Fodder tree and straw diets for sheep: intake, growth, digestibility and the effects of phenolics on nitrogen utilisation

Abstract

Intake, growth, digestibility and nitrogen (N) utilisation by sheep were investigated for seven experimental diets which consisted of teff straw ( Eragrostis abyssinica) fed ad libitum with phyllodes of Acacia cyanophylla (Diet 10, fruits of Acacia sieberiana (Diet 2), leaves of Acacia seyal (Diet 3) and Sesbania sesban (Diet 4), vetch hay ( Vicia dasycarpa) (Diet 5) and noug cake ( Guizotia abyssinica) (Diet 6). Diet 7 included urea to provide an intake of 1% N. A. cyanophylla had a high fibre and lignin content due to a high level of proanthocyanidins (condensed tannins). The three acacias had the highest content of soluble phenolics and insoluble proanthocyanidins. S. sesban had the lowest content of fibre and polyphenols. Diets containing acacias had the highest levels of feed refusals and the lowest intake of teff straw. However, the intake of A. seyal increased over time. Sheep fed S. sesban had the highest intake of teff straw. The growth rate was highest for Diets 4 and 5, reflecting high intakes, while sheep eating Diet 1 lost weight. Growth rates for sheep fed A. seyal were similar to those of sheep fed S. sesban and vetch hay during the last 7 weeks of the growth trial after intake had increased. Digestibility of fibre was lowest for diets containing acacias. Proanthocyanidins in A. cyanophylla may have complexed protein in the digestive tract causing negative digestion coefficients for lignin and N insoluble in neutral detergent. The content of faecal N that was soluble in neutral detergent was highest for sheep fed A. seyal, indicating higher excretion of microbial and endogenous N in the faeces. Urinary N loss was lowest and faecal N loss was highest for diets containing acacias. N balance was positive for sheep fed all diets except A. cyanophylla. The N fraction in diets containing acacias was not nutritionally uniform because of the influence of polyphenols on protein digestion and metabolism.