Archive: The two A's of reseeding

Published 1 August 14

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What are the two A’s of reseeding?

April and August are the two A’s of reseeding, and it’s all to do with timeliness and giving the plant every chance of success, says DairyCo technical extension officer Piers Badnell, as he discusses why and when to reseed.

Great Britain has a temperate climate which is absolutely ideal for growing and utilising grass and other forages. It is one of the economic advantages we have over our competitors. So, the aim should be to make use of this profit driver, irrespective of system or milk production.

Reseeding has clear economic benefits but does incur costs, and can put valuable grazing or silage ground out of production for a while. Ensuring you have an accurate picture of the fields which are underperforming, in terms of grass production, is crucial. The only way to do this is by measuring what is harvested in each field, either by the cows or by what goes in the silage pit. See Table 1, for an estimate on the density of a clamp and Table 2 for silage requirements of a dairy herd.

Table 1: Silage density guide (kg/m3)


Silage density guide

Source: DairyCo Feeding+

Table 2: Dairy herd silage requirements

Dairy herd silage requirements

Source: DairyCo Feeding+

If a field is underperforming is there something you can do about it without reseeding? You can start by checking if the pH or P&K levels are causing problems.  Also take a look at the soil health and make sure there is enough worm activity and plenty of worm casts. With recent dry weather, most worms will be inactive in dry soils, monitoring these is best done once we have had rain to moisten soils.  

Also, look for signs of healthy flora and fauna. Do dung pats disappear quickly as the soil life devours them, or do they remain for protracted periods of time?

When you have investigated all causes for a reduction in pasture performance, reseeding may be the solution. If thinking about reseeding have a look at the DairyCo new online tool which helps you select varieties suited for your system

Analysis and research from around the world, including DairyCo’s Milkbench+, show cost of production as the biggest driver of profit in dairy farming businesses (see Graph 1)

 Graph 1: Cost of production versus net margin

Cost of production

Source: Milkbench+ data

Reducing costs of production by increasing forage quality and quantity can make a huge difference to the bottom line. One way to help you reach this goal is by having a structured reseeding policy.

From the first day a new ley is established, there is a continual decline in the percentage of sown species as weed grasses and broad-leaved weeds move into the sward. Even under good management, it is likely that after six years a medium-term ley, based around intermediate perennials, could contain less than 60% ryegrass.

The impact of a decline in ryegrass content is a drop in yield and quality. A young perennial ryegrass ley should yield well in excess of 10t DM/ha/year in the first few years of its life and, if managed correctly, should maintain quality above 12ME and 20% crude protein.

Much of this yield and quality is driven by a high response to applied nutrients; for every kilogram of nitrogen applied (from bag/slurry or clover) you could see a 25kg DM response from a young vigorous sward at peak growing time. With this level of response, grass is cheap forage. See Table 3.

Table 3: An example of performance and costs of production of poor and good grass leys

example of performance and cost of production

For a full explanation of the costs associated with reseeding watch the DairyCo video Do you need to reseed?

The difference in terms of silage at 15kg DM/head is £75 for 100 cows per day

(Difference between the cost of silage from an old and a new ley = 5p/kg DN x 15 = 75p x 100 = £75)

The difference in terms of grazing at 15kg DM/head is £52.50 for 100 cows per day

(Difference between the cost of grazing an old and a new ley =3.5p/kg DM x15 = 52.5p x 100 =£52.50)

For arguments sake, if a full reseed costs £400/ha, in terms of silage, we have paid for one hectare to be reseeded in six days, and eight for grazing.

Table 4 below shows the value of reseeds. Another example comes from a DairyCo discussion group in July 2013. In a hot dry period, on a light land situation, old leys were growing at 5kg DM/ha/day and the new spring reseed grew at 45kg DM/ha/day.

Table 4: The value of new reseeds 

The value of new reseeds

Old leys may struggle to produce 6t DM/ha/year and, quite often, because of poor response rates, they may need greater nutrient inputs to achieve even that. The net result is less, poorer quality and more expensive grass.

There are two key times to look at reseeding in Britain, April and August, in order to ensure the best results. The decision about when to reseed needs to be made on a farm by farm, or even field by field, basis.



  • The seedbed has time to settle over the winter and will not be damaged by carrying stock in the spring
  • Good seed take if reseeding is done in time, and if the autumn has good growing conditions
  • Less impact on grazing stock as no land is taken out of production during peak growth times
  • Good weed control, if done early and you can still hit weeds when they are actively growing.



  • If you reseed too late, you may find weed growth stronger and outcompeting grass. You may spend a lot of money on reseeding only to have open swards colonised by meadow grass and chickweed.
  • If autumn conditions come early, reseeds will suffer.
  • You run the risk of poor seed take at this time of year.
  • The risk of weed ingression
  • Grass plants heads in the first season, and impact quality.
  • The later into August and September that you reseed the harder it is to get clover established.
  • With the current dry spell in some areas, soil moisture may be too low at present to support germination, delaying establishment.

Spring reseeding (April)


  • No heading in first season for perennial ryegrass
  • Opportunity to hit weeds before they get established. As grass is actively growing, it can better outcompete weed species
  • Minimal impact on grazing. Reseeding at the time of maximum grass growth may well mean you are not short of grazing elsewhere on the farm, so you can afford to have those areas out of production
  • Timeliness – an early autumn is not creeping up on you putting the reseed in jeopardy
  • The probability of a good take of seeds in ideal spring growing conditions
  • If spring reseeding conditions turn out to be disastrous, you have the opportunity of trying again in autumn and don’t have to wait another year.


  • Losing peak growth. By taking a field out at this time, you are losing grass production at its highest peak  
  • There is a possibility the soil in spring reseeded areas does not have time to settle before it needs to carry stock
  • If the field hasn’t been ploughed before reseeding, there may be competition from old grasses
  • If you don’t get good seed take, weeds have ideal growing conditions and can come to dominate sward.

Quality grass, be it in the form of grazing or silage, drives profit. To maximise this, we need productive leys fit to do the job. Assess which swards are performing and which are not and make a plan. It is reasonable to expect to reseed 10%-20% of the farm per year to achieve the farm’s potential and maximise our climatic advantage.

For further information on reseeding, see the DairyCo and British Grassland Society Factsheets.