weedy sward

29 June 2001

Reseed not always best for

weedy sward

By Richard Allison

CREEPING thistles and nettles are now a major problem in upland areas. But decisions on reseeding require careful thought, as it can be an expensive option, according to experts.

Poor weather has encoraged weeds, says Arthur Davies of the Institute of Grassland and Environmental Research. "The last three years have been particularly wet which, combined with poor grazing practice, has led to rapid infestation of many fields."

However, deciding whether to reseed weedy fields is not straight forward, stresses ADAS Bridgets agronomist, Martin Froment. Full reseeds are expensive and the short-term benefits from increased production barely cover the cost.

"A new MDC guide on grassland reseeding will help producers decide when it is cost effective to replace weedy swards. The first thing to consider is the extent of weed infestation and how much ryegrass is left in the sward.

"Research has shown that swards containing only 20% perennial ryegrass can be brought back to highly productive swards, containing 80-90% ryegrass within two to three years. This is achieved by controlling weeds, correcting the stocking rate and paying more attention to soil nutrient status," he explains.

Heavily poached areas in fields containing little perennial ryegrass with heavy weed infestations should be reseeded. Persistent perennial weeds can be controlled using glyphosphate and ploughing, says Mr Froment.

Many weeds in a field can indicate a soil deficiency, resulting in grass being unable to compete effectively with weeds, explains Mr Davies. "Soil pH is often overlooked and should be regularly checked and corrected when required."

Mr Froment adds that producers should check soil nutrient status before reseeding. Over the last 30 years, the phosphorus and potassium levels of grassland soils have fallen and sulphur deficiency is becoming widespread.

"Poor grassland performance can give the impression that a field needs reseeding, but it may need feeding, which is much cheaper to correct.

"Both over and under stocking grassland is a major factor leading to weed infestation due to the sward becoming more open. In addition, overstocking increases poaching. Where this occurs in patches, spray off weeds and reseed the affected area. This is cheaper than reseeding the whole field," he adds.

However, reseeding alone does not guarantee success and the factors which originally led to weed infestation should be tackled, says Mr Davies. "Many upland areas which were reseeded two to three years ago, are now infested with weeds because grassland management mistakes are being repeated.

The main benefit of reseeding is more grass, but for swards more than five years old with 50% of perennial ryegrass remaining this will only cover the costs of reseeding and lost production during establishment, says Mr Froment.

"For older, more weedy swards, the boost to grass production is considerable and equates to an extra £61/ha. This assumes savings on silage making costs from using newer varieties of grass which contain more sugar, eliminating the need for an additive."

To get the most from reseeding, Mr Davies advises using the best suited variety for the enterprise. In upland areas, choosing a red fescue and timothy sward can give 30-70% more lamb output/ha in spring than perennial ryegrass, because of earlier grass growth.

"Producers can reduce the cost of reseeding by using their own equipment. Broadcasting seed with a fertiliser spreader followed by Cambridge rollers is preferred to using a seed drill. With drilling, plants normally remain in rows resulting in a less dense sward.

"Timing of reseeding is also crucial. Leaving it too late in September will not allow sufficient time for grass to establish before winter. White clover is particularly slow to establish," warns Mr Davies. &#42


&#8226 Estimate weed and ryegrass content.

&#8226 Check soil fertility.

&#8226 Choose variety carefully.

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