Effect of Centaurea maculosa on sheep rumen microbial activity and mass in vitro

Spotted knapweed, Centaurea maculosa, a herbaceous weed from Eurasia, is altering the composition of native rangeland communities across western North America. Herbivore use of this plant is limited, possibly because glandular trichomes covering the epidermal surfaces of aerial tissues produce cnicin, a biologically active sesquiterpene lactone. We determined the concentrations of cnicin in plant parts from different growth stages (initial, mature, regrowth) of C. maculosa and the effects of these plant parts on sheep rumen microbial activity and mass (in vitro), when mixed in different proportions with grass hay. Leaves had higher crude protein and lower neutral and acid detergent fiber than stems or grass hay. Cnicin concentrations were highest in leaves, intermediate in flower buds, and lowest in stems. Cnicin concentrations in leaves increased from June to July, but decreased in stems. Regrowth had slightly lower cnicin concentrations that mature growth. High percentages (70% and 100%) of mature and regrowth leaves and flowers of C. maculosa in the mixtures depressed the rate and amount of microbial activity, whereas high percentages of stems from initial growth enhanced the rates of microbial activity. Microbial activity was more responsive to the different mixtures, plant parts, and growth stages than microbial mass, possibly because microbial populations cannot adjust rapidly to changes in diet. After cnicin was extracted from leaves, microbial activity was greater from these leaves than from grass hay. In contrast, after cnicin was extracted from flower buds, microbial activity from these flower buds was still depressed, indicating other compounds or the remaining cnicin were still affecting microbial activity. In summary, sheep rumen microbial activity was reduced significantly by mature and regrowth leaves that contained high concentrations of cnicin. Since most herbivores selectively graze leaves, the bitter-tasting cnicin could deter large ruminant feeding of C. maculosa by altering their behavior and/or by affecting rumen function.