Nitrogen Input Effects on Herbage Accumulation and Presence of Pasture Plant Species

Abstract

Long-term responses of pasture plant species to management strategies that vary amount and form of N inputs form a knowledge gap. Our objective was to determine how supplementation of grazing beef cattle (Bos taurus) with corn (Zea mays L.) dried distillers grains plus solubles (DDGS) in unfertilized pasture (SUPP) aff ects annual herbage accumulation and presence of plant species and functional groups relative to unsupplemented beef cattle on unfertilized (CONT) and N-fertilized (FERT) smooth bromegrass (Bromus inermis Leyss.) pasture. We addressed this objective in the sixth (2010) and seventh (2011) years of a long-term experiment in eastern Nebraska, where N input from DDGS supplementation and urea fertilizer averaged 0, 43, and 90 kg ha -1 yr -1 within CONT, SUPP, and FERT, respectively. For these years, annual herbage accumulation averaged 6.87, 6.80, and 10.58 Mg ha -1 (LSD = 1.49) and 66.5, 66.2, and 76.0% of herbage accumulated by 25 June (LSD = 5.3) in CONT, SUPP, and FERT, respectively. Smooth bromegrass occurred in 99.8% of 0.1-m 2 pasture quadrats regardless of treatment. Cessation of N fertilizer input, however, increased presence of annual foxtail (Setaria spp.), annual graminoids, and annuals among quadrats in CONT relative to FERT. Supplementation of DDGS, while shown in previous studies to improve weight gains and N use efficiency in cattle, supplied enough N through excretion to provide an intermediate level of resistance to annual weeds. Nitrogen excreted from cattle supplemented with DDGS, however, did not aff ect herbage accumulation in subsequent years.

Reduced Nitrogen Mineralization and Litter Decomposition in Unfertilized Smooth Bromegrass Pastures

“…Soil N mineralization of cool-season grass pastures with ³10 yr of N fertilization has been reported to be up to twofold greater than unfertilized pastures (Gill et al, 1995;Hatch et al, 2000;Ledgard et al, 1998;Parfitt et al, 2005). However, we measured a decline in soil N supply 8 yr after cessation of external N inputs along with a previously reported decline in herbage productivity in the CONT pastures (Guretzky et al, 2013). However, we measured a decline in soil N supply 8 yr after cessation of external N inputs along with a previously reported decline in herbage productivity in the CONT pastures (Guretzky et al, 2013).…”

“…On average, pasture soils contained 4.0% organic matter, a pH of 5.5, 24.2 g C kg -1 (39.6 Mg C ha -1 ), 2.34 g N kg -1 (3.83 Mg N ha -1 ), 753 mg K kg -1 , and 16 mg P kg -1 at the 0-to 15-cm depth (Guretzky et al, 2013). The soils are deep silt clay loams consisting of four soil series: Tomek (fine, smectitic, mesic Pachic Argialboll), Filbert and Filmore (both fine, smectitic, mesic Vertic Argialboll), and Yutan eroded (fine-silty, mixed, superactive, mesic Mollic Hapludalf ) (USDA-NRCS, 2012).…”

“…The grazing management is detailed in Greenquist et al (2009) and Guretzky et al (2013). The grazing management is detailed in Greenquist et al (2009) and Guretzky et al (2013).…”

“…In 2005, a long-term experiment was initiated on smooth bromegrass pasture in Nebraska to compare effects of management strategies that alter amount and form of N inputs in pasture on beef cattle gains and economics. Cessation of annual N fertilization in CONT and SUPP, however, decreased annual herbage accumulation (Guretzky et al, 2013), litter deposition (Guretzky et al, 2014a), and root and rhizome mass (Guretzky et al, 2014b) suggesting that cessation of annual N fertilization also may impact litter decomposition and soil N mineralization compared to the long-term FERT strategy. Cessation of annual N fertilization in CONT and SUPP, however, decreased annual herbage accumulation (Guretzky et al, 2013), litter deposition (Guretzky et al, 2014a), and root and rhizome mass (Guretzky et al, 2014b) suggesting that cessation of annual N fertilization also may impact litter decomposition and soil N mineralization compared to the long-term FERT strategy.…”

“…Livestock retain only a small fraction of forage N consumed with 75 to 90% being returned to the system in dung and urine (Haynes and Williams, 1993). Fertilization is often used to increase herbage productivity and N content of forages (Guretzky et al, 2013), with an indirect contribution to N cycling from increased N return in excreta (Orr et al, 2012). Fertilization is often used to increase herbage productivity and N content of forages (Guretzky et al, 2013), with an indirect contribution to N cycling from increased N return in excreta (Orr et al, 2012).…”