Effects of dietary crude protein level on the digestibility of nutrients, excretion of fecal N and urinary N-fractions and the kinetics of 15N labeled urea in growing male goat kids

A feeding trial was conducted with 16 male Saanen kids (4 kids/treatment) weighing 19.0 ± 1.8 kg in the middle of which they were subject to a metabolism trial and kept in cages for 14 days for a quantitative 10 days collection of excreta. Kids were fed a constant amount of the four diets formulated out of molassessed wheat straw [55 g crude protein (cp) per kg dry matter (dm)] and pelleted concentrates having 87, 117, 144 and 176 g cp/kg dm. They were injected about 100 mg of 15N-labeled urea into the jugular vein at the beginning of the collection period. Nutrient digestibility, fecal N and urinary N-fractions, N and 15N balances were measured; irreversible loss (il) of urea from the body urea pool (bup) and the kinetics of N in the body calculated. Increasing dietary cp increased digestibility of organic matter (om) from 71% to 80% and of crude fiber (cf) from 28% to 58%; excretion of fecal N by 56%; of urea N by 93%; proportion of N of urea, allantoin, creatinine, uric acid plus hypoxanthine in urinary N from 31% to 66%, 12% to 22%, 3% to 12%, and by 1.7%, respectively; irreversible iron (il) of N from 1.98 to 12.24 g/d; transfer of urea to gastro-intestinal tract (git) from 1.58 to 6.69 g/d (i.e., equivalent to 80% to 55% of il); recycling of the degraded urea N to metabolic pool from 1.22 to 6.12 g/d; but it has decreased the proportion of retained N of bup origin from 38 to 30% and excretion of urinary non-urea N from source other than bup was constant. Excretion of urea is the mechanism by which goat kids adapt to variable supply of dietary protein. Excretion of fecal N was more closely related to dm intake, while that of urea N to dietary cp level and of purine derivatives (pd) to digestible om intake. SINET: Ethiopian Journal of Science Volume 24, No. 2 (December 2001), pp. 265-282 Key words/phrases: Goat kids, 15N kinetics, N-metabolism, protein, ure