The effects of effluent from biodigesters loaded with pig or cow manures on soil fertility and forage yield of cassava grown as a perennial crop

Forage production of a cassava variety originating from Thailand was investigated using effluent from biodigesters charged with either cow (CM) or pig manure (PM), which provided 525 kg N ha(-1) 18 months(-1). The first harvest was done 90 days after planting and consecutive harvests (all material above 60 cm height) were taken every 60 days thereafter. At each harvest, leaves and stems plus petioles were weighed and recorded separately, and samples were analysed for dry matter and nitrogen. Soil fertility was assessed three times during the experimental period by a 'biological test' with maize grown in soil samples from the cassava plots. Soil samples were analysed for chemical composition at the beginning and the end of the experiment. The highest total forage DM and crude protein (CP) yields and leaf DM and CP yields, respectively, were obtained in PM plots. The leaf proportion of the harvested crop was around 50% and no treatment effect was observed. The CP content ranged from 19.9-26.4% and 6.29.1% in the leaves and stems plus petioles, respectively, and a significant harvest occasion effect on CP content on leaves was recorded. Both PM and CM treatments had a significant positive effect on maize biomass production at the final assessment. The total soil N removal by the forage crop differed significantly between treatments and the N uptake was 67 and 27% above the N inputs in treatments PM and CM, respectively. There was a decline in the concentration of some elements in all plots during the experimental period.