title: Effects of nitrogen fertilization and harvest date on yield, digestibility, fiber, and protein fractions of tropical grasses.
abstract: To evaluate the response of three tropical forage species to varying rates of nitrogen (N) fertilization [0, 39, 78, 118, 157 kg of N/(ha x cutting)] and five summer harvests, forage DM mass and nutritive value were evaluated in a randomized complete block design with a split-split plot arrangement of treatments. Plots (n = 60) were established in 1996, and five harvests were conducted every 28 d from June through September in 1997 and 1998, with fertilizer applications occuring after each harvest. Fertilization with 78 kg of N/(ha x cutting) increased forage mass in these grasses by 129% (P < 0.01) compared with no N fertilization. Additional N did not result in further increases of forage mass. Bermudagrass (Cynodon dactylon) produced more forage DM [P < 0.01; 1,536 +/- 43 kg/(ha x cutting)] than stargrass [Cynodon nlemfuensis; 1,403 +/- 43 kg/(ha x cutting)] or bahiagrass [Paspalum notatum; 1,297 +/- 43 kg/(ha x cutting)]. Peak forage mass for all species occurred in late June and July. In vitro organic matter digestibility (IVOMD) of stargrass increased (P < 0.01) linearly with fertilization. A quadratic response to N fertilization (P < 0.01) was noted in IVOMD of bermudagrass, whereas bahiagrass was not affected. Bermudagrass was more (P < 0.01) digestible (57.5 +/- 0.4) than stargrass (54.6 +/- 0.4) and bahiagrass (51.9 +/- 0.4%). As fertilization level increased, NDF decreased linearly (P < 0.01) in all three forages. Total N concentration increased (P < 0.01) linearly as N fertilization increased in all forages. Total N concentration was highest (P < 0.01) in stargrass (2.4%, DM basis) compared with bermudagrass (2.2%) and bahiagrass (2.0%). Total N concentration was depressed in all forages for late June and July harvests (P < 0.01). Fertilization increased (P < 0.05) the concentration (% of DM) of all protein fractions. In July and August, nonprotein N was reduced 11.8% (P < 0.01), whereas ADIN increased in July (P < 0.01). Bahiagrass had less N in cell contents than did bermudagrass and stargrass but had a greater concentration of N associated with the cell wall. Managerial factors, including rates of N fertilization and harvest dates, can have profound effects on the nutritional value of forage. An increased understanding of these effects is imperative to improve supplementation programs for ruminants.
authors.affiliation: Department of Animal Sciences, University of Florida, Gainesville 32611-0910, USA.
authors.forename: C R
authors.initials: CR
authors.lastname: Johnson
authors.forename: B A
authors.initials: BA
authors.lastname: Reiling
authors.forename: P
authors.initials: P
authors.lastname: Mislevy
authors.forename: M B
authors.initials: MB
authors.lastname: Hall
mesh_terms: D000821:Animal Feed / Q000032:analysis*; D000818:Animals; D004043:Dietary Fiber / Q000032:analysis; D004063:Digestion; D005308:Fertilizers; D009584:Nitrogen / Q000008:administration & dosage; D009753:Nutritive Value; D010940:Plant Proteins / Q000032:analysis; D006109:Poaceae / Q000737:chemistry* / Q000254:growth & development; D011897:Random Allocation; D012418:Ruminants
chemical_list: D004043:Dietary Fiber; D005308:Fertilizers; D010940:Plant Proteins; D009584:Nitrogen