Hippuric acid output of sheep fed four tropical grasses.

Abstract

Proc. Aust. Soc. Anim. Prod. Vol. 17 HIPPURIC ACID OUTPUT OF SHEEP FED FOUR TROPICAL GRASSES J.B. LOWRY and E.A. SUMPTER 433 Hippuric acid (benzoylglycine) occurs universally in the urine of mammalian herbivores. However, the dietary precursors have received little attention. Martin (1982) regarded various substituted cinnamic acids as the main source. Direct addition of p-coumaric acid and ferulic acid to the diet caused elevated excretion of hippurate (Lowry and Sumpter 1987). The low N levels in mature tropical grasses, together with the high levels of cell-wall phenolic acids (Ford and Elliot 1987), suggest that these compounds may impose a significant nitrogen loss through the requirement for glycine although they do not appear to adversely affect rumen function (Lowry and Sumpter 1987). This study examines this aspect for four tropical pasture grasses. Mature Merino wethers were supplied with the diet species as the sole diet for 7-21 days and then monitored for a period of 4-5 days with analyses as described by Lowry and Sumpter (1987). The species were spear grass (Heteropogon contortus), Rhodes grass (Chloris gayana), Urochloa (Urochloa mozambicensis) and forage sorghum (Sorghum sp.). Table 1. Nitrogen and phenolic acid content of feeds; and the digestibility data N, nitrogen (g/kg); PA, total phenolic acids (g/kg); n, number of sheep in each experiment; DMI, dry matter intake (g/day); DMD, dry matter disappearance (x); HA, hippuric acid output as g/kg DMI. Output of hippuric acid in terms of feed ingested varied from 6 to 12 g/kg, and was related to the level of phenolic acids naturally present in each species. These results are probably the first direct evidence of such a relationship. Between-animal variation in hippuric acid output was considerable, but can be partly accounted for by variation in digestibility. On the spear grass diet, excretion of N as hippurate (7.8% N) accounted for 16% of the total N supply in the feed. The effect on N requirement could be higher due to metabolic losses and a possible charge-balancing requirement for ammonium cation in the urine. It is clear that cell wall phenolic acids of some mature tropical grasses impose a direct N requirement that can be substantial in relation to available N and may provide more precise means for predicting the need for N supplementation. FORD, C.W. and ELLIOT, R. (1987). J. Agric. Sci. Camb. 108: 201. LOWRY, J.B. and SUMPTER, E.A. (1987). In 'Herbivore Nutrition Research', pp 51-52, editor Mary Rose. (Australian Society of Animal Production). MARTIN, A.K. (1982). Br. J. Nutr. 47: 155-164. CSIRO Division of Tropical Animal Science, Townsville, Qld 4814