Forage Testing

Forage quality won't be hitting it out of the park for dairy producers in many parts of Canada this year as an unusually wet growing season reduced nutrient and digestibility levels, writes Joel Bagg - Forage Specialist; and Tom Wright - Dairy Cattle Nutritionist, Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA).
calendar icon 21 March 2009
clock icon 5 minute read

New lab technique tells you what you need to adjust.

When your forage quality is suspect, appropriate laboratory analysis improves your ability to anticipate and adjust for production problems and better balance rations.

Accurate estimates of Net Energy (NE) and Total Digestible Nutrients (TDN) of forages give you more reliable ration formulation and more predictable livestock performance. Guessing at forage digestibility can result in cows just not milking the way you think they should.

For more than 30 years, Acid Detergent Fibre (ADF) has been used to predict NE and TDN, and Neutral Detergent Fibre (NDF) to predict feed intake. While ADF and NDF are good measures of forage fibre content, there are more accurate ways to tell how well your cows' rumens will digest that fibre.

Another method sometimes used to estimate fibre digestibility is a calculation from percentage analysis of lignin, the woody non-digestible part of the plant. However, research has demonstrated that this analysis does not account for enough of the variation in NDF Digestibility (NDFD).

Newer laboratory methods used to estimate digestible energy are becoming much more sophisticated and accurate. They can give you a better reading on NDFD to help formulate and balance your herd's diet.

In Vitro NDF Digestibility

Measuring NDFD during in vitro, or test tube, fermentation best determines fibre digestibility. This analysis uses incubated live buffered rumen fluid to measure the amount of feed digestion under simulated rumen conditions. Laboratories usually use 24, 30 or 48-hour time periods.

For dairy cattle nutrient requirements, the U.S. National Research Council has adopted this method of determining NE by using NDFD values from 48-hour incubations. While NDFD results can vary among laboratories and batches, the NDFD information is often more useful to nutritionists in troubleshooting than traditional energy and intake estimates. NDFD analysis is currently available from some Ontario commercial feed testing labs.

NIR Analysis

Researchers have also developed techniques for measuring NDFD using Near Infrared Reflectance Spectroscopy (NIR). It estimates forage nutrient composition from the reflectance of specific light wavelengths. This approach relies on the correlation between NDFD results obtained from in vitro digestibility and measures of the same samples analysed by NIR.

NIR is faster and cheaper than traditional wet chemistry and in vitro procedures. However, NIR analysis of NDFD is often considered less accurate than in vitro analysis. This is particularly true if the calibration equations the lab uses are based on a limited number of samples.

The inconsistency of NIR analysis is likely to be more noticeable with poor quality forages. As the NIR database improves, however, so will the accuracy of NDFD analysis. Meanwhile, poor quality forages are best analysed by the in vitro method for reliable results.

Improved Energy and Intake

NDFD for a particular forage type can range widely. NDFD provides more accurate estimates of TDN, NE and intake potential. Increased NDFD will result in higher energy values and, more importantly, increased forage intakes.

Relatively small fibre digestibility improvements can significantly in-crease dry matter intakes. Research has shown a one per cent unit increase in forage NDFD results in 0.37 pounds more dry matter intake, resulting in 0.55 pounds more milk per cow per day. Higher intakes of higher energy forage should enable fresh cows to maintain weight better and get back in calf sooner.

By measuring NDFD, ration balancing is more precise, with more predictable animal performance. It also gives you the tools to better compare different forages.

If you have two haylage samples testing 19 per cent CP, 31 per cent ADF and 42 per cent NDF, would you consider them equal? If NDF digestibility analysis results are available, and forage number one has NDFD of 40 per cent and forage number two has NDFD of 60 per cent, you would know forage number two would give you a lot more milk.

Maturity Affects NDFD

The NDFD of alfalfa haylage can range from 70 down to 35 per cent. Similar to NDF percentage and other quality measures, maturity and harvest timing have the greatest influence on NDFD. The amount and timing of rain greatly affected when you could harvest forage crops this year.

The NDF in leaves is more digestible than the NDF in stems. Lignification in mature stems significantly decreases fibre digestibility, since lignin is indigestible. As plants mature, the leaf-to-stem ratio declines and so does the NDFD.

Other Agronomic Factors Affecting NDFD

Grasses and corn silage harvested in a timely manner generally yield higher NDFD than legumes. This helps explain why dairy cows fed early-cut grass often perform better than an ADF-NDF analysis alone would suggest.

Early-cut grasses have high fibre digestibility, but digestibility declines more rapidly with advanced maturity at harvest. Fibre digestibility of pre-bud stage legumes is lower than that of grasses, but digestibility declines at a slower rate.

Growing conditions, such as temperature and moisture, also affect lignification and NDFD. First cuts tend to have higher fibre digestibility than later cuts.

Laboratory Analysis

Consider requesting NDFD 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.

Nutritionists should be assessing NDFD for its potential to affect rate of passage in the rumen and reduce forage and total dry matter intake of cows.

Both these factors will affect the energy content of the forage available to the cow. Information on NDFD values will add to the understanding of what your cows are eating, why production might be lagging and what options are available to optimize feed.

This article appeared in the November 2008 Ruminations column of the Ontario Milk Producer magazine.

March 2009

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