PA Corn Silage Hybrid Evaluation Program: The development of the OMD Index

Integrating organic matter digestibility data is a useful practice to gauge silage value and match corn hybrids to farm needs.

The Pennsylvania Corn Silage Hybrid Evaluation Program is a collaborative effort of The Pennsylvania State University and the Professional Dairy Managers of Pennsylvania (PDMP). The mission of the program is to provide reliable, third-party silage hybrid research in a customizable format so that producers can determine the best hybrids for their operations according to the unique characteristics that are important to their situations and objectives. The program is also intended to influence the decisions of seed companies to the plant breeding decision makers, so that developing highly digestible forage hybrids become a priority of the commercial seed industry (Source: Professional Dairy Managers of Pennsylvania).

Corn silage hybrid data are collected from 10-12 locations across the state, varying annually based on field availability and weather conditions, that exemplify a wide range of growing conditions found in Pennsylvania. Hybrids are classified and grown based on region and relative maturity. Field history, hybrid traits, and growing conditions are available at every location when results are reported.

Penn State’s Plant Sciences Department plants, maintains, and monitors the plots through the growing season and submits silage samples to an independent laboratory for testing. Results reporting is targeted for the first week of November.

Data Reporting
Results are available in Excel and PDF formats. The Excel database allows for the results to be “sortable” and user-friendly when sorting the data based on different parameters of the hybrids. The PDF version makes the data smart device-friendly, allowing for an easier option for viewing when using a phone or tablet.

For 2019 results, an index called Organic Matter Digestibility (OMD) was added to the output. The OMD calculation is using wet chemistry to determine NDFD30 and 4-h starch digestibility.

 

 

Parameters to be reported in 2020 include:

  • Dry Matter (DM, %)
  • Yield (tons/acre)
  • Organic Matter Yield (tons/ac)
  • Crude Protein (CP, % DM)
  • Neutral Detergent Insoluble Crude Protein (NDICP, % DM)
  • Neutral Detergent Fiber (aNDFom, % DM)
  • Lignin (% DM)
  • 240hr Undigested NDF (uNDF240, % DM)
  • Ash (% DM)
  • Starch (% DM)
  • Total Fatty Acids (TFA, % DM)
  • Starch Digestibility (StarchD, % starch)
  • 12hr NDF Digestibility (NDFD12, % NDF)
  • 30hr NDF Digestibility (NDFD30, % NDF)
  • 120hr NDF Digestibility (NDFD120, % NDF)
  • 240hr NDF Digestibility (NDFD240, % NDF)
  • Organic Matter Digestibility Index (OMDI, %)
  • Digestible Organic Matter Yield (DOM Yield, tons/ac)
  • Population (Plants/acre)
  • Relative Maturity

The OMD Index
The digestibility of nutrients in corn silage is paramount when determining its nutritional value. Starch and NDF are responsible for much of the digestible energy in corn silage. In order to give dairy producers and nutritionists a tool to evaluate corn silage hybrids, we developed a new quality index based on organic matter digestibility of protein, fat, NDF, and starch called the OMD Index.

The OMD Index is intended to represent the digestible portion of silage dry matter and is based on chemical analyses only. It does not predict dry matter intake or milk production of the cows, although numerous studies have clearly shown that digestibility of forage dry matter is directly related to lactational performance of dairy cows. The OMD index does not represent the absolute digestibility of silage OM, as this can be reliably determined only in experiments with live animals, but it is representative of the potentially digestible OM and can be used for comparison among silage hybrids. Simulation analyses using the Cornell Net Carbohydrate and Protein System (CNCPS v.7.0; Cornell University, Ithaca, NY) have shown a good relationship between corn silage OMD and model-predicted milk production of dairy cows fed a standard diet containing approx. 40% corn silage (dry matter basis).

How is the OMD Index Used?
Quality of corn silage is associated with digestibility of NDF and starch. A long-standing goal of PDMP is to create a single measure of nutritive value using several variables associated with digestibility. Traditional variables, crude protein, NDF, lignin, and fat, are combined with in vitro digestibility determinations for NDF (NDFD30) and starch (4-hour, 1-mm grind). Once combined, these digestibility coefficients sum to predict organic matter digestibility (OMD). The OMD Index equation is:

The OMD Index is calculated using the following equation: OMD Index (%) = {[(crude protein – NDFCP) × 0.89] + (fatty acids × 0.75) + (starch × starchD ÷ 100) + [(aNDFom – lignin) × NDFD30 ÷ 100)]} ÷ [(crude protein – NDFCP) + fatty acids + starch + (aNDFom – lignin)] × 100.

Where: OMD Index (%) is Organic Matter Digestibility Index; crude protein, fatty acids, starch, NDFCP (NDF-bound CP), aNDFom, and lignin (ash-free) are expressed as % of corn silage dry matter; 0.89 is assumed (based on literature data) coefficient of digestibility of silage crude protein; 0.75 is assumed (based on literature data) coefficient of digestibility of silage fatty acids; starchD is starch digestibility in vitro (by wet chemistry at 4-hour and sample ground through a 1-mm sieve) expressed as % of starch; and NDFD30 is NDF digestibility in vitro (by wet chemistry at 30 hour and sample ground through a 1-mm sieve) expressed as % of NDF.

Users can also calculate DOM yield per acre, with the following equation: DOM yield (t/ac) = [DM yield × (100 – ash) ÷ 100] × (OMDI ÷ 100)

Where: DOM yield is Digestible Organic Matter yield (t/ac); DM yield is silage dry matter yield (t/ac); ash is silage ash content as % of DM; and OMDI is Organic Matter Digestibility Index (%).

Use of OMD Index:
The OMD Index is intended to represent the digestible portion of silage dry matter and is based on chemical analyses only. It does not predict dry matter intake or milk production of the cows, although numerous studies have clearly shown that digestibility of forage dry matter is directly related to lactational performance of dairy cows. The OMD Index does not represent the absolute digestibility of silage OM, as this can be reliably determined only in experiments with live animals, but it is representative of the potentially digestible OM and can be used for comparison among silage hybrids. Simulation analyses using the Cornell Net Carbohydrate and Protein System (CNCPS v.7.0; Cornell University, Ithaca, NY) have shown good relationship between corn silage OMD and model-predicted milk production of dairy cows fed a standard diet containing approx. 40% corn silage (dry matter basis).

 

 

Use of DOM yield:
This value is an indication of the digestible organic matter yield per acre and is heavily influenced by the yield of silage dry matter per acre for a specific silage hybrid and test area. Unlike OMD Index, which strictly characterizes silage chemical composition and digestibility, the DOM value involves silage yield and is influenced by factors other than silage digestibility. If users are primarily interested in silage digestibility characteristics, they should use OMD Index to compare hybrids.

Conclusion
Organic matter digestibility is not a new measure. For years, researchers and nutritionists have used digestibility estimates to formulate rations for dairy cattle. Today, integrating these data is a useful practice to gauge silage value and match corn hybrids to farm needs. In the end, we hope the OMD Index serves to facilitate choice of corn silage hybrid by the dairy producer and fine-tune rations for dairy cows.

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