Low Light Intensity Effects on Grain Sorghum at Different Stages of Growth 1

‘Georgia 615’ grain sorghum ( Sorghum bicolor (L.) Moench) was shaded for weekly periods in three field experiments to determine if periods exist during the life cycle of the sorghum plant when low light intensities are deleterious to grain yield components. Black plastic fabric with 25% light transmission was used to shade plants. Grain yield per ha and per panicle, seed number per panicle, seed weight, and heat units from seeding to 50% anthesis were determined. Two stages of plant development most adversely affected by low light intensity were found. The first stage occurred from pre‐boot vegetative, when the last leaf was just visible in the whorl to full panicle expansion or during approximately the two‐week period just before 50% anthesis. Shading during this period resulted in reduced number of seed per panicle. The second stage was during the milk and dough stages of grain development or during approximately the three‐week period just after 50% anthesis. Shading during this period resulted in lighter sed with little or no change in number of seed per panicle. When number of seed per panicle was reduced by shading, heavier seed weight compensated for part, and sometimes all, of the potential yield decrease. Grain yields per panicle and per ha of shaded sorghum were generally lower than for the unshaded check.