The effects of energy and crude protein (CP) supplementation on diurnal and seasonal variations in rumen pH and concentrations of ammonia (NH3) in lactating ewes grazing native pasture (veld) were studied during two autumn (1981 and 1982) and two spring (1983 and 1984) lambing seasons. Different levels of energy and / or CP were provided daily via rumen fistulae to the ewes. Considerable diurnal and seasonal variation in rumen pH was observed between and within the winter (1981 and 1982) and summer (1983 and 1984) seasons. This was ascribed to seasonal differences in grazing pattern, herbage intake and ruminal activity of grazing sheep. In summer, ruminal pH was lower and increased between 08h00 and 12h00,while in winter it decreased during the same period. Prevailing ruminal pH of Dorper ewes was always lower than for Merino ewes. Supplementary CP and CP plus energy influenced ruminal pH. The small effect of supplementary energy on ruminal pH indicated that much of the maize starch was apparently digested postruminally. Concentration of rumen NH3 was higher in summer, but with no definite pattern of diurnal variation discernible within seasons. Prevailing concentrations of ruminal NH3 of Dorper ewes were lower during winter than those of Merinos. During winter (1981 and 1982), concentration of rumen NH3 was markedly increased by supplementary CP but not affected by energy, while CP plus energy resulted in intermediate NH3 levels. In summer (1983), rumen NH3 concentration was increased by CP and decreased by energy, while CP plus energy resulted in intermediate rumen NH3 levels. During the second trial in summer (1984), supplementary energy did not appear to affect rumen NH3 concentration. Diurnal and seasonal variation in ruminal pH and NH3 levels are discussed in relation to feed intake and animal performance.
