Effects of management on biodiversity and productivity of grazed grassland

Abstract

Grazing is seen as a promising tool to maintain and enhance biodiversity due to the creation of structurally diverse pastures. So far, grassland biodiversity has been studied in relation to various grazing management scenarios, but knowledge of grazing effects gained on experiments managed in the same way over various years is very rare. In the first experiment, we aimed to answer the question of which grazing intensity is suitable to benefit insect diversity. On a long-term grazing experiment two treatments of grazing intensity had been established in 2002, and an additional third treatment had been introduced in 2005. The influence on insect diversity in terms of grasshopper (Orthoptera) and butterfly (Lepidoptera) species richness and abundance was investigated from 2002 to 2004 and again in 2010 and 2011. Confirming our hypotheses, the most intensively grazed paddocks featured the lowest diversity. However, the most extensive treatment did not prove to be more favorable for insects than the medium one. We therefore suggest that continuous grazing of medium intensity is most appropriate to maintain and enhance insect diversity due to a sufficient average sward height and the creation of a patchy grazed sward. This grazing management offers several ecological niches beneficial for a wide array of species, in particular for less mobile herbivore groups like grasshoppers. When focusing grassland phytodiversity, it is also important to consider the type of grazing livestock. Grazer species differ with regard to their grazing behavior and can hence differently affect vegetation and sward structure. In a second experiment, we therefore used either cattle or sheep in mono grazing or both grazer species in mixed grazing. In order to analyze the effect of grazers on different sward types, swards were also manipulated with herbicides against dicotyledonous plants. Paddocks were grazed from 2007 to 2011 and vegetation composition was studied before the grazing season each year and additionally in 2012. Sward composition and diversity confirmed obvious differences between sward types. Grazer species, in contrast, had only negligible effects on vegetation development over years and sward structure. Cattle-grazed paddocks had a higher alpha-(species richness) and beta-diversity, but only for one year and one sward type. As conclusion, in rotational stocking the choice between cattle and/or sheep can only be used in a limited way to form vegetation composition and diversity on common agriculturally used grasslands with a very slight advantage for cattle grazing in terms of phytodiversity. On the same experiment, we also studied how plant species diversity and productivity in terms of herbage mass are related with livestock performance. Our results negated the productivity-diversity theory. Pastures might offer enough plant species for complementary resource use even on species-poor, herbicide treated swards. In contrast, forage of swards with a high species richness contained more crude protein and less ADF thereby leading to a higher nutritive value. However, these results might also be attributed to a legume effect due to the overlap of species richness with legume and forb abundance in this experiment. Pasture productivity and quality were both not influenced by grazer species. Considering all years together, findings point towards an enhanced livestock output on phytodiverse pastures reflected by daily live weight gains of lambs. Mixed grazing also seems promising to contribute to an increased animal performance of lambs and suckler cows. In conclusion, livestock grazing can be used to shape sward structure by the variation of grazing intensity with subsequent effects on biodiversity, whereas the type of livestock seems to have only minor effects, at least in the rotational grazing system studied here. Even though the relationship between species richness and herbage production was not found, a higher amount of dicotyledonous forbs and legumes can contribute to a better forage quality which translates also into higher livestock weight gains of lambs. In conclusion, grassland management can be used to enhance biodiversity and this in turn has the potential to improve livestock performance.