Submitted to: Journal of Animal Science
Publication Type: Abstract Only
Publication Acceptance Date: 7/1/2002
Publication Date: 7/1/2002
Citation: Coleman, S.W., Moore, J.E. 2002. Variability in relationships among forage intake, digestibility, NFD, and ADF [abstract]. Journal of Animal Science.85(1):94. Interpretive Summary:
Technical Abstract: Confusion exists about forage quality and in methods to measure or predict forage quality. The conventional wisdom is that there are close relationships between voluntary forage dry matter intake (DMI) and digestible dry matter (DDM) concentration, DMI and NDF, and DDM and ADF. Correlation coefficients were obtained from several publications on grasses and legumes, and from a test database of 75 grass hays fed to cattle for voluntary intake measurement and to cattle or sheep for digestibility. Published correlations (r) between DMI and DDM range from -0.32 to 0.86. In the test database, the correlation between DMI and DDM was 0.75. The conventional wisdom is that intake of NDF is a constant 1.2% of BW, a concept is based on work with mixed diets balanced for high producing dairy cows. Several publications, however, have not confirmed this concept, particularly for forages fed alone. In the test database, NDF intake ranged from 0.4 to 2.4% of BW. When DMI was estimated from the equation: DMI (% BW) = 120/NDF, DMI was underestimated in most cases except for protein-deficient native grasses where intake was overestimated. Many forage testing programs use simple regression equations to predict DMI from NDF and DDM from ADF. In the early research that serves as the basis for some of these programs, Van Soest (J. Anim. Sci. 24: 834,1965) reported r = -0.65 between DMI and NDF, and r = -0.74 between DDM and ADF (n=83, grasses and legumes), similar to that found in the test database (-0.55 and -0.71, respectively). Published r values have ranged from 0.03 to -0.90 between DMI and NDF, and from -0.39 to -0.93 between DDM and ADF. We conclude 1) that voluntary intake and digestibility are related but often independent components of forage quality; 2) that when grasses and legumes are fed alone for measurement of forage quality, NDF intake will not be constant across all forages; and 3) that conventional wisdom about the relationships between DMI and NDF, and DDM and ADF cannot be justified. Routine forage testing programs using only NDF and ADF may often provide unacceptable estimates of DMI and DDM, for both grasses and legumes.