Five tropical grasses, Cynodon dactylon (coastcross), Cynodon nlemfuensis (star grass), Cynodon plectostachyus (callie), Digitaria pentzii (slenderstem), and Panicum maximum (guinea) PRPI 12917, were plan fed for hay production on a Vertisol soil with irrigation in the Lajas valley. On the basis of all factors studied, guinea, star grass and slenderstem, harvested at 45-day intervals (DI), are the best for hay production under adequate irrigation and fertilizer use in Vertisol soils in the southern coast of Puerto Rico. The forages were harvested at 35-, 45- and 55 DI. The highest yields of hay were obtained from guinea at 35- and 45 (2069 and 2901 kg/ha, respectively), and from slenderstem at the 55-DI (3397 Icg/ha). The highest crude protein content was obtained with star grass at the 35-DI (18.6%) and with callie at 45- and 55-DI (14.0% and 11.5%, respectively). Crude protein yield at 35-DI was highest for star grass (356 kg/ha). At 45- and 55-DI slenderstem was higher (363 and 384 kg/ha, respectively). The highest digestion coefficients (DC) at the 45- and 55-DI were obtained with guinea (63.0 and 59.0, respectively), although not significantly different (P>0.05) from those of slenderstem and coastcross. At a 35-DI, coastcross averaged the highest DC (64.3%). The highest voluntary intake at the 35-DI was observed with coastcross (2.8% of body weight daily), but not more (P>0.05) than with slenderstem or star grass. At the 45- and 55-DI, the highest voluntary intake was observed with star grass and guinea (2.4% and 2.3% of body weight daily, respectively). The highest relative nutritive value (digestibility X voluntary intake) was obtained with coastcross (181) at the 35-DI and with guinea at the 45- and 55-DI (1.51 and 1.36, respectively).