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Forage digestibility – NDFd and uNDF – with corn silage hybrids

Bill Powel-Smith for Progressive Dairyman Published on 29 September 2017
Silage pit

When selecting corn hybrids, dairy farmers often consider a variety of factors that play a role in the nutritional quality for dairy rations. Today’s corn silage hybrids offer high yield potential but can vary substantially in nutritional quality depending on genetics and environmental factors.

Dual-purpose, leafy and BMR hybrids are the three main types of corn on dairy farms producing corn silage.

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Growers might consider fiber digestibility with hybrids. Neutral detergent fiber digestibility (NDFd) is a common measure of fiber digestibility but a poor predictor of intake and forage quality. Undigested neutral detergent fiber (uNDF) is the neutral detergent fiber not digested after so many hours incubated with rumen bacteria; uNDF improves predictions of dry matter intake and rumen function.

Modest NDFd differences do exist among conventional silage hybrids (two to three percentage points), but the biggest influence over neutral detergent fiber digestibility is the growing environment during the vegetative growth stage.

Fiber values, such as the quantity of acid detergent fiber, NDFd and uNDF, are important in total ration formulation. However, their importance in evaluating hybrid genetics is minimal because their absolute values are affected by environmental conditions and dilution by starch and sugar.

When evaluating uNDF, producers should note the incubation time point (e.g., 24 versus 30 versus 48 hours) when comparing values from different laboratory reports.

The role of BMR

However, our researchers have concluded growing environment is three times more influential over fiber digestibility than genetics. This is why the seed industry has resorted to the use of BMR genetics as the only practical approach to significantly improve uNDF.

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While BMR corn silage hybrids have been on the market for nearly two decades, BMR mutants were first discovered in 1924 at the University of Minnesota, and BMR genes have been used in sorghum, sudangrass, millet and corn.

BMR is named for the way plants display reddish-orange coloration on the underside of the leaf mid-vein (mid-rib) starting at the four- to six-leaf stage. BMR corn typically has 20 to 30 percent less lignin and reduced cross-linkages with other cell wall carbohydrates than dual-purpose or leafy hybrids.

The reduced lignin in BMR silage results in a 4- to 10-point higher NDFd 24-hour value when analyzed in the lab. However, in the lab, the BMR sample cannot escape the analysis vessel. In the cow, the net effect is a faster rate of neutral detergent fiber digestion with the more fragile BMR fiber exiting the rumen much quicker than non-BMR corn silage.

This typically results in higher intakes of the entire diet (especially important in transition and early lactation cows), which usually drives higher milk yields. The improved rate of digestion and feed passage allows for higher-forage diets, improved rumen health and the potential to remove some supplemental energy or protein from the diet.

The higher starch content in certain BMR hybrids will also act to dilute uNDF levels in the corn silage and in the diet. Lower levels of uNDF have also been associated with improved intake potential of forages.

The majority of BMR is grown in separate, high-fertility fields to minimize agronomic risks. BMR needs to be managed and harvested similar to conventional hybrids, ideally when the kernels are at a ½- to ¾-milkline maturity. BMR hybrids can vary in uNDF from year to year (or field to field) depending upon the unique growing environment.

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As with non-BMR hybrids, attention to kernel damage during harvest is critical to ensure maximum ruminal starch availability.

BMR silage tends to be prone to aerobic stability problems (heating) due to extremely high levels of sucrose in the stalk. This tendency for heating at feedout can be significantly reduced with the use of inoculants containing the Lactobacillus buchneri bacterium.

Ideally, BMR corn silage is stored in a separate silo or bunker so it can be fed to the animals that benefit the most from high fiber digestibility and high dry matter intake. If segregation is not an option, growers can still benefit by using a superior corn silage mixture of BMR products and conventional hybrids with their cows.

Other considerations

Some growers prefer to evaluate hybrids based on indexes such as net energy of lactation or the University of Wisconsin’s “milk per ton” and “milk per acre.”

While they can be useful in ranking hybrids, it is important to evaluate the absolute value of the traits (yield, starch, fiber digestibility) that influence these index values. Traits like crude protein and oil content are less important simply because there are minimal genetic differences between these traits among commercial hybrids.

Some nutritionists also request ruminal starch digestibility values on silage plot reports. Most university and seed company silage hybrid testing programs do not provide starch digestibility values, since starch digestibility, as influenced by the amount of hard or vitreous starch in the kernel, is a trait also lacking in significant variation among commercially available hybrids.

While differences clearly exist in the amount of vitreous starch among hybrids harvested at grain maturity, as evidenced by differences in test weights, there are minimal differences in the amount of vitreous starch among corn silage hybrids harvested when kernels are pre-black layer maturity (e.g., ½- to ¾-milkline).

Furthermore, the length of time silage kernels are exposed to the fermentation environment influences ruminal starch digestibility.

While starch digestibility is an important measurement for nutritionists switching from long-stored corn silage to new-crop silage, it is not a trait that should be given consideration when selecting silage hybrid genetics.

Research by corn breeders suggests that to be 95 percent confident in selecting the best hybrid for silage yield or nutritional traits, a minimum of 20 direct side-by-side comparisons in the same plot are recommended. Hybrids should also be compared within the same maturity, seed treatment, technology segment, planting population and chop height.

It is also desirable to compare hybrids in multiple environments and growing seasons to better understand hybrid stability when exposed to extremes in growing conditions.

Growers should secure as much information as possible on the performance of a silage hybrid and not be satisfied with catalog scores (e.g., 1 through 9).

Seed companies serious about silage will be able to provide absolute values (E = Excellent, V = Very Good, G = Good, M = Moderate, Blank = Not Labeled) for important silage traits compared against their own hybrids as well as competitors. Finally, growers should be cautious about putting too much credence in “beauty pageant” forage contests, where yield is not considered and there is no way to ensure all entries were chopped at the same height.  end mark

PHOTO: Choosing a silage hybrid right for your dairy involves considering growing environment as well as seed genetics and understanding the value of each. Staff photo.

Bill Powel-Smith
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