The performance potentials of six perennial ryegrass varieties for conservation or grazing use were compared between 1994 and 1996 in N. Ireland. They were maintained, either under constant lax defoliation (conservation) or constant severe defoliation (simulated grazing) management for 3 years, or annually alternated between these two managements with the changeover taking place either in autumn or at the first cut of the following year. Starting one set with lax defoliation in the first year and the other with severe defoliation created duplicates of these two alternating treatments. The results showed that variety yield differed depending on the season and the management imposed and there were differential responses to the various treatments. Conducting an alternating management system that implemented the management change in autumn provided a valid estimate of yield performance potential for both simulated grazing and conservation use, compared to constant management systems. Although significant differences in variety ranking between lax defoliation and severe defoliation management yields were only observed in the third year, the study showed that if the management change was implemented in autumn, the yield potential of varieties in the following year was not affected by the preceding year's management regime. Leaving a longer sward (6 cm) to over-winter increased the spring performance of the varieties in some years, compared to a shorter sward (3 cm), which may have implications for grazing management. There were also indications that imposing an alternating management system might alter the absolute magnitude of the sward density ratings relative to a constantly managed system, though variety rankings would remain unaffected.
It was concluded that the current alternating management testing system used to evaluate candidate varieties for UK National or regional recommended lists, does not cause any variety to be unfairly advantaged or disadvantaged.