Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2005
Publication Date: 3/2/2006
Citation: Skinner, R.H., Sanderson, M.A., Tracy, B.F., Dell, C.J. 2006. Above- and Belowground Productivity and Soil Carbon Dynamics of Pasture Mixtures. Agronomy Journal. 98:320-326. Interpretive Summary: There are few studies that have taken a practical approach to examining how well complex forage mixtures persist in intensively managed pastures. Little information is also available on how mixture complexity affects photosynthesis or the proportion of total inputs used for root production. We conducted an on-farm study to compare changes in yield, botanical composition, canopy photosynthetic rates, and root production of simple and complex forage mixtures managed for hay production or grazing. The most complex mixture yielded more forage dry matter than the simplest mixture, but this difference was due to the inclusion of a few highly productive forage species. Only about half of the species planted in the complex mixture persisted during the entire 6-year experiment. The most complex mixture also had more roots and they extended deeper into the soil than in the simplest mixture. Complex mixtures also had greater photosynthesis than the simple mixture during the summer when pastures were affected by drought. Greater root biomass combined with deeper penetration into the soil profile should improve productivity by increasing access to water in deep soil layers that would not otherwise be available. Our results suggest that planting a complex mixture of forages without regard to the identity of the species in the mixture is not wise. Producers should select species with specific desirable traits for inclusion in forage mixtures.
Technical Abstract: Increasing plant species diversity could enhance the ecosystem functions provided by grazed pastures such as forage yield, resistance to weed invasion, and soil carbon accumulation. Three forage mixtures (2-, 3-, or 11-species) were established on a farm in eastern Pennsylvania and grazed by dairy heifers or managed under a 3-cut hay system from 1999 to 2002. Net canopy photosynthesis was measured from early-April to early-October 2000 to 2002. Root distribution to a depth of 60 cm was measured in mid-September each year, and soil C and N concentrations to a 15-cm depth were determined in May 1999 and September 2002. The 11-species mixture yielded 43% more forage dry matter than the 2-species mixture. This difference was mainly due to the inclusion of a few highly productive forage species in the 11-species mixture. Canopy photosynthesis did not differ among mixtures in the spring, but in the summer was 50% greater in the 3- and 11- compared with the 2-species mixture. The 11-species mixture also had 30 to 62% greater root biomass than the other two mixtures and a greater proportion of roots in deeper soil layers. Soil carbon either remained unchanged or decreased, depending on species composition, with the greatest decrease occurring in the 11-species mixture. No relationship existed between changes in soil C concentration and either canopy photosynthesis or above- and below-ground productivity. Deeper rooting could reduce drought stress by increasing access to deep soil moisture. Selecting forage mixtures to include specific desirable traits, such as greater rooting depth, could result in improved pasture performance.