Assessing nutritional quality of trees and shrubs for livestock: What should be measured?

Abstract

The digestibility and protein availability of woody plants consumed by large herbivores, including livestock, is compromised by abundant secondary metabolites rarely found in grasses, such as condensed tannins, and lignin contents that are often higher than those of grasses. The purpose of this study was to determine the extent to which browse quality assessments published in the African Journal of Range & Forage Science over the past 25 years have measured such components, to highlight gaps, and to offer recommendations to guide sample collection, handling and analysis in future studies. Only 42.9% of 21 relevant papers included measurements of condensed tannin content. Most papers included acid detergent fibre (90.5%) and neutral detergent fibre (85.7%), while 57.1% included acid detergent lignin. Only two papers included digestible nitrogen or digestible CP, both showing that up to 40% of nitrogen could be unavailable. Prospective contributors of browse quality manuscripts should include analyses of condensed tannin content, available nitrogen (digestible protein) and all fibre fractions. Assessments can be enhanced further by adding metabolomic analyses when possible. Last, but not least, sufficient details of methods must be provided to allow for quality assurance and repeatability.