Some animal-related factors influencing the cycling of nitrogen in mixed farming systems in sub-Saharan Africa

Abstract

In sub-Saharan Africa the importance of nutrient cycling in mixed farming systems is poorly understood, and there is a paucity of information in the literature. The vital role played by livestock in regulating fluxes through nutrient cycles is often ignored in experimentation and crop modelling although, in farming practice, the animal is an integral part of the system. The aim of this review is to highlight animal-related factors which can influence the dynamics of nitrogen (N) cycling in mixed farming systems on the sub-continent. Marked changes in seasonal feed availability and nutritive value occur which affect both the quantity and quality of excreta. The partition of N excretion between urine and faeces can also fluctuate widely with changes in diet composition, and may be influenced by the degradability of the dietary protein in the rumen. The presence of tannins in feeds has been shown to be negatively correlated with protein degradability and digestibility. Different species and classes of livestock produce excreta with varying nutrient concentrations. By controlling animal movements, adjusting carrying capacity in a grazing situation, or storing and distributing excreta from animals kept in confinement, a farmer can influence the proportions of nutrients returned to the soil. Losses of nutrients in storage and on application can be appreciable. The management of the soil and the returned nutrients will determine the losses incurred after application and the quantity potentially available to the crop. The farmer will also decide on the crops to be grown and their management. Crops differ in their nutrient requirements and the efficiency of utilisation. The partition of nutrients between grain and the vegetative parts of the plant affect the nutritive value of the components. When residues are grazed in situ, consumption will depend on animal numbers and the length of time spent grazing. Animals select preferred plant parts, and appreciable losses due to trampling and soiling may occur. The many factors influencing nutrient cycling provide opportunities for decision-making by the farmer. This process can be helped by mathematical modelling where the effects of different management options can be simulated. In taking account of animal-related factors, more realistic simulation models can be developed for the study of crop production in mixed farming systems in sub-Saharan Africa.