Fibre Digestibility & Forage Quality
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Accurately estimating Net Energy (NE) and Total Digestible Nutrients (TDN) of forages results in more reliable ration formulation and more predictable livestock performance. When we poorly estimate the digestibility of forages cows just dont milk the way we think they should.
There is no single, easy way in the lab to directly measure digestible energy of forages. The methods used to estimate digestible energy are becoming much more sophisticated and accurate. For over 3 decades we have used Acid Detergent Fibre (ADF) to predict NE and TDN, and Neutral Detergent Fibre (NDF) to predict intake. While ADF and NDF are good measures of the amount of fibre in a forage, they do not necessarily do an accurate job of measuring how digestible that fibre is.
Another method used to estimate fibre digestibility is a calculation from % lignin analysis. However, research has demonstrated that % lignin does not account for enough of the variation in NDF digestibility.
Fibre digestibility is best determined by measuring the NDF digestibility (NDFD) during an in vitro fermentation. In vitro digestibility uses incubated live rumen fluid to measure the amount of digestion of feeds under simulated rumen conditions. Time periods used are usually 30 or 48 hours.
In vitro NDF digestibility analysis is currently available from some of the commercial feed testing labs in Ontario. Researchers have also developed techniques for measuring NDFD using Near Infrared Reflectance Sprectroscopy (NIRS). Many U.S. laboratories offer NDFD analysis using NIRS. While in vitro analysis is the most accurate, NIRS analysis is cheaper and faster..
NDF digestibilities of a forage type can range widely. NDF digestibility gives us more accurate estimates of TDN, NE and intake potential. Increased NDF digestibility will result in higher energy values, and perhaps more importantly, increased forage intakes. Relatively small improvements in fibre digestibility can significantly increase dry matter intakes. By measuring fibre digestibility, ration balancing is more precise, with more predictable animal performance. It also gives us the tools to better compare different forages.
If you have two haylage samples that both analyze 19% CP, 31% ADF and 42% NDF, would they be considered equal? If NDF digestibility analysis results are available, and Forage #1 has NDFD of 40%, and Forage #2 has NDFD of 60%, we know there is a lot more milk or gain in Forage #2.
For many years dairy producers have targeted alfalfa haylage that tests 20-30-40 - 20% Crude Protein (CP), 30% ADF and 40% NDF. Some nutritionists are suggesting this target goal might be replaced with 20-40-50 - 20% CP, 40% NDF and at least 50% NDFD.
The NDF digestibility of alfalfa haylage can range from 70 to 35 %. Similar to %NDF and other quality measures, maturity and harvest timing have the greatest influence on fibre digestibility. The NDF in leaves is more digestible than the NDF in stems. Lignification of the xylem tissue in mature stems significantly decreases fibre digestibility. As plants mature the leaf-to-stem ratio declines and so does the fibre digestibility. The NDFD of corn silage declines about 10 percentage units between half milk-line to black layer.
In general, timely harvested grasses and corn silage will have higher NDFD than legumes. This helps to explain why dairy cows fed early cut grass often perform better than an ADF/NDF analysis would indicate. Early cut grasses have very high fibre digestibilities, but this declines more rapidly with advanced maturity. Fibre digestibility of pre-bud stage legumes is lower than that of grasses, but legume fibre digestibility declines at a slower rate. Growing conditions, such as temperature and moisture will also affect lignification and NDF digestibility. First cuts tend to have higher fibre digestibility than later cuts.
Relative Feed Value (RFV) is a commonly used index of forage quality based on ADF and NDF. A new improved index has been developed called Relative Forage Quality (RFQ) that uses NDF and NDFD. This RFQ index more accurately reflects intake and TDN of a forage, particularly of higher quality grasses.
Consider requesting NDF digestibility when sending your forage samples away for analysis. Talk to your commercial forage testing lab and your nutritionist to discuss using this technology to improve forage quality, ration balancing and livestock performance.
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