Frequency of defoliation and nitrogen and phosphorus fertilization on Andropogon gayanus Kunth .1. Yield, crude protein content, and in vitro digestibility

An experiment was conducted in a semi-arid region located in the State of Zulia, western part of Venezuela (10 degrees 32'N and 71 degrees 42'W, 600 mm average annual rainfall), to evaluate dry matter (DM) yield and in vitro organic matter digestibility (IVOMD), and crude protein (CP) content of Andropogon gayanus Kunth as affected by three frequencies of defoliation (every 42, 63, and 84 days) and fertilization with three levels of nitrogen (N) (0, 100, and 200 kg N ha(-1) year(1)) and two levels of phosphorus (P) (0 and 75 kg P2O5 ha(-1) year(1)) in a factorial array using a split-split-plot experimental design with frequencies in the main plots, N in the sub-plots, and P in the sub-subplots with three replications. Soil was a sandy loam Aridisol with a pH of 5.5. Average soil calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), and P contents were 0.6, 0.3, 0.1 and 0.17 meq 100g(-1) soil, and 6 ppm, respectively. Data from six, four, and three cuttings for the frequencies of 42, 63, and 84 days were analyzed over the total duration of the study (252 days) as well as separately for periods with high (168 days, 384 mm) and low rainfall (84 days, 69 mm). Frequency of defoliation was the only factor that influenced (P10.05) the variables studied, In the overall analyses, the highest (P10.05) DM yield (3,656 kg ha(-1) cutting(-1)) was obtained with harvests every 63 days. Mean IVOMD decreased (P<0.05) from 54.2 to 51.7% with increasing harvest interval from 63 to 84 days, respectively. The average CP content was 7.9%. With low rainfall, mean DM yield was 2,209 kg ha(-1), CP content declined (P10.05) from 7.5 to 3.9% as the cutting interval increased, and IVOMD decreased (P10.05) between 42 and 63 days. With high rainfall, the highest (P<0.05) DM yield (4,872 kg ha(-1)) and IVOMD (56.5%) were found at 63 days of age. Mean CP content was 9.3%. These results confirm that A. gayanus is a highly productive forage grass. Lack of response to N and P fertilizers may be attributed partly to relatively low rainfall during the experiment, adaptation of the grass to low fertility soils, and long intervals between N applications and the next harvest.