Corn Silage Digestibility - Separating the Grain from the Chaff

Joanne Siciliano-Jones, Ph.D., F.A.R.M.E. Institute, Inc., Homer, New York (607) 749-5747


The digestibility of a given farm’s forages is of critical importance in determining the profitability of milk production. This being said, we must recognize that the digestibility of a given forage crop is the result of the combined effect of environment, harvest and ensiling management and genetics. As in most aspects of dairy farming, good genetics alone will not ensure success, but a combination of excellent management and good genetics will stack the deck in your favor.

To properly evaluate corn silage digestibility rankings requires two things. The first is an understanding of the various methods used in determining silage digestibility and what this means to your farm and your cows. The second is how the growing season and your management decisions will affect the digestibility of the resulting crop. The following information was developed to put these issues into perspective.

Digestibility Testing Methods

Historically, forage quality has been evaluated using acid detergent fiber (ADF, the fiber remaining after a forage is washed in an acidic detergent solution). While high ADF forages are generally lower in digestibility and energy content than are low ADF forages, research over the past two decades demonstrated that the relationship between forage digestibility and ADF levels varies considerably. Day length, in particular, can change the relationship between ADF and digestibility even on the same forage varieties. In addition, highly digestible forages can have the same ADF content as poorly digested forages. As a result of the problems with using ADF to predict forage digestibility, biologically based forage digestibility tests have been adopted.

There are 3 major types of digestibility tests. These are 1) animal digestion trials where a test feed is fed to a number of animals and differences in digestion are measured, 2) in vitro digestibility testing where a test feed is finely ground and disappearance of dry matter or fiber is measured in a buffered solution of ruminal fluid contained in a glass (or plastic) vessel, and 3) in situ digestibility testing where a test feed (either ground or as fed) is placed in a porous cloth bag and disappearance of dry matter or fiber is measured in the rumen of a ruminant animal.

Animal digestion trials are the most accurate method for measuring forage digestibility. However, these studies require large numbers of animals (>20 per test feed), large amounts of test feed, and substantial amounts of time. As a result, this type of testing is not feasible for evaluation of large numbers of hybrids in many locations. In vitro and in situ tests were developed as a way to obtain the same types of digestibility information with a lower investment of time and money. Both in vitro and in situ digestion are correlated with animal production. Each method has certain advantages and disadvantages that must be taken into account when evaluating silage digestibility tests.

In vitro digestibility testing was the standard testing method for many years because of consistency of the results and small sample size requirements. Generally, in vitro procedures use a buffered solution of ruminal fluid. However, some procedures use purified fiber digesting enzymes such as cellulases. A drawback of in vitro testing is that it requires the use of very finely ground (1 or 2 mm) samples. It is well known that finely ground feedstuffs are digested differently than coarsely chopped materials. Recent research has shown that finely grinding feedstuffs may erase differences in digestibility. This problem is demonstrated in a comparison of corn silage harvested at dent and black layer stages which was recently conducted by researchers at Washington State University and the University of Idaho. While there is no doubt that corn silage harvested at dent will be more digestible and support a higher level of milk production than black layered corn silage, the following figure shows how drying and finely grinding both corn silages removes digestibility differences as measured with in vitro testing. Recent research at the University of Minnesota suggests that in vitro testing may reflect starch content more than actual digestion.

DM Disappearance of Corn Silage Harvested at Dent or Black Layer (Hunt et al.)

Rankings based on in vitro fiber (NDF or cell wall) digestibility remove the starch concern. However, the research from Washington State University and the University of Idaho presented above suggests fiber digestion is also affected by grinding. We would expect that the less mature fiber in the dent silage would be more digestible than that in black layered silage. As shown below, we can detect the loss of fiber digestion due to increased maturity when the samples are not ground. Grinding the samples completely removes the effect of maturity on fiber digestion.

NDF Disappearance of Corn Silage Harvested at Dent or Black Layer (Hunt et al.)

