The relationship between DM yield/cutting and the fermentable organic matter (FOM) content of tropical grasses was appropriately investigated to re-assess optimal grass maturity to feed dairy cattle. Nine different grass species belonging to the genera Brachiaria spp. (Mulato II, Ruzi), Panicum spp. (Guinea, Hamil, Mombasa, TD58), and Pennisetum spp. (King, Napier, VA06) were chemically analysed and subjected to an in vitro gas production (IVGP) test. For 72 h, gas production (GP) was continuously recorded with fully automated equipment. A triphasic, nonlinear, regression procedure was applied to analyse GP profiles. Across all the grasses, it was found that the neutral detergent fibre (NDF) contents increased with increasing maturity of the grass while the CP contents decreased with increasing NDF contents. In all nine grasses, digestible organic matter (dOM) was significantly affected by the week of cutting but IVGP was similar between the weeks of cutting in Ruzi, Hamil, Mombasa, and Napier grasses. Except for Guinea grass, the lowest dOM values were found when the grasses were cut after ≥5 weeks of regrowth. Harvesting grass one or two weeks earlier than the normal cutting time is a practically relevant intervention in increasing forage quality and productivity of dOM and fermentation potential.

A study was conducted to estimate the optimal levels of energy, protein, and fiber in the diets of New Zealand White (NZW) growing rabbits fed on fresh green forage available in North Vietnam. Mathematical modelling of nutrient-response curves was applied for the study. A total of 125 male growing rabbits at 6 weeks of age were randomly divided into 25 groups of 5 each to be fed on diets with an array of different levels of metabolizable energy (ME), crude protein (CP), and acid detergent fiber (ADF) by means of varying the ratio between fiber rich grasses, viz. elephant grass (Pennisetum purpureum), Setaria grass (Setaria sphacelata), or Para grass (Brachiaria mutica), and protein rich forages, viz. water spinach (Ipomoea aquatica) vines, sweet potato (Ipomoea batatas) vines, or Trichantera leaves (Trichanthera gigantea), in the basal diets. The average daily gain (ADG) and feed conversion ratio (FCR) were used as key responses of the rabbits to varied levels of ME, CP, and ADF in the diets. The results showed that the levels of ME, CP, and ADF in the diets strongly affected the ADG and FCR of the rabbits following curvilinear patterns with the highest ADG and lowest FCR when the diet contained 2106-2162 Kcal ME kg-1 DM with 16.5-16.8% CP and 21.9-22.4% ADF. Therefore, these levels of nutrients were considered to be optimal in the diets of NZW growing rabbits fed on local forages.