Seeds pricey, so adjust rate to site, variety and pests
By Jonathan Riley
HIGH reseeding costs mean seed rates must be sufficient to ensure a dense sward is established and failure is avoided.
ADAS grassland agronomist Bob Granfield says reseeding costs of £200/ha (£80/acre) make the price of failure too high for producers to risk cutting seed rates.
He recommends an average seed rate of 32kg/ha using a ryegrass mix. This is provided producers are drilling in ideal sowing conditions during July to August on an ideal site – one where there is a fine tilth, warm weather and the promise of rain.
But, he says that where conditions are less than ideal, and when different grass varieties are used, seed rates must be adjusted upwards.
"A 32kg/ha rate on an ideal site is adequate for a mix containing diploid perennial seed. But more modern varieties have larger tetraploid and hybrid seeds, and rates must be raised to about 35kg/ha to ensure there are sufficient seed numbers," he says.
Specialist Italian ryegrass mixes, which have particularly large seed could need rates of 37kg/ha, he adds.
Aberdeen-based SAC grassland agronomist David Younie says that producers in colder locations should drill as early as possible, and increase seed rates still further.
He recommends swards should have 6000 tillers/sq m – which is about 450-500 plants/sq m- in the first spring after sowing.
"Weather must be taken into account. Colder conditions increase winter kill and slow down germination, leaving the seed vulnerable to pests and diseases for longer," explains Dr Younie.
He says seed rates in the north of Scotland should be about 10% higher than the average recommended for sites further south.
But for producers who aim to cut costs, Bob Clements, head of ecology and agronomy at the Institute of Grassland and Environmental Research, North Wyke, Devon says that dense, productive grass swards can be achieved with seed rates of only 20kg/ha, cutting £40/ha (£16/acre) from reseeding bills.
He believes some producers are too cautious about using low seed rates, fearing that reseeds will fail.
"For a dense, productive sward only 450 healthy plants/sq m are needed. And as almost 100% of grass seeds germinate, theoretically about 500 plants/sq m should be enough. This would require a seed rate of only 5-10kg/ha – well below the seed rates of up to 40kg/ha sometimes used.
"But, in practice, drought stress, poor seed-beds or disease result in losses of about 20%. The main losses occur because of fungal disease and pests.
"If these are controlled, even with severe plant losses taken into account, seed rates of only 20kg/ha should be sufficient for a good sward."
But regardless of seed rate, establishing a successful sward requires good pest control, says Dr Clements
Frit fly is the main pest, and autumn sowings are the most susceptible to damage from the flys larvae.
"The larvae can cause losses of 50%, but rather than increase seed rate it is more cost-effective to use the insecticide chlorpyrifos at half the recommended rate," advises Dr Clements.
He suggests adequate control is achieved using chlorpyrifos at 0.7 litres/ha. At this rate beetles and wasps – natural predators of the frit fly – survive and help to reduce larvae numbers.
"With leather jackets it may be unnecessary to apply full control measures, and so further cost cuts could be made. Monitoring numbers, particularly in a wet August, will allow the risk level to be established. Control should be geared towards numbers present."
• Consider conditions.
• Seed size influence.
• Control pests.