ALTHOUGH THERE is little alternative to a traditional autumn reseed for grass fields that are well past their sell-by date, a spring over-seed will give tired swards a useful flush of young grass and a boost to more productive species.

“By the time a long-term ley is about eight years old, more than 40% of the sward is likely to comprise weed grasses and broad-leaved weeds,” notes Germinal Holdings technical consultant, Dai Hides.

“These species are lower yielding, less palatable to livestock, of lower quality and less responsive to nitrogen fertiliser.”

Replacing them with modern varieties by re-seeding or over-seeding takes advantage of the agronomic developments achieved by plant breeders.

“Taking dry-matter production as an example, breeders make an improvement of around 0.5% per year on average,” says Mr Hides.

“So by the time a grazing ley has been down for 10 years, the latest varieties will offer a 5% improvement.”

Direct drilling or over-seeding costs at least half as much as the 175-190/ha (70-77/acre) cost of a wholesale re-seed, takes land out of production for only a few weeks and requires little in the way of preparation.

The main concern is to minimise competition with the new seedlings from the existing grass, which is best achieved by over-seeding after grazing the sward tightly or taking a cut for silage.

There are pros and cons with both direct drilling and broadcast over-seeding.

General-purpose disc drills like the Moore Uni-drill and Vaderstad Rapid have an output advantage over narrower – typically 2.5m – specialist implements like the Aitchison Seedmatic tine drill and are more practical for sowing cereals and other crops.

But the Seedmatic, with its flexible pigtail tines and “T” shaped boot, which places seed into an “inverted keyhole” slot, arguably creates a more favourable environment for seed.

Disc drills put seed into a narrow vee-shaped slot. The Aitchison”s Baker coulter point creates a tilth within an enclosed slot where, the manufacturer argues, there is more likely to be sufficient humidity in dry situations for the seed to germinate and grow away.

Trials certainly point to direct drilling having the advantage over broadcasting when it comes to establishing fresh supplies of ryegrass or clover in an existing sward.

Studies by the Institute of Grassland & Environmental Research – IGER – into the potential for red clover in upland pastures used a cultivator drill in comparison with plain and harrow broadcasting techniques.

“Direct drilling was an extremely effective method of establishing red clover in ryegrass,” says IGER”s Mick Fothergill.

“It gave 85% better establishment than broadcasting in the first season, which translated into a 46% higher annual yield of herbage.

“In the second year, however, the red clover plots established by broadcasting benefited from compensatory growth, with more comparable yields.”

Earlier IGER work at Aberystwyth compared the Aitchison tine direct drill with conventional plough-based re-seeding and different broadcasting techniques.

While the conventional approach achieved the best red clover establishment at just under 86 plants/sq m, direct drilling into a silage ley after first cut and in dry conditions came within 11%.

Broadcasting techniques – using a tine-harrow/seeder unit, spike and chain harrows – trailed this.

Nonetheless, even if flexible tine harrows like the Einbock and Opico implements lack precision when it comes to placing seed at an appropriate depth, their widespread use suggests that, on the whole, producers get satisfactory results.

The machines certainly have an output advantage given typical working widths of 4-6m and fast operating speeds. Likewise in terms of utilisation – routine grass grooming work is their primary role so any seeding capability is a bonus.

fwmachinery@rbi.co.uk