Why sward density is critical for overseeding success

Overseeding swards with a multispecies mix after grazing is significantly more effective than doing so on higher covers.

This is according to a Welsh study that shows this method almost doubles the establishment rate.

Wil Evans, who took part in the study, was looking for ways to reduce soil cultivation at Hendreseifion, his family’s 130ha (321-acre) beef and sheep farm near Machynlleth, Powys.

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He was awarded funding by the Farming Connect Try Out Fund to trial a range of options for broadcasting seed into existing swards, to establish which would generate the best results.

Wil used an electro-broadcaster mounted on a quad bike to spread grass, legume and herb seed mixes onto fields of permanent pasture.

Grass covers were at a density of either 1,900-2,500kg dry matter (DM)/ha pre-grazing or 1,200-1,400kg DM/ha post-grazing at the end of June 2025.

After broadcasting, cattle were turned onto the pasture to aid seed-to-soil contact.

On 3 September 2025, 14 weeks after the seed was sown, a survey of the field sown with the lower-density grass cover showed an establishment rate of 46% compared with 28% in the field with the higher one – a difference of 48.7%.

Sowing conditions

Independent agricultural consultant James Holloway, who advised Wil on soil fertility status pre-seeding, says this outcome reflects the better seed-to-soil contact and reduced canopy competition in lower density covers.

Grazing removes bulk and opens the sward, creating favourable conditions for surface-applied seed.

Changing this sequence increases the risk of seed becoming trapped in leaf litter or buried unevenly, and that reduces germination and establishment, James adds.

The post-grazing system aligns with best practice: tight grazing or cutting, followed by light harrowing to expose bare soil and remove thatch.

The trial also revealed that some species established significantly better than others.

Perennial ryegrass, white and red clovers, plantain and chicory were the most prolific.

These species are better suited to low-disturbance systems, offering rapid germination, competitive root development, and resilience under grazing, says James.

In contrast, yarrow, burnet, bird’s-foot trefoil and other herbs were less consistent in establishing.

These have a greater sensitivity to competition, making them less reliable if they are broadcast instead of being incorporated in the soil, he advises.

Some of the seed used was coated with a biological treatment.

This has yet to be launched commercially but has been shown in trials by the company developing it, Fielder Nutrition, to provide early vigour and resilience under variable conditions, allowing a more uniform emergence.

While the Farming Connect project showed the treatment had little benefit, Wil says the timing of its application to the seed may have influenced that.

Wil Evans © Debbie James

The recommendation is for the seed to be broadcast within 72 hours of treatment.

However, he was reluctant to do this in the very dry soil conditions, so the timeslot was missed except in one field.

In that field, emergence was three days earlier than in swards where untreated seed had been broadcast.

And sowing into post-grazing covers led to a 76% increase in species successfully established, compared with non-treated seeds.

James says greater efficacy may also be achieved from the biological treatment under drilled or cultivated seeding, options where seed placement and soil contact are more consistent.

Trampling

Post-broadcasting, cattle trampled the seed into the soil, mimicking the effect of double rolling and supporting establishment.

While this is cost-effective, it needs deliberate management, James cautions.

Trampling should be done within 24 hours of sowing and at a moderate stocking density – low enough to prevent compaction but sufficiently high to give adequate seed-to-soil contact.

Soil moisture content is important, too – it should be moist but not saturated, he says.

To optimise timing, the trial used LoraWan sensors.

These provide real-time data on soil moisture and temperature at different depths, as well as air temperature, before, during and after sowing.

Farming Connect’s red meat sector officer for mid-Wales, Owain Pugh, who oversaw the trial, says these data were invaluable.

“It helped correlate environmental factors with seed germination and establishment success,” he observes.

Practical advice for other farmers

• Weed control Broadleaved weeds such as docks, thistles and nettles should be controlled before overseeding, as limited herbicide options are available after sowing because of the plant diversity in multispecies mixes.
• Soil condition James advises targeting a soil pH of 6.5, a P index of 2 and a K index of 2-. “Assess soil structure and apply remedial actions where necessary before overseeding,” he says.
• Seed-bed preparation To prepare the seed-bed, the sward should be tightly grazed or cut followed by a light harrow, with seed-bed fertiliser and lime applied if needed, in line with RB209 recommendations.
• Species mix A specialised herbal ley mix containing species proven to perform when surface sowing should be used.
• Sowing method The mix should be broadcast evenly across the surface and followed with consolidation by either double rolling or controlled trampling. “If trampling, treat as a deliberate intervention, not incidental grazing,” James recommends.
• Aftercare After sowing, the ley should be rested for four to six weeks, followed by light, rotational grazing to encourage rooting and tillering.