Limitations of Silage Starch Digestibility Testing for Comparing Enogen to Non-Enogen Hybrids

• Four different methods are commonly used to measure silage starch digestibility, each with strengths and weaknesses.
• Near Infrared Reflectance Spectroscopy (NIRS) is one of the more common methods used to analyze silage hybrid evaluation trials.
• NIRS has limited ability to accurately predict Enogen^® hybrid performance.
• Enogen hybrid performance is better predicted using in-situ testing methods.

Corn silage starch content frequently ranges from 25-35% of the whole plant harvested, making it a significant contributor of energy to feed rations.¹ Increases in starch digestibility in whole plant corn silage is believed to improve dairy and beef cattle performance, often making it 1 of several valuable traits considered when evaluating and selecting corn silage hybrids.² Accurately measuring starch digestion in a way that best reflects cattle performance, and doing so quickly and economically, is challenging compared to other nutritive assessments. Inaccurate starch values may have additional implications because they are commonly used in calculations with other forage analyses to create a single value to estimate overall silage quality.

There are 4 approaches to characterizing starch digestion, each with advantages and disadvantages. Historically, almost all methods have been good enough for comparing relative differences between hybrids, but with the recent introduction of Enogen corn hybrids, which contain higher levels of alpha-amylase to improve starch breakdown, starch digestion values could be biased by certain testing methods.

Methods for Quantifying Starch Digestibility:

1. In-vitro: This lab-bench technique uses rumen fluid from donor cattle to simulate digestion while outside the intestine or rumen.

   • Weakness: Variable results can occur from different testing batches due to changes in donor cattle rumen fluid. Silage samples must be finely ground, reducing any natural differences in digestion due to particle size. Samples could potentially be only partially digested due to not being exposed to multiple digestive processes within a live cow.
   • Advantage: Samples can be processed quicker and at less cost than most other techniques.

2. In-situ: Feed sample inside a porous sachet is placed inside live cattle rumen and incubated for a set period before removing and measuring actual starch disappearance.

   • Weakness: Results may vary when repeating test due to differences in each cow’s rumen fluid. Samples may only be partially digested due to only being placed in 1 specific area of cow rumen.
   • Advantage: Believed to better reflect how cattle respond to hybrid differences than in-vitro method.³ Partially improved by using larger feed samples and particle size and replicating in multiple sachets and/or cows for more true assessment of hybrids.

3. In-vivo: The most comprehensive of all approaches, in-vivo measures the total mixed rations fed and what is passed through the animal in feeding

   • Weakness: Slow and expensive process that requires looking at impact of the total mixed ration. Difficult to use for screening significant number of hybrids.
   • Advantage: The most accurate of all testing procedures as it measures the exact intake and pass-through within a live animal.

4. Near Infrared Reflectance Spectroscopy (NIRS): A lab process in which an apparatus measures reflectance and absorbance of specific light wavelengths of a silage sample and compares it to samples with known values previously quantified using in-situ analysis methods. NIRS uses multiple historic known values to calibrate against.

   • Weakness: It is only a prediction based on previously known sample values. Lack of calibration for starch digestibility interactions with traits such as Enogen could result in predictions not accurately representing actual performance in live animals.
   • Advantage: Quickest and least costly approach to testing many hybrids for relative differences.

Comparing NIRS and In-situ Results

Hybrid silage evaluation trials commonly test multiple hybrids simultaneously for several quality trait characteristics. A mixture of Enogen and non-Enogen hybrids frequently are found within the same trial and are analyzed using NIRS methodology for quantifying starch and starch digestibility. A controlled study was set up to better understand the accuracy of results using NIRS as compared to in-situ testing methods. Silage samples were collected from Enogen hybrids and respective non-Enogen isolines. Subsamples were pulled from each sample and starch disappearance was measured at 0- and 7-hour timings via in-situ assays and using NIRS calibrations specific for the 2 selected timings.

NIRS results for both Enogen and non-Enogen hybrids showed little difference between each other at both 0- and 7-hour intervals (dashed lines in Graph 1). When using in-situ methods, results of Enogen traited hybrids had a higher rate of starch disappearance at both 0- and 7-hour timings (solid green line in Graph 1). Although NIRS is quicker and a lower cost testing method, it cannot effectively detect starch digestion differences contributed by the alpha-amylase present in Enogen-traited silage. NIRS has potential for screening large numbers of hybrids to understand genetic differences, although it may not be the most effective method for illustrating starch digestibility differences among hybrids with and without the Enogen trait.

References

¹NRC - Nutrient Requirements of Dairy Cattle. 7th Revision. 2001. Natl. Acad. Sci., Washington, DC.

²L.F. Ferraretto and R.D. Shaver. 2012. Meta-analysis: Impact of corn silage harvest practices on intake, digestion and milk production by dairy cows. Prof. Anim. Sci. 28: 141-149.

³Heuer, C.R. 2014. Ensiling and processing of corn silage and high moisture corns and laboratory method comparison of starch digestion in ruminants. M.S. Thesis. University of Wisconsin-Madison, Madison, USA.