Management Effects on Herbage Yield and Botanical Composition of Rhizoma Peanut-Mixed Grass Associations

Broadleaf weeds invade rhizoma peanut (Arachis glabrata Benth.) (RP) stands as they age, possibly because of decline in associated grasses, particularly bahiagrass (Paspalum notatum Flügge). Our objective was to assess the effect of spring N fertilization and summer stocking rate (1.5 vs. 2.5 animals ha⁻¹; Exp. 1) or spring N fertilization and presence or absence (no grazing until July) of spring grazing (Exp. 2) on the herbage yield and botanical composition of mixed RP-grass swards. In both experiments, pastures were fertilized with either no N (N-0) or 35 kg ha⁻¹ of N (N-35) in March or early April for 3 yr. Sward herbage mass (Mg ha⁻¹) and botanical composition were assessed in spring (April), summer (July), and fall (October) of 1995 and 1996 and spring of 1997. In Exp. 1, stocking rate did not affect herbage mass or botanical composition, but N fertilization did. By spring 1997, there was an eight percentage unit increase in grass contribution (0.30 Mg ha⁻¹ increase in dry matter) and a trend for greater decline in RP (five percentage units, but no change in RP dry matter) for N-35 vs. N-0. Other forbs, Mexican tea (Chenopodium ambrosoides L.), and cogongrass [Imperata cylindrica (L.) Räusch.] were not affected by N rate. Similar changes in herbage mass and RP percentage were noted for Exp. 2, but there was a stocking rate × N fertilization interaction for grasses because the percentage increase in grass was greatest when N was applied and grazing deferred in the spring. Forb content of the sward was not affected by N fertilization or spring grazing. Our results show that grass yield and content of RP-grass swards can be manipulated by N fertilization and deferral of spring grazing, but for the limited time period studied (3 yr), increased grass presence was not sufficient to inhibit encroachment of weedy forbs. Joint contribution of the USDA-ARS and the Florida Agric. Exp. Stn. Journal Series no. R-06159.