Chemical Composition and in Vitro Digestibility of Five Heavily Fertilized Tropical Grasses in Puerto Rico

Abstract

Samples from five tropical grass species: Guinea (Panicum maximum), Pangola (Digitaria decumbens), Congo (Brachiaria ruziziensis), Merker (Pennisetum purpureum), and Star (Cynodon nlemfuensis), were harvested during nine weeks at ages from 7 to 63 days in southwestern Puerto Rico. The grasses were fertilized at the rate of 4480 kg/ha per year with a 15-5-10 fertilizer. The crude protein content of all grasses exceeded 10 percent up through 28 days of regrowth. Grasses declined in crude protein from a mean of 18.1 percent at 7 days to 5.6 percent at 63 days. A narrower range was observed between grasses from 42 to 63 days. The five tropical grasses possessed high contents of structural carbohydrates, principally cellulose and lignin, which increased with age, except in Pangola grass. Silica did not change uniformly with age in any of the grasses while hemicellulose was characterized by marked fluctuations. Digestibility values were lower at all stages of growth than in temperate forages of similar ages. Cellulose was negatively correlated with in vitro digestibility in all grasses except Pangola. Lignin appeared to be the predominant factor in determining digestibility. The relationship of silica to digestibility varied between species, being positive in some (Guinea and Congo) and negative in others (Star, Pangola, and Merker). The grasses may be ranked as follows with regard to their chemical composition and digestibility: Merker > Congo > Star > Guinea > Pangola. Pangola grass, though lowest in in vitro digestibility, declined least with advancing age, thus maintaining a more constant quality for a longer period of time.

In vitro ruminal degradation of carbohydrate fractions in tropical grasses fertilized with nitrogen

“…Across these studies high amounts of ash in Megathyrsus, low lignin in Urochloa and low amounts of crude protein in Cynodon were consistently observed. The chemical constituent findings are consistent with other reports for Cynodon (34) , Megathyrsus (35) , Urochloa (36) and Andropogon (29) , suggesting potential inherent growth differences in their plant tissues (33) . Fertilization with N modified the amount and distribution pattern of the nutrients in these plants (Table 1).…”

“…Grasses varied in their distributions of chemical constituents, which reflects differences in morphology and physiology. Previous reports have indicated variations in the chemical composition of tropical grasses due to species (30) , season of year (31) and plant age (32,33) . Across these studies high amounts of ash in Megathyrsus, low lignin in Urochloa and low amounts of crude protein in Cynodon were consistently observed.…”

“…It is known that hemicellulose has more complex linkages with lignin than cellulose (37) . Therefore, hemicellulose should be less available, increasingly so as the plant cell wall matures from more linkages between hemicellulose and lignin (33) . NF grasses contained more hemicellulose and more mature cell walls than FE (44) .…”