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

Abstract

A feeding trial was conducted with 16 male Saanen'kids (4 kids/treatment) weighing 19.0 ± 1.8 kg in the mi!idle of which they were
subject to a metabolism trial and kept in cages for 14 days. £ona.
quantitative 10 days collection of eX'~Teta. Kids were fed a constant amount. of the four diets formulated out of molassesseq wheat straw [55 g crude protein (CP) per kg dry matter (OM)] and pelleted concentrptes having 87, 117, 144 and 176 g cp/kg OM. They were injected about 100 rr,tg of 15N_' labeled urea into the jugular vein at the beginning of the collection period. Nutrient digestibility, fecal N and urinaryN-fractions, Nand ISN balances were measured; irreversible loss (IL) of urea from the body urea pool (BUI') and the kinetics of N in the body calculated. Increasing dietary CI' 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 Nby 93%; proportion of N of urea, allantoin, creatinine, uric add' 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 gld (i.e;, equivalent to 80% to 55% of IL); recycling of the degraded urea l':-l 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 nonurea 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 OM intake, while that of urea N to dietary CP level and of purine derivatives (PO) to digestible OM intake.