S U M M A R YExtensive grazing experiments were conducted over four summer breeding seasons in north-west Queensland between 1984 and 1989. Ewes in the last third of pregnancy were grazed on Mitchell grass {Astrebla spp.) pastures of varying forb (herbaceous plant other than a grass) content. Pastures of low forb content (F~) were attained by stocking paddocks heavily with wethers prior to the experiment, or by selecting paddocks which already contained pastures of low forb content. Pastures of high forb content (F + ) were kept without animals prior to the experimental period. Pastures comprised a range of forb biomass (29-828 kg DM/ha) and percentage forbs in total biomass (3-49%). Lamb marking percentage was found to be dependent on the biomass of forbs and proportion of forbs in the pasture, in two experiments. Ewes which grazed F + pastures had lower lamb marking percentages than those grazing F" pastures.Twenty pasture species collected during the experiments were analysed for the ratio of 13 C: 12 C and photosynthetic pathway was determined. Five forb and all three grass species had C 4 -dicarboxylic acid pathways, ten forbs had Calvin-C 3 pathways and two forbs had crassulacean acid metabolism pathways.Estimates of the proportion of C 3 and C 4 species in the diet were determined by analysing the ratio of 13 C:' 2 C in faeces. The apparent high proportion of Calvin-C 3 pathway forbs in the diet of ewes compared to the proportion in the total pasture biomass indicated that ewes preferentially select C 3 forbs in the diet, although there was no dependence of lambing percentage on the estimated proportion of C 3 forbs in the diet. Management strategies that may improve lambing percentage in the Mitchell grass areas of north-west Queensland are discussed.
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