Three grain sorghum cultivars were grown in the open field and under seven - and towo-year old rubber trees at there levels of nitrogen fertilizer and plant population density, to determine their performance in both seed and atoon crops in terms of grain yeild and other agronomic characters, quality perameters like protein and total starch content of grains and their effects on stem girth and latex yield of rubber. Comparing the six cultural environments with respect to the parameters evaluated, those grown in the open field and under two-year old rubber trees nanmely, MIT open field seed crop. MIT-open field ratoon crop, Kidapawan-under rubber seed crop and Kidapawan-under rubber ratton crop reduced the number of days to blooming, and gave higher grain and total dry matter yields, 100-grain weight and number of grains per panicle. The same treatments gave higher starch but lower protein content in the grains compared to those grown under seven-year old rubber trees namely. MIT-under rubber seed crop and MIT-under rubber ratoon crop. The u=yield component that was consistently associated with grain yield was number of grains per panicle. Nitrogen treatments increased grain yield, total dry matter yield, plant height, leaf area index and number of grains per panicle. Plant population density reduced leaf size and panicle length in both seed and ratoon crops in all environments. The cultivars showed a wide diversity of performance in different culturtal envivornments. There were as inverse relationships (...).