Because of the problems in differentiating forage digestibility introduced by grinding forage samples, in situ digestibility evaluations using as fed forages are receiving more attention. The in situ technique requires more care in the laboratory than in vitro testing to ensure repeatable results; however, numerous University and private studies show that in situ results with as fed forages are a better method to predict animal performance. Some laboratories conduct in situ tests using finely ground forage samples to reduce sampling error. In this case, the same problems experienced with in vitro testing will be experienced. A potential drawback to in situ testing is that the testing should be done in the same type of animal that the results will be applied to. For instance, if silages are to be evaluated for feeding to lactating dairy cows, the in situ tests cannot be conducted in sheep or beef animals. In spite of the extra care required, several companies are evaluating in situ testing of silage hybrids to ensure that digestibility evaluations result in a real economic benefit to customers.

To improve speed and reduce costs, some companies predict digestibility (either in vitro or in situ) using near infrared reflectance (NIR). Put very simply, this technique detects particular chemical structures by the way in which a light beam is reflected off the sample. Patterns of light reflectance are then related to various levels of different nutrients. Nutrients with distinct chemical structures such as moisture and nitrogen are easier to predict than more complex structures like ADF and NDF. Because all nutrient levels are predicted from light reflectance which can vary with growing season and day length, NIR calibrations must be updated regularly to remove the influences of interfering factors such as pigments. With NIR, predicting digestion of finely ground materials has the same problems discussed for in vitro testing. In addition, there is no singe "digestibility" molecule or compound, which makes prediction of digestibility with NIR technology more difficult than standard nutrient analysis predictions.

All of the methods discussed above can be conducted with variations on the basic procedure. One of the most common differences among labs is length of the digestion period. This can vary from 24 to 48 hours. Differences in digestibility methods and lengths of digestion result in different baselines for digestibility results making it virtually impossible to compare one company’s digestibility results to those of another.

Effect of Silage Management on Corn Silage Digestibility

Proper management of the growing crop is essential to maximize returns on every seed investment. Harvest and ensiling must also be carefully managed to prevent unnecessary loss of dry matter quality. Environmental factors such as heat and soil moisture will affect fiber digestibility in corn silage by encouraging fiber maturation. Harvesting at the proper maturity can reduce the negative impact of environment on digestibility by avoiding an overly mature crop.

Plant appropriate relative maturity hybrids to accommodate your harvest schedule. For many farms, this will mean using the traditional guidelines on planting early, mid season and full season hybrids (for your farm) to spread your workload at harvest. For bunker silos, harvest whole plant corn silage at 1/3 to 1/2 milkline or about 72 to 65% moisture to capture optimum yield and digestibility. Both the planting and harvest schedule must be managed in conjunction with the relative maturities of the corn silage hybrids to minimize the risk of harvesting overly mature corn silage with low digestibility. For instance, avoid planting all long season hybrids on your best fields early in the season and all short season varieties later on the poorer soils so that all the corn silage fields are not ready for harvest at the same time. It is equally important to place hybrids were they can be harvested. Planting a hybrid on poorly drained soils which will prevent timely harvest will result in a low digestibility silage.

Kernel processing, before or after ensiling, can increase digestibility of grain and stover, thereby reducing differences between corn silages of differing maturities. Kernel processing prior to ensiling results in the greatest gains in silage digestibility as shown by Dr. Carl Hunt from the University of Idaho (unpublished data, 1996).

Effect of Processing of Corn Silage on Nutrient Digestibility In Situ.

Most of the research on kernel processing has been conducted on fairly mature corn silage. Currently, research is being conducted to determine the benefit of processing corn silage harvested at optimal maturity (1/3 - milkline). In addition to questions about the effect of kernel processing on less mature corn silage, it is unclear whether kernel processing will reduce hybrid digestibility differences. A caution about hybrid selection is in order here. If corn silage is not processed, a high digestibility corn hybrid with hard kernels or a fast grain dry down relative to the stalk may prove less digestible on the farm than expected from (in vitro) digestibility testing results.

Growing corn silage in narrow rows to boost nutrient uptake from the soils and forage yields is an area of growing interest. Limited amounts of research suggest that narrow rows can stress corn plants, thereby reducing silage digestibility. My preliminary studies indicate potential hybrid differences in the response of silage digestibility to narrow rows. Hybrids which suffer from reduced grain yield (ear flex) will produce less digestible silage when grown under narrow row conditions. However, hybrids which grow and yield well in narrow rows do not have reduced silage digestibility.

Once harvest is complete, your job is not yet over. Excellent feeding management after ensiling is necessary to ensure that the producer receives full benefit from the crop. Poor feeding management can reduce production by as much as 10 lbs. daily and could easily outweigh gains made by planting a highly digestible hybrid.

