected with higher producing cows due to the faster rate of passage.
On the cost side, several things must be considered. The most important consideration is the cost of added machinery. A roller mill attachment will add $9,000-15,000 to the cost of a forage chopper. If this is depreciated over five years, annual depreciation would be approximately $2,400/year. There is the possibility that fuel cost could be higher and throughput (capacity) of the chopper reduced when a processor is added to a chopper. However, if the theoretical length of chop is increased to 3/4 inch with the processor from the customary 3/8 inch length without a processor, the power requirement and machine capacity would be changed very little when the processor is added. Maintenance costs of the roller mill are not included but would generally be absorbed into depreciation.
Assuming the ownership and operational costs of a processor will be about $2,400 per year, then it will require slightly more than 130 cows to justify the overhead and equipment costs ($2,400/18 = 133). This may not appear attractive, especially for smaller dairy operations. However, the costs for a custom operator would be spread over many more cows, and a positive return could be realized.
How much can a dairy producer afford to pay a custom operator for processing corn silage? Assuming a high rate of corn silage feeding (2/3 of the forage dry matter), a cow might be expected to consume about 17 lb dry matter per day of corn silage over the 305-day lactation, or about 7.5 tons of 35% dry matter corn silage during the lactation. Based on a net benefit of $18 per cow per lactation, a dairy producer could pay up to $2.40 per ton more for custom harvest of processed corn silage. Most custom operators are charging less than this, with reports ranging from no additional cost to $1.50 additional cost per ton of harvested silage.
Conclusion
The example used for economic analysis is a conservative estimate based on the lactation trials to date. Processing of corn silage appears to be a viable service for custom operators to offer, and a profitable management option for large dairy operations. When processing corn silage, be mindful of the need to:
