Can lignin be accurately measured?

Forages serve an important role in providing nutrients to ruminants while providing positive benefits to the environment. Forage cell wall digestibility is incomplete because of several structural features within the wall, but digestion is mostly inversely correlated with the amount of lignification that has occurred during cell wall development. Lignin is a hydrophobic polymer formed through enzyme-mediated radical coupling of monolignols, mainly coniferyl and sinapyl alcohols. The polymer is highly resistant to degradation and generally passes through the ruminant unmodified. Though lignin is resistant to degradation, it is not easily quantified within various types of forages. Numerous methods have been developed over the years to measure lignin levels in different plant species. Most frequently used among workers involved with forage development or utilization are the acid detergent, Klason, and permanganate lignin methods. More recently, acetyl bromide has received attention as a possible lignin determination method. The acetyl bromide method is dependent on determining the absorbance of the extract in which all the lignin of a sample has been dissolved. Each of these methods gives different lignin values for the same type of forage sample. For example, acid detergent, Mason, permanganate, and acetyl bromide lignin methods give quite different values for alfalfa stems: 93, 145, 158, and 135 g lignin kg-1 cell wall, respectively. These differences can be even greater for grasses: 25, 77,45, and 92 g kg-1 cell wall from corn (Zea mays L.) stalks analyzed by acid detergent, Klason, permanganate, and acetyl bromide lignin methods, respectively. This paper will discuss the different lignin determination methods and highlight the advantages and disadvantages of each as they relate to forage sample analysis.