Relationships Between and Within Physical and Chemical Constituents and in Vitro True Digestibility in Tropical Forage Grasses

Abstract

All parameters of chemical composition and in vitro true digestibility (IVTD) were related by simple and/or multiple correlation and/or regression coefficients in 60 samples of forage grasses representing 10 forages at 6 stages of growth. Dry matter, neutral-detergent fiber (NDF), acid-detergent fiber (ADF), lignin (L), cellulose (C), silica (Si), crude fiber (CF), and nitrogen-free extract (NFE) were positively correlated while crude protein (CP), hemicellulose (H), ether extract (EE), and ash (A) were negatively correlated with age. Crude protein was negatively correlated with all parameters except H, EE, and A. Hemicellulose was nore significantly related to the Goering and Van Soest fractions than to the A.O.A.C. fractions. Silica was correlated in a highly significant way with age, DM, ADF, and H only. In vitro true digestibility was negatively correlated with age, NDF, ADF, L, C, Si, L/ADF, L/H, L/C, CF, and NFE and positively correlated with CP, H, H/C, EE, and A. Multiple regression analyses indicated that, of the total variance in IVTD, 85 percent was attributable to the influences of NDF (48%), L (19%), Si (10%), EE (2%), CP (4%), and ADF (2%). From this regression and from other regressions, it was established that CP and ADF, or L, C, and Si, exerted the greatest influence upon IVTD. In line with this, it was determined that 81 percent was attributable to the influences of C (41%), H (1%), L (30%), and Si (9%) and that 85 percent was attributable to the influences of CP (69%), C (3%), L (7%), and Si (6%). Finally, it was determined that CP and lignification explained 69 and 7 percent, respectively, of the influences in IVTD. These data suggested that IVTD was highly dependent not only upon CP and lignification but also upon C and Si, or ADF, as forage grasses advanced in maturity from 30 to 180 days of growth. The equation: % IVTD = 96.85 + 0.62 (% CP) - 0.51 (% C) - 2.59 (% L) - 2.34 (% Si), best explained and estimated IVTD from chemical composition. It may be concluded that various chemical components of the grasses, namely, CP, C, L, and Si, may be utilized to estimate IVTD, without sacrificing accuracy and thus reducing time otherwise required and the cost otherwise incurred.