Renewing Run-Out Pasture on a Steep, Erodible Paddock
Tania runs 900 Romney-cross ewes on 260 ha of summer-dry hill country inland from Gisborne. One 12 ha paddock, known as The Steep, faces north at about 22 degrees. Its 18-year-old ryegrass-dominant pasture is run out: about 30% bare ground, moss, weeds, and grass grub larvae found in a spade square. Summer growth there has fallen to roughly 8 kg DM/ha/day, while the flats produce 25. She wants to renew it. Quotes are: full cultivation (spray, cultivate, roll, drill) $1,450/ha; spray and direct drill $680/ha; direct drill without spray $460/ha. Autumn drilling is planned for March. The paddock has two stock-water troughs and is the main route between the woolshed and the back block. The bank has offered to extend the overdraft by $10,000. Tania does not want to lose grazing for more than about eight weeks, and remembers the gully at the bottom silting up after a cultivation trial on a neighbouring farm.
1. Recommend which renewal method Tania should use and justify it against cost, erosion risk, weed control and the loss of grazing.
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Recommendation: spray and direct drill at $680/ha - about $8,160 for the block - rather than full cultivation.
Reasoning
- Erosion risk rules out full cultivation. At 22 degrees on a north-facing hill, cultivation breaks the root network and soil structure, leaving loose soil exposed to autumn and winter rain. The neighbour's silted gully is exactly the evidence: sediment, phosphorus and weed seed end up in the waterway, and the paddock loses its topsoil. Full cultivation also costs $1,450/ha, which over 12 ha is $17,400 - beyond the $10,000 overdraft.
- Spraying first is essential. The paddock is 30% bare with moss and weeds and 18-year-old ryegrass. Direct drilling straight into that competition (the $460/ha option) gives the new seedlings too much competition for moisture and light on a dry north face, and the result is usually a thin, weedy strike - money spent twice. The $220/ha difference between the two direct-drill options buys the knockdown that makes the sowing work.
- Budget: $8,160 fits inside the $10,000 facility and leaves some headroom for grub control and a follow-up spray.
- Grazing loss: direct drilling avoids the 6-10 weeks of bare fallow that cultivation needs and keeps stock able to move through, which matters given the paddock is the route to the back block.
- Species choice matters as much as method. A mix including cocksfoot or tall fescue plus subterranean or white clover suits a summer-dry north face better than ryegrass alone and will persist longer.
2. Explain why the direct-drill method alone will not solve Tania's grass grub problem, and describe what she should do about it before and at sowing.
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Why direct drilling alone is not enough
Grass grub larvae feed on roots in the topsoil through autumn and winter. Direct drilling places small, tender seedlings into exactly that layer, so the grubs graze the new pasture off as it germinates. The paddock looks green for a few weeks and then thins in patches that pull out of the ground easily - often mistaken for drought or poor seed. On a steep slope the result is worse than a poor strike: grubbed plants lose their root anchorage, so the new pasture offers no erosion protection in its first winter.
What to do
- Confirm the level of infestation by digging several spade squares across the paddock and counting larvae per spade, not just the one patch she found. Thresholds vary, but more than about 3-5 grubs per spade square justifies control.
- Choose a registered control and apply it at drilling, or use treated seed where available, following label rates and withholding periods.
- Consider timing. March drilling puts seed straight into actively feeding larvae. Delaying sowing to spring, when larvae have moved deeper to pupate, reduces damage markedly - but spring sowing on a dry north face risks moisture failure.
- Hold the spray-out. Spraying out the old pasture a few weeks early and letting it green up again can attract adults and concentrate the problem; alternatively, the fallow period starves larvae of root feed.
- Do not gamble the whole block. Sowing 12 ha into a known grub population without control risks the full $8,160. Sowing half first and checking the strike before committing the rest is sound risk management.
3. Tania wants the paddock back in the grazing round within eight weeks. Evaluate whether that is realistic and describe how she should graze the new pasture in its first year.
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Is eight weeks realistic?
Not reliably. Eight weeks is achievable in ideal autumn conditions - warm soil, good moisture, a clean, fast-establishing species - but several factors push against it here:
- A north-facing, summer-dry slope often has low soil moisture in March, slowing germination.
- Grub pressure may thin the strike and delay canopy closure.
- Her requested species mix, if it includes cocksfoot or tall fescue for persistence, is slower to establish than ryegrass.
A more realistic plan is 10-12 weeks to the first grazing, with the option of a light, quick graze earlier rather than a full round.
First-year grazing management
- Don't hard-graze the first time. Wait until the new pasture has reached about 2,500-3,000 kg DM/ha and the seedlings are firmly rooted - pulling a seedling should meet resistance. A hard first grazing on shallow-rooted plants on a steep slope is how new pastures get pulled out and rills start.
- Use light stock or young stock for the first grazing, on a dry day, and move them on quickly.
- Spell longer than usual between grazings through the first year to let root systems deepen.
- Control weeds - a follow-up spray for broadleaf weeds and any surviving old pasture is far cheaper than re-sowing.
- Monitor the grub population in autumn and spring, and be prepared to act rather than assume establishment has solved the problem.