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Managing grazed grass

Course: Grassland management | Last Updates: 11th October 2015

There is more to grazing than opening the gate and letting animals in to eat the grass – indeed this would be an extremely inefficient use of a potentially high value, low-cost feed.

In the UK we can grow grass relatively easily – it is our natural and national crop. But yields are rarely optimised and utilisation is generally poor, particularly when grazed. Yet the results of costings consistently show producers who rely heavily on forage, including grazed grass, use less bought-in feed and make more money.


How can grass growth be optimised?

Grass is not an easy crop to manage. In the UK, farmers tend to grow mixtures of three or more different varieties of ryegrass with one or two varieties of white clover, each with its own particular attributes and characteristics.

The pattern of growth is also challenging, with a surge of growth in spring followed by significant tailing off in summer. There is a smaller growth flush in autumn, before falling temperatures close grass growth down for winter.

Well managed, correctly fertilised grassland containing modern varieties can produce 10-11t of dry matter/ha when grazed. This potential can be compromised by compaction, weeds or limited crop nutrition.

How do I know if soil nutrient supply is limiting?

If there is shortage of any key nutrient, pasture performance will be disappointing. A soil test will indicate what the nutrient levels are. Any shortfalls can be made up by applying manure, slurry or artificial fertilisers.

Soils should be sampled every four years, ideally six months after the last application of manure or fertiliser.

Fields that are under-performing, those due to be re-seeded, or any which receive a lot of muck and slurry should be targeted for testing first. Aim to maintain phosphorus, potassium and magnesium at Index 2.

Nitrogen fertiliser should be applied to stimulate early spring growth and then at regular intervals post-grazing. The amount applied should be based on crop demand taking account of nitrogen supplied by organic manures as well as any NVZ restrictions that apply. The Fertiliser Manual, DEFRA, RB209 and the nitrate and NVZ guidelines that apply in England and Wales, Scotland and Northern Ireland, provide more details.

Applying lime to soils with a pH of 6 or below is a simple, effective way to boost grassland productivity. Five tonnes of lime/ha should raise pH by 0.4 units, but bear in mind it takes between nine and 12 months for pH to stabilise. Do not apply more than 5t/ha lime in any one season, as too much can lock up essential minerals in the soil.

Is compaction a concern?

Compaction is where soil structure has been damaged to such an extent that a solid, impermeable band forms in the topsoil, preventing free movement of air, water and nutrients. Grass cannot put roots down, causing stress that reduces its response to applied nutrients; performance will definitely be compromised, and many of these nutrients could be lost through surface run-off.

On grazing pasture, poaching by stock trampling about in wet conditions is the most common cause of compaction.

Cattle can leave 10cm deep depressions in the surface, which fill up with water that never drains away creating anaerobic conditions that kill grass. When hollows do dry out, the resulting bare patches provide ideal germination sites for weeds.

Sheep damage grass by pounding the ground as a flock, producing a compacted layer over a wide area, 2-6cm below the surface.

Compaction can be reduced by using multiple entry/exit points to fields, using dedicated tracks to move cattle, only out-wintering on free-draining land, and using mobile rather than fixed feeding troughs.

How do I control weeds?

Where weeds grow, grass doesn’t. At any level of infestation, weeds such as docks and thistles pose a significant threat to grassland productivity as they compete directly for light, water, nutrients and space. SAC trials showed that a 10% infestation of docks causes 10% yield loss. Even situations where current weed numbers are low can quickly turn into a more serious problem.

The main challenge for dairy farmers are 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 soil. The seeds remain viable for up to 80 years.

Long-term control of docks can be achieved by using a translocated herbicide which works down into the long tap root killing the whole plant. Treat when docks are young, all at the same growth stage, actively growing and not frost or drought stressed.

Creeping and spear thistles are the main concern for beef and sheep farmers. Once established, the root mass of creeping thistle is greater than the plant above ground. Spear thistle is a biennial that grows from seed and often goes unnoticed in its first year. In year two, the plant can grow over one metre in diameter before flowering, posing a significant competitive threat to grass.

Long-term control is best achieved using a translocated herbicide dedicated to thistle control, which will kill 80% of a population in one application. Thistles often emerge at different times in the growing season. Topping two to three weeks before spraying will even up the growth stages for improved control.

Spot spraying, or using a weed wiper, helps tackle small areas of weeds or where a targeted approach is required. When using herbicides always read the label.

The most effective grassland herbicides will affect any clover in the sward, so where weed presence becomes a limiting factor, plan to re-introduce clover later in the season.

What grasses should I grow?

Consider re-seeding if less than 50% sown species remain in the pasture. Always select varieties on the Recommended Grass and Clover List as these have been rigorously and independently tested.

For grazing leys consider intermediate and late-heading perennial ryegrasses, timothy – good for early season grazing – and white clover, which suits mid-season production and has nitrogen-fixing abilities. If ploughing is not an option, introduce seed by over-sowing and manage the grass to maintain a tight, dense sward.

Overstocking in wet conditions and poor management of feeding and entry/exit points will result in poaching and compaction, that will hamper grass growth.

How should I manage my grazing?

The key to generating high production off grazed grass is to exert adequate grazing pressure, ie take individual grass plants down low enough to stimulate high quality leafy re-growth for the next time an animal comes to eat it.

Lax grazing encourages rejection of areas which wastes grass, and allows plants to head and set seed. This instigates a rapid fall in quality.

Measuring swards is useful for achieving efficient grazing. For beef and sheep farmers a sward stick can help decide when animals should go into and come out of a field. For example, at turnout spring-calving suckler cows, grazed on a rotational system, should go into a paddock when the sward is 10-14cm and come out once it is grazed down to 5-6cm high. Ewes and lambs should go into swards that are 8-10cm high and leave when it is grazed down to 4-5cm. Where animals are continuously grazed – the sward height should be maintained at 5-8cm for in-milk suckler cows and at 4cm for ewes with lambs at foot.

Dairy farmers who make good use of grazed grass use plate meters to measure the amount of dry matter production/ha/day, and use a feed wedge or bar chart to work out if too much or too little grass will be available in the days ahead. A general rule of thumb is: Cows go in when the grass is the height of a beer can; cows go out when it is the height of a golf ball.

Three golden rules

  • Make sure nothing limits grass growth, eg nutrient status, compaction, weeds.
  • Grow the right grasses for grazing.
  • Follow a grazing plan which produces high quality re-growth to sustain high production the next time the animals come to graze it.
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