Fermentation characteristics of tropical legumes and grasses of good and poor nutritional quality and the degradability of their neutral detergent fibre

Abstract

Context Tropical legumes are commonly assumed to share all positive attributes known from temperate legumes such as lucerne. However, increasing evidence exists on the differences between those forages, particularly in terms of their ruminal degradability. Aims Exploring the 24-h rumen in vitro-fermentation characteristics of tropical legumes, their direct comparison with lucerne, and their interactions with grasses depending on their nutritional quality. Methods Arachis and stylosanthes (tropical legumes), pennisetum and andropogon (tropical grasses), and lucerne (lucerne_21 and lucerne_35, harvested 21 and 35 days after emergence respectively) were used for the study. On the basis of the nitrogen and neutral detergent fibre (NDF) concentration, arachis and pennisetum were classified as of good quality, while stylosanthes and andropogon as of poor quality. The following four incubation series were performed: first, forages alone were incubated under iso-nitrogenous conditions; second, forages were incubated under iso-nitrogenous conditions with supplemented starch; third, NDF extract of each forage was incubated alone; fourth, NDF extract of tropical grasses and legumes was incubated combined in grass : legume proportions of 33 : 67 and 67 : 33. Key results When incubated alone, gas production (GP) and total short chain fatty acids were higher for temperate legumes, intermediate for tropical legumes and lowest for tropical grasses. Similar trends were observed for GP when the forages were incubated with starch, but the differences between arachis and lucerne_35 disappeared; short chain fatty acids did not differ among all tropical forages. Moreover, acetate : propionate ratio was highest for tropical legumes, intermediate for temperate legumes, and lowest for tropical grasses. Gas production of NDF extracts was highest for the lucerne samples and lowest for the tropical legumes. Improvements in GP were found when the NDF from the poor-quality grass (andropogon) was combined with the legumes, particularly the good-quality legume (arachis). Conclusions On the basis of the gas production, tropical legumes appear to have lower degradability than do temperate ones, while also showing a different fermentation pattern. Fibre of tropical legumes is less degradable than that of tropical grasses, but when combining both fibre sources, there seems to be a synergistic effect on degradability. Implications The current results give important insights on the fermentation characteristics of tropical legumes, helping better understand their role in ruminants’ nutrition, while giving inputs towards improving their utilisation.