Forage quality assessment

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Editor's note: This information was presented at the recent Cornell Nutrition Conference by Debbie Cherney and Larry Chase, animal science faculty members at Cornell University, along with Jerry Cherney in the Department of Crop and Soil Sciences at Cornell.


Models, such as the Cornell Net Carbohydrate and Protein System, and ration balancing programs are increasingly used by consultants and producers to predict animal performance. These models and programs rely heavily on chemical and in vitro characterization of feeds. Because forage plays such a large role in dairy rations, animal performance could be improved through timely and improved forage analyses.

Animal performance is a function of intake and digestibility. Attempts to accurately predict intake routinely have met with a lot of frustration, in part because routine laboratory methods do not measure the characteristics of forage which are the true determinants of differences. So what forage analyses would measure those forage characteristics that relate most closely to intake and digestibility? Chemical fractions that have been associated with intake and digestibility include fiber and lignin.

A symposium held at the recent Joint Meetings of the ASAS, ADSA and CSAS discussed some of these emerging techniques for predicting forage quality in terms of fiber and lignin. Methods to analyze for NDF and ADF have been around for about 50 years while methods to analyze for lignin and protein have existed for more than 100 years. So what's new with these analyses that can help improve animal performance?

Because different factors affect intake and digestibility, and intake and digestibility are not always well related, we really need separate equations for both intake and digestibility. New equations for intake and for total digestible nutrients, like those used in the Dairy NRC, have shown promising results. These equations rely on accurate analysis of NDF digestibility. NDF digestibility can be estimated by lignin concentration and in vitro NDF digestion.

Lignin's value is in its relationship to digestibility or indigestibility. Understanding the causal factors of the negative relationship of lignin to digestibility has proven difficult. Advances in understanding this causal relationship will help to improve predictions of intake, which will improve performance. NDF fermentation can be broken down into three components: a fast digestion pool, a slow digestible pool and an indigestible pool. Van Soest et. al., (2005) observed that estimation of NDF, lignin and fermentation at 24 hours would enable the prediction of extent of digestion at any time of fermentation from a single fermentation at 24 hours.

In vitro gas production techniques have received a lot of attention lately. They have been used with success for digestibility kinetics, but to date have not been shown to be any better than traditional measures for predicting intake. New research with combined traditional measures of digestibility with information on gas production rates show increased accuracy of prediction over either newer gas methods or conventional in vitro methods for predicting intake (Pell and Schofield, 1993). The initial gas box methods of Pell and Schofield have been commercialized and are being used world-wide to evaluate forages.

It is clear that fiber and lignin assays will continue to be important, due to their strong association with factors affecting animal performance. Improved understanding of relationships of chemical constituents to digestibility and intake will improve models used to predict performance and Cornell will continue to be at the forefront of these developments.



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