Variação estacional da composição química-bromatológica, do teor de macronutrientes minerais e da digestibilidade "in vitro" do capim elefante (Pennisetum purpureum, Schum.) variedade napier

Portuguese. Com a finalidade de estudar alguns aspectos do valor nutritivo do capim elefante (Pennisetum purpureum, Schum.) variedade Napier, procurou-se, no presente trabalho, determinar os constituintes químico-bromatológicos e 6 macronutrientes minerais (nitrogênio, fósforo, cálcio, enxofre, potássio e magnésio), assim como estimar a digestibilidade "in vitro" da matéria seca e da celulose. A matéria seca, a proteína bruta, a fibra bruta e a cinza bruta foram determinadas pelos métodos descritos pela A.O.A.C. (1965), e a celulose pelo processo descrito por CRAMPTON e MAYNARD (1938). Dos macronutrientes minerais estudados, o cálcio, o potássio e o magnésio foram determinados por espectrofotometria de absorção atômica, o fósforo pelo processo do vanádio-molibdato de amônio, por digestão nítrico-perclórica, e o enxofre pelo método gravimétrico do bário. A digestibilidade "in vitro" da matéria seca e da celulose foi determinada por um período de 48 horas, utilizando-se fluido ruminal de carneiro, conforme o método descrito por CARVALHO (1967) e as modificações introduzidas por SILVEIRA (1970). Evidenciou-se a influência da maturidade sobre os constituintes químico-bromatológicos, os macronutrientes minerais e a digestibilidade "in vitro" do capim Napier. Os teores de materia seca, de fibra bruta e de celulose se elevaram e os teores de proteína bruta e cinza bruta decresceram com a maturidade da planta. Os teores de nitrogênio, fósforo, enxofre e potássio declinaram, os teores de magnésio mostraram tendência a se elevar e os teores de cálcio apresentaram constante variação com o avançar da idade da forrageira. A digestibilidade "in vitro" da matéria seca e da celulose foi influenciada negativamente pela maturidade da planta. Foram estabelecidas correlações entre os constituintes químico-bromatológicos e a digestibilidade "in vitro" e entre esta e os macronutrientes minerais.

English. This paper deals with an experimental work carried out in order to estimate the chemical composition, the major minerals content and the "in vitro" dry matter and cellulose digestibility coefficients of napier grass (Pennisetum purpureum, Schum), during the year. Random blocks, including 15 m² plots with 10 treatments (cuttings) and 3 replications, was the experimental design used. The forage was harvested at 45, 75, 105, 135, 165, 195, 225, 255, 285 and 315 days of vegetative growth, and the samples collected in previously established dates, at the same day hour. It was observed a maturity effect on the chemical composition of napier grass. Higher dry matter contents were observed after the second cutting, which is considered the best stage of vegetative growth in relation to heigth of forage, for harvesting. However, after 75 days the crude protein contents were not enough to meet the minimum nutrient requirements of cattle under grazing management. The highest crude fiber contents were observed in the flowering stage. It was noted that nitrogen, phosphorus, sulfur and potassium concentrations became lower as maturity was reached. In the other hand, magnesium content showed a consistent tendency to increase in the latter cuttings. The highest calcium concentration was observed during the dry season of the year, when the forage was in the advanced stage of maturity. In relation to phosphorus, the lowest concentrations were also found during the dry season, and this fact confirms several previous observations on the seasonal defficiency of phosphorus in forage. Calcium, sulfur, potassium and magnesium contents could be considered enough to cover the nutrients requirements of cattle. However, as far as phosphorus is concerned, only during the first phase of vegetative growth (third cutting) the grass would have enough phosphorus to meet the minimum requirements of cattle. Dry matter, crude fiber and cellulose contents showed significative and negative correlations to nitrogen, phosphorus, sulfur and potassium concentrations, but to the magnesium content, the correlation was positive. On the other hand, crude protein and ash contents presented significative and positive correlations to nitrogen, phosphorus, sulfur and potassium contents, exception to magnesium content showed significative and negative correlation to ash. It was observed a negative effect of maturity on the digestibility of napier grass. "In vitro" dry matter digestibility coefficients declined from 71,64% in the first cutting at 45 days to 43,69% in the tenth cutting at 315 days, and in relation to cellulose the reduction was 78,10% to 35,47%, respectively. "In vitro" dry matter digestibility coefficients were negative and highly correlated to dry matter (r=-0,91), cellulose (r=-0,88) and crude fiber (r=-0,93) contents, and positively correlated to crude protein (r=0,88) and ash (r=0,89) contents. In the same way, it was noted a negative and high correlation between "in vitro" cellulose digestibility and dry matter (r =-0,93), cellulose (r = -0,85), and crude fiber (r=-0,91), and significative correlation to crude protein (r= 0,88) and ash (r= 0,88) contents. A positive and high correlation was observed between "in vitro" digestibility coefficients of cellulose and dry matter (r= 0,99). Significative correlation coefficients were found between "in vitro" dry matter digestibility and nitrogen (r = 0,88), phosphorus (r=0,66), sulfur (r=0,66) and potassium (r=0,90) contents, but in relation to magnesium it was observed a negative correlation (r=-0,43). The same situation was observed as "in vitro" cellulose digestibility is concerned, that is: nitrogen (r= 0,88), phosphorus (r=0,68), sulfur (r=0,68), potassium (r= 0,89) and magnesium (r=-0,41).