Effect of the Environment on Corn Silage Digestibility

It is well known that growing degree days (GDD) affect silage digestibility. As GDD increase, digestibility initially increases because of grain development. Further along in the maturation process, increases in GDD decrease digestibility. This loss in digestibility is due to drying and hardening of the grain, as well as maturation and lignification of the plant fiber. Losses in corn silage digestibility with increasing GDD can be avoided by selecting hybrids with above average digestibility rankings and harvesting the crop on time.

In addition to GDD, corn silage digestibility is affected by soil moisture during the growing season. As precipitation during the growing season increases, corn silage digestibility has been shown to decrease. Cornell researchers have shown that the effect of moisture on digestibility of corn silage is 3 times as great as that of GDD. Unfortunately, farms in the Northeast have little control over available moisture during the growing season. During wet growing seasons, timely harvest of corn silage becomes imperative to minimize digestibility losses. However, field evidence suggests that timely harvest alone will not prevent substantial losses in digestibility in some cases. As more information on the interaction between hybrid, growing conditions, and crop management becomes available, better recommendations may be available.

Hybrid Selection

First and foremost, you must select hybrids that can be grown and harvested at the appropriate stage of maturity on your farm. After this, forage inventories may dictate hybrid selection strategy. If corn silage supplies are generally tight, yield must be a primary consideration. If supplies are generally abundant, quality characteristics may be weighted more heavily. This must be a farm by farm decision. Growers are advised to use the following selection criteria (listed in approximate order of importance) in choosing suitable hybrids.

Unique Genetics are a special case. A number of companies market corn hybrids which are genetically very different from common commercial hybrids. The most important consideration in evaluating this type of hybrid is what information is available to demonstrate the hybrid’s advantages. On farm trials, unless conducted with split pens, are notoriously difficult to evaluate. Just think how many events on your farm can cause your milk production to change. The best evidence is a controlled research test - either a lactating cow trial or digestibility comparisons.

Almost equally important is expected yield. In some cases, increased digestibility is accompanied by poor yields or standability. Make sure you know any downside risks associated with a particular hybrid. Three of the more common cases of unique genetics are discussed below.

Starch variants (waxy corn, high sugar corns). A number of starch variants have been found in corn; many are used in human products. No conclusive research is available with these plants. In evaluating this type of hybrid, consider 3 issues. First, will you be feeding enough starch from corn silage that you will make a meaningful change in the types of starch/sugars reaching the rumen. If you currently feed more than 8-10 lbs. of corn meal and less than 25% of ration DM comes from corn silage, the answer is likely to be no. Second, don’t forget the stover. An improved starch package in the grain cannot make up for low fiber digestibility in the stover. Third, determine what will happen to the starch or sugars during ensiling. Will ensiling remove or reduce the unique starch characteristics provided by the hybrid.

High oil corn. High oil corn can be grown as a hybrid or using the DuPont TopCrossR system. TopCrossR produces a substantially higher oil level than currently available hybrids. To obtain meaningful gains in your feeding program, a high oil corn silage must be fed in fairly large quantities (> 50% of forage DM). Corn silage produced in a TopCrossR system can provide between 1and 1.5 lbs. of fat per day (16-25 lbs. of corn silage DM/day), replacing all supplemental fat, other than bypass fat products. When using this type of corn hybrid, the basal ration must be adjusted accordingly to prevent ruminal upset from too much fat. As stated for the starch variants, stover digestibility is important. In the TopCrossR system this information may already be available for the parent hybrid. The best choice for high oil corn silage is a hybrid with highly digestible stover and a high oil content.

Brown midrib corn. Brown midrib corn has a mutation which prevents complete lignification. This mutation has been studied for many years. Increases in digestibility and animal performance are well documented. Older hybrids were plagued by poor agronomic performance; less information is available on new hybrids. This is one case where documentation of the digestibility information is of less importance than documentation of agronomic performance.

Evaluating Digestibility Evaluations

Since there is no standard method for determining corn silage digestibility, hybrid digestibility tests must be approached with care. Various testing procedures can result in biases for certain types of hybrids. Four major areas are of importance.


Putting It To Work On Your Farm

Before deciding to include nutritional quality as one of the criteria to be used in hybrid selection, each farm operation should be thoroughly evaluated. The evaluation should cover the following areas: