Reducing Nitrogen Costs Using Clover - Teagasc

White clover has a number of useful attributes. Rhizobia bacteria live in nodules on the roots of white clover and have the capacity to make Nitrogen (N) available in the soil for pasture production, reports A.Boland, J. Humphreys and A. Lawless, Teagasc Moorepark and Johnstown Castle.
calendar icon 13 January 2009
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The quantity of N supplied can be as much as 120 units per acre (150 kg/ha). At present, the rising cost of oil and gas internationally is driving up the cost of fertilizer N. Furthermore under REPS-4 there are payments through a supplementary measure for establishing clover swards of up to €1,200 per farm. On many farms there is substantial potential to improve income by making more use of clover in swards.

White clover has the highest digestibility of any grassland species including perennial ryegrass. It is often difficult to maintain the digestibility of grass-only swards receiving low inputs of fertilizer N. White clover thrives under low fertilizer N input. Eight years of work at the dairy research farm in Solohead, County Tipperary has shown that clover-based swards receiving fertilizer N input of 72 units of fertilizer N per acre (90kg/ha) per year are able to carry up to 0.9 cows per acre (2.2 cows per ha) producing 1,200 gallons per acre, which is the equivalent of 14,000 litres of milk per ha.

This milk is being produced with less than half a ton of concentrate per cow. National survey data has shown that average stocking rates on dairy farms is a little under 0.8 cows per acre (1.9 cows per ha) producing 800 gallons per acre equivalent to 9,000 litres of milk per ha. National average fertilizer N input to dairy farms is 140 units per acre(175kg/ha). In other words, one and a half times national average milk production is being produced with clover-based swards at Solohead receiving half the national average fertilizer N input. Clover is best suited to ground that can be grazed over a long grazing season and tight grazing is important.

On-farm research has shown that where grassland has been converted over to clover-based swards on fairly intensively stocked dairy farms fertilizer N inputs have been halved while maintaining or increasing milk output. If you do not need to apply for derogation on the basis of stocking rate but you are buying a substantial amount of fertilizer N each year, you should be growing clover on at least part of your farm. This represents a large proportion of dairy and more intensive drystock farms in Ireland.

Growth Habits of White Clover

One of the big differences between ryegrasses and white clover is the effect that soil temperature has on growth. Substantial amounts of grass growth commence when temperatures at 10 cm depth in the soil reach around 6°C. The growth of clover does not really take off until soil temperatures reach around 9°C. This means that in spring there are generally no substantial amounts of grass growth until some stage during March, whereas the accumulation of substantial amounts of clover does not begin until late-April. Clover makes up less than 5 per cent of the total herbage in Spring and does not effect Spring growth. If clover based swards get the same amount of fertilizer N in Spring as a grass only sward there will be no difference in Spring growth. During the course of the year the clover content of swards oscillates between 5 to 10 per cent in March/April to 40 to 50 per cent in July/August.

White clover is a relatively shallow rooted species and therefore does well on fertile soils that maintain a relatively high soil moisture status during the summer months. It does not tolerate being water-logged very well and clover does not do well on very wet soils. Also poaching damage on wet soils can destroy clover stolons, which causes the rapid loss of clover from the sward. The Rhizobia bacteria associated with clover perform best in soils with a high lime status and therefore clover does not grow very well in acid and, in particular, peat soils.

Clover can be expected to perform best and to make the greatest contribution to pasture productivity on medium to free-draining loam soils which represent the majority of soils in Ireland. Grassland on these soils can be grazed over a long grazing season because they are less prone to poaching damage. Clover also makes a very valuable contribution to grassland productivity on heavier and wetter moderately drained soils similar to those found at Solohead. The clover swards at Solohead have preformed remarkably well over the last eight years. Clover is not perennial or permanent in grassland in the same sense as perennial ryegrass. Once introduced into a sward it generally has a life-span of three to eight years depending on factors such as the cultivar sown, soil conditions and grassland management. The loss of clover is a slow decline, unless the loss of clover is accelerated due to bad management, particularly inappropriate herbicide use, poaching damage or high covers over the winter

Increasing the White Clover content of Swards

White clover seed is often included in grass seed mixtures to no effect. The clover is usually killed off by one of a number of reasons such as the clover seed being buried too deep, sown too late in the year or inappropriate herbicide use following establishment. There is no point in sowing white clover in the seed mixture unless you are prepared to mind the clover and make it work for you. This means cutting back on fertilizer N from April on and allowing the clover to supply N via biological fixation. White Clover Cultivars The Irish recommended list is limited to five white clover cultivars (DAFF, 2008). The Northern Ireland recommended list is more comprehensive (DARD, 2006).

