Nonstructural Carbohydrate Residue, Neutral Detergent Fiber, and In Vitro Dry Matter Disappearance of Forages1

Abstract

AbstractTotal nonstructural carbohydrate residue (TNCR) has been considercd “structural carbohydrates” and assumed to be mainly cell wall constituents. Consequently, TNCR should have composition similar to other fiber residues, as neutral detergent fiber (NDF), and could be similarly correlated with dry matter digestibility. Such a relationship would permit the determination of the total non‐structural carbohydrate (TNC) status and the nutritive value of forages in the same analysis. The objective of this study was to determine the relationship between TNCR and NDF and to test the use of each to predict in vitro dry matter disappearance (IVDMD).Forages evaluated were timothy (Phleum pratense L.), tall fescue (Festuca arundinacea Schreb.), alfalfa (Medicago sativa L.), forage sorghum [Sorghum bicolor (L.) Moench] and Coastal bermudagrass [Cynodon dactylon (L.) Pers.]. Samples were analyzed for IVDMD, TNC, TNCR, total N, several fiber fractions, and water soluble carbohydrates (WSCHO).The TNCR was significantly (*, 0.05 and **, 0.01 levels) correlated with NDF in timothy (r = 0.98**), tall fescue (r = 0.98**), and sorghum (r = 0.86*), but not in alfalfa (r = 0.30) or Coastal bermudagrass (r = 0.51). The level of TNCR was similar to that of NDF in timothy, but averaged 7.7 to 15.3 percentage units higher in the other forages. TNCR was more closely correlated with IVDMD for sorghum (r = −0.94** vs. −0.75*) and for bermudagrass (r = −0.79∗ vs. −0.43) than was NDF. The IVDMD was best predicted for four of the five forages and for all five forages (in a combined analyses) when TNCR plus N composed the model compared with NDF + N. Coastal bermudagrass IVDMD was equally well predicted by a model containing TNC + N and not considering either NDF or TNCR. The TNCR was apparently as accurate as NDF for predicting IVDMD of the cool season grasses and possibly more accurate for the warm season grasses.