Effects of nitrogen fertilization and harvest date on yield, digestibility, fiber, and protein fractions of tropical grasses

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(.)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 w ere 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(.)cutting) increased forage mass in these grasses by 129% (P < 0.01) compared with no N fertilization. Additional N did not result in fur ther increases of forage mass. Bermudagrass (Cynodon dactylon) produced mor e forage DM [P < 0.01; 1,536 43 kg/(ha-cutting)] than stargrass [Cynodon nl emfuensis; 1,403 43 kg/(ha(.)cutting)] or bahiagrass [Paspalum notatum; 1,2 97 43 kg/(ha(.)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 bahia grass 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 fe rtilization level increased, NDF decreased linearly (P < 0.01) in all three forages. Total N concentration increased (P less than or equal to 0.01) li nearly as N fertilization increased in all forages. Total N concentration w as highest (P < 0.01) in stargrass (2.4%, DM basis) compared with bermudagr ass (2.2%) and bahiagrass (2.0%). Total N concentration was depressed in al l forages for late June and July harvests (P < 0.01). Fertilization increas ed (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 increa sed in July (P < 0.01). Bahiagrass had less N in cell contents than did ber mudagrass and stargrass but had a greater concentration of N associated wit h the cell wall. Managerial factors, including rates of N fertilization and harvest dates, can have profound effects on the nutritional value of forag e. An increased understanding of these effects is imperative to improve sup plementation programs for ruminants.