Clover cultivars are divided into three categories based on leaf size. In general, small-leaf cultivars are lower yielding but more persistent than large-leaf cultivars and vice versa. Medium-leaf cultivars, in general, are intermediate in terms of both yield and persistency. However, experience has shown that white clover cultivars, regardless of leaf size, do not persist indefinitely in swards even with good management. In general, the productivity of the clover will tend to decline after a period of around five years. Small-leaf cultivars are generally recommended for sheep because they are more tolerant of grazing by sheep. Cattle and dairy cows are far less selective grazers than sheep and they consume clover in proportion to its content in the sward.

They also graze less close to the ground than sheep. Therefore small-leaf cultivars are more at risk of being shaded out of a sward grazed by cattle than large-leaf cultivars. Large-leaf cultivars grow more aggressively and are better able to compete in swards grazed by cattle, albeit over a shorter lifetime. The problem of long-term persistency can be dealt with by over-sowing on a regular basis. If reseeding with the intention of promoting white clover in swards for beef and dairy cattle, a 50:50 mixture of large-leaf and medium-leaf cultivars is recommended, although the small-leaf cultivar Crusader has a lot to offer in terms of yield and persistence and has been used at Solohead for the last four years.

The recommended sowing rate of clover is 1.0 to 2.0 kg seed per acre (2.5 to 5.0 kg/ha). The lower rate is recommended following crops such as maize etc. The higher rate is recommended in grass-to-grass reseeds. The clover seed is sown with around 10 to 12 kg perennial ryegrass seed per acre (25 to 30 kg/ha).For REPS the minimum rate is 5 kg/ha

An important part of reseeding is deciding on which perennial ryegrasses to include in the seed mixture. Perennial ryegrasses show a progression of heading dates from around mid- May through to mid-June. Grass seed mixtures containing a high proportion of late-heading diploids are recommended for sowing with white clover. Clover fits in well on farms where it is possible to graze over a long grazing season, where cows are turned out to grass as they calve in February and remain at grass until the end of November or early December. In the past it has been suggested that tetraploid cultivars are more suited to white clover than diploids because tetraploids are more open and hence are less competitive with the clover. However, using tetraploids solely to facilitate the clover can result in very open swards that make them more prone to damage and lowers sward persistency. Ideally, the grass-clover mixture should contain between three and six different ryegrass cultivars and the majority of these cultivars should be diploids. Tetraploids can be included as a minor component.

Direct Reseeding

The least cost-effective way of introducing clover into swards is by direct reseeding, although it is necessary in many circumstances where the sward is run down and infested with undesirable grasses and weeds.There are a number of different ways of directly reseeding swards. This can involve the conventional approach where the old sward is burned off using a glyphosate-type product (Roundup, Gallup, Touchdown etc.) the old sward is grazed or cut off, the old sod may be ploughed down, or cultivated using a heavy disc or power harrow, lime is applied in line with recommendations, the ground is tilled ideally to create a relatively fine firm seedbed and sowing takes place using a seed drill or by broadcasting using a fertilizer spreader or seed barrow. There are many different variations of this approach.

However, it is important to remember that clover seed is very small. Ideally, it should be broadcast onto a fine firm seedbed. A safe approach is to cultivate and roll before dropping the seed on the soil surface. There is no advantage in burying the seed. Grass seed is mostly sold in mixtures that contain a small amount of clover; usually around 0.5 kg per acre (1.25 kg/ha) which is less than recommended for grass-to-grass reseeds outlined above. One approach is to sow the grass seed mixture using a conventional drill and to subsequently broadcast additional clover seed onto the soil surface before rolling in. The ideal time for sowing grass-clover swards is during April and May or during June and July. One of the biggest reasons for incurring the expense of reseeding is the deterioration of sward botanical composition due to loss of perennial ryegrass and ingress of troublesome weeds. Under normal circumstances, the best post-emergence sprays to use for reseeded swards are products such as Legumex, Nintex, Undersown, Acumen, Alistell etc.


Over-sowing was developed to maintain the long-term productivity of swards at Solohead. It is a necessary and integral component of the system. It is a low-cost method for introducing clover into permanent grassland without the need for cultivation. The strategy involves oversowing 20 per cent of existing clover swards each year on a five-year rotation.Over-sowing involves broad-casting clover seed over the sward after first-cut silage using a slug pellet applicator or by mixing it with a small amount of fertilizer such as 0:7:30 or Gran lime and spreading it with a fertilizer spreader. The total cost of this approach is around €25 per acre or, if spread over five years, around €5 per acre per year. This leaves a net benefit of up to €75 per acre per year. This approach works well for open swards when soil moisture is adequate and the sward is kept well grazed out for the remainder of the year. It is not suited to old butty swards or swards that are badly infested with weeds. In cases such as these it is probably better to burn off the sward with a glyphosate-type product (Roundup etc.) and carryout a full reseed.

Poor results have occurred when dry weather and soil conditions follow over-sowing.. The importance of tight grazing after over-sowing cannot be over-emphasised. The single most important recommendation, which can greatly improve the success of over-sowing is tight grazing for the remainder of the year.

December 2008

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