Sulfur amino acid metabolism and protein synthesis in young sheep fed ryegrass pasture and two Lotus cultivars containing condensed tannin

Abstract

Groups of sheep were fed either fresh ryegrass pasture, a low condensed tannin (CT) diet (Lotus corniculatus; or a medium CT diet (L. pedunculatus) to compare the effects of these feeds on methionine and cysteine metabolism, because of their significance in wool production, and on phenylalanine as an indicator of general protein metabolism. Dry matter intake (DMI) among groups, ryegrass pasture, Lotus corniculatus, L. pedunculatus, were significantly different, therefore the treatment means for metabolic measurements and wool growth were adjusted by covariate analysis using DMI. Neither diet nor DM1 had any overall effect on phenylalanine concentrations in whole blood or phenylalanine irrevirible loss rate (IRL). However there were significant (P < 0.001) diet effects on cysteine blood concentrations. Methionine effects paralleled those of cysteine, with higher concentrations in sheep fed L. pedunculatus relative to pasture fed animals. Sheep fed L. pedunculatus had much higher cysteine and methionine ILRs compared with either the pasture or L. corniculatus groups. Sheep fed the highest level of condensed tannin in their diet (l. pedunculatus) had significantly greater rates of transsulfuration, cysteine and methionine oxidation. Overall the results show that CT in L. pedunculatus gives a greater degree of 'protection' during digestion for cysteine, methionine and phenylalanine, while L. corniculatus resulted in very little advantage. Condensed tannin effects on wool production are marginal and secondary to whole body metabolic influences.