Selecting silage hybrids: a nutrition perspective

Long-term corn hybrid selection programs for grain yield potential have contributed to the tremendous progress in that trait observed by corn farmers. Corn silage is a combination of both grain and stover fractions in the whole-plant harvest, which makes selecting corn hybrids for silage production much more difficult for dairy farmers.

This greater complexity is mainly because both fractions contribute to whole-plant yield at harvest, the relative proportion of each fraction contained in the whole-plant harvest influences corn silage nutrient concentrations, and the digestibility of each fraction in corn silage can vary at feedout.

Starch is variable

The energy value of corn silage influences both level of milk production and supplemental grain amounts needed in rations. Starch, contained in the kernel (grain) fraction of corn silage, contributes about half of its energy value. So starch concentration measurements are commonly included in corn silage hybrid performance trials.

Even though corn hybrid influences grain yield potential and thus can influence the kernel to stover ratio and corn silage’s potential starch content, the actual starch content is largely uncontrolled because it varies greatly depending on crop growing conditions (for example, rainfall amounts and timing), timing of harvest relative to kernel maturity, and cutting height. All of these factors can also alter the kernel to stover ratio during the production and harvest of corn silage.

Therefore, corn silage starch content on dairy farms is highly variable as indicated by a normal (two-thirds of samples) range of 25 to 39 percent (dry matter [DM] basis; average of 32 percent) in a commercial lab survey data set of 300,000 corn silage samples analyzed at the four major U.S. commercial forage testing labs. This variation is not unexpected since, as previously described, the relative proportion of kernel and stover fractions contained in the whole-plant harvest, which is influenced by many factors, determines the starch content of corn silage.

The takeaway messages from the foregoing discussion are as follows:

1. Inclusion of starch concentration or grain yield in corn silage hybrid selection programs is important for setting the potential for high-starch content, but results will vary.

2. Focus on management of key corn silage production and harvest practices, as mentioned previously, to optimize starch content.

3. Possibly the most important step in optimizing the utilization of corn silage in dairy cattle rations is frequent and accurate sampling and analysis for starch content during feedout. The latter also applies to neutral detergent fiber (NDF) content, which essentially varies inversely with starch content.

A relatively hot research topic in recent years has been starch digestibility and the potential for altering kernel endosperm properties (reduced kernel hardness) to improve starch digestion. Incorporation of these corn endosperm properties into corn breeding or hybrid selection programs has been, and continues to be, slow to evolve.

Maintain feed carryover

Specific to corn silage, the practical importance of the corn genetic aspect of starch digestibility is tempered because the kernel should be less mature than black layer at harvest, processing of the kernel during harvest enhances starch digestibility, and starch digestibility elevates over time of storage in the silo through fermentation and proteolysis of the starch-protein matrix.

Furthermore, there is no standardized agreed-upon method for assessment of differences in starch digestibility among samples in the lab. I am unaware of starch digestibility being incorporated directly into any university-run corn silage hybrid performance trials at this time. Therefore, I do not currently recommend starch digestibility as a focal point in corn silage hybrid selection.

Focus more on proper harvest maturity, chop length and kernel processing, and having at least four months of carryover corn silage inventory for achieving high starch digestibility during feedout. Work with your nutritionist during corn silage feedout to assess starch digestibility and make necessary ration adjustments. This can be done either directly through rumen in vitro analysis or indirectly through analysis of kernel processing score and changes in soluble protein or ammonia-nitrogen over time in storage. Pay attention to future developments in the starch digestibility area as research continues.

Fiber differences

Corn silage is also a highly significant contributor of NDF in dairy cattle rations. Analysis of the commercial lab survey dataset showed an average NDF content of 41 percent dry matter. The NDF content of corn silage is most heavily influenced by the relative proportion of stover and grain fractions in the whole-plant harvest. Therefore, much of the attention in the fiber area with hybrid selection is on fiber digestibility, specifically measurement of rumen in vitro NDF digestibility (percent of NDF; ivNDFD).

Reduced corn silage lignin content and corresponding gains in ivNDFD, DM intake, and lactation performance have consistently been observed in research trials with brown midrib (bm3) mutant corn hybrids.

A 15-year data summary, from the UW-Madison Agronomy Department’s annual corn silage hybrid performance trials, indicated that ivNDFD for the bm3 hybrids was increased by 6 to 11 percentage units above the trial average. However, corn silage starch content and DM yield per acre trended lower for the bm3 hybrids that were included in these field plot trials. Agronomic, starch, and yield considerations need to be weighed when making decisions with regard to the bm3 hybrids.

Unfortunately, for conventional-type (nonbrown midrib) corn silage hybrids, progress in improving ivNDFD has been slow, and only small relative differences among these hybrids are often observed in commercial hybrid performance trials. Due to sampling and assay variation and with limited replication, ivNDFD (as a percent of NDF) differences between samples of only 3 percentage units or less may not be statistically different or repeatable. Therefore, the magnitude of the ivNDFD difference needs to be kept in perspective when using ivNDFD as a parameter for corn silage hybrid selection.

Don’t forget yield

Dry matter yield per acre is important and needs to be considered when selecting corn hybrids for silage production. How important yield drag is relative to enhanced quality parameters depends on several factors, including: per cow land availability, land price or rental cost, opportunity cost for any reduced grain harvest, proportion of corn silage used in rations, ability to target silages to different animal groups based on quality parameters, and the forage inventory situation in given year.

Given the complexity of corn silage and its components, involve both your nutritionist and agronomist in discussions of the quality versus yield tradeoff and how they factor into the corn silage hybrid selection. It is important to recognize that hybrid by environment interactions exist. This is why replicated data on corn silage hybrids measured across multiple locations over multiple years is required for valid comparisons.

This article appeared in the November 2016 issue of Hay & Forage Grower on page 14.

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