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

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.

Dairy cows fed on tropical legume forages: effects on milk yield, nutrients use efficiency and profitability

Two trials with multiparous dairy cows were conducted. Experiment 1 tested the effects of increasing forage proportion in the diet (500, 600, and 700 g/kg DM) when a mixed sorghum (Sorghum bicolor) and jackbean (Cannavalia ensiformis) silage was used as forage. Experiment 2 studied the substitution of sorghum silage and soybean meal by jackbean silage or fresh cowpea (Vigna unguiculata) forage in the diet. All diets were iso-energetic and iso-proteic. In each experiment, 30 cows were used and separated into three groups. In experiment 1, there were no differences in dry matter intake (DMI), milk yield (MY), or apparent total tract digestibility (aTTd) among the three diets, but milk fat content increased with increasing forage proportion, even though the similar neutral detergent fiber of all diets. Nitrogen use efficiency was highest in the diet containing 600 g forage/kg DM, and some evidence was observed for a better profitability with this forage proportion. In experiment 2, feeding legumes increased DMI despite no effects on aTTd. Milk yield increased in line with DMI, with a larger increase for the fresh cowpea. Nitrogen use efficiency and milk composition were not affected by the diets. The increased MY and lower feed costs increased the economic benefits when feeding legumes, particularly when feeding fresh cowpea. Feeding fresh cowpea or jackbean silage to dairy cows appears to be an alternative to soybean as protein source, ideally at a forage proportions of 600 g/kg DM, without altering milk yield and quality and increasing the farm profitability.

Chemical composition of rumen microbial fraction and fermentation parameters as affected by tannins and saponins using an in vitro rumen fermentation system

Castro-Montoya, J. M., Makkar, H. P. S. and Becker, K. 2011. Chemical composition of rumen microbial fraction and fermentation parameters as affected by tannins and saponins using an in vitro rumen fermentation system. Can. J. Anim. Sci. 91: 433–448. Post-rumen chemical composition of the microbial fraction is one of the factors that determines the nutrients absorbed and available for maintenance and production of the animal. The hypothesis was that tannins and saponins alter chemical composition of rumen microbes and fermentation parameters in the rumen. Purified quebracho, mimosa, chestnut and sumach tannins; and quillaja and gypsophilla saponins were incubated with 380 mg of substrate (hay:concentrate 70:30 wt/wt) for 24 h in an in vitro gas production system at concentrations from 0.25 to 1.25 mg mL−1. Saponins increased N and reduced sugar contents of the liquid-associated microbes. The ratio of crude protein to purine bases significantly increased on adding sumach and chestnut tannins and decreased on the addition of quebracho and mimosa tannins. Quebracho, mimosa and chestnut tannins reduced total short-chain fatty acid production. The acetate:propionate ratio decreased for all additives. Results suggest that in vitro (a) depending on the source and the concentration, tannins would have an effect on the nitrogen and sugar contents of the liquid associated microbes, (b) saponins are likely to increase N and reduce sugar contents of rumen liquid associated microbes, and (c) estimation of microbial protein synthesis based on purine bases may lead to under- or over-estimations in the presence of tannins and saponins. In vivo studies are required to validate these results.