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•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•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•cutting)] than stargrass [Cynodon nlemfuensis; 1,403 ± 43 kg/(ha•cutting)] or bahiagrass [Paspalum notatum; 1,297 ± 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 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.

Implications

Nitrogen fertilization affects forage mass and quality in bahiagrass, bermudagrass, and stargrass. Forage dry matter mass reached a plateau for these forages with application of 78 kg of N/(ha•cutting). Greatest total N concentration was achieved at the higher fertilization treatments in all forage species. The nitrogen pool available for rumen microbes to be utilized in microbial protein synthesis is increased in tropical forages as fertilization rates increase. The Cynodon grasses exhibited a larger percentage of fraction A and less fraction C, than bahiagrass, suggesting that these forages have a higher percentage of the forage N in a useable form for ruminants. Depressions in digestibility and soluble N fractions of tropical grasses during summer months suggest that supplementation may be an appropriate strategy at a time when forage quality may limit animal performance.

Key Words: Digestibility, Fertilization, Forage, Protein, Yields

© 2001, by the American Society of Animal Science. All rights reserved.

J. Anim. Sci. 2001. 79:2439-2448