ABSTRACT
Summary Seventy-six grass hays, harvested at 2, 4, 6, and 8 weeks of regrowth, were fed ad libitum to wethers in a digestibility and intake trial. Range of organic-matter digestibility (OMD) was 54.1 % to 72.3% and that of organic-matter intake (OMI) 17.3 to 27.4 g/kg. Lignin was the chemical component most highly related to OMD (r = - 0.72). Neutral detergent fiber (NDF) was better than acid detergent fiber (ADF) as a predictor of OMD (r = - 0.66 vs. - 0.45). In-vitro NDF digestibility (IVNDFD) was superior to in-vitro organic-matter digestibility (IVOMD) and all other analyses as a predictor of OMD (r = 0.84). OMI was best predicted by ADF (r = - 0.54). Lignin and p-coumaric acid were about equally well correlated with NDF digestibility, although p-coumaric acid represented an average of only 0.6% of NDF. Ferulic acid was not well related to any in-vivo measures of forage quality. In-vivo NDFD was more closely related to the amount of potentially digestible NDF than to NDFD predicted from estimated rates of passage and digestion (r = 0.89 vs. 0.75). We conclude from these studies that (1) the relationship of fiber fractions to forage quality in these tropical grasses differs from the conventionally accepted relationship; (2) the influence of rate of passage on variation in digestibility of tropical grass hays, fed ad libitum, may be marginal; (3) in-vitro techniques remain the most accurate method for estimating digestibility; and (4) further investigation into the nature of plant phenols and their relationship to forage quality is merited.
