The liability of seed crops of perennial ryegrass (Lolium perenne) to contamination by wind-borne pollen

Abstract

Some aspects of intervarietal crossing in Lolium perenne have been studied.Using a number of experimental designs with non-red plants as the tester variety, results have been obtained as to the effects of isolation distance on intervarietal crossing.In all cases contamination decreases rapidly, at first with increasing distance, but there is a progressive reduction in the rate of decrease produced by repeated increases in isolation distance. The actual rate of decrease varies with the number and arrangement of plots.The effect of intravarietal pollen in reducing intervarietal crossing appears similar to that of distance in its mode of action. Within short distances from the contaminant, additional rows of intervening plants are highly effective in reducing contamination. This suggests that pollination within a population of plants occurs chiefly between neighbouring plants.The flowering periods of the different strains of Lolium perenne and L. italicum overlap to varying degrees. Several early strains flower at the same time, e.g. British commercial, New Zealand Certified Mother Seed and Station-bred S24 perennial rye-grass; others, such as Kentish indigenous perennial ryegrass and strains of L. italicum, which are intermediate in date of flowering, overlap with both the early- and late-flowering strains.Intervarietal crossing is considerably reduced when varieties which differ widely in date- of flowering are grown in close proximity to one another, but even in the case of extreme early and late strains some spatial isolation, or discarding of border rows, is necessary for maintaining varietal purity.In certain instances when seed crops of early-flowering strains are endangered by late-flowering plants growing in adjoining leys, satisfactory isolation may be achieved through cutting back the contaminant crop just before flowering. When seed crops of late-flowering strains are grown alongside leys containing early-contaminant varieties, cutting back should be delayed until a few days prior to commencement of flowering in the seed crop.The practical application of the results to grass seed production has been discussed. Existent isolation requirements appear not too great for grass seed crops intended for further seed multiplication, but for commercial seed crops the spatial isolation distance generally recommended may be reduced to 100, or even 50 yd., in the case of large fields.