Selection for more persistent perennial ryegrass (Lolium perenne) cultivars for subtropical/warm temperate dairy regions of Australia

Abstract

Three experiments were conducted in subtropical/warm temperate dairy regions of Australia to relate persistence of perennial ryegrass (Lolium perenne) to their response to temperature and defoliation. Experiment 1 was a small plot cutting study on a Krasnozem soil with 2 perennial ryegrass lines: LP31, a New Zealand Agriseeds Ltd experimental line; and Kangaroo Valley (KV), adapted to the south coast of New South Wales. They were defoliated at 1 (L) or 3 (3 L) leaves/tiller regrowth stage and at 20 or 60 mm stubble height. When defoliated at 3 L, LP31 was most productive (8839 v. 8481 kg DM/ha) and persistent (57 v. 49 plants/m² surviving into the autumn of the second year) while the reverse was true at 1 L (5753 v. 6237 kg DM/ha and 27 and 54 plants/m², respectively).

In experiment 2, small plots were sown to 11 cultivars comprising the New Zealand Agriseeds Ltd experimental perennial ryegrass lines: F₂ LP93045, F₂ LP93026 (Dobson × Bronsyn), LP54, F₂ LP92039 (Dobson × Jumbo), LP30 and the commercially available varieties: Bronsyn, Meridian, Dobson, Yatsyn1 and Impact. The plots were 'rotationally' grazed by Friesian heifers. Over the 25 months of this experiment Bronsyn was the most productive (7173 v. 5310 kg DM/ha) and persistent (17 v. 4 plants/m² for Bronsyn and mean of the remainder, respectively) cultivar.

In experiment 3, the 3 most persistent cultivars from experiment 2 (labelled 'Cas'), and 8 other cultivars, were grown as a mini-sward in a temperature-controlled glasshouse. The cultivars were grouped on the basis of persistence, origin and response to defoliation interval and temperature as follows: group 1 (persistent): Bronsyn, Bronsyn (Cas), F₂ LP93026 (Dobson × Bronsyn) (Cas), Dobson; group 2 (persistent adaptive ecotype): Victorian; group 3 (not persistence): LP2551, Impact, F₂ LP92039 (Cas); group 4: European. The treatments were 28/17°C (HT) or 21/10°C (LT) (day/night) temperature and 1 (1 L) or 3 (3 L) leaves/tiller defoliation interval. Yield (1.28 v. 0.77 g DM/plant) and water-soluble carbohydrate (WSC) content (127 v. 15 mg WSC/plant) were substantially higher in plants defoliated at 3 L compared with 1 L, respectively. The effect of temperature on yield was not significant but more WSC plant reserves accumulated (92 v. 60 mg/plant) at LT than HT. There was a significant (P<0.001) interaction between defoliation interval and cultivar group, with the cultivar, Victorian, being the highest yielding at a short defoliation interval (1.04 v. 0.71 g DM/plant for Victorian and the mean of the remaining cultivars, respectively) while group 1 cultivars (most persistent) were highest yielding at 3 L (1.45 v. 1.08 g DM/plant) and HT. In contrast, group 3 cultivars, were least productive at high temperatures, irrespective of defoliation interval.

The results showed that the Australian adapted ecotypes Victorian and KV, which are not responsive to defoliation frequency, were only average yielding. In contrast, the newer cultivars such as Dobson and Bronsyn or crosses thereof are also persistent and are more productive but only when defoliated at an appropriate interval (3 leaves/tiller). In this group, growth was reduced at the higher temperature under frequent defoliation but promoted at infrequent defoliation. The more productive and persistent cultivars tended to have a higher content of WSC plant reserves.