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Abstract Annual ryegrass (Lolium multiflorum Lam.) is grown on more than one million ha in the south-eastern USA each year. Recommended and actual seeding rates vary substantially within the region. The objective of this study was to evaluate variation in seed weight, germination, seedling vigour and seasonal yield performance among annual ryegrass cultivars. During 1997, 1998 and 1999, seed from fourteen commercial cultivars was weighed and germinated to determine numbers of pure live seed (PLS) m-2 before yield evaluation at four locations. Seed from ten cultivars was planted at 0·7 and 2·0 cm depth in a greenhouse study to evaluate relative seedling vigour. Cultivar mean single-seed weight ranged from 2·4 to 4·8 mg in 1997, 1·8 to 4·5 mg in 1998, and 2·6 to 4·6 mg in 1999. Seed germination ranged from 78·8% to 98·0% in 1997, 82·3 to 98·3% in 1998 and 77·8 to 98·3% in 1999. Seed number, PLS m-2 , ranged from 675 to 1289 in 1997, 710 to 1550 in 1998, and 717 to 1179 in 1999. Among the ten cultivars evaluated for seedling vigour, seedling weight differed between planting depths and a significant cultivar by year interaction was observed. Seedling weight was highly correlated with seed weight at each seeding depth. The effect of increasing number of PLS m-2 on subsequent yield performance, although small, was consistently negative. These results indicate that target plant populations may be obtained more economically by adjusting seeding rates for seed size differences among cultivars and seed lots of annual ryegrass. [ABSTRACT FROM AUTHOR]