The potential of pods from tree legumes as supplement to low quality roughages for ruminants.
The goal of the study was to examine the use of pods from tree legumes as supplements to poor quality roughage-based diets. Trials were carried out to address issues related to the nutrient content of the pods and their limitations as supplements due to the presence of anti-nutritional factors. In the first trial (Chapter 3), the chemical, mineral and amino acid compositions of pods from six tree species were examined. The rumen degradation of the dry matter, nitrogen and cell wall constituents of the pods were evaluated, using the nylon bag technique. In trial 2 (Chapter 4), different rumen ecologies were created in fistulated sheep by feeding pasture hay in combination with different pod meals and alfalfa (50:50), in order to examine the effects of anti-nutritional factors (present in the pods) on the degradation of dry matter and fibre constituents by ruminal microorganisms. Trial 3 (Chapter 5) further examined the effects of anti-nutritional factors on the production of volatile fatty acids (VFAs) and on the activities of microbial enzymes in the rumen. In trial 4 (Chapter 6), ensilage was examined as a means of detoxification of cyanogenic glycosides in the pods of Acacia sieberiana and molasses as well urea were evaluated as preservatives that could improve the aerobic stability of the silage produced from the pods of A. sieberiana. In trial 5 (Chapter 7), the silage and four other feeds (3 pod meals and alfalfa) were used in a choice feeding trial to study the effect of anti-nutritional factors on the palatability and intake of the feeds by goats and sheep, using hay as the standard feed for comparison. Trial 6 (Chapter 8), evaluated the use of the silage with or without wheat bran as supplements to a roughage basal diet fed to lambs. Intake, digestibility of dry matter, organic matter, cell wall constituents, nitrogen retention and weight gain were considered as indices for examining the potential of the supplements. Chapter 9 presents a general discussion on the attributes and limitations of pods as supplements. It also leads to a conclusion on the importance of browse in tropics and raises the need for further research on this class of feeds. The results of the work show that pods from tree legumes are rich in nitrogen and minerals and may provide sufficient ammonia (71-85 mg/l of rumen fluid) in the rumen that could enhance the growth and/or activity of rumen microorganisms. The maintenance of rumen pH by the pod meals at a range of 6.2-6.4 gives an additional advantage over other supplements that contain high concentrations of soluble carbohydrates because cellulolytic activity by rumen microbes is said to be optimal around this pH range. The importance of pods as supplements was however, reduced by the presence of anti-nutritional factors (especially condensed tannins) which had a very high concentration (28%QE) in the pods of Acacia sieberiana. The results of the feeding trial showed a positive correlation with the intake of the basal diet and weight gain when pods meals were included in the diet at moderate levels (30% of total dry matter intake). This is an indication that the pod diets were able to maintain a conducive rumen environment for microbial activity and at the same time, provided by-pass protein through the formation of protein-tannin complexes which leave the rumen at near neutral pH but in the abomasum (pH 3-4), the protein was liberated and digested by gastric enzymes to provide amino acids that were utilized by the host animal. However, when the pod meal was increased to 50% of total intake (Chapters 4 and 5), the concentration of condensed tannins in the diet depressed the degradation of cell wall constituents, production of VFAs and reduction in the activity of fibrolytic enzymes in the rumen. Ensilage was found to be effective in reducing the concentration of cyanogenic glycosides by 80% when the pods of A. sieberiana were ensiled for a period of 35 days. The resulting silage was relished by both goats and sheep in a choice feeding trial, an indication that besides the benefit of reducing this important anti-nutritional factor, ensilage enhanced the intake and palatability of these pods. Molasses and urea improved the fermentation process, the nutritive quality and the aerobic stability of the silage. The important conclusions drawn from the results of this work are that, at low concentration, tannins are beneficial to ruminants by protecting plant proteins from excessive degradation in the rumen thus preventing bloat and increasing the quantity of dietary protein reaching the lower gastro-intestinal (GI) tract. However, at higher concentrations, the effect of condensed tannins is rather detrimental, both to the ruminal microorganisms and the host animal, mainly through their binding effect with proteins and structural carbohydrates and precipitation of both microbial and gastric enzymes, with the net effect of reducing the digestibility of the roughage-based diet. Further research is necessary to better quantify the concentration of anti-nutritional factors in this class of feeds in order to optimize their utilization by rumen microorganisms and host animal, taking into account the fact that the concentration varies according to the plant species, stage of maturity of the plant and the environment in which the plant is found. It should also be born in mind that the biological effects of different tannins depend on the characteristics of the specific tannin (condensed or hydrolysable), the animal species and possibly the nutritional status of the animal, particularly whether the animal is tannin-naive or tannin-adapted.