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PASTURE MANAGEMENT

Late Summer Management Can Improve Forage Yield Distribution and Nutritive Value in Temperate Grassland

Facultad de Agronomía, Univ. Nacional del Centro de la Provincia de Buenos Aires, B7300, Azul, Argentina

^* Corresponding author (marzadun{at}faa.unicen.edu.ar).

Chemical defoliation (glyphosate spraying) at the end of summer and fertilization are practices proposed to improve winter forage availability in native grasslands of the Flooding Pampa of Argentina. However, there is no preceding work comparing the effect of either practice on the yield and botanical composition of these grasslands. In 2002 and 2003, three defoliation treatments were applied in mid-February: control (C) without summer defoliation; severe summer defoliation (D) to 2- to 3-cm stubble height and glyphosate, isopropylamine salt of N-[phosphonomethyl] glycine, G, applied at 2.2 kg a.i. ha^–1. Additionally, the effect of three rates of N, 0, 35, and 70 kg N ha^–1and P, 0, 11, and 22 kg P ha^–1 applied in combination with the three late summer defoliation treatments were researched. Early changes during fall in the biomass components ryegrass (Lolium multiflorum Lam.), warm-season grasses (WSG), dead matter, and litter were evaluated after defoliation treatments. The effect of defoliation treatments along with N and P fertilization was followed on seasonal and annual forage yield, botanical composition, in vitro dry matter digestibility (IVDMD), and crude protein (CP). Glyphosate application increased annual ryegrass contribution while reducing WSG and legumes, mostly narrow leaf trefoil (Lotus tenuis Waldst & Kit.) and annually yielded 29% less forage dry matter (DM) than the intense mechanical defoliation. Forage IVDMD and CP were increased by D and G in fall the first year. In the second year CP was low in G in winter and spring. Changes in botanical composition can explain the differences observed in quality DM traits. Fertilizer increased forage yield 2 to 10 kg DM per kg N and 8 to 24 kg DM per kg P applied. Severe defoliation applied in late summer increased fall and winter forage availability and quality without the negative effect on legumes observed with glyphosate that had a negative impact on DM accumulation and quality in spring.

Abbreviations: C, control • CP, crude protein • CSG, cool-season grass • D, defoliation • DM, dry matter • G, glyphosate • IVDMD, in vitro dry matter digestibility • WSG, warm-season grass

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Received for publication July 9, 2007.