Pasture management effects on diet composition and cattle performance on continuously stocked rhizoma peanut-mixed grass swards

In Florida, rhizoma peanut (RP; Arachis glabrata Benth.), a tropical legume , combines the attributes of excellent nutritive value, competitive ability with tropical grasses, and high animal performance. The objective of this study was to determine the effects of spring N fertilization (0 vs 35 kg/ha ) and summer stocking rate (1.5 and 2.5 bulls/ha) on herbage mass, nutritiv e value, herbage allowance, and diet botanical composition of grazed RP-gra ss swards and their interaction with growth and development of bulls (Senep ol, and Brahman or Angus). The study was conducted in 1995 and 1996 at the USDA, ARS, Subtropical Agriculture Research Station in Brooksville, FL. Nit rogen was applied in April of each year, and all pastures were stocked with 1.5 bulls/ha until approximately July of each year, when stocking rate was increased on half the pastures to 2.5 bulls/ha. Herbage mass (HM, kg/ha), herbage allowance (HA, kg/kg BW), nutritive value (CP and in vitro organic matter digestibility [IVOMD]), and diet botanical composition (fecal microh istological) readings were determined. Animal measurements included total a nd seasonal (spring vs summer), ADG, hip height (cm), scrotal circumference (SC, cm), and plasma urea nitrogen (PUN, mg/dL). Herbage mass (3.0 +/- 0.1 2 Mg/ha and 3.4 +/- 0.13 Mg/ha in 1995 and 1996, respectively) was not affe cted by nitrogen fertilization or stocking rate but was affected by season (P < 0.05) due to increased plant growth rate associated with summer rainfa ll. Stocking rate did affect herbage availability, but it never fell below 3 kg/kg BW, indicating herbage availability was never limiting. Crude prote in (200 to 140 g/kg) and IVOMD (650 to 540 g/kg) were not affected by treat ment, but declined (P < 0.001) from spring until fall. Treatments also had no effect on diet botanical composition. Summer ADG averaged about 0.2 kg/d lower than spring ADG, due, in part, to seasonal declines in nutritive val ue. Because herbage allowance was never limiting, full-season ADG was not a ffected by stocking rate or N fertilization and averaged 0.61 +/- 0.03 and 0.60 +/- 0.02 kg/d in 1995 and 1996, respectively. There were season x bree d interactions (P < 0.05) for ADG due to greater declines during the summer for Angus than for Senepol or Brahman. There were no differences in final BW, SC, BCS, hip height, or PUN due to treatments, but breed differences we re noted (P < 0.05) for all measures except BCS.