Critical factors influencing no-till establishment of short-term ryegrass (Lolium multiflorum) into a kikuyu (Pennisetum clandestinum) pasture

Abstract

In the subtropical dairy region of Australia, poor establishment of short-term ryegrass (Lolium multiflorum) oversown into tropical grass pastures is a common occurrence requiring re-sowing. A survey of subtropical dairy farmers was undertaken to relate management practices used in oversowing ryegrass to sward establishment and subsequent growth. Two glasshouse studies were also conducted to examine (1) the effect of temperature, ploidy, seeding depth, and mulch cover on ryegrass emergence and (2) the effect of temperature and ploidy on growth and development of ryegrass seedlings. Subtropical dairy farmers only used grazing management to control the growth and residue levels of the tropical grass pasture before oversowing. The average residue was 1200 kg DM/ha to a 5 cm height, and where the residue amount and height were higher, the ryegrass failed to establish. Tetraploid cultivars were preferred in early sowings and diploid cultivars were favoured in later sowings. When direct-drilled, either seed type was sown to a depth of 1–3 cm. A 20–30 kg/ha sowing rate was common for diploids and was at least 2× that for tetraploids. A seedling count <600 plants/m2 resulted in 1317 tillers/m2 in spring compared with 1886 tillers/m2 for a count >600 plants/m2. The ryegrass seedling emergence study was conducted at 25/15°C (day 0600–1800 hours)/(night 1800–0600 hours) for 14 days after sowing and then repeated at 20/10°C. The treatment combinations were 2 seed types (tetraploid or diploid) × 4 sowing depths (0, 1, 3, or 6 cm) × 3 mulch heights (1, 5, or 10 cm above surface). The main effects, seed type, sowing depth, and mulch height had significant (P < 0.05) effects on seedling emergence, irrespective of temperature, and all interactions were significant (P < 0.05) with the exception of the seed type × mulch height interaction. At the higher temperature the proportion of emerged seedlings declined from 0.52 to 0.16 with increasing mulch cover, from 0.43 to 0.29 with increasing sowing depth, and was higher for tetraploid than for diploid cultivars (0.44 v. 0.26, respectively). At the lower temperature the proportion of emerged seedlings declined from 0.85 to 0.20 with increasing mulch cover, from 0.62 to 0.39 with increasing sowing depth, and was higher for tetraploid than for diploid cultivars (0.63 v. 0.52, respectively). The ryegrass seedling study used treatment combinations of 3 temperature regimes (25/15°C, 20/10°C, or 15/5°C) × 2 seed types (tetraploid or diploid) × 5 harvest times (3, 4, 5, 6, and 8 weeks after sowing). At 8 weeks after sowing tetraploid top DM was significantly (P < 0.05) higher than diploid top DM at low (4100 v. 3040 mg/plant) and medium (5370 v. 2600 mg/plant), but not high (2460 v. 2780 mg/plant) temperatures. Tetraploid tiller and leaf numbers were substantially reduced by high temperature but not for diploid cultivars at 8 weeks. Tetraploid root DM at 8 weeks was highest (2360 mg/plant) and lowest (1200 mg/plant) at medium and low temperatures, respectively, while diploid root DM (mean = 1440 mg/plant) was not affected by temperature. Top growth was most rapid at 6–8 weeks (700–3392 mg/plant) and even more so for root growth (260–1617 mg/plant). These results indicate that when oversowing, ryegrass establishment will be most successful if the ryegrass seed is not sown below 3 cm but, more importantly, if the tropical grass residue is restricted to a 5 cm height. Further, sowing a tetraploid cultivar may be preferable to a diploid cultivar, with its superior emergence and seedling growth over a range of temperatures and sowing conditions.