Ecological and economic implications of plant diversity and grazing in pasture systems

摘要

Converting exotic forages to native warm-season grasses (NWSG) such as big bluestem (Andropogon gerardii), little bluestem ( Schizachyrium scoparium), and Indian grass (Sorghastrum nutans ) offers a sustainable alternative because NWSG may yield comparative livestock gains with less fertilizer, as well as offer habitat for arthropods and declining grassland bird species such as Dickcissels (Spiza americana ). In the Southeastern United States, the predominant forage species are exotics such as bermudagrass (Cynodon dactylon) and tall fescue (Schedonorus arundinaceus), so NWSG conversion could substantially improve sustainability and wildlife habitat on private lands in the region. In 2011 and 2012, I studied response of Dickcissels and arthropods to forage origin, diversity, and grazing at the Mississippi State University Prairie Research Unit in Monroe Co., MS, USA. Four treatments were established among 12 pastures representing a gradient in management intensity, including a grazed bermudagrass and tall fescue mix (GMEP), grazed Indian grass monoculture (GINP), grazed mixed native pasture (GMNP), and a non-grazed mixed native pasture (NMNP). Grazed treatments were stocked with steers from May through August each year. I also evaluated the economic implications of each grazing system.;In general, there was a positive response to native grasses among Dickcissels and arthropods and a negative effect from grazing. Dickcissel productivity (fledgling/ha) was highest in NMNP and lowest in GMEP, whereas productivity was intermediate and similar among grazed native pastures. This pattern was attributable to availability of suitable nest sites because nest survival and brood size did not vary by treatment. Several arthropod taxa responded positively to greater vegetation density in NMNP, but communities were largely similar among grazed pastures irrespective of forage origin or diversity, suggesting little short-term response to NWSG. In spite of unfavorable growing conditions due to drought, cattle grazing NWSG consistently outperformed conspecifics in GMEP, with 45--72% higher average daily gain. Partial budget analysis indicated that NWSG pastures yielded up to 36% marginal rates of return despite establishment costs. These results suggest NWSG conversion can benefit tall grass specialists such as Dickcissels while offering competitive alternatives to exotic forages, resulting in net benefits for both conservationists and producers.