Development of an in vitro method to determine rumen undigested aNDFom for use in feed evaluation

A portion of the forage cell wall, defined as neutral detergent fiber (NDF), is indigestible to anaerobic microbial digestion in ruminants. This fraction has been characterized by surface area relationships between acid detergent lignin, but recently, data have been published describing the dynamic nature of this relationship. In situ approaches have been described to estimate indigestible NDF, recovering the undigested NDF after long-term fermentations (uNDF). To be applicable to nutritionists and diet formulation, determining uNDF needs to be conducted in a commercial laboratory similar to other routine analyses of forage chemistry. A series of studies were conducted to evaluate an in vitro approach, to describe uNDF, which is repeatable and adaptable for routine feed evaluation. One hundred and two forages of several species were analyzed for NDF, acid detergent lignin, and uNDF. The uNDF was estimated by several approaches involving long-term fermentations and filtration steps to evaluate the length of time necessary to exhaust the digestible NDF and a filtration method necessary to maintain sample integrity by ensuring low sample loss and uniform recovery with residues from long-term in vitro fermentation. To determine uNDF, in vitro fermentations were conducted on 0.50 or 0.75 g of dry matter samples, in triplicate, at multiple time points up to 504 h and initially used Gooch crucibles with Celite (Thermo Fisher Scientific, Waltham, MA) as a filtering aid. The final method utilized a 1.5-µm pore size glass microfiber filter, which allowed for increased repeatability and improved sample recovery (lowest standard deviation). In this study, in vitro fermentations of 240 h were adequate to characterize and identify uNDF, which was repeatable among conventional forages provided the samples, after NDF analyses, were filtered through the same glass fiber filter. This approach could be adapted by commercial laboratories and would provide opportunities to develop near-infrared reflectance spectroscopy equations and calibrations.