Effekt av ras och säsong på nötkreaturs avbetningsgrad av konkurrenskraftiga betesmarksväxter

Abstract: Grasslands which have been actively managed for a long time and which in modern times have not been subject to cultivation such as fertilization are called semi-natural grasslands. These rare and species-rich areas hold an ecological, biological and historical value and if the management stops these areas will likely diminish. More effective agricultural systems and a declining number of grazing animals have in Sweden, among other countries, made semi-natural grasslands less valuable as foraging areas. Shrubs and fast growing plants rapidly colonize abandoned semi-natural grasslands, threatening biodiversity. The objective of this experiment was to determine the effects of breed and season on diet composition, with special reference to four competitive species found in wet areas – in cattle grazing heterogeneous semi-natural grasslands – and thereby contribute to improved management of these areas. During the grazing period of 2004 semi-natural grasslands were grazed by heifers of the traditional Swedish breed Väneko and by the commercial Continental breed Charolais. Defoliation of F. ulmaria (Meadowsweet), J. effusus (Soft rush), A. glutinosa (Alder) and D. cespitosa (Tufted hairgrass) were investigated in moist to wet areas during spring, summer and autumn. The heifers were allocated into six enclosures with four heifers of one of the two breeds in each enclosure. In the moist areas the degree of defoliation were examined in 0.5 × 0.5 meter plots, with 15 F. ulmaria plots, 15 J. effusus plots, 15 A. glutinosa plots and 30 D. cespitosa plots. The proportion defoliated plots were analyzed using a logistic model with overdispersion and Pearson's chi-square test. The proportion and height of defoliated shoots in the plots were examined in a nested model with repeated measurement. A majority of F. ulmaria (98%) and A. glutinosa (87%) plots and half of the J. effusus (51%) and D. cespitosa (42%) plots were defoliated some time during the grazing period. In spring, Väneko heifers foraged a higher proportion of F. ulmaria plots than Charolais heifers did (P = 0.0038) and they also foraged a higher proportion of F. ulmaria shoots in the plots than Charolais heifers did. Further there was a tendency for Väneko heifers to forage more D. cespitosa plots during summer (P = 0.0781), compared to Charolais heifers. More F. ulmaria and A. glutinosa plots were foraged during autumn than during summer (P = 0.0396 and P = 0.0345 respectively) and the proportion defoliated shoots of both F. ulmaria and A. glutinosa were higher during autumn than during spring (P < 0.0001). The study did not show any main effects of breed concerning the heifers´ defoliation of the four species, but a few interactions of breed and season were established even though there was extremely good access to forage. Like other studies of grazing preferences in cattle, this study also shows differences in preference over time. Plots with root shoots from A. glutinosa were preferred by both breeds during the later half of the grazing season when cattle even with similar access to forage, grazed more A. glutinosa than during spring. F. ulmaria appeared to be relatively desirable to the heifers and to a certain degree even A. glutinosa, while J. effusus and D. cespitosa were not grazed to the same extent in moist areas. The study shows that the variation in forage among cattle grazing semi-natural grasslands depends more on season than on breed and that F. ulmaria and A. glutinosa are more desirable during autumn than during spring.