Sulfur and methionine metabolism in sheep. III.* Excretion and retention of dietary and supplemented sulfur, and production responses to intraruminal infusions of DL-methionine

Abstract

Two experiments were carried out to study the excretion and retention of nitrogen and sulfur when intraruminal infusions of DL-methionine were given to sheep. Merino wethers were fed on a 50:50 oaten chaff: lucerne chaff ration at different levels of dry matter intake (DMI), either as chaff or ground and pelleted, and with or without intraruminal infusions of DL-methionine. Liveweight gain in experiment 1, and the rate of wool growth and sulfur incorporation into wool were also measured. [35S]L-methionine was used to trace sulfur excretion from the supplement. Increasing the DMI increased (P < 0.001) nitrogen and sulfur excretion in both faeces and urine, but also increased (P < 0.001) the apparent retention of nitrogen and sulfur. The excretion of nitrogen and sulfur was not significantly affected by the form in which the basal ration was supplied, but sulfur retention did improve (P < 0.05) when the ration was given as chaff. DL-methionine supplements resulted in large increases (P < 0.001) in the excretion of total and inorganic sulfate sulfur in urine, and there was a tendency for urinary neutral sulfur excretion to increase. The amino acid supplement did not affect faecal sulfur excretion except at the lower level of feed intake in experiment 2. Differences in the amount of sulfur, from DL-methionine and [35S]L- methionine injections given into the rumen, excreted in faeces and urine indicate possible differences in the metabolism of the amino acid isomers. Increasing DMI resulted in improved (P < 0.001) rates of liveweight gain and wool growth; DL-methionine infusions had no effect. Further, estimates of sulfur incorporation into wool indicate that very little supplementary methionine was utilized at the follicular sites despite increased sulfur retention. The patterns of sulfur excretion suggest that the added methionine was rapidly and virtually completely dissimilated in the rumen. It is concluded that production responses to dietary methionine supplementation occur only under sulfur-deficient conditions or occasionally where degradation is avoided. _____________________ *Part II, Aust. J. Biol. Sci., 32: 65 (1979).