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.