In order to determine the effect of the frequency of pruning and nitrogen fertilization on two varieties of king grass (Pennisetum purpureum), an experiment was conducted at the EARTH University, in Las Mercedes, Guácimo, Limón. The experimental design was a completely randomized block with six repetitions. The treatments were: four frequencies of pruning (7, 10, 14 and 17 weeks), two varieties (green and purple) and fertilization (with and without 100 kg/ha of N, 50 kg/ha of P₂O₅ and 50 kg/ha of K₂O, per year). No significant differences were found between varieties. The frequency of pruning was the variable that presented the most effect (p<0.01). Aerial biomass production accumulated during 14 weeks was 13 830 kg/ha of dry matter. This was not significantly different from the production at 17 weeks. The fertilized plants grew 20 cm more than those which were not fertilized and after 17 weeks reached a height of 295 cm. The frequency of pruning at 17 weeks showed better results (p<0.01), with 52 % ground cover. The regrowth and subsequent accumulated production indicated that it is appropriate to use frequency of pruning at 14 weeks for plants grown as an animal feed source.