Estimating the voluntary intake by sheep of tropical grasses from digestibility, regrowth-age and leaf content: a meta-analysis

Abstract

Context The voluntary intake (VI) of forages by ruminants is usually estimated from diet DM digestibility (DMD), but may be related also to the age of regrowth and the leaf blade content (Leaf) of the forage. Aim To examine the reliability of the prediction of the VI of tropical grasses by sheep from the DMD, Leaf and Regrowth-age characteristics of the forage. Methods Data from eight experiments with mature sheep fed tropical grass hay diets (n = 229) were used to explore prediction of VI of DM (VIDM), digestible DM (DDM) (VIDDM) and estimated metabolisable energy from the DMD, Leaf and Regrowth-age of the forage. Key results The variables were generally correlated. In data pooled across experiments the VI (g/kg W0.75.day) of DM was poorly correlated with DMD, Leaf or Regrowth-age (r = 0.30–0.52). The regressions between VI and each of the variables differed among experiments in elevation (P < 0.001) but generally not in slope. When ‘experiment’ was included as a factor the VIDM (g/kg W0.75.day) = K + 0.0912 × DMD (R2 0.80; r.s.d. 6.8; K range –26.0 to +7.8; P < 0.001). Also VIDM = K + 0.069 × DMD + 0.020 × Leaf (R2 0.88; r.s.d. 5.4; DMD and Leaf, P < 0.001); thus inclusion of Leaf reduced the r.s.d. while K ranged widely (–20.5 to +12.0). The voluntary intake of digestible DM (VIDDM) = K + 0.081 × DMD + 0.011 × Leaf (R2 0.89; r.s.d. 3.2; DMD and Leaf P < 0.001; K range –35.0 to –16.3). Regrowth-age was correlated with both Leaf and DMD, and VIDM was predicted by Regrowth-age or Leaf with comparable error. Because numerous factors alter the composition of grasses at a specific Regrowth-age the DMD should be a more generally suitable variable to predict intakes of forage DM and DDM. Conclusions The estimation of the VI of ruminants ingesting tropical grass forages can be improved if the diet Leaf is included with diet DMD as a predictor. However, the general prediction of VI of sheep may involve large errors. Implications Knowledge of the leaf content as well as the digestibility should improve estimation of VI of tropical grasses by ruminants.