Forage production can vary substantially during the annual cycle in the tropical islands of Puerto Rico and St. Croix. Cool temperatures, low levels of solar radiation, and low rainfall in December and January have been hypothesized to cause decrease in forage growth. A forage growth model was used to simulate yield in different environments in order to examine these hypotheses quantitatively. Weather data were obtained over a period of three to six years from three locations in Puerto Rico and one location in St. Croix. Minimum temperatures were always near or above 20° C and, consequently, did not appear to cause serious losses in forage production. The forage model predicted a decrease in forage production during the winter months due to decreased levels of solar radiation; however, yields were estimated to be approximately 70 to 80% of summer yields. Whereas shallow rooting depth of 45 cm could cause decreased yields in some situations, inadequate rainfall could not explain large yield decreases in winter months. This research indicates that a factor in addition to the ones tested contributes to the loss in winter forage yield. It is speculated that short day lengths directly influence the regulation of plant growth such that forage yield is decreased in winter months.