Differences in nitrogen and urea metabolism between goats bred for fibre production (Angora goat) or meat production (Boer goat)

Abstract

This experiment was conducted to determine whether selection for fleece weight is accompanied by changes in the efficiency of nitrogen utilization, using Angora and Boer goats as models of animals bred for fleece or meat production respectively. A diet, containing a protein: energy ratio of 12 g CP/ MJ ME, was fed at either 77, 88.5, 100, 111.5 or 123% of estimated maintenance energy requirements. Mean N digestibility was 5% lower in Angora goats (P < 0.01), but Angoras retained 33% more N (P < 0.01). N retention increased quadratically (P < 0.01) with level of feed, but there was no interaction between diet and breed (P> 0.05). When expressed on a metabolic weight basis, urea N plasma flux rate was 14% higher in Angoras (P < 0.05), but 18% less urea N was excreted in the urine (P < 0.01). Urea N recycling was 48% higher in Angoras (P < 0.01). Angoras partitioned 62% of flux to recycling and 38% to excretion; in Boer goats proportionately less was recycled (48%) and more was excreted (52%) (P < 0.01). Urea N flux and recycling rates (g N/d) increased (P < 0.05) as the amount of diet fed was increased. It is suggested that higher N retention and urea recycling could be an adaptative mechanism consequent to increasing the demand for amino acids via selection for fleece production.

Hierdie proef is uitgevoer om te bepaal of seleksie vir vagmassa met veranderings in die doeltreffendheid van stikstofbenutting gepaard gaan. Die Angorabok en die Boerbok is gebruik as modelle van diere wat vir vag- of vleisproduksie onderskeidelik geteel is. 'n Dieet met 'n proteien: energie-verhouding van 12 g RP/MJ ME is teen peile van 77, 88.5, 100, 111.5 of 123% van beraamde onderhoudsenergiebehoefte gevoer. Gemiddelde N-verteerbaarheid was 5% laer by die Angora (P < 0.01), maar N-retensie was 33% hoër (P < 0.01). N-retensie het kwadraties verhoog (P < 0.01) met dieetvlak, maar geen interaksie tussen dieet en ras (P > 0.05) is gevind nie. Plasma-ureumomsettempo (uitgedruk op 'n metaboliese massabasis) was 14% hoër by Angoras (P < 0.05), maar 18% minder ureum-N is in die uriene uitgeskei (P < 0.01). Hersirkulering van ureum-N was 48% hoër by die Angora (P < 0.01). By die Angora is 62% van die gesintetiseerde ureum hergesirkuleer en 38% uitgeskei; by die Boerbok is minder hergesirkuleer (48%) en meer uitgeskei (52%) (P < 0.01). Ureum-N-omset- en hersirkuleringstempo's (g N/d) het toegeneem (P < 0.05) namate die dieetvlak toegeneem het. Daar word voorgestel dat hoër N-retensie en ureumhersirkulasie moontlik 'n aanpassingsmeganisme kan wees in reaksie op 'n verhoogde aminosuurbehoefte geassosieer met seleksie vir vagproduksie.

Keywords: Angora, goat, matabolism, nitrogen, nutrition, recycling, retention, ruminant, urea.