15 April 2023

Tips on establishing white clover in grassland swards

The benefit of clover in grassland has previously been reported through increased animal performance, increased herbage production and potential saving in chemical nitrogen (N) fertiliser, as well improved N use efficiency and N farm gate surplus.

If Irish farms are to successfully establish clover as part of their grazing system, it will need to be undertaken over a period of time (3 to 4 years) and take a combination of methods. Michael Egan, Teagasc Grassland Researcher Officer, Teagasc Moorepark, outlines the steps in establishing white clover in this article.

Soil fertility is a crucial factor in establishing and maintaining adequate clover (average 22%) on farm. Clover requires fertile soils for establishment and persistence, with a soil pH 6.5, and > index 3 for P and K. Rhizobia bacteria that fix N in association with clover are more productive in soils with a pH of 6.5.

The phosphorus content of the soil is also important when establishing a clover sward. White clover seeds are very small and clover seedlings tend to be relatively fragile. Seedling vigour is favoured by having plenty of P in the vicinity of the establishing seedling. It is usually recommended that clover seed is sown with a fertiliser that contains P compound, as this will favour establishment.

When selecting clover cultivars to sown, use the DAFM Recommended List. Small and medium-leaved cultivars are best suited to intensive grazing systems, with large leaf clovers more suited to silage-based systems. Clover should be sown ideally when soils are warm and moist – ideally in late April / May. Sowing in the autumn can reduce chances of a successful establishment, as soil temperatures are on the decline so it is more difficult for clover to compete with the grass. White clover can be established on your farm using two methods: 1) direct reseeding; 2) over-sowing.

1. Direct reseeding – key steps involved in a full reseed

Aim to reseed as early in the year as possible (April, May, June), when soil temperatures are high and increasing, and there is adequate opportunity for weed control.
Soil sample for P, K and pH.
Spray off the old pasture with a minimum of 5L/ha of glyphosate; allow a minimum of 7 to 10 days after spraying before cultivating.
Prepare a fine, firm seedbed.
Use grass and white clover varieties from the Irish Recommended List.
Cattle sowing rate – 28-30kg/ha of grass plus 3.5-5.0kg of medium leaved clover.
Sheep sowing rate – 25-28kg/ha of grass plus 5.0-6.0kg of small leaved clover.
Avoid sowing white clover seed too deep – sowing depth approx. 10mm.
Apply 40kg N/ha (30 units N/acre) at reseeding.
Apply P and K fertiliser as required.
Roll well to ensure good contact between the seed and the soil.
Apply 25kg N/ha (20 units N/acre) 4-6 weeks post sowing.

2. Over-sowing

Over-sowing is a simple and low cost method of introducing clover onto your farm. Success is very much dependent on weather conditions around sowing and post sowing grazing management; therefore there is a certain amount of risk associated with this approach and it should be undertaken in the early part of the year (early April to late May).

Key steps involved with over-sowing white clover:

Do not over-sow old ‘butty’ swards with a low content of perennial ryegrass – white clover will not establish well in these.
Control weeds before over-sowing white clover as weed control options afterwards are more limited. Some herbicides have a residue of up to four months – always check the residual time on the label of the product or seek advice on a suitable weed control product.
Take a representative soil sample for P, K and pH analysis and correct soil fertility prior to over-sowing. Optimum soil fertility when over-sowing will help increase the chances of success.
White clover seed can be broadcast onto the sward or stitched in using a suitable machine.
If broadcasting with a fertiliser spreader: Mix clover seed with 0:7:30 fertiliser and only add white clover to the spreader when you are in the field to avoid white clover settling at the base of the spreader, with a max of 1ha at a time.
Stitching in white clover seed with a drill/harrow ensures better seed to soil contact. Stitching must be used for oversowing sheep grazed swards due to the denser nature of swards.
Over-sow directly after grazing (≤ 4cm post-grazing sward height) or after cutting the paddock for surplus bales – ideally only over-sow three to four paddocks at a time.
Sow at a rate of 5.0-6.0kg of white clover seed/ha.
Soil contact post over-sowing is one of the most crucial factors affecting germination.
Reduce N fertiliser post over-sowing to 15kg N/ha per month (12 units N/acre) for two months.

White clover establishment blueprint

A targeted multi-year approach should be used in establishing a white clover system – a combination of reseeding and over-sowing

Reseed approx. 10% per year.
Over sow approx. 15% per year.

Year 1 – reseed 10% and over sow 15% = 25%;
Yearr 2- reseed 10% and over sow 15% = 25% (50%);
Year 3 – reseed 10% and over sow 15% = 25% (75%);
Year 4 – reseed 10% and over sow 15% = 25% (100%) – on going there after.

Paddocks for a full reseed should be identified as early as possible in the process to avoid over-sowing clover on these (poor performing, age of sward, weed content etc.).

Select paddocks for over sowing to give the best chance of establishment:

Optimal soil fertility (index 3 or > for P and K, soil pH 6.5);
High perennial ryegrass content;
Open/low density swards – dense swards prevent light getting to new clover plant, hindering establishment;
Low weed content.

Any paddocks that are not suited for over-sowing in the first year (but not ear marked for reseeding) should have any issues corrected and over sown the following year. These include the correction of soil fertility issues and the spraying of any weeds well in advance of over-sowing to allow for residual time frame.

Management of grass-clover swards after over-sowing

Poor establishment results have been obtained where grass gets too strong after over-sowing. This is the single biggest reason for failure that lies within the farmer’s control. Swards need to be grazed tight after over-sowing clover, for the first three grazings post sowing, both for direct reseeding and over-sowing (<1,200kg DM and grazing to ≤ 4 cm). By doing this, it allows light to penetrate to the base of the sward which is essential for clover establishment.

Weed control is an essential element in both direct reseeding and over-sowing. Weeds in new reseeds are best controlled when grass is at the 2-3 leaf stage and clover is at the first trifoliate stage, with a clover-safe herbicide. When over-sowing clover into existing grass swards, it may be better to control more established weeds before over-sowing white clover into the sward.

For more information on white clover and the benefits it can bring to your farming system, click here.