1. Forage yield from cassava grown as a perennial crop fertilised with effluent from biodigesters loaded with pig or cow manure and the effect on soil fertility.

Authors: Khieu Borin, Frankow-Lindberg, B.E., 2006.
Biological Agriculture and Horticulture 2006, 24: pp 91-104

Summary: The experiment was established on 440 m2, of which 270 m2 were used for experimental plots and 170 m2 served as borders between plots and as paths for access. The soil is a sandy loam classified as Sretayen Kampong Speu Province (National Soil Classification) and consisting of clay (19.6%), fine silt (12.5%), coarse silt (8.2%), fine sand (28.2%) and coarse sand (28.1%). The soil was of moderate fertility with a pH of 6.1-6.9. A cassava variety originally developed in Thailand was used for the experiment. The variety is presently extensively cultivated in the Northeast provinces of Cambodia because of its high tuber yields. Middle parts of cassava stems (20-25 cm long) were planted in a slanted position, two per drill, with a distance between drills of 50 cm. A total of 84 stems were planted in each experimental plot of 30 m2. The experiment was irrigated once daily during establishment (<30 cm height), whereafter the frequency of irrigation was regulated according to rainfall. Weeding was done frequently by hand during the establishment period (three months) and less frequently thereafter. A total of 350 kg N ha-1 year-1 (or 525 kg N ha-1 over the whole experimental period) was applied in weekly dressings of effluent from biodigesters loaded with either pig (PM) or cow (CM) manure. A control treatment (C) received no N. The effluent was pumped into cement tanks and sampled after each filling into the cement tank for the analysis of NH3, total N and pH. The first harvest was done when the cassava had a height of about 150 cm (three months after planting) when all plant material 60 cm above-ground was cut. Thereafter, the plants were harvested every 60 days. At each harvest, the plant material was manually partitioned into leaves and stems plus petioles, which were dried and weighed separately for the determination of the leaf proportion. Soil fertility was assessed three times during 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.

Abstract: 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 6.2-9.1% 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 of some elements in all plots during the experimental period.