Sewage-grown algae as a source of supplementary nitrogen for ruminants

SUMMARY A sheep-feeding experiment was carried out to assess the nutritional value of algae meal as a protein supplement in ruminant diets. The algae, mainlyChlorella, were grown in sewage ponds, harvested by flocculation with alumina (Al2(SO4)3.18H2O), and then drum-dried. Eight young rams, divided into two equal groups, were offered a basal concentrated diet, to which algae meal or soya-bean meal (SBM) was added to contribute 50% of the dietary nitrogen (N). The digestibility values of dry matter (D.M.) were 69·3 and 79·3%, and those of organic matter (OM) were 75·3 and 82·2% for the algae diet and the SBM diet, respectively. Nitrogen digestibility of the algae diet was 16% lower than that of the SBM diet and the calculated value of N digestibility in algae meal was 61·7%. However, there was no difference between treatments in the daily amount of retained-N or in the proportion of digested N which was retained in the body. The algae meal contained 5·7% aluminium. This was probably the reason for the much lower P absorption in the algae diet (6·67%) as compared with the SBM diet (29·5%). Ammonia-N concentration was lower and total volatile fatty acids (VFA) concentration was higher in the rumen of the algae diet-fed sheep. However, the VFA profile was similar for both diets. Sewage-grown algae could perhaps be used as a protein supplement for ruminants, provided that the harvesting technology is directed to produce a low-mineral, low-aluminium, young biomass, which would be highly digestible and would not interfere with P absorption.