There is a significant yield response to be obtained from reseeding grassland, with research from Moorepark in the Republic of Ireland showing that newly reseeded paddocks grow 2.7 more tonnes per hectare of dry matter (DM) than old paddocks dominated by weed grasses.

At a reseeding event organised by Gortavoy Feeds, Holland Agri Contracts and Germinal in Pomeroy last week, CAFRE beef and sheep adviser Brian Hanthorn said that farmers who reseed typically get a 10-15% boost in yields, which in some cases can be up to 25%.

Where other issues – such as land drainage, phosphorus (P) and potassium (K) deficiencies and low soil pH – are also corrected at the time of reseeding, presumably the response can be even greater.

However, there is little point going to the cost of reseeding (around £250 per acre where land has been ploughed) if the underlying soil remains infertile – the weed grasses will quickly return.

Also soil pH should be at least 6. “I see a lot of soil samples with a pH of 5 to 5.5. It is crazy. N, P and K uptakes are significantly reduced. People are just throwing fertiliser away,” said Hanthorn.

As well as a yield response, reseeded grasses tend to have higher sugar levels, which should result in improved silage quality and be more responsive to fertiliser inputs.

But heavy cuts of silage can leave a newly reseeded sward more open and increase the opportunity for weed grasses to return.

“A heavy cut is hard on a reseed. New grasses tend to prefer frequent cutting,” said Hanthorn.

It was a point also made by Dr Mary McEvoy from Germinal. She advised farmers not to take a silage cut in the first year, if possible, as grazing thickens the sward, helping it to persist into the longer term.

When selecting new grasses, she encouraged farmers to make sure the varieties are tested in NI and listed on the DAERA recommended list, available from the AFBI website.

In a seed mix, she said that heading date for each variety should be similar (a range no more than 10 days in a grazing mix, or seven days in a silage mix).

Also, tetraploid grasses tend to more palatable to livestock, and produce higher yields than diploids, but have a more open growth habit, so are not as suited to more marginal land, suggested McEvoy.

Ideally a mix should contain around 30 to 35% tetraploids, and 65 to 70% diploids, but on wetter land, the proportion of tetraploids in the mix should be reduced.

In a newly established reseed this autumn, she said it is good practice to ensure a couple of light grazings to encourage plant tillering.

When deciding whether the sward is ready for grazing she advised using the “pull-test”: if the roots come away with the plant, then the grass needs longer to establish.