Abstract. The literature suggests that small genomes promote invasion in plants, but little is known about the interaction of genome size with other traits or about the role of genome size during different phases of the invasion process. By intercontinental comparison of native and invasive populations of the common reed Phragmites australis, we revealed a distinct relationship between genome size and invasiveness at the intraspecific level. Monoploid genome size was the only significant variable that clearly separated the North American native plants from those of European origin. The mean Cx value (the amount of DNA in one chromosome set) for source European native populations was 0.490 AE 0.007 (mean AE SD), for North American invasive 0.506 AE 0.020, and for North American native 0.543 AE 0.021. Relative to native populations, the European populations that successfully invaded North America had a smaller genome that was associated with plant traits favoring invasiveness (long rhizomes, early emerging abundant shoots, resistance to aphid attack, and low C:N ratio). The knowledge that invasive populations within species can be identified based on genome size can be applied to screen potentially invasive populations of Phragmites in other parts of the world where they could grow in mixed stands with native plants, as well as to other plant species with intraspecific variation in invasion potential. Moreover, as small genomes are better equipped to respond to extreme environmental conditions such as drought, the mechanism reported here may represent an emerging driver for future invasions and range expansions.

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In the Czech Republic yeast cultures have recently become an occasional component of feeding rations for cattle. Some papers informed about significant effects of yeast cultures on fermentation under specific anaerobic conditions existing in rumen. Numerous studies (Sune et al., 1998;Alshaikh et al., 2002; Lila et al., 2004 and others) documented positive effects of yeast cultures not only on the rumen environment of dairy cows but also on the improvement of microbial activities. According to Blake (1993), yeast cultures obviously improve the cellulolytic activities of rumen microorganisms in such a way that they increase their total numbers, improve fibre digestion, reduce lactate accumulation, reduce the concentration of oxygen in rumen fluid and improve utilisation of starch supplied in the feeding ration. In this way they influence (inhibit) the rate of VFA production and, thus, increase the stability of rumen environment and improve the intensity of digestion. Sullivan and Martin (1999) also reported that the supplement of a Saccharomyces cerevisiae yeast culture into the diet of dairy cows improved the utilisation of lactate and digestion of cellulose. Lyons (1993) and Strohlein (2003) stated that some yeast strains showed a better capability to use lactate because they stimulated its utilization by propionic acid bacteria. The utilisation of lactate by these bacte- ABSTRACT: This study presents the results of an experiment in which the effect of addition of a yeast culture (Saccharomyces cerevisiae, Strain 47) on rumen fermentation was studied in thirty-six dairy cows of Holstein breed. The animals were divided into one control and five experimental groups. Each group involved 6 individuals. The animals received a diet consisting of good maize silage with a higher dry matter content (16 kg), clover-grass haylage (l6 kg), meadow hay (3 kg) and supplementary feed mixture (7.5 kg). The rations were fed to cows as total mixed ration (TMR). In experimental groups, the yeast culture was added into the feed mixture in amounts of 2, 4, 6, 8, 10 g per day and animal. Samples of rumen fluid were taken perorally 3-4 hours after feeding. The obtained results indicated that the addition of a Saccharomyces cerevisiae SC-47 culture in recommended doses showed a positive effect on ruminal digestion. As compared with control, the addition of all aforementioned amounts of the yeast culture into the feeding ration resulted in all cases in a statistically significant (P < 0.01) decrease in pH and fluctuated near the lower limit of the reference values. As compared with control, the yeast culture supplementation showed a positive effect (P < 0.01) on production of volatile fatty acids (VFA) (127.6 vs. 84.0 mmol/l). The utilisation of ammonia was higher (P < 0.01) in experimental groups (8.12, resp. 8.68 mmol/l) than in controls (9.06 mmol/l). The difference in protozoa numbers in rumens of dairy cows in the control and experimental groups was statistically highly significantly (P < 0.01) different. There was a close relation...