Whole-plant corn harvested as silage provides a major portion of the forage inventory on many dairy farms. Corn silage is normally a high-energy forage with tremendous per acre dry matter (DM) yield potential relative to other forage crops. The nutritive value of corn silage can vary depending upon several factors including:

• Proportion of grain relative to stalk, cob, and leaves.

• Fiber content of the stalk, cob, and leaves.

• Digestibility of the fiber.

• Starch content of the grain.

• Digestibility of the starch.

• Oil content.

• Protein content.

In the past, corn hybrids selected for grain production (i.e. high-grain yield) were simply grown for harvest as corn silage. If harvested at the proper stage of maturity, this practice usually produced corn silage with a high proportion of grain and a high starch content. However, this approach may not optimize fiber or starch digestibility. Recently, seed companies have placed increased emphasis on the selection of corn hybrids specifically for silage production. This has increased the attention given to the fiber content of corn silage and its digestibility. The rate of kernel dry-down relative to stalk dry-down and kernel hardness at a specific kernel milk-line position and their impact on starch digestibility has sparked recent interest. Whether it is harvest timing, crop processing, or hybrid selection, improving starch digestibility is a major focal point of corn silage research and management recommendations.

Following is a brief overview of several corn silage hybrids currently on the market and their impact on dairy production.

Brown-Midrib
Brown-midrib (BMR) corn contains less lignin in its stalks and leaves compared to normal corn plants. Because lignin is the indigestible component of fiber and its presence lowers cellulose digestibility, fiber digestibility is increased with BMR corn silage. This was shown in research trials many years ago, along with the potential for improved animal performance. Until recently, commercial production was limited due to agronomic problems. The first commercially available BMR corn was FullTime,™* a hybrid marketed by Cargill Hybrid Seeds.

University of Wisconsin research evaluated FullTime BMR 657 versus a corn grain hybrid in a feeding trial utilizing total mixed rations with 26 Holstein cows (120 DIM at trial start-up) over 16 weeks of lactation. Because of the lower neutral detergent fiber (NDF) content of the BMR silage (41% versus 46% NDF) and its higher NDF digestibility (56% versus 47%, 30-hour in vitro NDF digestibility), the BMR diet provided more forage than the control diet (60% versus 45% forage in ration DM). The forage DM was comprised of ²/3 corn silage and ¹/3 alfalfa silage. The higher fiber digestibility of the BMR silage allowed for similar DM intakes for both diets, even though cows fed the BMR diet with 15% units more forage DM consumed 6 lb/cow/day less corn DM. Milk production was three lb/cow/day lower (98 versus 95 lb). But, milk fat yield was higher (3.24 versus 3.06 lb/cow/day) for the BMR diet.

Michigan State researchers reported BMR silage increased milk production 6.0 lb/cow/day over its high lignin counterpart when fed in high-forage diets. In a later trial, researchers reported BMR silage increased milk production 7 lb/cow/day in both typical and high NDF diets. But, feeding BMR in the normal NDF diet reduced milk fat test 0.4 percentage units. Comparison of a control silage (normal NDF diet) versus BMR silage (high NDF diet) showed similar milk yields, but higher milk fat percentage and yield for the BMR silage. Results from this comparison were similar to findings at the University of Wisconsin.

More research with various types of diets is needed to optimize the utilization of BMR corn silage by lactating dairy cows. To maintain milk fat test and optimize fat production when feeding a large proportion of BMR corn silage, a higher forage or fiber diet will usually need to be fed and ration particle length needs to be of sufficient length. When considering BMR corn silage, the potential for lactation response or feed cost savings should be weighed against the extra seed cost and any yield reduction.

Leafy
Leafy corn plants are comprised of more leaves above the ear compared to standard corn hybrids. Leafy-corn hybrids are promoted for whole-plant silage production within the seed-corn industry due to their greater tonnage potential. Wisconsin researchers evaluated a leafy hybrid (Mycogen TMF 106) versus a grain hybrid in a feeding trial with Holstein cows that averaged 75 DIM at trial start-up. All diets were fed as a TMR containing 50% forage (²/3 corn silage and ¹/3 alfalfa silage–DM basis). Whole-plant silage made from the leafy hybrid contained four percentage units more moisture than silage made from the grain hybrid, even though both hybrids were harvested at ¹/2 milk-line stage of maturity. Silage nutrient composition was similar for both hybrids.

Lactation performance did not differ between the silages. Total tract digestibility of organic matter and fiber were reduced and starch was increased for the leafy silage. An increased 24-hour in situ ruminal starch degradation for the leafy silage was observed, which could be related to the softer kernel texture of the leafy hybrid. This may be an important characteristic, especially when silage is harvested too dry and a crop processor is not used. Under the harvest conditions of the Wisconsin study, silage made from the leafy hybrid did not improve lactation performance. Minnesota researchers also reported no improvement in lactation performance from feeding a leafy silage hybrid.

High-Oil
The 2% difference in oil content between typical dent corn and high-oil corn kernels (4% versus 6%) contributes very little to the overall oil content of silage made from high-oil corn hybrid plants. Research trials have been conducted with high-oil corn hybrids produced by traditional corn breeding techniques.

Recent research trials were conducted with high-oil corn produced via the Topcross^®* system of Dupont Optimum Quality Grains. This system uses male-sterile plants of top hybrids seeded with 5%-10% high-oil pollinator plants. The purpose of this system is to produce corn with greater oil content and less yield reduction than conventional high-oil hybrids. This system is available commercially and is rapidly gaining popularity as the preferred method for producing high-oil corn.

In general, research trials show milk production was increased when diets containing high-oil corn grain were fed. However, little benefit to feeding high-oil corn silage has been observed. This response could be related to the smaller change in oil and energy concentrations seen with high-oil corn silage compared to typical corn silage hybrids. Some of the disadvantages associated with growing high-oil corn include: yield reduction, reduced oil content due to lack of isolation, effect of drought on pollinator plants, effect of emergence problems on pollinator plants, and higher seed costs. At this time, the disadvantages of using high-oil corn hybrids for silage appear to outweigh the benefits.

Waxy
Normal-dent corn contains 75% amylopectin (branched glucose chains) and 25% amylose (straight glucose chains), while waxy corn contains 100% amylopectin. Based on glucose structure (branched-chain form of starch), amylopectin was believed to be more digestible. However, digestion trials with beef and dairy cattle have not substantiated this as actually occurring. New York researchers evaluated waxy corn silage versus normal-dent corn silage. The consensus was no benefit was achieved when waxy corn silage was fed to lactating Holstein and Jersey cows. Agronomic performance of waxy corn may be lower than standard commercial hybrids from which it was developed.

Harvest Management
While hybrid selection is an important factor in the production of high-quality corn silage, good harvest-management practices are vital to help ensure high-quality silage is delivered to the cow. The best lactation performance by dairy cows has been shown to occur at 65% to 70% whole-plant moisture. Once most of the kernels are dented and the milk-line is visible, moisture content should be assessed. Whole-plant moisture content should be the trigger for harvesting corn silage.

The general recommendation for corn silage harvested with a conventional harvester (without a crop processor) is ³/8 inch theoretical length of cut (TLC). Based on Wisconsin research, the recommended chop length for corn silage harvested with a harvester fitted with a crop processor is ³/4 inch TLC. The recommended roll clearance ranges from ¹/16 to ¹/8 inch (1 to 3 millimeters).

When selecting a corn hybrid for silage, consider consulting a crop advisor to help evaluate the hybrid's DM yield and other agronomic factors. Regardless of the corn hybrid used for silage, remember to use silage inoculants properly, pack well, and cover securely to minimize storage losses.

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