- This study was undertaken to obtain information on the quality of grass seed when windrowed at the high moisture levels necessary for obtaining maximum yields. A secondary goal was to monitor moisture and temperature fluctuations in the windrow and to relate their effects to development and deterioration of the seed. Three species of perennial grasses were included in the study: tall fescue (Festuca arundinacea Schreb.) variety "Fawn", orchardgrass (Dactylis glomerata L.) variety "Potomac", and perennial ryegrass (Lolium perenne L.) variety "Linn". The effect of heading date on seed development at the recommended windrowing time was evaluated with individually tagged panicles. Tall fescue was windrowed on 28 June, 01 July and 06 July; orchardgrass on 28 June, 01 July and 06 July; and perennial ryegrass on 01 July, 06 July and 16 July. These dates were chosen to provide windrows mowed at moisture levels higher than, equal to, and lower than the optimum for maximum yields of pure live seed. Moisture and temperature fluctuations in the windrow were monitored and changes in seed development and deterioration were evaluated. Heading date influenced several seed quality characteristics at the recommended time for windrowing. Early inflorescences were lowest in seed moisture content. Tall fescue and perennial ryegrass seeds from the Late group, and orchardgrass seeds from the Intermediate and Late groups had not attained maximum seed weight. Seedlings of tall fescue and orchardgrass produced by seeds from the Late group had not attained maximum shoot weight. Seeds of all inflorescence age groups reached maximum viability several days before optimum windrowing date. Seed moisture contents when windrowed were 51.6, 41.5, and 15.9% in tall fescue, 44.8, 41.4 and 30.7% in orchardgrass, and 49.8, 28.5 and 19.1% in perennial ryegrass. During periods without rainfall, seed moisture decreased rapidly during the day, increased during the night, and decreased to a new low the following day. Several rains occurred during this period, raising seed moisture levels above 40%. During the drying period, seed moisture in the lower part of the perrenial ryegrass windrow was as much as 8% higher than seed in the upper part. The average maximum temperatures in the upper and lower portions of the perennial ryegrass windrow were 38.3 and 24. 3°C during July and 31.1 and 19. 2⁰C during August. The maximum temperature reached in the upper part was 47°C on 28 and 29 July. Each weeks' delay in windrowing resulted in improved seed quality, with the greatest improvement occurring between the first and second cutting, However, the data support Klein and Harmond's recommendations for windrowing tall fescue at 43% moisture, orchardgrass at 44% moisture, and perennial ryegrass at 35% moisture. Perennial ryegrass seed quality was not improved by delaying windrowing, while orchardgrass and tall fescue seeds increased somewhat in weight. Because of the risk of increased shattering losses, delaying windrowing to improve seed quality does not appear justified. A small amount of seed development took place while the seeds were in the windrow. Seed weight of tall fescue harvested at 51.6% moisture increased from 250.6 to 279.6 mg/ 100 seeds. No increase in seed weight occurred in the other species or windrowing dates. After several days in the windrow, speed of germination increased in all three species. Germination percentage, seedling emergence, and shoot weight did not improve after windrowing. Very little seed deterioration occurred during the first month in the windrow. A perceptible drop in germination of orchardgrass and perennial ryegrass occurred during the 2 weeks prior to the 19 August sampling date. Seed weight declined by the end of the experimental period in all three crops. Some germinated seeds were found in the perennial ryegras windrows by the end of August. No reduction in speed of germination or shoot weight occurred, while there was a trend toward reduced seedling emergence near the end of the experimental period. There were no indications that the stage of maturity when harvested influenced the rate of seed deterioration in the windrow. The probability of seed deterioration occurring at a faster rate than in 1976 is not great since a total of 75.69 mm (2.98 in) of rain fell during the experimental period, compared to a normal rainfall of 27.43 mm (1.08 in).