Remove weeds to boost production

UK - February 24, 2011

An increase of just 1p/kg DM in concentrate cost will add £80/year to the feed bill for each cow in the herd. Figures like these are stimulating a determination to produce as much milk from grazed and conserved grass this season, says GrassMaster Charlie Morgan.

Grass is the cheapest feed on the farm – it really is. After all it is our national crop and our maritime climate is perfect for growing decent tonnages of high quality, nutritious ruminant feed.

However, it is not an easy crop to manage. In the UK mixtures of three or more varieties of ryegrass are grown with one or two varieties of clover, each with its own attributes and characteristics.

The pattern of growth is also challenging, with the growth surge in spring followed by a tailing off in summer with perhaps a small autumn flush.

Well managed, correctly fertilised grass containing modern varieties can produce up to 14t DM/ha. But this potential can be compromised by limited crop nutrition, soil compaction and weeds.

Grow grass not weeds
Where weeds grow, grass doesn’t. At any level of infestation, weeds such as docks, thistles and nettles pose a significant threat to grassland productivity as they compete directly for light, water, nutrients and space. Even situations where current weed numbers are low can quickly turn into a much bigger problem, given favourable conditions for weed ingress.

SAC trials showed that a 10% infestation of docks causes 10% yield loss of grass.  Further research carried out by IGER compared spraying plots with a 20% dock content, with Pastor, a broad-spectrum translocated herbicide from Dow AgroSciences, with plots that had no docks. The results showed that getting rid of the weeds increased grass yields annually by an average of 2.6t DM/ha/year, over three years. 

At 2011 costs of around 5p/kg DM for grazed grass, and 10p/kg/DM for silage (depending on the system), this extra feed could be worth £130/ha or £260/ha respectively.

The main weed challenge for dairy farmers is docks which thrive in moist, fertile soils where moderate to high levels of nitrogen have been applied. Broad-leaved and curled docks are perennial weeds that grow from seed (up to 70,000/plant produced a year) or root sections in the ground. The seeds can remain viable for up to 80 years.

As well as reducing grass yield, docks only have 60% of the nutrient value of grass, so will pull down the overall feed value of silage in the clamp.

Stop weeds establishing
The best defence against weeds is to stop seedlings from gaining a foothold in the first place. Over or under-grazed, gappy leys, and areas that have badly poached in wet weather, will present bare patches of soil that are ideal germinating stations for weeds. A dense, well-tillered sward growing on well-structured, non-compacted soil will reduce the opportunities

Make sure that the nutrient status of the soil is correct for growing grass. Out of all the soil samples tested last year at one UK laboratory, 44% from grassland were above the ideal index 2 for phosphate; 29% were above the target index 2 for potash. Applying more fertiliser in these instances is clearly a waste of money, but will also encourage weeds like docks which love nutrient-rich soils, as will the over-application of manures.

Where weeds have established, there is a problem to fix. Getting the topper out when the sight of weeds gets too much to bear is an option – but is really a cosmetic solution that does not get down to the root of the problem. The weeds are still alive in the soil and retain the resources to re-emerge.

A longer term solution is to use a targeted translocated herbicide, such as Doxstar for docks or Thistlex for thistles, which move down into the roots and affect a more complete kill. An added advantage of this type of herbicide is it will not hold back grass growth in any way – this was also proven in the IGER trials.

As with all chemical products, producers should read the product label and follow the manufacturer’s recommendations carefully in terms of rates and water volumes, as these have been carefully tested and calculated for optimum control.

For best results sprays should be applied when the weeds are healthy, not ravished by disease or insects, actively growing – not stressed by frost or drought - and at the perfect growth stage; docks at rosette stage up to 25 cm high or across, and thistles at rosette stage up to 20cm high or across. Silage fields should be treated at least three to four weeks before cutting to allow the herbicides to work to its full extent before defoliation.

Controlling weeds in clover

Achieving 30% clover in the dry matter yield of a field over a season saves on fertiliser costs, increases animal feed intakes, provides late season cover, and improves soil structure.

However, being broad-leaved it is susceptible to herbicides that knock out any broad-leaved plants. And with the withdrawal of Alistell, a leading ‘clover-safe’ product in the UK market – farmers will need to find other solutions to the clover or weeds dilemma.

Which is more important?

If weeds are a problem and/or an embarrassment, farmers have to decide whether maintaining the clover or controlling the weeds is more important.
Where infestations are small or concentrated in particular areas, applying herbicides as a spot treatment or through a weed wiper may kill the weeds, leaving the clover relatively unscathed.

But when weeds account for 20% or more of the field, there may not be enough saving in bagged nitrogen fertiliser to be worth saving the clover.
In these situations be brave, take the clover out using a good translocated herbicide and reintroduce it later.

Over-sowing – follow a plan
Encouraging clover to germinate and grow away in established grass is tricky. The key is allowing the clover seed to fall on bare, moist soil so it can germinate and establish. This can only be done with a combination of tight grazing and grass harrowing or direct drilling. 

On cutting ground:

• Check and correct pH, P and K in the previous autumn
• Spray with an appropriate translocated herbicide at least three or four weeks before cutting
• After cutting, stock with dry ewes or youngstock. Do not apply N
• Grass harrow – two to six passes to create 25% bare ground
• Broadcast clover seed and grass harrow
• Roll and/or stock with dry ewes/youngstock for six to ten days
• Remove stock and rest for three to four weeks, then mob stock graze and repeat until winter.

On grazing ground only:

• Check and correct pH, P and K in the previous autumn
• Spray with an appropriate translocated herbicide in the autumn
• Grass harrow dense swards previous spring and autumn
• Graze tight March/April (for April sowing), but sowing preferable in July
• Grass harrow – two to six passes to create 25% bare ground
• Broadcast clover seed and grass harrow
• Roll and/or stock with dry ewes/youngstock for six to ten days
• Remove stock and rest for three to four weeks, then mob stock graze and repeat until clover has established.