A comparison of the fibre and seed development in fibre flax and linseed types of Linum usitatissimum (L.) was carried out in replicated field plot experiments in 1993, 1994 and 1996 in Northern Ireland. Flax cultivar Ariane, and linseed cultivars Barbara (1993 and 1996) and Flanders (1994) were each grown at seed rates of 500, 1000 and 1500 seeds/m2 and sampled at 7-day intervals from the start of flowering until harvest. The number of buds, flowers and capsules per plant were counted and straw height and total, seed, straw, total fibre, long fibre and short fibre dry matter ( DM) yields determined for each sample date. Fibre yields were determined after water retting the straw for 7 days at 20°C and extracting the fibre by scutching and hackling.

The emergence dates were similar in all years, but site and season interactions resulted in later flower and capsule development in 1993. Although the total biomass of flax and linseed were similar at the start of flowering, flax maintained a more rapid straw growth rate and linseed a more rapid seed growth rate giving final fibre yields that were 1300 kg/ha higher in flax than linseed and seed yields that were 800 kg/ha greater in linseed than flax. The proportion of fibre in the straw was higher in flax than linseed and the proportion of long fibre within the total fibre fraction was also greater. Final seed yields were highest in 1993, and fibre yields from flax were higher in 1994 than in the other years. Seed rate had no effect on seed yield in either flax or linseed but at the lowest seed rate, which gave plant counts of about 400 plants/m2 , the total fibre yield was significantly reduced compared with the higher seed rates with plant counts of about 1200 plants/m2 . It is concluded that decreasing fibre flax seed rates from 1500 to 500 seeds/m2 will reduce potential fibre yields by 25% but have little effect on seed yield, whilst increasing linseed seed rates from 500 to 1500 seeds/m2 would only result in marginal increases in the potential yield of fibre.