The use of Dichrostachys cinerea pods
Studies were conducted to investigate the potential of Dichrostachys cinerea pods as protein supplements. Bulk D. cinerea, Acacia erioloba, A. erubiscens, Piliostigma thonningii and A. nilotica pods harvested in August 2000 were used for chemical analyses and an in vitro tannin bioassay. Chemical analyses were carried out to estimate the level of N, tannins and fibre in the pods. The pods were then evaluated using the in vitro gas production technique as a tannin bioassay, by incubating pods, with or without a tannin-binding agent, polyethylene glycol (PEG). Increase in gas production upon adding PEG was used as a measure of tannin anti-nutrient activity.
Dichrostachys cinerea pods gave the highest response to tannin inactivation (over 200% increase in cumulative gas production), followed by A. nilotica. Dichrostachys cinerea and A. nilotica pods were selected for further evaluation of tannin inactivation methods in vivo, however, due to limited resources only one species could be evaluated at a time. This paper reports the in vivo evaluation of D. cinerea. Alkaline and PEG treatments were evaluated in sacco and in a nitrogen balance trial with goats offered D. cinerea pods. Milled (2 mm screen size) treated and untreated pods were incubated in nylon bags in the rumens of goats fed on a grass hay diet supplemented with mixed pods. Treated pods had significantly higher soluble fractions, rates of degradation and effective degradabilities of nitrogen (outflow rate 0.05) compared to the untreated pods. Treated pods were used as supplements for goats fed on a basal diet of grass hay. The supplements were compared to a commercial goat feed (goat meal) and a control treatment where goats were unsupplemented. Supplementation significantly increased both the intake of hay and N retention. The control group were in negative N balance. Supplementing with PEG treated pods significantly increased hay intake compared to the untreated pods, while there was no difference between the alkaline treated pods and the untreated ones. However, untreated pods gave a higher N retention value compared to the treated pods and this value was the same as that with goat meal. This suggests that there is no need to ameliorate the tannins in D. cinerea pods before feeding, as these are beneficial to the protein nutrition of the animal. PEG treatment may have resulted in excessive protein degradation in the rumen and increased N loss through the urine.
The use of wood ash
Dry and mature tree fruits from Acacia and other tree species are potential protein sources for goats in semiarid areas of Zimbabwe . However, the presence of high levels of phenolics limits their utilization. Tannins are known antinutritional protein binding secondary plant compounds, which reduce availability of dietary protein. To improve the utilization of tanniniferous tree fruits, tannin-inactivating treatments are required. The use of polyethylene glycol is well documented but not affordable for subsistence farmers. In addition the effect of PEG-tannin complexes on the soil is largely unknown, hence caution is required where PEG is used on a large scale. Work on high-tannin sorghum has provided evidence that alkali treatments can be used to inactivate tannins. Sources of alkali include expensive, and relatively dangerous, sodium hydroxide and ammonium hydroxide as well as inexpensive wood ash and sodic soils. Wood ash is noncorrosive and subsistence farmers have experience in handling it as most of their fuel is provided by firewood. The use of wood ash solution to treat tree fruits can provide a cheap and safe way of improving their utilization as protein supplements during the dry season.
Wood ash effectively reduced the detrimental effect of tannins on in vitro fermentation. However, wood ash is less effective when compared with NaOH presumably because of the lower caustic properties of wood ash. Higher concentrations of wood ash may be required for improved effectiveness. There is potential for wood ash to be used by farmers to detannify tree fruits before feeding goats in order to improve utilization and protect animals from long-term effects of phenolic compounds. There is a strong possibility that alkali treatment enhances oxidation of low molecular weight phenolics which may not be bound by PEG but are still toxic to rumen microbes and impair post-rumen digestion and absorption as well as nutrient metabolism.
Fruits of A. nilotica and A. erioloba may require treatments that inactivate low molecular weight phenolics when being fed in large quantities or as a sole supplement. Alkaline treatments are potential candidates for this role.
Fruits from D. cinerea and P. thonningii contain appreciable quantities of condensed tannins, which may not be metabolised in the rumen and may thus require treatments.
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