Intake characteristics of perennial ryegrass varieties when grazed by sheep under continuous stocking management

Fifteen intermediate-heading varieties of perennial ryegrass were continuously-stocked with sheep during two years and sward surface height was maintained in the range of 4 to 5 cm. Large differences in daily dry matter intake ewe-1 were observed between the varieties suggesting that there is considerable scope for breeding improved varieties for grazing. Compared with the mean in each year, the ranges in values were proportionately 0.81 to 1.24 in 1998 and 0.80 to 1.19 in 1999. Highest intakes were measured for sheep grazing 2 tetraploid hybrid varieties which also had markedly higher liveweight gains ewe-1 and ha-1 compared with those grazing the 9 diploid and 4 tetraploid varieties examined. Daily intakes were not correlated with eating time but were correlated with intake rate min-1 eating (r = 0.73 and 0.88 in 1998 and 1999, respectively). Animals appeared unable to compensate fully for low intake rate by increasing eating time. Intake rates were not correlated with green leaf mass, the number of grass tillers m^-2 or water soluble carbohydrate concentrations in grass snips from the grazed horizon. High intake rates were achieved not only by sheep grazing highly-tillered diploid varieties but also by those grazing tetraploid hybrids with fewer tillers m^-2. Other plant morphological and chemical attributes need to be explored to determine what factors might influence the intake characteristics of varieties destined for grazing so that new target traits can be identified and improved methods can be developed to screen rapidly for these traits during breeding programmes and during variety evaluation.